in-situ neutron diffraction study of microstructure evolution

7/30/2019 In-situ Neutron Diffraction Study of Microstructure Evolution

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IN-SITU NEUTRON DIFFRACTION STUDY OF MICROSTRUCTURE EVOLUTION

AND THE DEVELOPMENT OF A MICROSTRUCTURE-BASED GENERAL

CONSTITUTIVE LAW OF SOLIDS FOR COMPUTATIONAL MATERIAL SCIENCE

Jorge Cisneros, Ph.D. Student

Dr. Xin Wu, Associate Professor

Wayne State University, Detroit, MI 48202

In Collaboration with

Oak Ridge National Lab, Oak Ridge, TN 37831

1.INTRODUCTIONOver the last decade, the manufacture of advanced high strength steels for increased fuel effiencyand saftey of automobiles has been a primary concer for the automotive industry. Dual-phase

(DP) steels show an advantage over traditional carbon steel, because of the high strength andsufficient plasticity. However, production with these new steels is difficult. Due to the strengthand higher work hardening rate, stamping presses appraoch their tonnage limit quickly and there

are also problems with spring back. Better prediction of microstructure during the manufacturing

of the material could eleviate these problems. DP steels reley on micro alloyed composition

which include and allotropic phase transformation of austenite to various low tempuraturephases to achieve a balance between strength and ductility. During deformation retained

austenite is transformed by strain martensite. In recent year many studies have been conducted

on the effects of DP steel microstucture from: various heating rates, stress inducedtransformation , and heat affected zone (HAZ) from welding. A general constitutive law

describing the plasticity of solids does not exist (unlike gas and fluid). Progress in

thermodynamic treatment has been made to introduce microstructure entropy into the materialequation as a state parameter and make the deformation process to be a closed loop form, enableto address the problems in classical plasticity such as the material behavior under non-linear

strain paths and with loading/unloading/reverse loading. The experimental measurements of

microstructural entropy under various processes are critically needed

2.BACKGROUND OF THE PROJECT2.1.Current DP steel manufacturing techniqueTypical DP steels are produced from a low carbon steel with small amounts of micro alloyed

element, which is intercritically heated followed by a specified amount of cold working. The

chemical composition for DP980 is shown in Table 1. This results in hard-phase martensite

grains embedded in a soft ferrite matrix. The micrograph of DP980 is shown in Figure 1 asobserved by scanning electron microscope (SEM). The volume fraction of martensite is 32 pct,

which was determined through image analysis.

Table 1. Chemical composition (Weight Percent) of DP980 Steel


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Figure 1. SEM micrograph of DP980 steel. "F" and "M" represent ferriteand martensite, respectively.

2.2.DP property testingTo evaluate the performance DP980, various testing and evaluation methods need to be used, and

the correlation between heat treatment and resulting performance is needed. The commonly usedmechanical testing methods are tension, and hardness testing. A samples strip is heated throughcontrolled induction heating then tested once cooled. These tests are compared to the values of

an unaltered sample strip. Results of the some tensile test and hardness test are shown in figure 2

and 3 respectively. Optical microscopy is then used to analyze the micro structure and determine

the volume fraction. Once the sample has been reheated and cooled it is impossible to distinguishretained austenite from ferrite. An in-situ monitoring technique is highly desired. High energy X-

ray diffraction can be used to identify the microstructure composition in-situ during heating and

cooling. One drawback of this technique is the low frame rate of observation, which is to slow toobserve the grow rate of martensite of a rapidly cooled sample.

Time constrains when using in-situ diffraction techniques, results in limited tests having beenpreformed under cyclic loading. This test could also give important insight to the fatiguemechanics of DP980 steel.


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Figure 2. True stress vs. strain for DP980 with various maximum reheating tempuratures.

Figure 3. Hardness vs. Peak heating temp for various cooling rates.

3.IDETIFIED PROBLEMS OF INTEREST AND THE NECESSITY OF NUETRONBEAM DIFFRACTION ANALYSIS

1. Importance of understanding microstructure: The microstructure of metal isresponsible for the strength and ductility properties. Dp 980s microstructure typically

consist of martensite and ferrite. During deformation the soft ferrite plastically deformsbefore martensite. Depending on the size of the typical unit cell and the volume fraction

of each phase are the most important parameters for the material. After reheating

depending on the cooling rate it is possible to have retained austenite, which transformfrom stress during deformation to martensite. This transformation results in a increase in

ductility.


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2. Develop understanding of effect from thermodynamic treatments: Induction heatingis currently the leading method for re-heatment treatment to increase the formability of

DP980. It is preferred for the short period of time needed to heat the metal enough to

change the microstructure. Preliminary studies have shown a increase in formability.However, without in-situ testing understanding the exact variables which allow the metal

to be tailored to sny specification for manufacturing.3. Non-destructive examination on bulk specimens for in-situ testing: With the use of high

energy beam to penetrate bulk polycrystalline solids, the modern Neutron Diffractiontechnique allow in-situ examination of interior crystallographic evolution, so that the

microstructural entropy and its evolution during thermomechanical processes can be

measured, which otherwise can not be obtained otherwise by any existing traditionalmetallurgical approach such as surface XRD and electron microscopy. ORNL has recently

enhanced the beam sampling rate that allows obtaining detailed microstructure information

during phase transformation and during large plastic deformation of various mode

combinations, critical to establish the experimental foundation for the theoreticaldevelopment..

4.EXPERIMENTAL APPROACH FOR THE PROPOSED STUDYAdvanced testing on coupled mechanical, thermal and electrical resistance testing will be used

under static and fatigue loading, while using neutron diffraction as a non-destructive bulk

material examination technique is to be used for in-situ monitoring the evolution in the test.

(1)Heating test set upFor heated test, the specimen is mounted inside of a induction coil with a attached thermo

couple(Fig. 4). A micro controller is used to control the heating rate and air convection is used to

control the cooling rate. This test will also be compared to the results of resistive heating furnace,

which has control of heating and cooling rate and a thermocouple for recording temperature. A

third method for heating the sample is using hot air gun and convection. A thermal profile isdecided based off previous analysis. The heating time and holding time are the same for all

sample and the cooling rate will be varied (Fig. 5).


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Figure 4: Induction heating setup. Micro controller, AC power supply, and heating coil.

Figure 5: Thermal history for test sample.

(2)Tensile test setupAny existing load frame can be suitable for the ranges of load and velocity, and additional

sensors for load, displacement, and temperature distribution will be prepared for enhancing theexisting measurement or for back up.

An infrared image system will be used, and the local temperature distribution will be recorded at

8 frame per second to for a back up verification of thermo couples.


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Figure 6: Thermal images from infrared camera.

Fatigue test setup:

The PI designed and fabricated a high frequency fatigue testing system that uses a voice coil

actuator for axial loading and a motor for rotational actuator to produce multi-axial load. A 3Dwireless sensor is mounted for dynamic testing. The high-frequency actuator can provide 100Hz

at 1mm amplitude, or even higher frequency at reduced traveling, see Fig 7. The system can be

reconfigured for current tab tension-shear fatigue.

For testing thermal-mechanical coupled loading and damaging, the existing resistance furnacewill be replaced by a liquid-tube that contains two heating wires with independent close-loop

heating control, a technique recently developed by PI for obtaining microstructural energy

(entropy) of deformation from total applied energy and adiabatic heat [ ]. The foundation of the

thermodynamics of microstructure entropy on constitutive law has been established byBerdichevsky [4] who is a colleague of the PI. The experimental technique developed is well

suited for the proposed study to obtain uniform heating, and the applied heat can be recorded for

energy balance analysis as a means of damage evolution.


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A high-frequency testing system. It has the flexibility for testing in air or in furnace,

under static and fatigue load, and capable of providing a high frequency fatigue at

100Hz and 1mm amplitude.

Sensing: Utilize tools available at ORNL neutron beam line to record the evolution of

microstructure during testing.

5.PROPOSED TASKSIn this study thermal treatment and mechanical processes will be performed on DP steel at

ORNL, for the following tests:

5.1.Task 1. Set up portable multi-purpose testingsystem at ORNL:The system has a mechanical load-frame with various fixtures and an environment

chamber, capable to provide deformation in tensile, compression, bending, under static andfatigue loading and at room or elevated temperatures - available at WSU.

5.2.Task 2. In-situ measurement of statistical and spacial microstructuresunder various loading conditions:

a) Static grain growth under various constant temperatures, anddynamic grain growth under various strain rates and temperatures: the

statistical crystal orientation evolution, (not sure if can also do

beam scan to obtain spacial crystal orientation evolution - more

interest here). The results will be used to develop grain growth model

and the model for grain boundary energy.b) In-situ phase transformation and kinetics: for recentnano-martensite steels and for advanced high strength steels (AHSS,e.g. DP980, TRIP780). measure phase transformation starting and ending

temperature during heating and cooling at various heating/cooling

rate, with and without deformation, including cooling by spraying

liquid nitrogen for understanding the nano-martensite formationmechanism.

0

1000

2000

3000

4000

5000

0 100 200 300 400 500

Time (ms)

Accelaration(2.5micro-g)

Channel 1

Channel 2

Channel 3

100 Hz/1mm


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c) Strain hardening: continuous straining, straining with non-linearstrain path, and in-situ measurement of microstructure evolution for

monolithic metals and for duplex metals. The strain hardening process

is measured by statistical measurement of crystal orientationdistribution, the lattice constant changes and the degree of

order/disorder of crystals. If possible, develop a beam scan techniqueor mechanical shadowing mechanism for obtaining additional 3D spacialmicrostructure information.

d) Loading/unloading behavior and internal stress development: bycyclic straining under tension and bending the in-situ latticeconstant evolution can be measured and related to internal stress

development among different phases/constituents (for metals with

non-uniform microstructures), as well as macroscopic residual stress

development, important for failure behavior and springback prediction,

6.SCHEDULEBased on above defined on-line testing the total beam line time requested are given:

Before 5/2013: preparation of systems and pre-testing

During the summer intership:

I. Setup testing system and training for high-energy beam application.(Note: the applicant has prior experience with the use of ANL highenergy XRD)

II. Thermal test: isothermal, heating and cooling under a constant rate.III. Static testing of above mentioned mechanical testingIV. Dynamic testing combination of thermomechanical testing: under different strain

rate and temperatures.

7.SIGNIFICANCE OF THIS STUDY

7.1.Scientific and Engineering Relevance of This Study:Develop a framework of microstructure-based constitutive equation for solids, and applied to

computer simulation of microstructure entropy and large plastic deformation behavior undervarious thermomechanical processes, being of great interest in current manufacturing process

simulation.

in-situ neutron diffraction study of microstructure evolution

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