Detwinning process and its anisotropy in shape memory alloys

Detwinning process and its anisotropy in shape memory alloys Yong Liu∗ School of Mechanical and Production Engineering Nanyang 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. It is 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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Smart Materials, Proceedings of SPIE, Vol. 4234 (2001)

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 of Ms, while upon heating a reverse transformation takes place at a critical temperature of As. 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.

0.4

Figure 1. Phase transformation in a NiTi SMA as “seen” by Differential Scanning Calorimeter. During cooling, cubic austenite transforming to monoclinic martensite begins at Ms (martensite starting temperature) and completes at Mf (martensite finishing temperature). Upon heating, martensite transforming to austenite begins at As and completed at Af temperatures. Forward transformation releases heat and results in an exothermic peak, while reverse transformation absorbs heat and leads to an endothermic peak.

Martensite

Heat Flow, W/g

0.2

Mf

Ms

Cooling

0.0 Heating

As

Af

-0.2

Austenite -0.4

-30

0

30

60

90

120

Temperature, °C

Martensite is a “soft” phase and austenite is a “hard” phase. When in martensitic state, the SMA containing 100% of lattice twins 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 to As 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 austenite phase 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 be referred for further information1-12.

(a)

MaCROSCOPIC CHANGE

A → M

σ

(b)

Superelasticity Cooling

Deforming

Heating

MiCROSCOPIC CHANGE Ferroelasticity

As Austenite

Twinned Martensite

De-twinned Martensite

ε

Shape memory

Af Austenite

T

A ← M

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 deformation can 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) (2) (3) (4)

Detwinning process under monotonic tension. Anisotropy of detwinning and shape recovery between tension and compression. Anisotropy of detwinning due to textures. Prediction of the detwinning and martensite deformation process. PROPERTIES

Shape Memory Alloys

MECHANISMS

Shape memory effect

Detwinning and thermally induced PT*

High damping capacity

Detwinning and stress induced PT

Wear resistance


Fatigue resistance


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 end of 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.

Tension Stress

Fully detwinned martensite Dislocation generation

Partially detwinned martensite

Further detwinning

Onset of detwinning/ variant reorientation Self-accommodated martensite containing 100% twins

0

Tension Strain

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 different detwinning 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 characterized by 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 internal stress 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 developed during 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.

85


2000

NiTi bar

1600

Stress, MPa

Compression 1200

800

Tension

Formation of high density dislocations; Detwinning is insignificant Further detwinning

400

Formation of high density dislocations 0

Detwinning

0

5

10

15

20

Strain, % 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 area

(a)

detwinned area

Initial plate boundary

(b)

Figure 6. (a) A partial detwinning and reorientation of martensite twins under tension of a NiTi SMA (annealed at 600°C) 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 SMAs33. 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 200°C 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 twoway 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 125°C which is about 50°C higher

86


than that of undeformed condition, and the (Ms-Mf) is about 18°C. For Sample B, the reverse transformation begins at about 110°C, however, the (Ms-Mf) is about 65°C. In addition, the two-way memory strain developed in Sample A is 2.7%, while in 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.

1

NiTi bar, predeformed under tension to 18% strain

(a)

Shape Recovery Strain, %

1 0

As = 125 oC

3

-1

As = 75 oC Mf = 27 oC

-2

εtw = 2.7%

-3

Af = 142 oC

Af = 93 oC

εr = 4%

-4 Ms = 45 oC

2

-5 -50

0

50

100

150

Temperature,

4

(b)

o

200

250

C

NiTi bar, predeformed under compression to 20% strain

Shape Recovery Strain, %

2

Ms = 31 oC

εr = 3.5%

Af = 85 oC

3

εtw = 0.8%

Mf = 3 oC

Af = 175 oC

o

2

As = 57 C

3

1 As = 110 oC 1

0 -50

0

50

100

150

Temperature,

200 o

250

300

C

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 are Ms =59°C, Mf = 39°C, As = 74°C and Af = 93°C, respectively.

87


Table 1. Transformation temperatures of a NiTi SMA before and after tension and compression deformations. NiTi bar, 600°C WQ As-annealed Tensioned to 18.5% strain Compressed to 20% strain

Ms °C 59 45

Mf °C 39 27

∆M °C 20 18

As °C 74 75

Af °C 93 93

∆A °C 19 18

∆T °C 35 48

As, °C 1st heat 74 125

Af, °C 1st heat 93 142

∆A °C 19 17

31

3

28

57

85

28

54

110

175

65

Ms and Mf are martensite start and finish temperatures, respectively. As and Af are 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 are polycrystalline 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.

600

NiTi rolled sheet

RD

TD 6.07%

Generation of dislocations within type II twins and a partial de-twinning of (001) compound twins

Stress, MPa

450

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 transverse direction (TD) leads to formation of dislocations and partial detwinning of (001) compound twins.

300

Low shape recovery 6.16%

150

De-twinning of type II twins

0

0

1

2

High shape recovery 5% 3

4

5

5.7%

6

7

Strain, %

88


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 to dislocation 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 during deformation along the TD. The reorientation and detwinning of (001) compound twins is responsible for the observed nonflat 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.

A C B Detwinned

Detwinned

(a)

(b) Figure 9. (a) Detwinning of martensitic NiTi rolled sheet (annealed at 600°C) 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 twins has 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

89


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 is considered 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. SD of type II twin

RD [010] / [001] 41.7° 95°/90.2°

48.3°

RD [010] / [001]

TD [100] / [14 0 1] SD of (001) Compd. twin

TD [100] / [14 0 1]

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]M direction 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) compound twins 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 the experimental 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 of texture 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. The deformation 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 phase transformation, 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.

90


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 the detwinning 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. ND

TD

β

L02

TD

RD

TD

0

L 10

β

0

Figure 11. Schematic representation of two types of lattices, L1 and L2 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 of L01 and L02 are assigned so that (001) and (010) directions are coincidence with RD and ND for lattice L01 , and with ND and RD for lattice L02 , respectively.

L →

a cos β    100 = a sin β ,    0 

L →

 0    100 = − a sin β ,   − a cos β 

0 1

0 2

0   010 = 0 ,   b  b    010 = 0 ,   0

c    001 = 0   0 0   001 = 0   − c 

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, γ, as twinning as shown 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 lattices having 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 Κ1, shear strain γ and shear direction η1 are intrinsically related to the lattice structure as well as the twinning type.

91


resolved shear stress

Κ1

shearable

γ

unshearable

η1 1

η1

η1 Κ2

Κ2 Κ1

(b) (c) (a) 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 process will 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 is interrelated 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. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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J. Perkins, Shape Memory Effects in Alloys, The Metallurgical Society of AIME, Plenum Press, New York, 1975. K. Otsuka and K. Shimizu, Int. Metals. Rev., 31, No. 3, 93, 1986. H. Funakubo, Shape Memory Alloys, Gordon and Breach Science Publishers, Amsterdam, 1987. T.W. Duerig, K. N. Melton, D. Stöckel and C. M. Wayman, Engineering Aspects of Shape Memory Alloys, Butterworth-Heinemann, London, 1990. R. Gotthardt and J. Van Humbeeck, Proc. of ICOMAT-95, J. de Physique IV, vol. 5, C8, Suppl. au J. de Physique III, No 12, 1995. M. Fremond and S. Miyazaki, Shape Memory Alloys, Springer-Verlag, 1996. K. Inoue, K. Mukherjee, K. Otsuka, and H. Chen, Proc. of Int. Conf. on Displacive Phase Transformations and Their Applications in Materials Engineering, 1996. J. Beyer, A. Böttger and J. H. Mulder, Proc. of ESOMAT-97, J. de Physique IV, C5, Suppl. au J. de Physique III, No 11, 1997. G. Airoldi, I.Muller and S. Miyazaki, Shape Momory Alloys-From Microstructure to Macroscopic Propertiee, Trans Tech Publications, 1997. K. Otsuka and C. M. Wayman, Shape Memory Materials, Cambridge University Press, London, 1998.


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