fatigue behavior studies of niti shape memory alloys using bulge test_swapnil dhakate_10311017

7/27/2019 Fatigue Behavior Studies of Niti Shape Memory Alloys Using Bulge Test_swapnil Dhakate_10311017

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FATIGUE BEHAVIOR STUDIES OF

Ni-Ti SHAPE MEMORY ALLOYS USING BULGE TEST

Masters Degree Dissertation

Submitted in Partial Fulfilment of Requirements for the Degree of

Master of Technology

In

Metallurgical Engineering and Materials Science

by

Swapnil N Dhakate

Roll No: 10311017

Under the supervision of

Prof. Prita Pant

Dept. of Metallurgical Engineering and Materials Science

Indian Institute of Technology Bombay

June, 2012


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Acknowledgement

Foremost, I would like to express my sincere gratitude to my guide, Prof. Prita Pant, for

the opportunity provided to work on such an interesting topic. It has been a memorable learning

experience because of the freedom she allowed for doing work.

I would also like to acknowledge Dr. Madangopal K. for his extended support and

guidance. I would also like to thank OIM National Facility and Metal Forming Lab for allowing

use of their facilities.

I extend special thanks to Nishant Shelkar for helping me with all electronics related

queries. I am very grateful to Jaiveer Singh for his timely help with the EBSD and SEM

instruments. A very special thanks to my peers and fellow MEMS M.Tech students, andespecially my group members for their friendship, particularly, the distractions they provided. I

would also like to thank all those who directly or indirectly helped me during duration of my

project and made possible whatever I was able to do.

Yours Truly

Swapnil Dhakate


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Abstract

An experimental investigation in martensitic transformation behaviour during fatigue of

NiTi shape memory alloys using bulge test was undertaken. In order to carry out the fatigue, we

have designed a set-up that allows us to carry out Bulge tests on miniature samples so that a

marked region of the sample can be studied after repeated loading. The set-up was automated so

that the precise and accurate strains can be provided and large number of fatigue cycles can be

carried out. This set-up was then used to bring out the microstructural changes that occur during

fatigue in NiTiCuCr. EBSD results show that material can withstand large number of cycles (50)

at lower strain of ~10% whereas at higher strain of ~25% the material starts deteriorating at very

low cylces (5). The martensite retained after 50 cycles at 10% and 25% strains is 3% and 37%

respectively, which is large variation with strain. At higher strain, the orientation (001) shows

higher retained martensite as compared to (111) and (101) orientations. XRD showed that after

same number of cycles of fatigue on NiTi and NiTiCuCr, there was not much difference in the

amount of martensite formed but the orientations at which the martensites appear were different.


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Table of Contents

Abstract.....i

Table of Contents ..ii

List of Figures..iii

List of Tables..v

1. Introduction12. Literature survey

2.1. Introduction to SMA..32.2. Martensitic transformation..52.3. Pseudoelasticity and Superelasticity....92.4. Mechanical fatigue in NiTi and NiTiCu alloys.112.5. Factors affecting stress-strain response..132.6. Microstructure during fatigue.172.7. Characterization techniques and testing methods

2.7.1.DSC..192.7.2.Bulge Testing.192.7.3.EBSD212.7.4.Aramis21

3. Experimental Work3.1. Sample Fabrication..233.2. Bulge Fatigue Testing 25

3.2.1.Device Design253.2.2.Test Procedure.263.2.3.Automatization of set-up.263.2.4.Microprocessor coding..30

4. Bulge Fatigue Results4.1. Microstructual Correlation using EBSD.334.2. XRD Analysis.394.3. Strain Measurements Using GOM41

5. Conclusions and Future Work.446. Bilblography.467. Appendix.50


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List of figures

Fig 2.1 Crystal structure of (a) cubic austenite phase (b) monoclinic martensite phase..3

Fig 2.2 Schematic illustration of shape memory effect4

Fig 2.3 Bain Distortion.5

Fig 2.4 Lattice invariant deformations (b) Slip (c) Twin6

Fig.2.5 Schematic representation of the interface, habit plane7

Fig 2.6 Crystallographic steps for the B2 to Bl9 transformation in NiTi...7

Fig 2.7 Phase Transformation paths of NiTi alloys.8

Fig 2.8 (a) 3 possible lattice correspondences in the reverse transformation. 9

Fig.2.9 Different stress-strain path followed by superelasticity and pseudoelasticity...10

Fig 2.10 Stress-strain curves (hysteresis curves) during cyclic loading with number of cycles11

Fig 2.11 Stress-strain curve for same no. of cycles of loading but at different stress amplitudes.12

Fig 2.12 Accumulated plastic strain with increasing no. of cycles for different stresses..13

Fig 2.13 TTT diagram describing aging effect on alloy Ni52Ti48..16

Fig 2.14 Quasi-static Stress-Strain Curve for NiTi....17

Fig 2.15 Accumulation of localized plastic deformation within the grains upon cyclic load18

Fig 2.16 Microscope images at strain levels 0%, 2%, 4%, 10% showing full transformation...18

Fig 2.17 A typical DSC scan on a sample of Nitinol 19

Fig 2.18 Schematic representation of the bulge occurring during bulge testing.. 20

Fig 3.1 Assembled, dissembled, cross-sectional view of the bulge testing device25

Fig 3.2 Bulge Fatigue testing arrangement components27

Fig 3.3 Circuit diagram for the microcontroller circuit for controlling the stepper motor28


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Fig 3.4 Control pins on the circuit board29

Fig 3.5 Screenshot of the Wiz-C while setting up pin controls of the microcontroller..30

Fig 4.1 IPF images & Phase maps for NiTiCuCr for: 0, 5, 10, 30 & 50 cycles @ ~10% strain35

Fig 4.2 IPF maps for B19 in NiTiCuCr for 0, 5, 10, 30, 50 cycles resp. @ ~10% strain.36

Fig 4.3 IPF images & Phase maps for NiTiCuCr for 0, 5, 10, 30 & 50 cycles @ ~25% strain.37

Fig 4.4 IPF maps for B19 in NiTiCuCr for 0, 5, 10, 30, 50 cycles resp. @ ~25% strain.38

Fig 4.5 IPF maps for B2 in NiTiCuCr for 0, 5, 10, 30, 50 cycles resp. @ ~25% strain39

Fig 4.6 Grey Scale maps for NiTiCuCr for 0, 5, 10cycles resp.@ ~25% strain39

Fig 4.7 Variation in fraction of martensite for different strains.40

Fig 4.8 X-ray graphs at ~10% strain for various cycles & at ~25% strain for 20 cycles ...41

Fig 4.9 Screenshot of the interface while using GOM...43


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List of Tables

Table 3.1 Compositions of the alloys being used...24

Table 3.2 Specification of the device.27

Table 4.1 Deformation data obtained from ARAMIS44


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Chapter 1

Introduction

The development of new materials is critical for applications that need properties, or

combinations of properties not available in existing materials. Shape memory alloys (SMA)

are a good example of such novel materials, where the functional characteristics, such as

shape memory effect and superelasticity, are exploited through the interplay of structure and

properties. Shape memory alloys represent one subset of a class of materials called

smart/active materials, where, their functional capabilities arise due to distinctive

combination of various physical parameters, such as mechanical, thermal, optical, electro-

magnetic, and are usually accompanied by a change in crystal structure.

The three major types of shape memory alloys are the copper-zinc-aluminum-nickel,

copper-aluminium-nickel, and nickel-titanium (NiTi) alloys but Shape-memory alloys

(SMAs) can also be created by alloying zinc, copper, gold, and iron. They have the ability to

return to a previously defined shape when heated past a set transformation temperature

following deformation. Many SMAs also display superelastic or pseudoelastic behavior

where large deformation can be recovered on unloading. The NiTi alloys have greater shape

memory strain (up to 8 % versus 45% for the copperbased alloys), tend to be much more

thermally stable, have excellent corrosion resistance and also higher ductility. These

properties make SMAs, and in particular nickeltitanium (Nitinol), attractive for a variety of

applications. Important amongst these are actuators, stents, guidewires and other biomedical

applications in which fine and intricate design structures are subjected to rather complex

deformations[1].

The deformation is recovered in SMAs because it is accommodated by martensitic

phase transformation and transformation twinning/detwinning instead of crystallographic

slip. They undergo a martensitic phase transformation from a high-temperature, high-

symmetry austenite state (cubic) to a low-temperature, low symmetry martensite

(monoclinic) state. The change of symmetry gives rise to multiple variants of martensite in

order to maintain zero distortion of the habit plane and formation of transformation twins

during the process. Superelastic behavior arises when the material is deformed at a

temperature sufficiently above the martensite to austenite transformation temperature. The

applied stress causes a shift in martensite start temperature, and deformation is caused by

stress-induced austenite to martensite transformation. However, martensite is unstable at this

temperature and transforms back to austenite on unloading, hence recovering the


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deformation. This mechanism of transformation is reasonably well understood in single

crystals but is poorly understood in polycrystals.

Though extensive research has been carried out on fatigue behavior of SMAs, only

few studies have made an attempt to find the exact microstructural changes and the phases

accumulating as a result of irreversible processes during the process of transformation duringmechanical fatigue. Keeping this in view, mechanical fatigue experiments on a NiTi alloy

were conducted so as to obtain the microstructural evolution in fatigue of NiTi alloys.

Though austenite to martensite transformation during loading is expected to get completely

reversed during unloading, accumulation of defects in the lattice hampers reversible phase

transformation thus leading to deteriorating superelastic or shape memory response [2]. Our

objective was to monitor changes in microstructure that occur with increasing cycles, and

correlate them with residual martensite content in unloaded samples. This would help us

understand the effect of microstructure on fatigue behaviour of shape memory alloys.

The experimental work was carried out in three different stages. Firstly, the miniature

bulge fatigue testing device, developed earlier by Edul Patel [3], was automated so as to

provide control over the strain applied, and to make it feasible to subject samples to a large

number of loading-unloading cycles. Secondly, bulge fatigue testing and microstructural

evolution studies, using EBSD and XRD, were done by obtaining microstructure with

increasing cyclic loading at constant strain. Thirdly, strain measurements were calculated for

the varying applied strains and number of cycles of fatigue using GOM (ARAMIS) software.


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Chapter 2

Literature Survey

2.1 Introduction to Ni-Ti SMAs

The NiTi shape memory alloy (SMA), Nitinol, is widely used in biomedical,

structural, aerospace engineering etc due to its superior performance relative to other SMAs.

There are two characteristic effects exhibited by SMAs viz. shape memory effect (SME) and

superelasticity (SE) or pseudoelasticity (PE). Because of its shape memory properties,

Nitinol has been used for applications such as pipe couplings, actuators, dental & medical

applications like stents, surgical instruments, orthopedic components, etc. All these

applications involve repetitive loading, hence it becomes important to have knowledge

regarding Nitinol fatigue, factors affecting it, and ways to improve fatigue-life of the

material.

Shape memory effect and superelasticity are unique properties of certain alloys

systems that arise from a (reversible) thermo-elastic behavior of the alloy that has its basis in

temperature-induced diffusionless, solid-solid, first-order transformation (high-temperature

austenite low-temperature martensite) involving a change in crystalline symmetry

Fig 2.1 Crystal structure of (a) cubic parent austenite phase (after the British scientist Sir W.

C. Roberts-Austen, 1898) and (b) monoclinic low temperature martensite phase (after the

German Scientist Adolf Martens, 1890) [4]


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Fig 2.2 Schematic illustration of SME. Af & Mf are austenite & martensite finish temps. [5]

Above the austenite finish temperature (Af), the parent material is called austenite

and has a cubic structure. Upon cooling below the martensite start temperature (Ms), the

crystal structure changes to a low-symmetry, monoclinic martensite structure; this

martensitic transformation is reversible upon heating. Below the martensite finish

temperature (Mf), the alloy is completely martensitic and is termed as thermally-induced

martensite (TIM). TIM is also called self-accomodating martensite because it has a twinned

structure so as to have zero macroscopic strains on average. When the TIM is deformed by

an external stress, the shape change is accomplished by detwinning i.e. by the

transformation of twinned variants to a common variant favoured by the applied stress. Upon

heating above the austenite finish temperature, Af, martensite transforms back to the

austenite phase, in the process reverting to the original (undeformed) shape as shown in

schematically in Fig 2.2. Thus the material exhibits shape memory effect. In contrast, when

the austenite phase is deformed by external stress within the temperature range Af < T < Md,

the austenite undergoes a stress-induced martensite (SIM) transformation. Upon removal of

the load, however, the martensite structure reverts back to the austenite phase, since austenite

is the more stable phase within this temperature window. This is termed as the

superelasticity if the specimen recovers all of the loading strains; partial recovery of strain is

termed pseudoelasticity [5].

In principle superelastic SMAs would be ideal for repetitive loading-unloading

applications, provided the strain is small enough so that martensite does not undergo plastic

deformation. But in practice, Nitinol devices suffer from several forms of fatigue damage

that range from component shape change to catastrophic fractures. It has been found that

mechanical working and heat treatment significantly affect the behavior of Ni-Ti SMAs

implying that there exists a correlation between the microstructure and shape memory effect


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exhibited. Hence, microstructure studies for enhanced commercial applications remains a

good motivation for understanding the deformation mechanism in these materials during

fatigue.

2.2 Martensitic Transformation

Martensitic transformations are displacive, diffusionless transformations during

which the atoms execute a small, well-defined movement, resulting in a change in lattice

structure and a shape change. The lattice structure of the resulting martensite has orientation

relationship to the lattice structure of the parent phase. The austenite martensite interface

plane has been shown to be an undistorted plane, called the habit plane.

2.2.1 Diffusionless nature of the transformation

In a martensitic transformation, individual atoms execute well-defined and correlated

movements, where the displacement of each atom is less than one interatomic distance. The

martensitic structure is the result of a lattice transformation entirely without atomic diffusion.

This is completely different from some diffusion-controlled solid state transformations in

which atoms undergo random diffusion movements of a relatively long range nature [6].

It has been well established by Bain in 1924 that upon biaxial expansion of the lattice

constant in the (100) plane, a FCC solid may transform very rapidly into a BCC or BCT

phase through relaxation of the interlayer spacing along the perpendicular axis c, as shown in

figure 2.3. This is known as Bain distortion.

Fig 2.3 Bain Distortion in FCC to form BCT structure [6]


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(T) orientations are opposite in sense thus resulting in an undistorted plane in an average

sense. In BCC SMAs, 24 variants are possible.

Fig.2.5 Schematic representation of the interface. The microscopic strains in the lamellae

(M) and (T) are in opposite senses[6]

Fig 2.6 Crystallographic steps for the B2 to Bl9 transformation in NiTi. (a) B2 cells; (b) B2

BCT cell; (c) Orthorombic distortion; (d) (100)B2-[011]B2 shear to monoclinic;

(e) (011)B2[011]B2 planar shuffle. [8]


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The Austenite to Martensite transformation in Ni-Ti occurs in 3 ways as shown in the figure:

Fig 2.7 Phase Transformation paths of NiTi alloys for different alloy conditions [6]

The one step transition from B2 to B19 has been reported as first order thermo -

elastic one. The R-phase transformation occurs in binary TiNi alloys, if they are cold-

worked followed by annealing at proper temperatures, so that high densities of rearranged

dislocations are left in specimens. The conditions, under which the R-phase transformation

occurs i.e two step transformation, are to suppress B19 transformation relative to R-phase

transformation by introducing precipitates or dislocations, etc. The third type of

transformation is obtained when Ni is substituted by Cu in a binary TiNi alloy. The

martensite upon the first transformation is called B19 (orthorhombic), and the second

transformation represents the one from B19 to B19.

2.2.3 Reorientation and reverse transformation

During transformation various variants are formed depending on the stress applied

however it is commonly found that these variants eventually converge to one single most

stable dominant variant. During the elimination of variants little macroscopic strain is

generated. The positions of the martensite interfaces change under the influence of stress,

creating a balance of variants whose shears best accommodate the direction of applied strain.

The interfaces between variants move to grow the most favourably oriented variants and

shrink the least favourably oriented. Some variants become dominant in the configuration.

This process creates a macroscopic strain, which is recoverable as the crystal structure

reverts to austenite during reverse transformation [8]. Figure 2.8 illustrates how the

crystallographic reversibility is automatically guaranteed by superlattice structures.


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Fig 2.8 (a) Three possible lattice correspondences in the reverse transformation of the B2 to

B19 transformation; (b) parent phase crystal structure resulting from lattice correspondence

A: a B2 superlattice the same as the pretransformation structure; (c) parent phase crystal

structure resulting from lattice correspondence B: completely different from a B2

structure[8].

Generally, the crystal structure of martensite is relatively less symmetric compared to

that of the parent phase. For this reason, the kinds of lattice correspondences between the

phases involved in the reverse transformation are restricted. In figure there are, if we ignore

the ordered arrangement of the atoms, three equivalent lattice correspondences, represented

by the rectangles marked A, B and C. Since the changes in crystal structure shown in (c)

would raise the free energy, reverse transformations along path B or C are impossible. Thus

the orientation of the parent phase crystal is automatically preserved by its ordered structure.

2.3 Pseudoelasticity and Superslasticity

Superelasticity typically occurs when a material is deformed above As, where

external applied stress causes the transformation of the parent austenite phase to a

martensitic phase. Normally, austenite is the stable phase under stress-free conditions.

However, upon the application of a critical stress, austenite starts transforming to martensite

at a constant stress, thus producing a stress plateau. The plateau (points B to C in Fig 2.9) is

a consequence of the martensite phase variants formation during transformation. The

selection of the variants depends on the loading direction and the crystal orientation in case

of a polycrystalline specimen. At a certain strain value, the stress plateau terminates, and the


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new martensitic phase now elastically deforms. However, this martensite becomes unstable

upon the removal of the stress; hence during unloading, the martensite phase reverse

transforms to the austenite phase and at zero loads the material should be entirely austenitic

with the specimen recovering its original undeformed form [9,10]. Superelasticity typically

occurs if the specimen temperature is between Af < T < Md, where Md is the highest

temperature at which stress induced martensite forms for particular value of stress. If the

temperature is above Md, the stress level needed to induce martensite is greater than the

stress required to introduce slip in austenite and hence SIM does not occur.

Fig.2.9 Different stress-strain path followed by superelasticity and pseudoelasticity

Any incomplete recovery during unloading in the stress-strain curve is termed

pseudoelasticity. In some materials, deformation of the austenite phase well above Md

(where SIM is not possible) leads to the formation of twins that are unstable and hence there

is a driving force to return to the original condition by the shrinking or disappearance of the

twins during unloading. This type of mechanical twinning disturbs the lattice ordering and is

therefore different from conventional twinning; it is termed pseudotwinning. Upon

unloading, the material attempts to return to the preferred lattice ordering by shrinking of the

twins [11,12]. This is depicted by the non-linear unloading of the stress-strain curve in

Figure. Few other materials that are in their martensitic state also exhibit twinning

pseudoelasticity (martensitic pseudoelasticity), where the deformation occurs by the motion

of the martensite twin boundaries.


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2.4 Mechanical Fatigue of NiTi Alloy

The mechanical fatigue behavior of Ti-Ni and Ti-Ni Cu shape memory alloys can largely be

said to depend on the stability of their pseudoelasticity (PE) curves during mechanical

cycling. The stability of PE can be evaluated or characterized by the residual elongation

remaining after the specimen is unloaded, the critical stress needed for martensite to start

forming, and the hysteresis curve which is a measure of the energy dissipated in one cycle

[13]. The stability of these characteristics is affected by the test temperature, the nickel

content and the previous heat treatment of the alloy. If the test temperature is chosen close to

Af, stability of PE stress-strain diagram is achieved easily because the residual deformation

after unloading is lowest. Also, if the critical stress for slip is high and the alloy readily

undergoes cyclic strain hardening, then residual elongation after unloading decreases as the

number of cycles increase, and hence the cyclic stress-strain diagram is more rapidlystabilized. For increasing the critical stress for slip, higher nickel content and suitable heat

treatment can induce dislocations and precipitation hardening can be done resulting in

reduced residual deformation during fatigue.

Fig 2.10 Stress-strain curves (hysteresis curves) during cyclic loading with number of cycles

The hysteresis loop observed in stress-strain diagram has been said to be result of

dissipation of the non-chemical irreversible energy that arises due to the interfacial friction

between adjacent martensite variants during their reorientation process [14]. When stress is

applied, there are certain martensitic variants that are favored for growth whereas others are

reoriented, leading to gradual dislocation accumulation at the interface between different

martensite colonies. The hardening observed during cyclic loading is related to this

accumulation. With increasing cycles the width of the hysteresis loop decreases, indicating

decrease in ability of material to undergo reversible transformations. The narrowing of loops


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represents decrease in the area inside the loop which is the energy dissipated in each cycle

[15]. The reason stated for this is that with increase in stress range the volume fraction of

detwinned martensite increases and the proportional energy loss of the detwinned martensite

is only half of the twinned martensite, as was observed by Huang and Lim.

Fig 2.11 Stress-strain curve for same no. of cycles of loading but at different stresses [15]

For comparison of effect of stress amplitude, Fig 2.11 shows the stabilized stress

strain hysteresis loops generated at various stress amplitudes, all at the 100th fatigue cycle. At

all the stress ranges, the loops have relatively narrow width with near-linear loading andunloading response. With increasing stress amplitude, the hysteresis loops move to the right

on the strain axis thus indicating higher amounts of strains accumulated in the SMA [10].

This shows that with increasing the stress amplitude the amount of irreversible martensite

also increases leading to strain accumulations.

Fig 2.12 Accumulated plastic strain with increasing no. of cycles for different stresses[15]


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Consider Fig 2.12 where the effect of stress amplitude on the accumulated plastic

strain, as a function of the number of fatigue cycles is shown. It can be clearly seen that the

strains are much higher for same number of cycles for higher stress amplitude as has been

discussed earlier. It can be seen that plastic strain increases rapidly between 10th and 20th

cycles before reaching a plateau value that is maintained until failure. The initial strain

results from a combination of stress-induced growth of one martensite variant at the expense

of an adjacent, unfavorably oriented one, as well as stress-induced reorientation of

martensite and twin boundary migration within a martensite variant. Due to such

adjustments, dislocation density increases during cyclic deformation. It is believed that

internal plastic deformation is necessary for martensite reorientation in polycrystalline

matrices because orientation mismatch exists among the preferential variants of martensite in

neighboring grains [16]. Therefore, the interface between any two martensite variants

becomes highly strained, which in turn makes dislocation based deformation difficult. This is

possible reason stated for the observed stabilization at higher N.

It has been observed that the energy dissipated i.e. area under the hysteresis loop

goes on decreasing with number of cycles which implies that the damping capacity of the

material goes on decreasing with progressive cycling. Also, decrease in recoverable energy

i.e. the area within the reverse transformation curve decreases with increase in the number of

cycles.

2.5 Factors Affecting Mechanical Fatigue Behavior

2.5.1 Composition Effect

As the nickel content increases, Ms decreases and the transition from shape memory

effect to pseudoelasticity shifts towards the lower temperature region. Also, the critical stress

for slip, for inducing martensite in austenite, rises with increasing Ni content and results in

reducing the growth rate of residual deformation due to retained martensite as the number of

cycles increases and support stabilization of the cyclic stress-strain diagram. This increase is

attributed to solid solution hardening and to some extent also to the precipitation of fine

Ti3Ni4 particles [18]. The greater deformations in lower nickel alloys i.e. less hardened

alloys are ascribed to the fact that that their dislocations offers only slight resistance againstthe growth of preferentially oriented martensite variants.


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It has been stated that Cu addition to NiTi alloy prevents Ti3Ni4 precipitation, and

results in a narrow transformation hysteresis, more constant Ms temperature and superelastic

plateau. The austenitic transformation (Af) of nickel/titanium alloys generally increases with

copper content. A disadvantage of most Ti-Ni-Cu alloys is that the transformation

temperatures do not decrease below room temperature. The Ms above room temperature

does not allow complete Austenite at room temperature. In order to decrease the austenitic

transformation temperature, without substantially affecting its superelasticity and SME, the

addition of Cr, W, Mn, Fe, Co and Al is done [17]. It is believed that the addition of 0.5 to

2% Fe and upto 1% Cr will reduce the Af temp of a NiTiCu alloy to below 30oC. Following

is the relationship for obtaining ternary alloys of NiTiCu while retaining superelastic

properties at temperatures below 35oC. The amount of the additional element X is not greater

than approximately 5%, preferably not greater than about 3%.

(Ti50-a/Ni50-b/Cua+b)100-c/Xc

where, a=0-10%, b=0-20%, c=0-5%, X= V, Cr, Mn, Fe, Co, W, Al

2.5.2 Effect of Thermo-mechanical Treatment and Temperature

Any solution annealed microstructure in cyclic loading does not exhibit fullyrecoverable superelastic strains, i.e. it exhibits irreversible strains that cannot be recovered

either by unloading or heating. This has been shown to be due to the ease of slip in the

austenite phase that leads to partial pseudoelasticity. Thus, in order to improve the SE

properties, the critical stress for slip has to be increased through any of the following

methods: 1. work hardening, 2. grain-refinement, 3. precipitation-hardening or a combination

of these. The grain-size effect on the pseudoelastic behavior of NiTi shows that refinement

of grain size improves pseudoelasticity. Following are the thermo-mechanical treatment used

for NiTi alloys:

Cold-work + Stress relieving Annealing:

It has been seen that samples cold worked and annealed at 400oC not only exhibit large

superelastic strains, but also the best SE and SM characteristics. The cold-worked alloy

exhibits large amount of tangled dislocation networks. The specimen annealed at 400oC

shows re-arranged dislocations, i.e., recovery takes place. However, the annealing treatment

at 400oC does not recrystallize the alloy. These heat treated samples exhibit a clean

martensitic transformation and also maintain sufficient strength due to the re-arranged


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dislocations. The specimens annealed at 400oC exhibited superior superelasticity than the

specimens annealed at 500oC, in which recrystallization takes place. The specimens annealed

at 600oC exhibits grain growth following recrystallization, and superelastic behavior is

inferior [18].

Precipitation hardening:

A solution-annealed alloy aged at about 600oC exhibits superelasticity, but in a very

narrow temperature range. In comparison, specimen aged at 500oC exhibits superior SM and

SE characteristics, with a large superelastic temperature range. This can be ascribed to a

high-density of precipitates, increasing the critical stress for slip.

Aging at 500oC produces high density of small sized Ti11Ni14 precipitates, whereas at

600oC, the precipitates are larger and density is lower. At 700oC, the precipitates are not

observed at all. is the transformation order for

precipitates, but since Ti11Ni14 phase is the only desired precipitate, because of its coherency

with the parent phase matrix, for increasing the pseudoelasticity, time of the transformation

must be taken care of [18].

Fig 2.13 TTT diagram describing aging effect on alloy Ni52Ti48 [18]


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Work-hardening + precipitation-hardening:

Both work-hardening and precipitation hardening has been shown to improve the SE and

SM characteristics. It has been further shown that combined usage of both types of hardening

mechanisms is more effective to improve the critical stress for slip. This treatment is

provided when SE is to be achieved at higher stress levels. The recovery temperature forcold-worked NiTi alloys is between 400oC and 500oC. Aging heat treatment of a cold

worked NiTi alloy is likely to produce a complex microstructure containing large densities of

dislocations and precipitates that are interwoven in such a way that precipitations act as

obstacles for dislocation movement and may hinder the recovery process [18].

2.5.3 Effect of Mean Strain

Mean stress and mean-strain determine the phase of the material in superelastic

specimens. Depending on the mean strain, a specimen may contain austenite (A), martensite

(M), or both phases and different deformation modes are associated with each of these cases.

Fig 2.14 Quasi-static Stress-Strain Curve for NiTi [22]

During loading of specimen, austenite phase exists at strains lower than f. As the

strain reaches upper plateau region transformation from austenite to martensite occurs. So a

mixture of austenite and martensite exists till strain reaches f+t. Further straining causes

complete transformation to martensite followed by elastic and plastic deformation. During

unloading, martensite remains stable till strain is reduced to r and then transformation to

austenite starts for strains in the lower plateau region. The difference in the mixture of

martensite and austenite in this region as compared to that in upper plateau is the volume

fraction of the phases present. Transformation to austenite occurs with some residual

martensite at strains lower than r-t. Now if a small value of strain is superimposed on a

fixed value of strain then it can either trigger forward or backward transformation.


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2.6 Microstructure during Fatigue

Limited work has been done on the evolution of microstructure during fatigue of

SMAs. Though some work has been reported on the microstructure, it has focused on the

study of growth of residual martensite phase during cyclic deformation.

From the microstructures shown in figure, it was concluded that some residual

variants of martensite occurs after cyclic loading in some grains but not in others. The reason

for such a behavior has not been mentioned. It was found that the same variants of martensite

get activated in each cycle but the spatial position varies from cycle to cycle. Different

variants of martensite may form in each cycle and it may form in different grains in

successive cycles.

(a)

(b)

Fig 2.15 (a) Accumulation of localized plastic deformation within the grains upon cyclic

load. Identical locations imaged on virgin specimen and after four different cycles, specimen

fully unloaded. (b) Micrographs of activated variants at 2% strain in one set of grains at

different cycle numbers (cycle 1, 6 and 10 from left to right) [14]


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Fig 2.16 Microscope images at strains 0%, 2%, 4%, 10% showing full transformation [14]

The above microstructure shows that even at 100% transformation it is not necessary

that there will be 100% martensite in the grain that are favourable for martensitic

transformation. (*The 100% transformation is said to be the point at which plateau of the

stress-strain curve cease to be flat and rises upwards due to plastic deformation of

martensites formed.) Also the grains that are unfavourably oriented never transform. As the

transformation begins in a grain martensite variants increase but at some higher stress, a

grain adjacent to the transformed grain gets activated due to local increase in stress and

hence instead of further transformation in the first grain, transformation to martensite begins

in adjacent grain. Due to this sequenced transformation behavior, variants are restricted in

each grain in turn and unable to transform fully, in contrast to what is seen typically in single

crystals.

2.7 Characterization techniques and testing methods

2.7.1 Differential Scanning Calorimetry

DSC is a thermal analysis technique in which the difference in the amount

ofheat required to increase the temperature of a sample and reference is measured as a

function of temperature. The basic principle underlying this technique is that when the

sample undergoes a physical transformation such as phase transitions, more or less heat will

need to flow to it than the reference to maintain both at the same temperature. By observing

the difference in heat flow between the sample and reference, differential
http://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Phase_transitionhttp://en.wikipedia.org/wiki/Phase_transitionhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Heat


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scanning calorimeters are able to identify transformations. The result of a DSC experiment is

a curve of heat flux versus temperature or versus time.

Fig 2.17 Shows a typical DSC scan on a sample of Nitinol [24]

The data is analyzed to report the Austenite start, peak and finish (As, Ap, Af) plus

the Martensite start, peak and finish (Ms, Mp, Mf). Typical data in fig. show that the

transition is both reversible and hysteretic. The transformation temperature hysteresis in the

transition on heating and cooling depends on the exact percentages of nickel and titanium in

the alloy. Typically, the magnitude of the hysteresis is reported as the difference between Ap

and Mp and in this case is 30.14C. Typical values of the hysteresis for binary Nitinol

samples are between 25-50C [24]

2.7.2 Bulge Testing

Bulge test is a versatile characterization method capable of determining a complete

set of material properties of thin films under various stress conditions. It is based onmeasuring the deformation of a thin specimen under an applied pressure. The obtained

pressure-deformation behavior is then utilized to extract meachanical properties of thin film

samples. In this method, a thin specimen is deformed by means of pressure application

which can be done using various available methods like hydroforming using oil or water, by

means of punch or in case of miniature bulge testing using a spherical headed screw which

acts like a punch. This method can also be applied to SMAs in order to study the biaxial

stress-strain behavior as well as fatigue.
http://en.wikipedia.org/wiki/Calorimeterhttp://en.wikipedia.org/wiki/Calorimeter


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Two major advantages associated with bulge test are the ease of specimen handling

and the capability of imposing various loading conditions. These cannot be obtained by other

thin film testing methods such as nanoindentation, substrate-curvature technique, and

microtensile testing.

Fig 2.18 Schematic representation of the bulge occurring during bulge testing and direction

in which pressure is exerted [27]

If we assume that the sample bulges spherically, the stress and strain in the thin specimen

may be considered uniform across the width, independent of whether the film deforms

elastically or plastically, and are given by

Elastic and plastic global stress:

Eq.1

Elastic and plastic global strain:

Eq.2

When the deflection is much smaller than the membrane width, i.e. h


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2.7.3 Electron backscatter diffraction (EBSD)

Electron backscattered diffraction (EBSD), also known as backscattered Kikuchi

diffraction (BKD) is a microstructural-crystallographic technique used to examine the

crystallographic orientation of many materials, which can be used to reveal texture or

preferred orientation of any crystalline or polycrystalline material.

Atomic layers in crystalline materials diffract the accelerated electrons from the

primary beam of scanning electron microscope. These diffracted electrons can be detected

when they impinge on a phosphor screen and generate visible lines, called Kikuchi band.

These band pattern are effectively projections of the geometry of the lattice planes in the

crystal and are capable of providing direct information about the crystalline structure and

crystallographic orientation of the grain from which they originate. This pattern is then

compared with the crystallographic information for phases of interest from the database.

Indexing of the lines then gives us the phases present and also allows us to perform analyses

on polycrystalline aggregates. It can also be applied to crystal orientation mapping, defect

studies, phase identification, grain boundary and morphology studies, regional heterogeneity

investigations, material discrimination, microstrain mapping, and physico-chemical

identification [28].

2.7.4 GOM (ARAMIS) Software:

Aramis is an optical measurement technique used for 3D deformation analysis.

ARAMIS is a non-contact and material independent measuring system providing for static or

dynamically loaded test objects, accurate 3D surface coordinates, 3D displacements and

velocities, surface strain values (major and minor strain, thickness reduction), etc.

In Aramis, CCD cameras are used for recording images of a sample undergoingdeformation. For each stage of load, this system calculates 3D coordinates of the object

surface on the basis of digital image processing and thus calculates 3D displacements and

strain.

In this 3D image correlation technology, two cameras catch the same image and by

the combination and correlation of these two images 3D strains are calculated. A random or

regular pattern with good contrast is applied to the surface of the test object, which deforms

along with the object. The deformation of this structure under different load conditions is

recorded by the CCD cameras and evaluated. The initial image processing defines unique
http://en.wikipedia.org/wiki/Crystallographyhttp://en.wikipedia.org/wiki/Crystallinehttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Crystallinehttp://en.wikipedia.org/wiki/Crystallography


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correlation areas known as macro-image facets, typically 5-20 pixels square, across entire

surface area [29]. The center of each facet is a measurement point that can be thought of as

an extensometer and strain start point. These facets are tracked in each successive image

with sub-pixel accuracy. Then the coordinates of the entire surface are precisely calculated

and compared with the undeformed image to get the 3D shape of the component, the 3D

displacements, and the plane strain. Also, in cases where images from the same region are

available with sufficient surface features to meet required conditions, there is a provision in

Aramis to upload image series in various stages so as to calculate displacements and strains

in the specimen.


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Chapter 3

Experimental work and Results

3.1 Material Preparation

Two NiTi alloys have been chosen for the study viz. NiTi and NiTiCuCr. It has already been

discussed that the replacement of Ni with Cu increases the pseudoelasticity of the material

and that of Cr decreases the austenite formation temperature, hence such a combination was

chosen for studies. The composition of these alloys is as shown below:

Table 3.1. Compositions of the alloys being used:

(in atomic %) Ni Ti Cu Cr

Alloy 1 50.8 49.2 - -

Alloy 2 45 49.7 5 0.3

Alloy Preparation: Alloys were prepared by mixing appropriate amounts of the

elements in their pure form and melting in vacuum arc furnace. Repeated melting i.e. about

6-7 times was carried out to ensure homogeneity of the alloys. The material obtained was in

the form of buttons of 50 gms, each of which was then hot rolled at 950o

C to form strips.

Rolling and Heat treatment: Reduction in thickness from 10 mm to 0.7 mm was carried out

in about 24 passes with approximately 9% reduction in each pass at a temperature of around

950oC. Solution annealing was then carried out in a muffle furnace at 950oC for 20 minutes

to obtain completely homogenized stress free alloys. The samples were water quenched after

annealing.

Further heat treatment, on an experimental basis, was carried out on Ni50.8Ti49.2 alloy in orderto precipitate Ni14Ti11 with smaller size and higher densities, as per the TTT diagram by

Nishida, Wayman and Honma [18]. The solution annealed alloy was aged at 550oC for 1hr

followed by water quenching. Since this treatment has been earlier tried and tested and has

worked as expected, it was carried out on half of the Ni50.8Ti49.2 material in inert gas

environment by encapsulating the material in quartz tube.


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Thus the samples used for studies are

(a) Ni50.8Ti49.2, solution annealed

(b) Ni50.8Ti49.2, solution annealed + precipitation hardened (aged, 550oC, 1Hr)

(c) Ni45Ti49.7Cu5Cr0.3, solution annealed

NiTiCuCr was not aged as there will be no changes in the phases present i.e. no precipitation

will be observed in alloy even after ageing because there is no excess of Ni available in the

alloy to form precipitates.

Specimen Preparation

From the rolled strips, 7 mm X 7 mm pieces were cut. For metallographic studies as

well as for mechanical fatigue experiments, the thickness was reduced from 0.7 mm to 0.25

mm by grinding and polishing. Polishing of samples was carried out using SiC emery papers

of different grits and diamond polish followed by electrochemical polishing in order to get a

highly polished mirror-like surface free from scratches and pits. These samples were

electropolished using Lectropol polishing machine at 16 Volt for 15 seconds at 0C in a 20%

perchloric acid solution in methanol.

Characterizarion Instruments used:

All Electron backscattered diffraction (EBSD) measurements were carried out on a

FEI Quanta-200 HV scanning electron microscope (SEM) equipped with a TSL OIM

analysis unit. The Kikuchi patterns were generated at an acceleration voltage of 20 kV, and

recorded by means of a camera. A crystallographic orientation map is produced by scanning

over a selected region of the sample by an electron beam. The resulting Kikuchi patterns are

indexed and analysed automatically. An image quality (IQ) parameter and a confidence

index (CI) is recorded for each Kikuchi pattern. All strain measurements were carried out

using ARAMIS, an online measurement technique from GOM software. The subset size and

subset step were carefully selected for the measurements. XRD patterns were obtained using

Pananalytical Xpert X-ray diffractometer. Analysing was done using Xpert Highscore and

Xpert Stress Viewer.


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3.2 Bulge Fatigue Testing

3.2.1 Device Design

The bulge test, in which a thin specimen is pressurized and its out-of-plane deformation or

bulge is monitored, is a well-accepted methodology for obtaining the stress-strain behaviour

of thin samples subjected to biaxial loading. The main advantage of using bulge test to study

fatigue in shape memory alloys is that the setup that we have designed allows us to test a

sample that is small enough that microstructure of a pre-defined region of the sample can be

seen repeatedly after cyclic loading. This gave us direct observation of microstructure

evolution with fatigue cycles, and we could correlate it with the residual strain. Also, it is

possible to estimate the total strain from bulge height when loaded and residual strain from

the height of unloaded sample assuming spherical bulge, and thus we can even correlate the

recovery after desired number of cycles. The device used for the bulge fatigue testing was a

modification of the one designed and developed by Edul Patel [3].

(a) (b)

(c) (d)

Fig 3.1 (a) Assembled, (b) dissembled, (c) cross-sectional view

and (d) photograph of bulge testing device

As can be seen from the diagram, it consists of a nut, two washers, bolt having hole

with threading on the inside and screw. The sample with thickness around 0.25 mm is

clamped by means of two washers. We found that the tip of the screw, though relatively

blunt, was still sharp enough that after repeated loading cycles, the sample had undergone

severe plastic deformation at the point of contact instead of uniform bulging. So, it was

replaced by another screw having a steel ball at its tip, which ensured that bulging of the


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sample takes place. This kind of screw, with steel ball and without plunger inside, though is

available in market but is scarce.

Table 3.2: Specification of the device

Specifications

Original Earlier Modified

Max sample size 7 mm sq 7 mm sq

Screw pitch .6 mm .7 mm

Least bulge height

attainable .1 mm adjustable

Bulge Diameter 2 mm 2 mm

Screw Diameter 2 mm 3 mm

3.2.2 Procedure for the bulge test

1. Sample was first cut into a 7 mm x 7 mm piece, thinned to 0.3 mm and then electropolished.

2. Area of 300 x 400 microns was marked on the specimen using micro-indenter3. The marked area in the undeformed sample was scanned using the EBSD system4. Sample was fitted in the bulge device and desired strain applied for 5 cycles5. Sample was removed from the device and the same marked area was scanned using

EBSD to observe changes in microstructure

6. Steps 4 and 5 were repeated for 10, 30 and 50 cycles

3.2.3 Automation of the set-up

In the first design of the device, the bulging screw was rotated manually to impose

the desired strain. But this gave poor control over the amount of strain applied, and was not

suited for multiple cycling experiments. Hence the set up was automated so the magnitude

of strain could be kept constant over multiple cycles.


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In automated set-up, cyclic loading is effected by means of a screw driven by stepper motor.

Details of the setup are shown in fig 3.2. Coupling the motor with the screw was a major

concern while developing the set-up. The important considerations were: easy detachability

of the screw from the motor, efficient transfer of load from motor to screw and ease of

handling. This difficulty was overcome by developing a custom-made head that was attached

to the shaft of the motor which could be fixed in the notch provided at the tail-end of the

screw. Sufficient space is provided for the in and out movement of the shaft-head in the

notch. This arrangement works as coupling between the mechanical part of the setup and

electronically controlled part. The motor is in turn controlled by a microcontroller.

Fig 3.2 (a) Coupling arrangement between the stepper motor and mechnical arrangement,

(b) Bulge Fatigue testing arrangement components

The head of motor shafts induces forward and backward movements in the screw. Now in

order to control the strain and number of cycles during fatigue, the movement of the motor

shaft is controlled using a microcontroller. The microcontroller circuit, for controlling motor

rotations, consists of PIC 16F877A, custom made driver A3982 and pull up resistor

arrangements. PIC 16F877A is a 40 pin, CMOS FLASH-based 8-bit microcontroller and 8

channels 10-bit Analog-to-Digital (A/D) converter, which works on 14-bit instruction

programming language. Earlier another IC i.e. PIC 16F84 was tried but due to technical

difficulties like requirement of another circuitry for analog to digital conversions, etc.

designing was complex. The circuit diagram for microcontroller circuit and schematic design

for entire setup is as follows:


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Fig 3.3 Circuit diagram for the microcontroller circuit for controlling the stepper motor

In the present case, a program for controlling the number of rotations and number of

cycles (for both forward and backward direction) has been fed in the PIC by means of an IC

programmer. Since there are no standard drivers available for the stepper motor circuit, a

custom driver for providing actual control signals to the stepper motor is used, which was

developed by Nex-Robotics. This is of great importance in the working of the circuit as a

precisely controllable signal is to be provided. With the help of switch provided inputs for

number of rotations and number of cycles can be fed, in binary form. This can even be

converted into decimal digital input The stepper motor that has been used has specifications

of 200 steps per rotation i.e. 1.8 degree of turn per step and torque ratings of 1.3 kg. Thepower supply for the entire assembly is provided using 450 W SMPS.

Fig 3.4 Control pins on the circuit board


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Program used for controlling the microprocessor is written in Wiz-C. WIZ-C

Professional is a complete PIC microcontroller (MCU) development, compiler, assembler

and simulation package for the C language. In this we have to select the kind of

microprocessor we have to program. Each microprocessor has a number of components

within it that have to be connected to the other components of the circuit. When each

component is selected it can be connected to the MCU pins by clicking the picture of the

component pin and then clicking the MCU pin - they will be joined. Pins can be user named

as well, to make the code easier to understand. Some components can only be connected to

certain pins - they will connect themselves automatically.

Each component has a list of parameters. In the case of our serial element this includes the

bit rate. The bit rate is selected from a drop down list, now when the application is generated

it will automatically configure the serial element for this bit rate regardless of the MCU type

used or oscillator frequency. Finally according to the connections made, a program is

required for the entire assembly to operate.

Fig 3.5 Screenshot of the Wiz-C while setting up pin controls of the microcontroller


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3.2.4 Microcontroller Programs:

Program written to control all switches on the microcontroller is given in Appendix

as Stepper_main. Program written at user interface is as follows:

#include "C:\\Swapnil\\Wiz-C Projects\\Stepper\\stepper_Auto.h"

//****************************************************************************

// Interrupt handling code written here enables user to initiate and terminate the program whenever

desired by providing suitable inputs

// Note quick interrupts are used so code must be simple

// defining the variables for the program

int one_cycle; // one cycles is the degree of rotation fixed for input 1

int fr_cycle_number; // Desired no. of one cycle (i.e. 60 degrees in this case) rotations beforereversing

int fr_cycles; // Desired number of forward cycles

int revcount; // Number of reverse cycles completed

int fr_cycle_count; // Number of forward cycles completed

int cycle_count; // total of forward and reverse rotations completed

int stopflag; // Stop rotations

void UserInterrupt()

{

// Insert your code here

#asmline SETPCLATH UserIntReturn,-1 ; SETPCLATH for interrupt routine

#asmline goto UserIntReturn ; Assembler - go back to interrupt routine

}

//**************************************************************************

// Initialisation code here.

// Note that when this routine is called Interrupts will not be enabled - the

// User will enable them before the main loop

void UserInitialise()

{

//LCDClear();


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one_cycle=34; // Since motor used have unit rotation of 2 degrees hence 34 input means that

when

program is called with '1' as input it rotates by 2*34=68 degrees precisely!!!

fr_cycle_number=1; // sets the initial values as 1 to start the rotation in forward direction

fr_cycles=1;

revcount=0; // reverse rotation does not begin

fr_cycle_count=0;

cycle_count=0;

stopflag = 1; // stopflag = 1 means 'stop' command for motor

ST=0;

DI=0;

}

************************************************************************

// Insert your main loop code if required here. This routine will be called

// as part of the main loop code

void UserLoop()

{

if(stopflag==0)

{

ST=1; // command on the MCS to stop

Wait(10); // after stopping wait for 10 microseconds

ST=0; // command to enable start of motor in reverse direction than the

current one

revcount = revcount + 1;

if(revcount==one_cycle) // loop to keep increasing the number of forward cycle counts

{

revcount = 0;

fr_cycle_count = fr_cycle_count + 1;

}

if(fr_cycle_count==fr_cycle_number) // condition to terminate forward cycle rotation

{


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fr_cycle_count = 0;

cycle_count = cycle_count + 1;

DI = !DI;

}

if(cycle_count==2*fr_cycles) stopflag = 1; // condition to terminate entire program

Wait(10);

}

}

****************************************************************************

// User occurrence code is code to initiate the motor

void startmot()

{

revcount=0; // setting initial variable values

fr_cycle_count=0;

cycle_count=0;

stopflag = !(stopflag);

}

****************************************************************************

// Write values code to convert binary values and input in decimal form for no. of one cycle rotations

before reversing and no. of such sets of forward and reverse cycles

void writevalues()

{

if(fr_t)

fr_cycle_number= int(bit0+bit1*2+bit2*4+bit3*8+bit4*16+bit5*32+bit6*64+bit7*128);

else

fr_cycles=int(bit0+bit1*2+bit2*4+bit3*8+bit4*16+bit5*32+bit6*64+bit7*128);

}


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Chapter 4

Bulge Fatigue Testing Results

4.1 Microstructural Correlation During Fatigue Using EBSD

In order to study the effect of fatigue loading on the microstructure of NiTiCuCr, fatigue

tests were carried out using the set-up described in section 3.2.3. The microstructure was

obtained using EBSD in deformed conditions after varying number of cycles and was

compared with initial microstructure in undeformed condition. The study of these

microstructures was done to get an insight into accumulation of residual martensite after

cyclic loading.

The sample with thickness around 0.25 mm in electropolished condition was prepared for

the test. In order to mark a specific area for repeated observations, microindentation was

done. It was ensured that the area selected is not marred by any abnormality like dents, pits,

etc. By repeatedly scanning the same region of the sample, we expected to identify where

residual martensite forms during cyclic loading and how does the microstructure change

during fatigue cycles. Also we expected to identify whether martensite forms preferentially

in certain orientations of austenite, and whether the martensite formed has any preferred

orientations.

For performing fatigue tests on NiTiCuCr samples, constant strain and strain rate

condition was maintained. Experiments were carried out at two different strains of about

10% and 25% by controlling the amount of rotation on the screw driven by motor. The strain

rate was constant as it is defined by the speed of rotation of the motor, which was constant.

The samples were fatigued for certain number of cycles viz. 5, 10, 30, 50. Since two

different strains were used, every other condition remaining same, attempt has been made to

compare martensitic formation with increasing number of cycles of fatigue as well as for

different strains.


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(a) (b) (c)

(d) (e)

(f) (g) (h)

(i) (j)

Fig 4.1 Color coded IPFmaps (colours correspond to orientations as per the IPF shown)

(a)(b)(c)(d)(e) & Phase maps (f)(g)(h)(i)(j) for NiTiCuCr for: 0, 5, 10, 30 and 50 cycles

respectively @ ~10% strain


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Fig 4.1 shows the inverse pole figure and phase maps for the NiTiCuCr samples taken from

same area after increasing number of cycles of fatigue for strain of approx. 10%. It can be

seen that there is there is change in shape of grains gradually along with slight change in

orientation (observed by comparing color code for IPF). It can also be observed that

martensite phase formed in first few cycles was near grain boundary area but as the cycling

progressed, uniform distribution was observed throughout the area irrespective of grain

orientation or size.

(a) (b) (c)

(d) (e)

Fig 4.2 IPF maps for B19 in NiTiCuCr for 0, 5, 10, 30, 50 cycles resp. @ ~10% strain

Fig 4.2 shows the IPF of retained martensite. It appears that martensite in the

undeformed sample has orientations around (001) crystallographic axis. After 10th

cycle, a

random distribution of orientations is observed. If we compare this to the phase map in Fig

4.1, there is almost no new martensite formation after 5 cycles. After 10 cycles, martensiteis no longer present only along grain boundaries, but distributed all over the scanned region.

And this new retained martensite appears to have no preferred orien tation.

Main observation:

Amount of residual martensite formed is extremely less (1-3%) even after 50 cycles

of fatigue. This indicates that if any stress-induced martensite formation occurred

during loading, it was reverting to austenite during unloading and no significant

accumulation of defects had occurred after 50 cycles.


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(a) (b) (c)

(d) (e)

(a) (b) (c)

(d) (e)

Fig 4.3 Color coded IPF images (a)(b)(c)(d)(e) & Phase maps (f)(g)(h)(i)(j) for

NiTiCuCr for undeformed, 5, 10, 30 and 50 cycles respectively @ ~25% strain

For understanding the effect of strain on residual martensite, similar fatigue cycles

were done at a significantly higher strain, approx. 25%. The same area was observed for

grain orientation and phase changes occurring as a result of fatigue. Since there was a slight

shift in the marked area of observation and the centre of the bulge, the deformed region in

slightly shifted from the centre towards right. IPF maps in Fig 4.3 clearly show extensive

grain fragmentation after just 5 cycles. Looking at the phase maps for the same region, it is

clear that most of the fragmented grains consist of retained martensite. After 5 cycles of


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deformation, the percentage fraction of martensite is about 19% as compared to about 1% at

earlier strain (~10%). This fraction increases to 37% as the number of cycles increases to 50.

Another important observation is that some original austenite grains are still retained

after 50 cycles, and these grains have blue (111), green (101) or nearby colors (orientations).

Grains with red (001) or its nearby colors (orientations) are first ones to form retained

martensite after the first 5 cycles. The (111) and (101) orientations of the austenite have the

ability to accommodate more strain as compared to (001) orientation. Hence the martensite

variants formed in (111) and (101) orientations is able to reorient and grow but in (001) the

martensitic variants undergo plastic deformation for the same strain, thus, resulting in

retained martensite in (001) orientation whereas reversible martensitic transformation in

(111) and (101). It may be speculated that at high strains, martensite formed in (001)

oriented austenite grains is more likely to be heavily deformed and hence retained upon

unloading.

(a) (b) (c)

(d) (e)


It can be seen that martensite has random texture, with a slight preference for (001)

orientation seen after 5 cycles. Austenite texture also appears to be more randomized after

deformation. It is speculated that the deformed region have grains of very small sizes which

are being considered while forming the IPFs. Therefore, it can be seen that the entire IPF is

covered even though only a few big grains can be seen in IPF maps.


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. (a) (b) (c)

(d) (e)


(a) (b) (c)

(d) (e)

Fig 4.6 (a),(b)&(c) Grey Scale maps for NiTiCuCr for 0, 5, 10cycles resp.@ ~25%

strain; (d) & (e) are surface relief due to martensitic formation


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The grey scale maps for different cycles of fatigue are shown in Fig 4.6. With increasing

deformation, the bands of martensite are observed to be increasing, this is evident from the

surface relief cause on the sample surface after deformation. The density of the martensitic

needles can be seen to increase with increasing fatigue.

Fig 4.7 Variation in fraction of martensite for different strains

In current study, the amount of martensites formed during the two different strains

was a good indication of how the martensite transformation varies with varying strain. It was

found that at strain of ~10% the retained martensite in the material was very less and rate of

increase with subsequent cycling was also low (ranged in order of few percents). Whereas

when the strain increased to ~25%, the martensite percent showed a steep rise even at low

number of cycling. This suggests that upto 10% strain, samples can undergo repeated cycling

without much change in the microstructure. This is very desirable in actuators that have to

undergo many cycles. At higher strain (25%), however, even after 5 cycles there was almost

19% retained martensite indicating that reversibility of phase transformation had been

significantly impaired at higher strains.

4.2 X-Ray Analysis

While EBSD scans are a good way to see change in microstructure, there is still a

limitation on the area that can be scanned. So X-ray diffraction was used to get volume

averaged data regarding the crystal structures present in the NiTiCuCr and NiTi deformed to

10% strain. Fig 4.8 shows Intensity vs. 2 plot for samples after 0, 5, 10, 20 cycles. In Cycle


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0 (undeformed sample) only austenite peaks are seen. With increasing number of cycles,

intensity of all martensite peaks increases implying an increase in martensite content in the

sample. Other peaks such as the M peaks of (220) and (132) appear only after 10 cycles of

deformation. The martensite amount is very low even after 20 cycles in both the alloys. The

peak A(220) has decrease from undeformed to 10 cycles deformation and then further

increased suggesting martensite M(220) formation at the same orientation. Also, graphs

below suggest that more orientations of martensite form in NiTiCuCr as compared to NiTi at

lower strains but almost same at higher strains.

(a) (b)

(c)

Fig 4.8 (a)&(b) X-ray graphs for NiTi and NiTiCuCr at ~10% strain for various cycles

of fatigue; (c) X-ray graph for NiTi and NiTiCuCr at ~25% strain and 20 cycles

From the graphs, it can be seen that the martensite contents can also be seen to

increase as the amount of strain increases. When NiTi and NiTiCuCr are compared for the

amount of martensite formed seems similar but at different orientations. Thus, we can

conclude that martensite content increase with strain but the orientations of the retained

martensite varies with the alloy.


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4.3 Strain Measurements using GOM

For calculating the amount of strain caused during fatigue with varying number of

cycles, we used ARAMISa GOM software which is an image processing software capable

of optical measurements. In order to obtain strain, extremely good quality images of the

same region on the sample surface are required before and after subsequent cycles of fatigue

deformation. These images are required to have sufficient features on the surface like second

phase, precipitates, grain boundary or even scratches, etc. The contrast of these features must

stand out when compared to the parent phase or image background.

In NiTiCuCr as well as NiTi, we already had these features in the form of precipitates

randomly distributed throughout the specimen surface. In order to create the contrast, mild

etching was done using H2O:HNO3:HF:: 5:4:1 volume fraction. Microindentation was done

so as to mark the same area. Then SEM images of the undeformed sample were taken at

1200X. Care should be taken while taking the SEM images for the deformed samples atvarious cycles that the brightness and contrast of the sample should be adjusted to be as close

as possible to undeformed image. Failure to do this will mean that the software cannot detect

facet points on the image which are required for strain calculation. As a result the entire

surface of the image will not be considered for calculating strain, only the detected region

will be. Another important factor in using ARAMIS for strain calculations by SEM images is

that for the kind of surface features your sample shows, facet point and facet steps are to be

decided which can only be determined by trials. For the images used in current project, the

facet size used was 30 with step size 13 with accuracy of 67% at computation steps 3.

The steps to use ARAMIS for strain calculations are: 1) Start New project from

File on the Access Toolbar at the top of screen 2) Select 2D project; by default it is 3D

3) Enter the description of the project 4) Set the Facet Size, Facet Step, Accuracy ,

Computation Steps, etc as project parameters 5) Once the project is formed, go to Stage

in the Access toolbar on top of screen and select Input Images via Image Series 6) Press

the arrow sign to select and input image (this should be your undeformed image and should

get set as Stage0) 7) Similarly input other deformed images in order of fatigue cycle 8) Set

Stage0 as start point using Auto start point or Add Start point 9) Compute the project by

going to Project on Access toolbar and then Compute Project .


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Fig 4.9 (a) Screenshot of the interface while using GOM; (b)(c)&(d) Color maps

showing major, minor strain and thickness variation respectively. It should be noted

that software is unable to detect entire surface area. Hence calculations are from the

detected area only.

Strain calculations were done for both NiTi and NiTiCuCr at different strains viz.

0.11mm bulge height for 50cycles, 0.23mm bulge height for 10, 20 and 50 cycles.

Table 4.1: Deformation data obtained from ARAMIS

NT,

0.11, 50

NT,

0.22, 10

NT,

0.22, 20

NT,

0.22, 50

NTC,

0.11, 50

NTC,

0.22, 10

NTC,

0.22, 20

NTC,

0.22, 50

Major Strain 8.84 11.9 20.36 16.24 8.01 10.49 16.78 17.64

Minor Strain 6.01 7.47 18.45 17.041 5.78 11.04 18.45 16.76

Thickness

reduction

1.012 1.35 1.95 1.78 1.06% 1.18 1.69 1.65

(NTNiTi, NTCNiTiCuCr, 0.11&0.22 are bulge height; 10, 20, 50 are no. of cycles)


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From the table, it can be seen that the strain values obtained for both the alloys at

0.11 mm bulge, 50 cycles is lower than that at 0.22 mm bulge, 10 cycles. Therefore, it can be

interpreted that the residual martensite formed under latter conditions is higher, which was

also evident from earlier studies. Also, in general, there is an increase in strains as the

number of cycles increased for both the alloys when bulged to 0.22 mm, except for NiTi

when number of cycles is increased from 20 to 50. It is highly likely that this is because the

area chosen for studies in 50 cycles sample was away from the bulge. In the limited data that

is available, major and minor strains appear to be not too different, especially for the

NiTiCuCr samples. This sort of a spherical shape of bulge is what we had hoped to achieve

from the setup.

NiTi is showing higher percentage of deformation as compared to NiTiCuCr at

similar conditions. The irregularity in the rate of increase and decrease of strain can highly

be attributed to personal inability to decide exactly the area which is going to get bulge.

Also, the images used cannot contain the features distributed evenly throughout the surface;

hence the images used for two different deformations contains different fraction of area

detectable by the software. These are the hindrances to effective use of the method which

needs to be sorted out. But this method can be used to make the FLDs for the alloy precisely.


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Chapter 5

Conclusions and Future Scope

Conclusions

1. Automation of the bulge fatigue set-up has been successfully carried out. Now,fatigue can be carried out for desired number of cycles at desired strain with

precision and control over the deformation process.

2. The EBSD study of the NiTiCuCr alloys shows much higher amount of martensitefor higher strain when the two strains 10% and 25% are compared. Even for 5cycles

at 25% strain the amount of martensite formed is much more than for 50 cycles at

10% strain. This implies that material is safe at lower strain of 10% for 50 cycles

3. It was observed that at higher strain the grains that seem to resist deformation andresidual martensite formation are mostly 2 orientations (111) and (101). With low

confidence it can be said that the orientation (001) is the preferred orientation for

formation of martensite.

4. GOM, image processing software, has been successfully used for determination ofstrain. This method, though is showing some limitations in softwares ability to

detect entire surface area, can be effectively used for calculating strains from SEM

images when compared to strain calculations from bulge height.


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Future Scope

1. Comparative Bulge Fatigue BehaviorA comparative study on bulge fatigue microstructure can be done for better

understanding of evolution of microstructure esp. martensite accumulation in all the alloys

and effect of various factors like composition, heat treatment, etc.

2. Martensite Orientation StudyMartensitic variants formed during fatigue can be identified using SEM images by

comparing with those which have earlier been identified experimentally by Nishida et.al.[41,42]. Also this can then be further applied in identifying preferred orientation of

martensite formed; even the preferred grain orientation can be found through this analysis for

martensitic formation. If the same region after increasing cycle of deformation is observed

then reorientation, if any, can be observed.

3. Developing Forming Limit DiagramWith the use of GOM (Aramis), FLDs can be formed for different SMAs. Samples

with different dimensions are required to be made such that all strain paths can be achieved

(as is done for sheet metal forming).

4. Identifying the effect of Strain rateBy small modification in the program and circuit designed, it can effectively be used

at varying strain rates as well. Hence there is scope for carrying out all the Bulge Fatigue

Studies at different strain rate.


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12.B. Strnade, S. Ohashi, H. Ohtsuka, S. Miyazaki, T. Ishihara, Effect of mechanical cycling onthe pseudoelasticity characteristics of Ti-Ni and Ti-Ni-Cu alloys, Mat. Sci. and Engg., 1995,

A 203, 187 196

13.G. Eggeler, E. Hornbogen, A. Yawny, A. Heckmann, M. Wagner, Structural and functionalfatigue of NiTi shape memory alloys, Materials Science and Engineering, 2004, A 378, 24

33

14.L. Catherine Brinsona; Schmidt, Rolf Lammering, Stress-induced transformation behavior ofa polycrystalline NiTi shape memory alloy, J.Mech. and Phy. Solids, 2004, 52, 15491571

15.Niraj Nayana, V. Buravallab, U. Ramamurty, Effect of mechanical cycling on the stressstrain response of a martensitic Nitinol shape memory alloy, Mat. Sci. and Engg A, 2009,

A525, 6067

16.R. J. Wasilewski, On the Nature Transformation of the Martensitic, Met. Trans. A, 1975, 6A,1405

17.Sachdeva, Miyazaki and Nia, US Patent No.5044947, Date:3rd Sep, 1991

18.M. Nishida, C.M. Wayman, and T. Honma, Precipitation Processes in Near-Equiatomic TiNiShape Memory Alloys, Met. Trans A, 1986, 17A, 1505,

19.M. Paryab, Asghar Nasr, Omid Bayat, VahidAbouei, Amin Eshraghi, Effect of heat treatmenton the Microstructural and Superelastic behavior of NiTi Alloy with 58.5wt% Ni, MJoM,

2010, 16 (2), 123-131

20.B. Strenade, S. Ohashi, H. Ohtsuka, T. Ishihara, S. Miyazaki, Cyclic stress-straincharacteristics of Ti-Ni and Ti-Ni-Cu shape memory alloys, Mat. Sci. and Engg., 1995,

A202, 148 156

21.S. Miyazaki, T. Imai, Y. Igo, and K. Otsuka, Effect of Cyclic Deformation on thePseudoelasticity, Met. Trans. A, 1986, 17A, 115

22.R.M.Tabanli T, N.K. Simha, and B.T. Berg, Mean Strain Effects on the Fatigue Properties ofSuperelastic NiTi , Met. And Mat. Trans. A, 2001, 32A, 1866


57/64

48

23.T. Duerig , A. Pelton, D. Stockel, An overview of nitinol medical applications,Materials Science and Engineering,1999, A273275, 149160

24.Carlton G. Slough, A Study of the Nitinol Solid-Solid Transition by DSC, TA Instruments

25.R. van Eijden, Numerical-experimental analysis of an improved bulge test for thin films,Master Thesis, Eindhoven University of Technology, Department of Mechanical

Engineering

26.Y. Xiang, Plasticity in Cu thin films: an experimental investigation of the effect ofmicrostructure, Ph.D. Dissertation, Harvard University, 2005, 54

27.L.B. Freund and S. Suresh: Thin Film Materials: Stress, Defect Formation, and SurfaceEvolution , Cambridge University Press, 2003, p.6

28.Tim Maitland and Scott Sitzman, Electron Backscatter Diffraction (EBSD)Technique and Materials Characterization Examples

29.J Kang1, M Jain2, D S Wilkinson1, and J D Embury, Microscopic strain mappingusing scanning electron microscopy topography image correlation at large strain, J.

Strain Analysis, 2005, 40-6

30.A.L. McKelvey and R.O. Ritchie, Fatigue-Crack Growth Behavior in the Superelastic andShape-Memory Alloy Nitinol, Met. And Mat. Trans., 2001, 32A, 731

31.S. Miyazaki a, K. Mizukoshi a, T. Ueki b, T. Sakuma c, Fatigue life of Ti50 at.% Ni and Ti40Ni10Cu (at.%) shape memory alloy wires, Materials Science and Engineering, 1999,

A273275, 658663

32.Ana Maria Figueiredo a, Paulo Modenesi b, Vicente Buono b, Low-cycle fatigue life ofsuperelastic NiTi wires, International Journal of Fatigue, 2009, 31, 751758

33.Wang, Z.-l. and Z.C. Kang, Functional and Smart Materials: Structural Evolution andStructural Analysis, Springer 1998.

34.Tadaki, K. Otsuka, and K. Shimizu, Shape Memory Alloys. Ann. Rev. Mater. Sci, 1988, 18,25-45


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fatigue behavior studies of niti shape memory alloys using bulge test_swapnil dhakate_10311017

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