ani sot ropy of detwinning in smas

8/4/2019 Ani Sot Ropy of Detwinning in SMAs

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Smart Materials, Proceedings of SPIE, Vol. 4234 (2001)82

Detwinning process and its anisotropy in shape memory alloys

Yong Liu

School of Mechanical and Production EngineeringNanyang Technological University

Nanyang Avenue, Singapore

ABSTRACT

Detwinning in crystalline solids is a unique deformation mechanism partially responsible for the shape memory effect in

addition to phase transformation. Owing to an insignificant dislocation process during detwinning leading to inelastic

deformation, the residual strain can be recovered through a reverse transformation. The maximum shape recovery strain is

intrinsically related to the lattice geometry and twinning mode. While the magnitude of shape recovery strain is related to a

competition of detwinning versus dislocation generation responsible for the macroscopically observed martensite

deformation. The detwinning magnitude is directional, and in the polycrystalline materials, it is related to the textures.Without textures, the detwinning process in polycrystalline solid is isotropic. With textures, the detwinning process is

enhanced for certain directions and reduced for other directions and so do the shape recovery strain. The anisotropy in

detwinning process allows the possibility of maximizing the potential of the polycrystalline shape memory alloys. This

paper presents recent results on the anisotropy of detwinning as a function of loading mode and texture orientation. The

anisotropy in detwinning process is also responsible for the direction-dependence of the shape recovery strain. The

fundamental reason responsible for this detwinning anisotropy is associated with the combination of twinning types, texture

orientation and loading direction, which can be further treated mathematically based on a physical model.

Keywords: shape memory alloy, twinning, detwinning, dislocation, martensite, deformation, shape recovery, modeling,

textures.

1. INTRODUCTION

In the last a few decades, a great variety of new materials have been invented which show non-traditional functions and the

list of new materials increases quickly in length. Among the newly developed functional materials, a class of materials

termed smart materials have been attracted special attention. These materials are found to be able to transform energy from

one form to another, especially transform thermo/magnetic/electric energy to mechanical energy or, sometimes, vise versa.

These materials include shape memory alloys (thermo-to-mechanical), piezoelectric materials (electric-to-mechanical),

magnetostrictive materials (magnetic-to-mechanical), etc. In other words, these materials exhibit either thermo-mechanical

or electro-mechanical or magneto-mechanical property, and so on. New materials have been invented in such a fast pace

that it hardly has time to accurately define them accordingly. As a result, several other new materials are also called smart

materials, e.g., electro-rheological materials, magneto-rheological materials, etc. Smart materials are sometimes also called

active materials. Categorizing materials according to their function and corresponding mechanism is beyond the scope of

this paper. In this paper, we will concentrate on materials that exhibit ability to convert thermo-energy to mechanical energy

through mechanisms associated with phase transformation and microstructural reconfigurations, i.e. shape memory alloys

(SMAs). The deformation mechanism of SMAs is found to differ significantly from the classic deformation mechanism. Itis associated with domain switching/reorientation rather than dislocation mechanism that has been widely found in

structural materials while their mechanical properties are of primary concern. Understanding this unique deformation

mechanism will significantly contribute to the property optimization and effective fabrication of these materials as well as

their performance prediction. This article will present the recent results on the deformation mechanism of SMAs. Major

factors affecting the deformation mechanism and their significance in the subsequent shape recovery process will also be

described. This knowledge may provide reference to the understanding of the deformation mechanism of other types of

smart materials having similar domain structures in microscopic scale.

Correspondence: Email: [email protected].


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1.1 Background Knowledge

The microstructural mechanisms responsible for the shape memory effect are martensite detwinning and phase

transformation. In general, two major phases exist in SMAs, low temperature martensite and high temperature austenite.

Austenite has a cubic lattice (B2) while martensite has a monoclinic lattice (B19) and consists of 100% lattice twins. During

cooling, austenite transforms to martensite starting from a critical temperature ofMs, while upon heating a reversetransformation takes place at a critical temperature ofAs. Transformation starting and finishing temperatures are important

parameters of SMAs and can be determined by several experimental techniques. Among them Differential Scanning

Calorimeter (DSC) is one of most commonly used and a typical DSC result is shown in Figure 1.

-30 0 30 60 90 120

-0.4

-0.2

0.0

0.2

0.4

Martensite

Austenite

AfAs

Mf Ms

Heating

Cooling

HeatF

low,

W/g

Temperature, C

Figure 1. Phase transformation in a NiTi SMA as

seen by Differential Scanning Calorimeter.

During cooling, cubic austenite transforming to

monoclinic martensite begins atMs (martensite

starting temperature) and completes atMf(martensite finishing temperature). Upon heating,

martensite transforming to austenite begins atAs

and completed atAftemperatures. Forwardtransformation releases heat and results in an

exothermic peak, while reverse transformation

absorbs heat and leads to an endothermic peak.

Martensite is a soft phase and austenite is a hard phase. When in martensitic state, the SMA containing 100% of latticetwins can be easily deformed through variant reorientation/detwinning process. The deformation process and corresponding

microstructural change are schematically illustrated in Figure 2a. When martensite is deformed, a residual strain will remain

until heated toAs temperature where the residual strain can be recovered through a reverse transformation. However, when

the SMA is deformed in its austenitic state, superelasticity will be obtained due to thermodynamic equilibrium of austenitephase at the testing temperature. A schematic illustration of the superelasticity and stress-strain-temperature behavior of

ferroelastic SMA is shown in Figure 2b. Several good books and review papers on SMAs have been published and can bereferred for further information1-12.

AusteniteTwinnedMartensite Austenite

De-twinnedMartensite

Cooling Deforming Heating

MaCROSCOPIC CHANGE

MiCROSCOPIC CHANGE

(a)

As

Af

T

Superelasticity

Ferroelasticity

Shape memory

A M

A M

(b)

Figure 2. (a) Schematic illustration of the mechanism of martensite deformation in SMAs and shape memory effect. (b) Superelasticity

and stress-strain-temperature behavior of shape memory effect.


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Figures 2a shows the detwinning process and the shape memory effect in an "ideal" case, meaning that (1) during martensite

deformation process no dislocations are generated and, as a result, (2) upon reverse phase transformation, the deformationcan be recovered completely. However, in reality, several factors can affect the detwinning process and often "deviate" it

from the "ideal" situation, which in turn affect the magnitude of the subsequent shape recovery. These factors include test

temperature13-15

, annealing condition16-18

, grain size19,20

, stress mode21-24

, and microstructure especially textures25-29

.

1.2. Detwinning of Shape Memory Alloys

Deformation of structural materials especially steels involves significant dislocation generations. Deformation in SMAs,

however, proceeds mainly through a martensite variant reorientation or detwinning of twins in the early stage rather than

dislocation generations. The detwinning process in SMAs is of primary importance for the occurrence of shape memory

effect. In order to obtain a good shape memory effect, during deformation of SMAs, dislocation generation is not needed but

to be avoided.

What makes SMAs so attractive is their unique combination of various novel properties including shape memory effect,

superelasticity, high damping capacity, good fatigue and wear resistance, the highest kinetic output per unit volume among

all other materials and, of significant importance, an excellent biocompatibility for NiTi SMAs. As listed in Figure 3, most

of these properties are associated with the detwinning process. In the following sections, the detwinning of SMAs, its

importance in understanding the process of shape recovery, and its influencing factors including loading mode and texture

distribution will be presented in the following order.

(1) Detwinning process under monotonic tension.(2) Anisotropy of detwinning and shape recovery between tension and compression.(3) Anisotropy of detwinning due to textures.(4) Prediction of the detwinning and martensite deformation process.

PROPERTIES MECHANISMS

Shape memoryeffect Detwinning and thermally induced PT*

High damping capacity Detwinning and stress induced PT

Wear resistance Detwinning and stress induced PT

Shape Memory Alloys Fatigue resistance Detwinning and stress induced PT

High kinetic output Detwinning and thermally induced PT

Superelasticity Stress induced PT

Biocompatibility Inert titanium oxide layer

* PT - phase transformation

Figure 3. Shape memory alloys especially NiTi SMAs possess an unique combination of various novel properties highly attractive for

various applications including biomedical, MEMS, sensors & actuators, energy dissipation and vibration suppression, etc. Most of the

properties of SMAs are related to their deformation mechanisms of both detwinning and stress-induced phase transformation.

2. DEFORMATION OF SMAs UNDER MONOTONIC TENSION

When deform a martensitic SMA under tension, SMAs initially yield at a relatively low stress (around 100 MPa for NiTi

SMAs) and followed by a stress-plateau extending to about 6% strain as schematically illustrated in Figure 4. Starting from

the end of the stress-plateau, the deformation behavior of SMAs becomes similar to that of traditional structural materials. It

was later found that most of the secret of SMAs lies in the stress-plateau.

The onset of the stress-drop has been generally recognized to be the onset of the martensite reorientation/detwinning

process. The stress-plateau is a result of detwinning/variant reorientation process where dislocation generation is

insignificant. Within the plateau region, the detwinning process proceeds unevenly throughout the testing sample, meaning,


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at given time and external load, the deformation amplitude is different from region to region30,31

. Macroscopically, it seems

that a deformation band propagates throughout the sample during the deformation process. In polycrystalline SMAs, the endof stress-plateau is not the end of the detwinning process. By further deformation into the region beyond the stress-plateau,

further reorientation and detwinning of martensite twins, which are less favorable to the applied force, can be expected. In

this region, deformation of martensite twins is accompanied by a further increase in the applied stress. In addition, a high

density of dislocations has been generated. Plastic deformation will be induced in the martensite plates having orientations

unfavorable to the applied stress31

.

0

Self-accommodated martensitecontaining 100% twins

Partially detwinnedmartensite

Fully detwinnedmartensite

Dislocation generation

Further detwinning

Tension Strain

TensionStress

Onset of detwinning/

variant reorientation

Figure. 4. Current understanding of the martensite deformation process in shape memory alloys. Thermally formed martensite in SMAs

consists of 100% of lattice twins. Under tension, the lattice twins are detwinned leading to macroscopic deformation up to 8% strain.

Further deformation is realized through dislocation generations.

In a recent research32

on the mechanism of anisotropy in detwinning process due to textures, it is found that two differentdetwinning mechanisms may exist in SMAs, namely, domino detwinning and assisted detwinning. These two detwinning

mechanisms define the macroscopically observed mechanical behavior of the material.Domino detwinning is characterizedby a self-propagating manner of detwinning under constant load, i.e., when the resolved shear stress reaches a critical value

for some favorably oriented martensite twins, detwinning of these twins will take place. Along with this initial detwinning,

neighboring martensite twins of less favorably oriented are triggered to detwin due to increase in the localized internal

stress. This detwinning process will continue until a limit of the orientation is reached where the increase in the internalstress will be unable to further induce the detwinning of the remaining twins. In this case, an increase in the external load is

required for a further detwinning process to take place and this type of detwinning is proposed termed as assisted

detwinning. The domino detwinning seems related to the occurrence of the stress-plateau while the assisted detwinning is

related to a further detwinning beyond the stress-plateau region. Along with the assisted detwinning process, dislocation

generation will take place as soon as a critical resolve shear stress for dislocation generation is reached.

3. ANISOTROPY OF DETWINNING PROCESS BETWEEN TENSION AND COMPRESSION

For polycrystalline SMAs, the stress-strain curves are different between tension and compression exhibiting an "asymmetry"as shown typically in Figure 5. Different from that of tension, under compression, the SMA is quickly strain hardened and

no flat stress-plateau is observed. This result clearly suggests that, when application of SMAs is concern, attention should be

paid to the loading mode of the SMAs during operation from both design and performance prediction point of view.

The mechanism of the mechanical asymmetry is further revealed based on a TEM study of the microstructure developedduring deformation24. Figures 6a and 6b compare the microstructure of NiTi samples deformed to 4% strain under tension

and compression respectively. Clearly, specimen deformed under compression load consists of a high density of

dislocations and no significant detwinning of martensite twins has been found. The different deformation mechanisms

between tension and compression further lead to differences in the thermo-mechanical properties.


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0 5 10 15 20

0

400

800

1200

1600

2000

Tension

Compression

Stress

MPa

Strain, %

NiTi bar

Detwinning

Formation of high

density dislocations;Detwinning is

insignificant

Formation of high

density dislocations

Further

detwinning

Figure 5. Anisotropy of martensite deformation process between tension and compression. Microstructure study shows that tension

deformation leads to martensite detwinning while compression deformation results in formation of a high density of dislocations and no

significant detwinning process took place.

type II twin

detwinned areadetwinned area

Initial plateboundary(a)

(b)

Figure 6. (a) A partial detwinning and reorientation of martensite twins under tension of a NiTi SMA (annealed at 600C) to 4% strain.

(b) Dislocations formed in martensite twin bands in NiTi specimens compressed to 4% strain. For details refer to Ref. 24.

As has been reported, when deformed in the martensitic state, a two-way memory effect can be developed in the SMAs

33

.Due to the different deformation mechanism between tension and compression, the resulted two-way memory effect is

found also different. Figure 7 highlights the results obtained from two pre-deformed NiTi samples, one was under tension to

18.5 % strain (Sample A) and the other was under compression to 20% strain (Sample B). After unloading to a stress free

state, both samples were then heated to above 200C and their shape recovery processes were recorded. Upon heating,

Sample A contracts while Sample B elongates. As shown in Figure 7, for both samples, pre-deformation leads to a shift of

the austenite transformation temperatures to higher temperature range, i.e., the martensite phase is stabilized. For both

samples, a two-way memory effect was developed, however, differs in magnitude of both the shape recovery and the two-

way memory strain. In addition, the magnitude of martensite stabilization and the reverse transformation features are also

different between samples under different deformation modes. During the 1st heating step, Sample A recovers 4% while

Sample B recovers about 3.5%. For Sample A, the reverse transformation begins at about 125C which is about 50C higher


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than that of undeformed condition, and the (Ms-Mf) is about 18C. For Sample B, the reverse transformation begins at about

110C, however, the (Ms-Mf) is about 65C. In addition, the two-way memory strain developed in Sample A is 2.7%, whilein Sample B it is only 0.8%. It is also noted that the transformation hysteresis is much narrower for Sample A than for

Sample B as listed in Table 1. Figures 5-7 clearly show that the deformation mode not only affects the detwinning process

and the resulted mechanical behavior of the SMAs, but also strongly affects the subsequent shape recovery behavior and

transformation characteristics of SMAs. Detwinning corresponds to a better shape recovery and higher two-way memory

strain, while a heavy dislocation generation corresponds to a lower shape recovery and poor two-way memory strain.

-50 0 50 100 150 200 250

-5

-4

-3

-2

-1

0

1 (a)

3

2

1

tw = 2.7%

r = 4%

Mf = 27

oC

Ms = 45

oC

As = 75

oC

Af = 93

oC

As = 125

oC

Af = 142

oC

NiTi bar, predeformed under tension to 18% strain

ShapeRecoveryStrain,

%

Temperature, oC

-50 0 50 100 150 200 250 300

0

1

2

3

4(b)

3

2

1

Ms = 31

oC

Mf = 3

oC

Af = 85

oC

As = 57

oC

Af = 175

oC

As = 110oC

tw = 0.8%

r = 3.5%

NiTi bar, predeformed under compression to 20% strain

ShapeRecoveryStrain,

%

Temperature, oC

Figure. 7. Shape recovery and two-way memory effect developed in a NiTi SMA by martensite deformation under (a) tension to 18.5%

strain and (b) compression to 20% strain. Anisotropy in both shape recovery and two-way memory strain is found between tension and

compression. The transformation temperatures are roughly estimated by a slope line method in the graph and can be different from the

values determined by other methods, e.g. DSC. In the undeformed condition the transformation temperatures areMs =59C,Mf= 39C,As= 74C andAf= 93C, respectively.


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Table 1. Transformation temperatures of a NiTi SMA before and after tension and compression deformations.

NiTi bar,

600C WQMsC

MfC

MC

AsC

AfC

AC

TC

As, C

1st heatAf, C

1st heatAC

As-annealed 59 39 20 74 93 19 35 74 93 19

Tensioned to

18.5% strain

45 27 18 75 93 18 48 125 142 17

Compressed

to 20% strain

31 3 28 57 85 28 54 110 175 65

Ms andMfare martensite start and finish temperatures, respectively.

As andAfare austenite start and finish temperatures, respectively.

M = Ms - Mf, A = Af- As, T = As Mf

4. ANISOTROPY OF DETWINNING PROCESS DUE TO TEXTURES

Various results25-29 have shown that, textured SMA rolled sheet exhibits a considerable difference in plateau-strain and

shape recovery strain when tested along different directions. Figure 8 summarizes the mechanical behavior of martensitic

SMA and corresponding microstructure changes as a function of loading direction relative to the texture orientation in a

rolled sheet material. Tension along rolling direction (RD) leads to a stress-plateau and martensite detwinning (Figure 9a).While tension along transverse direction (TD) results in dislocation generation (Figure 9b) and strain hardening. In general,

when tested along TD the shape recovery strain is the lowest if compared to measurement along other directions27. For a

rolled sheet, the direction where the highest shape recovery strain is obtained corresponds to the direction where the longest

stress-plateau-strain is obtained during the martensite deformation. While the test direction where a poor shape recovery is

obtained corresponds to a martensite stress-strain curve with no flat stress-plateau.

From an application point of view, prediction of the stress-strain relation and shape recovery strain under given texture(s),

loading direction and deformation amplitude is of primary importance. Understanding the rule of textures in the anisotropy

of various properties of SMAs can also lead to the optimization of the properties for desired applications by optimization of

the material processing procedures. Recent results29

suggest that the texture orientation relative to the shear direction of

lattice twins play a critical role in the overall performance of the SMAs. In most cases, SMAs in operation arepolycrystalline materials prepared by either rolling or drawing or other heavy deformations. Thus, textures exist in most of

the SMAs being used. In general, possess of textures is a wanted microstructural state if the material is used properly.However, it can also lead to unwanted poor performance if the textured materials are used improperly.

0 1 2 3 4 5 6 7

0

150

300

450

600

High shape recovery

Low shape recovery

Generation of dislocations within

type II twins and a partial

de-twinning of (001) compound twins

De-twinning of

type II twins5%

6.07%

5.7%

6.16%

NiTi rolled sheet RD TD

S

tress,

MPa

Strain, %

Figure 8. Anisotropy of martensite

deformation process in a textured rolled NiTi

SMA sheet. Tension along rolling direction

(RD) leads to detwinning of type II

twins. While tension along transversedirection (TD) leads to formation of

dislocations and partial detwinning of (001)

compound twins.


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Based on a systematic study29

on the anisotropy in detwinning process of a NiTi rolled sheet, it was found that, loading

along RD leads to re-orientation and detwinning of mainly type II martensite twins. While loading along TD leads todislocation generation and detwinning of (001) compound twins. The stress-plateau occurring during deformation along the

RD is related to the detwinning of mainly the type II twins. The detwinning of (001) compound twins is favorable to

the loading along TD. Although some martensite plates are detwinned by deformation to 6% along the TD direction, the

observed type II and (11 1 ) type I twins are not de-twinned. In addition, a large amount of plastic deformation has

been introduced in these twin bands especially type II twins. This is responsible for the strain hardening duringdeformation along the TD. The reorientation and detwinning of (001) compound twins is responsible for the observed non-

flat stress-plateau/stress-transition in the stress-strain curves. This observation can be further understood based on a

crystallographic analysis as schematically shown in Figure 10.

Detwinned

Detwinned

A

B

C

(a)

(b)

Figure 9. (a) Detwinning of martensitic NiTi rolled sheet (annealed at 600C) under tension to 6% strain along rolling direction. (b)

Dislocations formed in NiTi martensite under tension to 6% strain along transverse direction. For details refer to Ref. 29.

For two major martensite texture components, (010)[001]M and (001)[010]M, the shear direction (SD) of type II twinshas angles of respectively 41.7 and 48.3 to the RD of the specimen, while it has respectively 90.2 and 95 angles to the

TD. For the (001) compound twins, the SD has an angle of 6.56 and 0 to the TD, while it has 96.8 and 90 angle to the

RD for the (010)[001]M and (001)[010]M texture components, respectively. Thus, deformation along RD is easier to shear


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the type II twins and, in contrast, they are difficult to detwin when loading along TD. However, for the (001)

compound twins, the case is opposite, deformation along TD will easily shear the (001) compound twins. This relation isconsidered to be responsible for the differences in the microstructural changes during deformation along both directions and

is also responsible for the difference in the respective stress-strain curves.

RD [010] / [001]

TD [100] / [14 0 1]

SD of

type II twin

RD [010] / [001]

TD [100] / [14 0 1]

48.3

41.7

95/90.2

SD of (001)

Compd. twin

Figure 10. Schematic representation of the relationship among shear direction (SD) of twins, rolling direction (RD) and transverse

direction (TD) for two martensite textures, (001)[010]M and(010)[001]M. SD of type II martensite twins is parallel to [011]Mdirection which has 48.3/41.7 angle to RD and 95/90.2 angle to TD for (001) [010]M/(010)[001]M texture. SD of (001) compoundtwins is nearly parallel to the TD and perpendicular to the RD for both (001)[010]M and (010)[001]M textures.

5. PREDICTION OF THE DETWINNING PROCESS

To predict the shape recovery strain as a function of loading direction for textured SMAs, efforts have been made from an

approach of lattice correspondence between martensite and austenite by taking into account the transformation strain as a

function of lattice orientation26,34-36

. The calculated results show some consistency in tendency if compared to theexperimental data but have significant difference in magnitude. In the previous research, the details of martensite

detwinning prior to reverse transformation have not been considered instead of idealized. The anisotropy of martensite

deformation mechanism due to textures has in fact largely been ignored. Formation of a high density of dislocations during

martensite deformation will inevitably reduce the shape recovery strain upon subsequent heating. Thus, for a thorough

understanding of the anisotropy of shape recovery behavior, an understanding of the anisotropic deformation behavior of

martensite twins prior to heating is of significant importance. So far, no effort has been made on modeling the effect oftexture orientation on the anisotropy in detwinning process. The orientation dependence of the barrier stress for martensite

detwinning, the orientation dependence of the length of plateau strain and the shape of the stress-strain curves in general

may all be related to a single factor, i.e., the orientation dependence of the martensite detwinning process.

Based on above understanding of the anisotropy in martensite detwinning process, we may consider a different approach in

predicting the shape recovery strain as a function of texture orientation. The underlined principle is very simple: the

magnitude of shape recovery strain is intrinsically related to the magnitude of martensite detwinning process. Thedeformation of martensite consists of two mechanisms in microstructural scale, detwinning and dislocation generations.

Both mechanisms operate during martensite deformation but apply opposite effect on the subsequent shape recovery

process. Detwinning does not introduce permanent deformation to the material rather than introducing merely inelastic

deformation with atoms migrate less than an atomic distance and act synergistically. While dislocation generation introduce

permanent deformation to the materials with atoms migrate more than one atomic distance. Thus, upon reverse phasetransformation, the deformation associated with detwinning will be recovered while the deformation associated with

dislocation process will remain permanently. In order to obtain a high recovery strain, one should promote detwinning

process and suppress dislocation process during martensite deformation.


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In a recent research, Zheng and the present author have tried to predict the martensite detwinning process by taking into

consideration of texture distribution, twinning shear and loading direction32

. Based on a proposed physical model of thedetwinning process, a crystallographic analysis was performed on the resolved shear stress along the shear direction of

type II and (001) compound twins for two textures, namely, (010)[001]M and (001)[010]M textures. Further, the

deformation kinetics of both types of twins for loading along different directions was examined by assuming several

Orientation Distribution Functions (ODFs) of the textures. Finally, the simulation results of the stress-strain curves under

deformation along both RD and TD are compared to experimental results.

In order to describe the relation between texture orientation, twinning shear and loading direction, a Cartersian coordinate

system was introduced as shown in Figure 11. Where axes x1, x2 and x3 coincide with the RD, TD and the normal direction

(ND) of the rolled sheet, respectively.

TDND

RD

02L

01L

TDTD

Figure 11. Schematic representation of two types of lattices,0

1L and

0

2L lattices, in accordance with two textures, (010)[001]M and

(001)[010]M textures, respectively.

In conjunction with the (010)[001]M and (001)[010]M textures, two lattices of0

1L and

0

2L are assigned so that (001) and (010)

directions are coincidence with RD and ND for lattice0

1L , and with ND and RD for lattice

0

2L , respectively.

===

0

0001,0

0

010,

0

sin

cos

1000

1

c

b

a

a

L

==

=

c

b

a

aL 0

0

001,

0

0010,

cos

sin

0

1000

2

A schematic representation of the atomic arrangement of lattice twins is shown in Figure 12a. As schematically illustrated,

detwinning proceeds through shearing of twins relative to matrix (see Figure 12b). The lattice shear is taken place opposite

to the shear direction of twins and the shearing plane is parallel to the twinning plane. Consider that the detwinning process

is proceeded in the opposite direction of twinning by a lattice shear of the same magnitude of shear strain, , as twinning asshown in Figure 12c. It is obvious that, during the detwinning process, ideally, only a half of the lattices can shear from one

position (having mirror plane symmetry) to the other position (loss of mirror plane symmetry). Thus, only those latticeshaving an angle less than 90 degree inclined to the externally applied shear stress are able to shear (shearable}, otherwise,

they are unable to be sheared (unshearable) as also illustrated in Figure 12c. Clearly, only these two possibilities exist under

shear stress and the detwinning process is due to the lattice shear of the shearable portion of the twined lattices. Twinning

plane 1, shear strain and shear direction 1 are intrinsically related to the lattice structure as well as the twinning type.


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1

1

(a) (b) (c)

unshearable

resolved shear stress

shearable

22

11

11

Figure 12. Schematic illustration of the detwinning process. (a) Lattice twins formed thermally as a result of phase transformation and (b)

detwinning through shear along shear direction of twins. (c) Under given loading mode, only half of the paired lattice (twinned lattice)

can be (need to be) sheared in order to detwin.

Analysis of the detwinning process suggests that the anisotropy of detwinning process in textured SMAs is responsible for

the anisotropy of their mechanical behavior. The anisotropy of detwinning process is due to a combination of texture

distribution and twinning type. In general, the stronger is the texture the stronger is the anisotropy of the detwinning

process. The resolved shear stress along the shear direction of the thermally formed twins is the driving force for their

detwinning. As soon as the resolved shear stress in some grain(s) of twins reaches the critical value, the detwinning processwill begin. The barrier stress of the detwinning process is dependent on the twinning type and is a function of its twinning

shear strain. For further details in analysis please refer Ref. 32.

6. MAJOR CONCLUSIONS

Details of martensite detwinning process affect the shape recovery magnitude of SMAs. Detwinning promotes shape

recovery while dislocation generation during martensite deformation suppress the shape recovery. In polycrystalline SMAs,martensite deformation is a competition between detwinning and dislocation generation. This process is strongly directional

and is associated with texture orientation and loading direction. For a polycrystalline SMA drawn bar material, tension

promote detwinning, shape recovery strain and two-way memory strain, while compression leads to dislocation formation

which in turns results in poor two-way memory effect and less shape recovery strain. For a textured sheet material, the

relation among shear direction of twins, the texture orientation and testing direction plays a critical role in the martensite

detwinning process and the subsequent shape recovery process. The detwinning process and shape recovery magnitude isinterrelated and should be predictable based on physical model.

ACKNOWLEDGEMENTS

The author wishes to thank former colleagues, Z. L. Xie, J. Van Humbeeck and L. Delaey for a collaboration on research

of shape memory alloys, based on which a further understanding on the deformation process of SMAs is

achieved.

REFERENCES

1. J. Perkins, Shape Memory Effects in Alloys, The Metallurgical Society of AIME, Plenum Press, New York, 1975.

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