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Effect of deformation by stress-induced martensitic transformation on the transformation behaviour of NiTi
Intermetallics 8 (2000) 67±75
Eect of deformation by stress-induced martensitic transformation on the transformation behaviour of NiTi Yinong Liu*, G.S. Tan Department of Mechanical and Materials Engineering, The University of Western Australia, Nedlands, WA 6907, Australia Received 9 November 1998; received in revised form 3 August 1999; accepted 19 August 1999
Abstract It has been reported that martensite in near-equiatomic NiTi is thermally stabilised after a moderate deformation via martensite reorientation. This work continues the study by investigating the eect of deformation via stress-induced martensitic transformation on the transformation behaviour of the alloy. It was observed that the stress-induced martensite was also stabilised relative to the thermal martensite formed on cooling, as indicated by an increase in the critical temperature for the reverse transformation. Associated with the stabilisation, the heat eect, as determined by dierential scanning calorimetry, and the temperature interval of the reverse transformation, were measured and found to decrease with increasing level of deformation. The experimental results also demonstrated that the stress-induced martensitic transformation was microscopically localised, as expected for a ®rst-order phase transformation. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Martensitic transformations; B. Shape-memory eects
1. Introduction It has been reported in the literature that thermoelastic martensite in shape-memory alloys can be thermally stabilised by deformation, as indicated by an increase in the critical temperature for the reverse transformation of the deformed martensite relative to the thermal martensite [1±3]. The stabilisation eect is observed to be a one-time eect, which vanishes once the deformed martensite has reverted back to austenite on heating. Lin and colleagues investigated the eect of cold rolling on the transformation behaviour of binary NiTi alloys [1]. They observed that the critical temperature for the reverse transformation increased by 120 K after a severe deformation to 40% thickness reduction. They proposed that deformation-induced defects, including both dislocations and vacancies, were responsible for this stabilisation eect by hindering the movement of phase boundaries for the reverse transformation. The disappearance of the stabilisation eect in subsequent
* Corresponding author. Tel.: +61-8-9380-3132; fax: +64-8-93801024. E-mail address: [email protected] (Y. Liu).
transformation cycles was attributed to the annihilation of vacancies upon overheating and the formation of dierent martensite variants which are free from the hindrance of the deformation-induced dislocations in subsequent cycles. Whereas the explanations may seem reasonable for the case of deformation by cold rolling, where plastic deformation is inevitable even at low strain levels, they appear inadequate in explaining the stabilisation eect caused by mild deformation via martensite reorientation under monodirectional loading conditions [2,4]. Under these conditions, the stabilisation eect has been observed to be only a few degrees for deformation to low strain levels. Overheating by a couple of decades of degrees, as a thermal process, is hardly enough to cause any real change in the metallurgical conditions of a specimen. Furthermore, the selfaccommodating thermal martensite in a fully annealed polycrystalline NiTi deforms at a typical stress level of 120 MPa and the deformation is often observed to be fully recoverable up to a tensile strain of 6%. In this case, the production of structural defects during the process of martensite reorientation needs to be veri®ed. To clarify these uncertainties on the causes of the stabilisation eect, systematic investigations on the eect of mild deformation via martensite reorientation have been
0966-9795/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00079-5
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Y. Liu, G.S. Tan / Intermetallics 8 (2000) 67±75
carried out [2,5,6]. Piao and colleagues studied the transformation behaviour of both single-crystal and polycrystalline specimens of three alloys, including CuZnAl, NiTi and NiTiNb, by electrical resistance measurement and microscopic examination [2]. They proposed that the relaxation of the elastic strain energy stored in multiple-variant martensite is responsible for the stabilisation eect. For single crystals the relaxation occurs during deformation by martensite variant reorientation without plastic deformation. For polycrystalline specimens the relaxation of internal elastic energy is only achieved during deformation with some plastic deformation, due to the constraint of grain boundaries. Liu and colleagues studied the eect of deformation via martensite reorientation on the transformation behaviour of polycrystalline near-equiatomic NiTi alloys by dierential scanning calorimetry (DSC) measurement [6] and thermal dilatation measurement [5]. In addition to con®rming the eect of deformation on transformation temperatures, they also observed that the heat eect associated with the reverse transformation of the reoriented martensite increased as relative to the thermally induced martensite [6] and that a two-way memory eect developed after the deformation [5]. The magnitude of the increase of the endothermic heat for the reverse transformation of the reoriented martensite was found too high to be accounted for by the stored elastic strain energy of the thermal martensite. The occurrence of the two-way memory eect, as high as 4% in tensile strain, is indicative of a directional internal stress ®eld created by the reverse transformation of the reoriented martensite. On the basis of these observations, Liti and Favier proposed that both elastic energy and irreversible energy contribute to the stabilisation eect in polycrystalline materials [6]. Deformation by martensite reorientation not only relaxes the internal elastic energy that is stored in the thermal martensite, but also creates an internal elastic energy in the reoriented martensite that opposes its reverse transformation. Furthermore, internal plastic deformation is suggested to be a necessary condition for martensite reorientation in a polycrystalline matrix to co-ordinate the orientation mismatch of preferential variants for this neighbouring grains [6,7]. To verify these hypotheses, or to fully understand the mechanisms responsible for this phenomenon, more studies are needed, including new experimental evidence, quantitative analysis and theoretical modelling. This paper reports on an experimental investigation on the eect of deformation via stress-induced martensitic transformation on the transformation behaviour of a near-equiatomic NiTi alloy, as a continuation of the work on the deformation via martensite reorientation. Whilst not attempting to reach a conclusive explanation to the mechanism of the stabilisation eect, this paper provides a discussion of the hypotheses proposed in the literature.
2. Experimental procedure The material used in this study was a commercial Ti50.2 at%Ni alloy supplied by Shanghai Iron and Steel Institute. The as-received material was in wire form 1.5 mm in diameter. The wire was cold-rolled in multiple steps into a ribbon of 2.10.75 mm in cross-section. The coldworked alloy was annealed at 1073 K for 1.8 ks, followed by quenching into water. After the above treatment the alloy exhibited single-step transformation behaviour between the austenite and martensite. The critical temperatures of the transformations were determined by differential scanning calorimetric measurement to be Ms 308 K, Mf 295 K, As 327 K and Af 343 K. Straight specimens of 50 mm in length were cut from the ribbon for tensile deformation. The deformation was carried out using an Instron 8501 testing machine with a 10 kN load cell. Compression grips were used. The specimens were deformed in austenitic state at 321 K, which was 13 K above the Ms temperature and 6 K below the As temperature. At this temperature both the stress-induced martensite and the residual austenite after deformation are stable. The temperature was approached by cooling the specimen from above 373 K. The testing temperature was controlled with an accuracy of 0.1 K using a liquid bath, in which the tensile specimen was immersed. The use of the liquid bath prevented the direct measurement of local deformation. Instead, a local strain using an extensometer of 12.5 mm gauge length was measured on a rig connected to the grips. For comparison with the specimens deformed in austenitic state, another specimen was deformed in martensitic state by shear. This specimen had a nominal composition of Ti-50.15 at%Ni and it was annealed at 978 K after a cold work of 40% thickness reduction. The transformation behaviour of the specimens after deformation were studied by dierential scanning calorimetry (DSC) using a Perkin±Elmer DSC4 dierential scanning calorimeter in an argon atmosphere. The DSC measurement was conducted with a cooling/heating rate of 10 K/min. Samples for DSC measurement were cut from the gauge length of each deformed specimen using a low speed diamond saw to avoid extra deformation. For two specimens, randomly chosen multiple DSC samples were taken for transformation measurement to con®rm the homogeneity of the deformation. All the cutting and specimen handling after the deformation were carried out at below 300 K to avoid any undesired phase transformation prior to the DSC measurement. 3. Results Fig. 1 shows the stress±strain curves of the specimens deformed in tension to dierent levels of strain. The
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stress±strain curves exhibited a continuously increasing stress during the stress-induced martensitic transformation and a gradual transition from stress-induced martensitic transformation of the austenite to plastic deformation of the stress-induced martensite. This suggests a macroscopically homogeneous deformation during the process of the transformation. The homogeneity of the deformation was con®rmed by DSC measurements of the transformation behaviour of samples taken from dierent locations within the gauge length of the specimens. The measurements were found to be practically identical. Fig. 2 shows the DSC measurement of the transformation behaviour of the specimen deformed to 6%. Prior to the measurement the DSC sample was cooled to 273 K. The measurement was conducted in two full transformation cycles, starting from heating, as indicated in the ®gure. It is seen that two transformations were recorded on the ®rst heating. The ®rst reverse transformation at 335 K, denoted MTh ! A, is identi®ed as the reverse transformation of a thermal martensite, which was formed
Fig. 1. Tensile deformation behaviour of austenite NiTi.
Fig. 2. Eect of tensile deformation via stress-induced martenistic transformation on thermal transformation behaviour.
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from the residual austenite on cooling after the incomplete stress-induced martensitic transformation. The second reverse transformation at 349 K, denoted MSI ! A, is the reverse transformation of the stressinduced martensite (SIM). The heating process in the second transformation cycle, however, exhibited one transformation, which corresponded in temperature to the MTh ! A transformation on the ®rst heating. The cooling transformations were practically identical between the two transformation cycles. The transformation parameters are indicated in the ®gure, with the superscripts denoting the order of cycle and the subscripts denoting the transformation. Fig. 3 shows the DSC measurement for all specimens, (a) showing the transformations on the ®rst heating, (b) showing the transformations on the ®rst cooling and (c) showing the transformations on the second heating. It is seen in Fig. 3(a) that the specimens deformed to strains between 3 and 8% exhibited a two-step reverse transformation whereas the rest showed a single-step reverse transformation. With increasing strain in the range of 3±8%, the heat ¯ow intensity of the MSI ! A transformation increased progressively at the expense of that of the MTh ! A transformation. The heat ¯ow intensity of the MSI ! A transformation also appeared to increase with increasing strain with a narrow transformation temperature interval at higher strain levels. This is in contrast to the forward transformation on cooling and the reverse transformation on the second heating, where the maximum heat ¯ow intensity decreased with increasing level of deformation. The characteristic temperatures of the transformations measured at the maximum heat ¯ow on the DSC curves, as indicated in Fig. 2, are shown in Fig. 4. The characteristic temperature of the MSI ! A transformation, T 1SIM ! A , increased progressively with increasing strain. The characteristic temperature of the MTh ! A transformation, T 1ThM ! A [1], showed a slight increase with strain at low strain levels and remained approximately constant afterwards. The initial increase seemed to follow the trend of T 1SIM ! A . This is believed to be due to the overlapping of MSI ! A on MTh ! A in this strain range. T 2M ! A on the second heating followed the same trend as T 1ThM ! A at low strain levels and showed a slight decrease at high strain levels. The characteristic temperatures for the cooling transformation in both cycles decreased gradually with increasing strain. Fig. 5 shows the measurement of transformation temperature intervals, where T 1H ; T 1C and T 2H denote the temperature intervals for the reverse transformation on the ®rst heating, the ®rst forward transformation on cooling after the ®rst reversion, and the reverse transformation on the second heating, respectively. The values were determined using the tangential line method. For T 1H , values for strains in the range
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Fig. 3. (a) Transformation behaviour on the ®rst heating after deformation to various strains. (b) Forward transformation on cooling after the ®rst reversion. (c) Eect of deformation on the reverse transformation on the second heating.
between 3 and 8% were not measured due to the splitting of the reverse transformation. It is seen that whereas T 1C and T 2H followed the same trend, increasing with increasing strain, T 1H decreased signi®cantly after deformation. By 20% strain, T 1H decreased to 5 K whereas T 1C and T 2H increased to 25 K, in comparison of 15 K for the undeformed specimen. Measurements of the heat of transformation after the deformation are shown in Fig. 6, with (a) showing the
heat eect associated with the transformations on the ®rst heating and (b) showing the transformation heat for the transformation cycle after the ®rst heating. In Fig. 6(a), QThM [1] is the heat eect of the MTh ! A transformation and Q1SIM is the MSIM ! A transformation on the ®rst heating. In the strain range of 4±8% where the two reverse transformations coexisted, the two values were determined by dividing the spectrum at the minimum between the two peaks. Q1h is the total
Y. Liu, G.S. Tan / Intermetallics 8 (2000) 67±75
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heat eect of the reverse transformation(s) on the ®rst heating, which equals Q1ThM at low strain levels, Q1SIM at high strain levels and the sum of Q1ThM and Q1SIM at intermediate strain levels. Q1h appeared to exhibit a general decrease with increasing strain. In Fig. 6(b) Q1c is the heat of the forward transformation on the ®rst cooling and Q2h is the heat of the transformation on the second heating. In this ®gure Q1h is also shown for comparison. The transformation heat in the subsequent cycle decreased with increasing deformation. For comparison with the tensile deformation of austenite via stress-induced martensitic transformation, deformation in shear in the martensitic state via martensite reorientation was also investigated [4]. Fig. 7 shows the DSC measurements of the transformation behaviour of two specimens after the shear deformation. Specimen (a) was deformed by 4%, a strain well within the deformation limit for reorientation of the self-accommodating martensite in NiTi. Specimen (b)
was deformed to 17%, a strain well beyond the limit for martensite reorientation. It was observed that the reverse transformation on the ®rst heating occurred in one step for all specimens deformed in the entire range of strain up to 20%, with the characteristic temperature, TM ! A [1], increasing progressively with increasing strain. For specimen (a) shown in the ®gure, the ®rst reverse transformation occurred at a temperature 10 K above the reverse transformation in the following transformation cycle. Associated with the increase in the characteristic transformation temperature, the heat eect associated with the reverse transformation on the ®rst heating was also measured to have increased to 27.2 J/g after the deformation, by 4.1 J/g compared to the heat of the reverse transformation in the second cycle. For specimen (b) the R phase transition appeared on cooling, as a result of the excessive plastic deformation. The characteristic temperature and the heat eect of the reverse transformation on the ®rst heating were measured to have
Fig. 4. Eect of deformation on transformation temperatures.
Fig. 5. Eect of deformation on transformation temperature intervals.
Fig. 6. (a) Eect of deformation on the heat eect of the reverse transformation on the ®rst heating. (b) Eect of deformation on the heat eect of the forward transformation.
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Fig. 7. Eect of shear deformation via martensite reorientation on transformation behaviour.
increased by 33 K and 10.1 J/g, respectively, relative to the reverse transformation in the subsequent transformation cycle. 4. Discussion 4.1. Stabilisation of martensite The increase in the characteristic temperature for the reverse transformation in the deformed specimens is an indication of a stabilisation eect to the stress-induced martensite. The phenomenon of martensite stabilisation in near-equiatomic NiTi alloys has been observed under a variety of experimental conditions, including dierent loading modes [1,2,6], deformation via either martensite reorientation [2,5] or stress-induced martensitic transformation, dierent types of alloys [2], and single-crystal and polycrystalline materials [2]. The stabilisation eect is also observed to be accompanied by several other eects, including changes in transformation heat eect [6], decrease of transformation temperature interval [6] and the development of two-way memory eect [5]. Two fundamental mechanisms have been proposed for the stabilisation eect. For the case of severe deformation where a large number of dislocations are introduced, it is proposed that the reverse transformation experiences an increased internal frictional resistance to the transformation interface movement due to a pinning eect of the defects on transformation interfaces, resulting in an increase in the demand for driving force for the reverse transformation [1,8]. It has been proposed that in a polycrystalline matrix it is impossible to achieve a fully oriented martensite, either by martensite reorientation or stress-induced martensitic transformation, without internal plastic deformation. This is owing to the orientation mismatch among the preferentially
oriented variants in neighbouring grains [6,7]. Some experimental evidences, obtained by either transmission electron microscopic examinations of specimens after deformation [2,9], or synchrotron X-ray topographic observations of specimens after transformation cycling under stress [7,10], of the occurrence of this internal plastic deformation have been published in the literature. Whilst the hypothesis of deformation-induced defects is supported by these experimental evidences, there remain a few uncertainties in this analysis. The temperature of the reverse transformation of the stabilised martensite hardly exceeds 473 K in NiTi alloys. Below this temperature the deformation-induced dislocations are most likely to remain after the ®rst reverse transformation whilst the stabilisation eect has vanished. The roles of the dislocations to the stabilisation eect and in subsequent thermal transformation cycles are not clear. In specimens subjected to low levels of deformation, the critical temperature of the ®rst reverse transformation may only be several degrees higher than that of an undeformed specimen. The vacancy annihilation hypothesis can hardly be rationalised in this case. The in¯uence of the change in the variant accommodation structure is also ignored. In the second hypothesis, the stabilisation eect is attributed to variations in the internal elastic strain energy [2]. In a thermally induced martensite, the lattice distortion of the transformation creates an internal elastic stress ®eld, which opposes the lattice strain of the martensite. Piao and colleagues suggest that for a single crystal specimen, deformation by martensite reorientation relaxes this internal elastic stress ®eld. The loss of the internal stress as a driving force for the reverse transformation results in the stabilisation eect. For polycrystalline materials, the relaxation of the internal stress ®eld is restricted by grain boundaries, and is achieved only in conjunction with some internal plastic deformation. However, there also remain several questions regarding this hypothesis. First, the maximum elastic strain energy that can be stored in a thermoelastic martensite in NiTi, as estimated by 2 = 2E, can hardly exceed 0.5 J/g. The DSC measurement of the endothermic heat eect associated with the reverse transformation increased by 4.1 J/g after a shear deformation by martensite reorientation (Fig. 7), which is 10 times larger in magnitude than the maximum elastic energy. Secondly, if the resistive elastic strain energy in the self-accommodating martensite is relaxed by the deformation, another elastic energy resistive to the reverse transformation must be created by the reverse transformation, in an identical way to the creation of the internal elastic energy during a thermal transformation. Thirdly, the role of the internal plastic deformation that occurs during the process of martensite reorientation deformation for polycrystalline specimens is not explained in this hypothesis. Furthermore, by
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either of the two mechanisms, the endothermic heat eect associated with the reverse transformation of the deformed martensite is expected to increase, due to either the increase of the frictional energy as a resistance or the loss of the elastic energy as a driving force. The experimental results shown in Fig. 6(b) demonstrate that the transformation heats, whilst they showed a small increase with deformation up to 2%, exhibited a general trend of decrease with deformation for stress-induced martensite, in contradiction to the expectation. In the case of stress-induced martensitic transformation, a deformation is imposed to the matrix, which guides the formation of an oriented martensite. During such a process, an internal plastic deformation is expected to occur, as a co-ordination mechanism for the strain mismatch among the preferential variants in neighbouring grains. This deformation will create an internal elastic stress ®eld in the direction of the lattice strain of the stress-induced martensite. This elastic stress ®eld opposes the reversion of the stress-induced martensite, causing the stabilisation eect on the ®rst heating, and prevails in the austenite after the ®rst reversion, resulting in the two-way memory eect. In this hypothesis, the deformation-induced defects are envisaged to have two eects: (1) as obstacles to the movement of transformation phase boundaries to increase the irreversible energy and (2) as a source of internal stresses oriented with the oriented variants and alters the elastic energy. The veri®cation of this hypothesis requires further work on quantitative analysis and modelling. 4.2. Eect of deformation on heat of transformation As a general trend, the transformation heat was observed to decrease as a result of the deformation via stress-induced martensitic transformation, as shown in Fig. 6. By 20% deformation, the heat eect of the reverse transformation on the ®rst heating decreased by 6 J/g and the heat of the transformations in the following transformation cycle decreased by 12 J/g, as compared to that of the undeformed specimen. This is in agreement with a previous study on cold rolling [8] whilst in contrast to the eect of deformation by martensite reorientation (Fig. 7) [6]. Deformation of martensite by variant reorientation was found to increase the heat of the reverse transformation on the ®rst heating whilst to have little in¯uence on the heat of the transformations in subsequent cycles. It is known that the DSC measurement of transformation heat for a thermoelastic martensitic transformation includes contributions from the elastic energy and irreversible energy in addition to the enthalpy energy [11,12]. A variation in the DSC measurement of transformation heat caused by the deformation is attributed to variations in the elastic and irreversible energies, since the enthalpy energy is determined only by the chemistry and crystallography of the transformation. For any of the hypotheses discussed
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above, the endothermic heat associated with the reverse transformation of the deformed martensite is expected to be higher than that of a thermal martensite. Whereas the increase in the endothermic heat for the reverse transformation of the reoriented martensite appears to agree with the expectation, the decrease of the heat of the reverse transformation of the stressinduced martensite is yet to be explained. 4.3. Eect of deformation on transformation interval The temperature interval of the reverse transformation of the stress-induced martensite was measured to be smaller than that of the thermal martensite. This is in agreement with a previous study on deformation via martensite reorientation [6]. It is established that the temperature interval of a thermoelastic martensitic transformation is determined by the dierence in the elastic energy associated with the transformation at the beginning and the end of the process for homogeneous matrices [13]. The decrease of T1H is an indication that the variation of the elastic energy component of the free energy change of the reverse transformation is reduced. The temperature intervals of the transformations in subsequent cycles were increased by the deformation. This is attributed to the mechanical inhomogeneity of the matrix of a sample after deformation due to the occurrence of internal plastic deformation. This inhomogeneity leads to the nucleation and completion of the thermal transformations occurring at dierent temperatures at dierent locations within the sample. 4.4. Uniform and localised deformation Another dierence between the eects of the deformation by stress-induced martensitic transformation and the deformation by martensite reorientation can be seen by comparing the transformation behaviour of specimen (a) shown in Fig. 7 and those of specimens deformed to intermediate strain levels in Fig. 3. The specimen deformed by martensite reorientation exhibited one single reverse transformation whereas the specimens deformed by stress-induced martensitic transformation exhibited two separate reverse transformations, indicating dierent microscopic mechanisms of deformation for the two processes. There exist two possible micromechanisms for the process of deformation, uniform deformation and localised deformation. The two mechanisms are schematically illustrated in Fig. 8, with (a) for martensite reorientation and (b) for stress-induced martensitic transformation. In the uniform reorientation scheme, as shown in Fig. 8(a), all self-accommodation domains of the thermal martensite in a polycrystalline specimen deform partially and uniformly via reorientation. The total deformation of the specimen is the accumulation of the microstrains from each self-accommodation domain and an increase in deformation is
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Fig. 8. (a) Schematic illustration of uniform and localised mechanisms for martensite reorientation deformation in polycrystalline NiTi. (b) Schematic illustration of deformation localisation of stress-induced martenistic transformation in polycrystalline NiTi.
realised by an increase in the degree of reorientation in each domain. In the localised scheme, some of the selfaccommodation domains reorient to the full degree whilst the rest remains intact. The total deformation is the accumulation of the microstrains from the fully reoriented domains and an increase in deformation is realised by an increase in the number of fully reoriented domains. For a uniform deformation it is expected that the reverse transformation occurs in one step and that its temperature varies continuously with the degree of deformation. For a localised deformation it is expected that the reverse transformation occurs in two separate steps corresponding to the two dierent martensites and that the relative (DSC) intensity of the transformation of the deformed martensite increases continuously with increasing deformation at the expense of that of the thermal martensite. The experimental results shown in Fig. 7 indicate that the deformation process of martensite reorientation was microscopically uniform. Transmission electron microscopic examinations of specimens deformed by martensite reorientation con®rm this [1,2,9]. In the case of deformation by stress-induced martensitic transformation, as evident in Fig. 3(a), the deformation process was microscopically inhomogeneous. The microscopic inhomogeneity of the deformation by stress-induced martensitic transformation conforms the expectation for a ®rst order transformation, although macroscopically the deformation process can well be uniform. 5. Conclusions 1. Deformation via stress-induced martensitic transformation causes stabilisation to the stress-induced martensite, as indicated by the increase of the characteristic temperature for the reverse transformation.
Whilst the deformation-induced defects and the alteration of the variant structure are believed to be responsible for this stabilisation eect, a detailed mechanism is yet to be established. 2. The transformation heat eect associated with the reversion of the stress-induced martensite was found to decrease with increasing strain. This is in contrast to the observation of the eect of deformation by martensite reorientation on transformation heat. An explanation of this observation is yet to be achieved. 3. The temperature interval of the reverse transformation of the stress-induced martensite was reduced by the deformation. This is indicative of a reduction in the variation of the elastic energy during the process of the transformation. The increase of the temperature interval for the transformations in subsequent cycles is attributed to the in¯uence of deformationinduced structural defects. 4. The experimental evidence suggests that the deformation process of martensite reorientation was microscopically uniform, whereas the stressinduced martensitic transformation was microscopically inhomogeneous in polycrystalline NiTi. Acknowledgements The authors wish to thank Professor Kozuhiro Otsuka for directing their attention to the systematic work on this topic published in Ref. [2]. References [1] Lin HC, Wu SK, Chou TS, Kao HP. Acta Metall Mater 1991;39:2069. [2] Piao M, Otsuka K, Miyazaki S, Horikawa H. Materials Transactions, JIM 1993;34:919.
Y. Liu, G.S. Tan / Intermetallics 8 (2000) 67±75 [3] [4] [5] [6] [7]
Liu Y, Galvin SP. Acta Materialia 1997;45:4431. Liu Y, Liu Y, Van Humbeeck J. Scripta Materialia 1998;39:1047. Liu Y, Liu Y, Van Humbeeck J. Acta Materialia 1998;47:199. Liu Y, Favier D. Acta Materialia; submitted. Jourdan C, Gastaldi J, Grange G, Belkahla S, Gudnin G. Acta Metall Mater 1995;43:4213. [8] Lin HC, Wu SK. Metall Trans A 1993;24A:293.
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[9] Xie Z, Liu Y, Van Humbeeck J. Acta Materialia 1998;46:1989. [10] Jourdan C, GueÂnin G, Belkahla S, Gastaldi J, Grange G. J de Phys IV 1997;7:C5±489. [11] Salzbrenner RJ, Cohen M. Acta Metall 1979;27:739. [12] Ortin J, Planes A. Acta Metall 1988;36:1873. [13] Wollants P, Roos JR, Delaey L. Progress in Materials Science 1993;37:227.
Eect of deformation by stress-induced martensitic transformation on the transformation behaviour of NiTi Yinong Liu*, G.S. Tan Department of Mechanical and Materials Engineering, The University of Western Australia, Nedlands, WA 6907, Australia Received 9 November 1998; received in revised form 3 August 1999; accepted 19 August 1999
Abstract It has been reported that martensite in near-equiatomic NiTi is thermally stabilised after a moderate deformation via martensite reorientation. This work continues the study by investigating the eect of deformation via stress-induced martensitic transformation on the transformation behaviour of the alloy. It was observed that the stress-induced martensite was also stabilised relative to the thermal martensite formed on cooling, as indicated by an increase in the critical temperature for the reverse transformation. Associated with the stabilisation, the heat eect, as determined by dierential scanning calorimetry, and the temperature interval of the reverse transformation, were measured and found to decrease with increasing level of deformation. The experimental results also demonstrated that the stress-induced martensitic transformation was microscopically localised, as expected for a ®rst-order phase transformation. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Martensitic transformations; B. Shape-memory eects
1. Introduction It has been reported in the literature that thermoelastic martensite in shape-memory alloys can be thermally stabilised by deformation, as indicated by an increase in the critical temperature for the reverse transformation of the deformed martensite relative to the thermal martensite [1±3]. The stabilisation eect is observed to be a one-time eect, which vanishes once the deformed martensite has reverted back to austenite on heating. Lin and colleagues investigated the eect of cold rolling on the transformation behaviour of binary NiTi alloys [1]. They observed that the critical temperature for the reverse transformation increased by 120 K after a severe deformation to 40% thickness reduction. They proposed that deformation-induced defects, including both dislocations and vacancies, were responsible for this stabilisation eect by hindering the movement of phase boundaries for the reverse transformation. The disappearance of the stabilisation eect in subsequent
* Corresponding author. Tel.: +61-8-9380-3132; fax: +64-8-93801024. E-mail address: [email protected] (Y. Liu).
transformation cycles was attributed to the annihilation of vacancies upon overheating and the formation of dierent martensite variants which are free from the hindrance of the deformation-induced dislocations in subsequent cycles. Whereas the explanations may seem reasonable for the case of deformation by cold rolling, where plastic deformation is inevitable even at low strain levels, they appear inadequate in explaining the stabilisation eect caused by mild deformation via martensite reorientation under monodirectional loading conditions [2,4]. Under these conditions, the stabilisation eect has been observed to be only a few degrees for deformation to low strain levels. Overheating by a couple of decades of degrees, as a thermal process, is hardly enough to cause any real change in the metallurgical conditions of a specimen. Furthermore, the selfaccommodating thermal martensite in a fully annealed polycrystalline NiTi deforms at a typical stress level of 120 MPa and the deformation is often observed to be fully recoverable up to a tensile strain of 6%. In this case, the production of structural defects during the process of martensite reorientation needs to be veri®ed. To clarify these uncertainties on the causes of the stabilisation eect, systematic investigations on the eect of mild deformation via martensite reorientation have been
0966-9795/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00079-5
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carried out [2,5,6]. Piao and colleagues studied the transformation behaviour of both single-crystal and polycrystalline specimens of three alloys, including CuZnAl, NiTi and NiTiNb, by electrical resistance measurement and microscopic examination [2]. They proposed that the relaxation of the elastic strain energy stored in multiple-variant martensite is responsible for the stabilisation eect. For single crystals the relaxation occurs during deformation by martensite variant reorientation without plastic deformation. For polycrystalline specimens the relaxation of internal elastic energy is only achieved during deformation with some plastic deformation, due to the constraint of grain boundaries. Liu and colleagues studied the eect of deformation via martensite reorientation on the transformation behaviour of polycrystalline near-equiatomic NiTi alloys by dierential scanning calorimetry (DSC) measurement [6] and thermal dilatation measurement [5]. In addition to con®rming the eect of deformation on transformation temperatures, they also observed that the heat eect associated with the reverse transformation of the reoriented martensite increased as relative to the thermally induced martensite [6] and that a two-way memory eect developed after the deformation [5]. The magnitude of the increase of the endothermic heat for the reverse transformation of the reoriented martensite was found too high to be accounted for by the stored elastic strain energy of the thermal martensite. The occurrence of the two-way memory eect, as high as 4% in tensile strain, is indicative of a directional internal stress ®eld created by the reverse transformation of the reoriented martensite. On the basis of these observations, Liti and Favier proposed that both elastic energy and irreversible energy contribute to the stabilisation eect in polycrystalline materials [6]. Deformation by martensite reorientation not only relaxes the internal elastic energy that is stored in the thermal martensite, but also creates an internal elastic energy in the reoriented martensite that opposes its reverse transformation. Furthermore, internal plastic deformation is suggested to be a necessary condition for martensite reorientation in a polycrystalline matrix to co-ordinate the orientation mismatch of preferential variants for this neighbouring grains [6,7]. To verify these hypotheses, or to fully understand the mechanisms responsible for this phenomenon, more studies are needed, including new experimental evidence, quantitative analysis and theoretical modelling. This paper reports on an experimental investigation on the eect of deformation via stress-induced martensitic transformation on the transformation behaviour of a near-equiatomic NiTi alloy, as a continuation of the work on the deformation via martensite reorientation. Whilst not attempting to reach a conclusive explanation to the mechanism of the stabilisation eect, this paper provides a discussion of the hypotheses proposed in the literature.
2. Experimental procedure The material used in this study was a commercial Ti50.2 at%Ni alloy supplied by Shanghai Iron and Steel Institute. The as-received material was in wire form 1.5 mm in diameter. The wire was cold-rolled in multiple steps into a ribbon of 2.10.75 mm in cross-section. The coldworked alloy was annealed at 1073 K for 1.8 ks, followed by quenching into water. After the above treatment the alloy exhibited single-step transformation behaviour between the austenite and martensite. The critical temperatures of the transformations were determined by differential scanning calorimetric measurement to be Ms 308 K, Mf 295 K, As 327 K and Af 343 K. Straight specimens of 50 mm in length were cut from the ribbon for tensile deformation. The deformation was carried out using an Instron 8501 testing machine with a 10 kN load cell. Compression grips were used. The specimens were deformed in austenitic state at 321 K, which was 13 K above the Ms temperature and 6 K below the As temperature. At this temperature both the stress-induced martensite and the residual austenite after deformation are stable. The temperature was approached by cooling the specimen from above 373 K. The testing temperature was controlled with an accuracy of 0.1 K using a liquid bath, in which the tensile specimen was immersed. The use of the liquid bath prevented the direct measurement of local deformation. Instead, a local strain using an extensometer of 12.5 mm gauge length was measured on a rig connected to the grips. For comparison with the specimens deformed in austenitic state, another specimen was deformed in martensitic state by shear. This specimen had a nominal composition of Ti-50.15 at%Ni and it was annealed at 978 K after a cold work of 40% thickness reduction. The transformation behaviour of the specimens after deformation were studied by dierential scanning calorimetry (DSC) using a Perkin±Elmer DSC4 dierential scanning calorimeter in an argon atmosphere. The DSC measurement was conducted with a cooling/heating rate of 10 K/min. Samples for DSC measurement were cut from the gauge length of each deformed specimen using a low speed diamond saw to avoid extra deformation. For two specimens, randomly chosen multiple DSC samples were taken for transformation measurement to con®rm the homogeneity of the deformation. All the cutting and specimen handling after the deformation were carried out at below 300 K to avoid any undesired phase transformation prior to the DSC measurement. 3. Results Fig. 1 shows the stress±strain curves of the specimens deformed in tension to dierent levels of strain. The
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stress±strain curves exhibited a continuously increasing stress during the stress-induced martensitic transformation and a gradual transition from stress-induced martensitic transformation of the austenite to plastic deformation of the stress-induced martensite. This suggests a macroscopically homogeneous deformation during the process of the transformation. The homogeneity of the deformation was con®rmed by DSC measurements of the transformation behaviour of samples taken from dierent locations within the gauge length of the specimens. The measurements were found to be practically identical. Fig. 2 shows the DSC measurement of the transformation behaviour of the specimen deformed to 6%. Prior to the measurement the DSC sample was cooled to 273 K. The measurement was conducted in two full transformation cycles, starting from heating, as indicated in the ®gure. It is seen that two transformations were recorded on the ®rst heating. The ®rst reverse transformation at 335 K, denoted MTh ! A, is identi®ed as the reverse transformation of a thermal martensite, which was formed
Fig. 1. Tensile deformation behaviour of austenite NiTi.
Fig. 2. Eect of tensile deformation via stress-induced martenistic transformation on thermal transformation behaviour.
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from the residual austenite on cooling after the incomplete stress-induced martensitic transformation. The second reverse transformation at 349 K, denoted MSI ! A, is the reverse transformation of the stressinduced martensite (SIM). The heating process in the second transformation cycle, however, exhibited one transformation, which corresponded in temperature to the MTh ! A transformation on the ®rst heating. The cooling transformations were practically identical between the two transformation cycles. The transformation parameters are indicated in the ®gure, with the superscripts denoting the order of cycle and the subscripts denoting the transformation. Fig. 3 shows the DSC measurement for all specimens, (a) showing the transformations on the ®rst heating, (b) showing the transformations on the ®rst cooling and (c) showing the transformations on the second heating. It is seen in Fig. 3(a) that the specimens deformed to strains between 3 and 8% exhibited a two-step reverse transformation whereas the rest showed a single-step reverse transformation. With increasing strain in the range of 3±8%, the heat ¯ow intensity of the MSI ! A transformation increased progressively at the expense of that of the MTh ! A transformation. The heat ¯ow intensity of the MSI ! A transformation also appeared to increase with increasing strain with a narrow transformation temperature interval at higher strain levels. This is in contrast to the forward transformation on cooling and the reverse transformation on the second heating, where the maximum heat ¯ow intensity decreased with increasing level of deformation. The characteristic temperatures of the transformations measured at the maximum heat ¯ow on the DSC curves, as indicated in Fig. 2, are shown in Fig. 4. The characteristic temperature of the MSI ! A transformation, T 1SIM ! A , increased progressively with increasing strain. The characteristic temperature of the MTh ! A transformation, T 1ThM ! A [1], showed a slight increase with strain at low strain levels and remained approximately constant afterwards. The initial increase seemed to follow the trend of T 1SIM ! A . This is believed to be due to the overlapping of MSI ! A on MTh ! A in this strain range. T 2M ! A on the second heating followed the same trend as T 1ThM ! A at low strain levels and showed a slight decrease at high strain levels. The characteristic temperatures for the cooling transformation in both cycles decreased gradually with increasing strain. Fig. 5 shows the measurement of transformation temperature intervals, where T 1H ; T 1C and T 2H denote the temperature intervals for the reverse transformation on the ®rst heating, the ®rst forward transformation on cooling after the ®rst reversion, and the reverse transformation on the second heating, respectively. The values were determined using the tangential line method. For T 1H , values for strains in the range
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Fig. 3. (a) Transformation behaviour on the ®rst heating after deformation to various strains. (b) Forward transformation on cooling after the ®rst reversion. (c) Eect of deformation on the reverse transformation on the second heating.
between 3 and 8% were not measured due to the splitting of the reverse transformation. It is seen that whereas T 1C and T 2H followed the same trend, increasing with increasing strain, T 1H decreased signi®cantly after deformation. By 20% strain, T 1H decreased to 5 K whereas T 1C and T 2H increased to 25 K, in comparison of 15 K for the undeformed specimen. Measurements of the heat of transformation after the deformation are shown in Fig. 6, with (a) showing the
heat eect associated with the transformations on the ®rst heating and (b) showing the transformation heat for the transformation cycle after the ®rst heating. In Fig. 6(a), QThM [1] is the heat eect of the MTh ! A transformation and Q1SIM is the MSIM ! A transformation on the ®rst heating. In the strain range of 4±8% where the two reverse transformations coexisted, the two values were determined by dividing the spectrum at the minimum between the two peaks. Q1h is the total
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heat eect of the reverse transformation(s) on the ®rst heating, which equals Q1ThM at low strain levels, Q1SIM at high strain levels and the sum of Q1ThM and Q1SIM at intermediate strain levels. Q1h appeared to exhibit a general decrease with increasing strain. In Fig. 6(b) Q1c is the heat of the forward transformation on the ®rst cooling and Q2h is the heat of the transformation on the second heating. In this ®gure Q1h is also shown for comparison. The transformation heat in the subsequent cycle decreased with increasing deformation. For comparison with the tensile deformation of austenite via stress-induced martensitic transformation, deformation in shear in the martensitic state via martensite reorientation was also investigated [4]. Fig. 7 shows the DSC measurements of the transformation behaviour of two specimens after the shear deformation. Specimen (a) was deformed by 4%, a strain well within the deformation limit for reorientation of the self-accommodating martensite in NiTi. Specimen (b)
was deformed to 17%, a strain well beyond the limit for martensite reorientation. It was observed that the reverse transformation on the ®rst heating occurred in one step for all specimens deformed in the entire range of strain up to 20%, with the characteristic temperature, TM ! A [1], increasing progressively with increasing strain. For specimen (a) shown in the ®gure, the ®rst reverse transformation occurred at a temperature 10 K above the reverse transformation in the following transformation cycle. Associated with the increase in the characteristic transformation temperature, the heat eect associated with the reverse transformation on the ®rst heating was also measured to have increased to 27.2 J/g after the deformation, by 4.1 J/g compared to the heat of the reverse transformation in the second cycle. For specimen (b) the R phase transition appeared on cooling, as a result of the excessive plastic deformation. The characteristic temperature and the heat eect of the reverse transformation on the ®rst heating were measured to have
Fig. 4. Eect of deformation on transformation temperatures.
Fig. 5. Eect of deformation on transformation temperature intervals.
Fig. 6. (a) Eect of deformation on the heat eect of the reverse transformation on the ®rst heating. (b) Eect of deformation on the heat eect of the forward transformation.
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Fig. 7. Eect of shear deformation via martensite reorientation on transformation behaviour.
increased by 33 K and 10.1 J/g, respectively, relative to the reverse transformation in the subsequent transformation cycle. 4. Discussion 4.1. Stabilisation of martensite The increase in the characteristic temperature for the reverse transformation in the deformed specimens is an indication of a stabilisation eect to the stress-induced martensite. The phenomenon of martensite stabilisation in near-equiatomic NiTi alloys has been observed under a variety of experimental conditions, including dierent loading modes [1,2,6], deformation via either martensite reorientation [2,5] or stress-induced martensitic transformation, dierent types of alloys [2], and single-crystal and polycrystalline materials [2]. The stabilisation eect is also observed to be accompanied by several other eects, including changes in transformation heat eect [6], decrease of transformation temperature interval [6] and the development of two-way memory eect [5]. Two fundamental mechanisms have been proposed for the stabilisation eect. For the case of severe deformation where a large number of dislocations are introduced, it is proposed that the reverse transformation experiences an increased internal frictional resistance to the transformation interface movement due to a pinning eect of the defects on transformation interfaces, resulting in an increase in the demand for driving force for the reverse transformation [1,8]. It has been proposed that in a polycrystalline matrix it is impossible to achieve a fully oriented martensite, either by martensite reorientation or stress-induced martensitic transformation, without internal plastic deformation. This is owing to the orientation mismatch among the preferentially
oriented variants in neighbouring grains [6,7]. Some experimental evidences, obtained by either transmission electron microscopic examinations of specimens after deformation [2,9], or synchrotron X-ray topographic observations of specimens after transformation cycling under stress [7,10], of the occurrence of this internal plastic deformation have been published in the literature. Whilst the hypothesis of deformation-induced defects is supported by these experimental evidences, there remain a few uncertainties in this analysis. The temperature of the reverse transformation of the stabilised martensite hardly exceeds 473 K in NiTi alloys. Below this temperature the deformation-induced dislocations are most likely to remain after the ®rst reverse transformation whilst the stabilisation eect has vanished. The roles of the dislocations to the stabilisation eect and in subsequent thermal transformation cycles are not clear. In specimens subjected to low levels of deformation, the critical temperature of the ®rst reverse transformation may only be several degrees higher than that of an undeformed specimen. The vacancy annihilation hypothesis can hardly be rationalised in this case. The in¯uence of the change in the variant accommodation structure is also ignored. In the second hypothesis, the stabilisation eect is attributed to variations in the internal elastic strain energy [2]. In a thermally induced martensite, the lattice distortion of the transformation creates an internal elastic stress ®eld, which opposes the lattice strain of the martensite. Piao and colleagues suggest that for a single crystal specimen, deformation by martensite reorientation relaxes this internal elastic stress ®eld. The loss of the internal stress as a driving force for the reverse transformation results in the stabilisation eect. For polycrystalline materials, the relaxation of the internal stress ®eld is restricted by grain boundaries, and is achieved only in conjunction with some internal plastic deformation. However, there also remain several questions regarding this hypothesis. First, the maximum elastic strain energy that can be stored in a thermoelastic martensite in NiTi, as estimated by 2 = 2E, can hardly exceed 0.5 J/g. The DSC measurement of the endothermic heat eect associated with the reverse transformation increased by 4.1 J/g after a shear deformation by martensite reorientation (Fig. 7), which is 10 times larger in magnitude than the maximum elastic energy. Secondly, if the resistive elastic strain energy in the self-accommodating martensite is relaxed by the deformation, another elastic energy resistive to the reverse transformation must be created by the reverse transformation, in an identical way to the creation of the internal elastic energy during a thermal transformation. Thirdly, the role of the internal plastic deformation that occurs during the process of martensite reorientation deformation for polycrystalline specimens is not explained in this hypothesis. Furthermore, by
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either of the two mechanisms, the endothermic heat eect associated with the reverse transformation of the deformed martensite is expected to increase, due to either the increase of the frictional energy as a resistance or the loss of the elastic energy as a driving force. The experimental results shown in Fig. 6(b) demonstrate that the transformation heats, whilst they showed a small increase with deformation up to 2%, exhibited a general trend of decrease with deformation for stress-induced martensite, in contradiction to the expectation. In the case of stress-induced martensitic transformation, a deformation is imposed to the matrix, which guides the formation of an oriented martensite. During such a process, an internal plastic deformation is expected to occur, as a co-ordination mechanism for the strain mismatch among the preferential variants in neighbouring grains. This deformation will create an internal elastic stress ®eld in the direction of the lattice strain of the stress-induced martensite. This elastic stress ®eld opposes the reversion of the stress-induced martensite, causing the stabilisation eect on the ®rst heating, and prevails in the austenite after the ®rst reversion, resulting in the two-way memory eect. In this hypothesis, the deformation-induced defects are envisaged to have two eects: (1) as obstacles to the movement of transformation phase boundaries to increase the irreversible energy and (2) as a source of internal stresses oriented with the oriented variants and alters the elastic energy. The veri®cation of this hypothesis requires further work on quantitative analysis and modelling. 4.2. Eect of deformation on heat of transformation As a general trend, the transformation heat was observed to decrease as a result of the deformation via stress-induced martensitic transformation, as shown in Fig. 6. By 20% deformation, the heat eect of the reverse transformation on the ®rst heating decreased by 6 J/g and the heat of the transformations in the following transformation cycle decreased by 12 J/g, as compared to that of the undeformed specimen. This is in agreement with a previous study on cold rolling [8] whilst in contrast to the eect of deformation by martensite reorientation (Fig. 7) [6]. Deformation of martensite by variant reorientation was found to increase the heat of the reverse transformation on the ®rst heating whilst to have little in¯uence on the heat of the transformations in subsequent cycles. It is known that the DSC measurement of transformation heat for a thermoelastic martensitic transformation includes contributions from the elastic energy and irreversible energy in addition to the enthalpy energy [11,12]. A variation in the DSC measurement of transformation heat caused by the deformation is attributed to variations in the elastic and irreversible energies, since the enthalpy energy is determined only by the chemistry and crystallography of the transformation. For any of the hypotheses discussed
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above, the endothermic heat associated with the reverse transformation of the deformed martensite is expected to be higher than that of a thermal martensite. Whereas the increase in the endothermic heat for the reverse transformation of the reoriented martensite appears to agree with the expectation, the decrease of the heat of the reverse transformation of the stressinduced martensite is yet to be explained. 4.3. Eect of deformation on transformation interval The temperature interval of the reverse transformation of the stress-induced martensite was measured to be smaller than that of the thermal martensite. This is in agreement with a previous study on deformation via martensite reorientation [6]. It is established that the temperature interval of a thermoelastic martensitic transformation is determined by the dierence in the elastic energy associated with the transformation at the beginning and the end of the process for homogeneous matrices [13]. The decrease of T1H is an indication that the variation of the elastic energy component of the free energy change of the reverse transformation is reduced. The temperature intervals of the transformations in subsequent cycles were increased by the deformation. This is attributed to the mechanical inhomogeneity of the matrix of a sample after deformation due to the occurrence of internal plastic deformation. This inhomogeneity leads to the nucleation and completion of the thermal transformations occurring at dierent temperatures at dierent locations within the sample. 4.4. Uniform and localised deformation Another dierence between the eects of the deformation by stress-induced martensitic transformation and the deformation by martensite reorientation can be seen by comparing the transformation behaviour of specimen (a) shown in Fig. 7 and those of specimens deformed to intermediate strain levels in Fig. 3. The specimen deformed by martensite reorientation exhibited one single reverse transformation whereas the specimens deformed by stress-induced martensitic transformation exhibited two separate reverse transformations, indicating dierent microscopic mechanisms of deformation for the two processes. There exist two possible micromechanisms for the process of deformation, uniform deformation and localised deformation. The two mechanisms are schematically illustrated in Fig. 8, with (a) for martensite reorientation and (b) for stress-induced martensitic transformation. In the uniform reorientation scheme, as shown in Fig. 8(a), all self-accommodation domains of the thermal martensite in a polycrystalline specimen deform partially and uniformly via reorientation. The total deformation of the specimen is the accumulation of the microstrains from each self-accommodation domain and an increase in deformation is
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Fig. 8. (a) Schematic illustration of uniform and localised mechanisms for martensite reorientation deformation in polycrystalline NiTi. (b) Schematic illustration of deformation localisation of stress-induced martenistic transformation in polycrystalline NiTi.
realised by an increase in the degree of reorientation in each domain. In the localised scheme, some of the selfaccommodation domains reorient to the full degree whilst the rest remains intact. The total deformation is the accumulation of the microstrains from the fully reoriented domains and an increase in deformation is realised by an increase in the number of fully reoriented domains. For a uniform deformation it is expected that the reverse transformation occurs in one step and that its temperature varies continuously with the degree of deformation. For a localised deformation it is expected that the reverse transformation occurs in two separate steps corresponding to the two dierent martensites and that the relative (DSC) intensity of the transformation of the deformed martensite increases continuously with increasing deformation at the expense of that of the thermal martensite. The experimental results shown in Fig. 7 indicate that the deformation process of martensite reorientation was microscopically uniform. Transmission electron microscopic examinations of specimens deformed by martensite reorientation con®rm this [1,2,9]. In the case of deformation by stress-induced martensitic transformation, as evident in Fig. 3(a), the deformation process was microscopically inhomogeneous. The microscopic inhomogeneity of the deformation by stress-induced martensitic transformation conforms the expectation for a ®rst order transformation, although macroscopically the deformation process can well be uniform. 5. Conclusions 1. Deformation via stress-induced martensitic transformation causes stabilisation to the stress-induced martensite, as indicated by the increase of the characteristic temperature for the reverse transformation.
Whilst the deformation-induced defects and the alteration of the variant structure are believed to be responsible for this stabilisation eect, a detailed mechanism is yet to be established. 2. The transformation heat eect associated with the reversion of the stress-induced martensite was found to decrease with increasing strain. This is in contrast to the observation of the eect of deformation by martensite reorientation on transformation heat. An explanation of this observation is yet to be achieved. 3. The temperature interval of the reverse transformation of the stress-induced martensite was reduced by the deformation. This is indicative of a reduction in the variation of the elastic energy during the process of the transformation. The increase of the temperature interval for the transformations in subsequent cycles is attributed to the in¯uence of deformationinduced structural defects. 4. The experimental evidence suggests that the deformation process of martensite reorientation was microscopically uniform, whereas the stressinduced martensitic transformation was microscopically inhomogeneous in polycrystalline NiTi. Acknowledgements The authors wish to thank Professor Kozuhiro Otsuka for directing their attention to the systematic work on this topic published in Ref. [2]. References [1] Lin HC, Wu SK, Chou TS, Kao HP. Acta Metall Mater 1991;39:2069. [2] Piao M, Otsuka K, Miyazaki S, Horikawa H. Materials Transactions, JIM 1993;34:919.
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Liu Y, Galvin SP. Acta Materialia 1997;45:4431. Liu Y, Liu Y, Van Humbeeck J. Scripta Materialia 1998;39:1047. Liu Y, Liu Y, Van Humbeeck J. Acta Materialia 1998;47:199. Liu Y, Favier D. Acta Materialia; submitted. Jourdan C, Gastaldi J, Grange G, Belkahla S, Gudnin G. Acta Metall Mater 1995;43:4213. [8] Lin HC, Wu SK. Metall Trans A 1993;24A:293.
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[9] Xie Z, Liu Y, Van Humbeeck J. Acta Materialia 1998;46:1989. [10] Jourdan C, GueÂnin G, Belkahla S, Gastaldi J, Grange G. J de Phys IV 1997;7:C5±489. [11] Salzbrenner RJ, Cohen M. Acta Metall 1979;27:739. [12] Ortin J, Planes A. Acta Metall 1988;36:1873. [13] Wollants P, Roos JR, Delaey L. Progress in Materials Science 1993;37:227.