Two-way shape memory effect developed by martensite deformation in NiTi
Introduction
It is known that a two-way shape memory effect may be developed in near-equiatomic NiTi alloys through certain thermomechanical treatments, which are commonly known as training procedures1, 2, 3. Common procedures of two-way memory training include shape memory training, pseudoelastic training, and thermal cycling training under a constant stress. Shape memory training involves repeated cycles of deformation of the alloy by a stress-induced martensitic transformation and recovery of the deformation by a reverse transformation induced by heating under no stress4, 5. Pseudoelastic training is simply a mechanical cycling process in pseudoelasticity through the stress-induced martensitic transformation and the reverse transformation that proceeds against the external stress[6]. Thermal cycling training involves subjecting the alloy to repeated thermal transformation cycles under the influence of an external bias stress7, 8. Among these three training procedures constant-stress thermal cycling has appeared to be the most effective method in introducing two-way memory effect6, 7.
These three training procedures all involve phase transformations between the austenite and martensite. It is commonly accepted that through the training procedures an anisotropic dislocation structure is developed in the austenitic matrix4, 5, 6, 7. This dislocation structure creates an anisotropic stress field in the matrix, which guides the formation of martensite into variants of preferential orientations in relation to the deformation adopted in the training procedure, thus resulting in a macroscopic shape change during subsequent thermal transformation cycles.
It has been envisaged that deformation via martensite reorientation in a polycrystalline matrix is also effective in introducing internal plastic deformation[9]. It is suggested that this internal plastic deformation is a necessary mechanism of deformation to compensate for the orientation mismatch among the preferentially oriented martensite variants in neighbouring grains10, 11. Considering that this internal plastic deformation is introduced by a uniaxial external stress, it is likely that the internal stress field created by this internal plastic deformation is directional, thus having the potential to induce a two-way memory effect. This study was aimed at investigating the effect of deformation by martensite reorientation on the development of a two-way memory effect in a NiTi alloy.
Section snippets
Experimental procedure
The material used was a commercial NiTi alloy supplied by Company AMT (Belgium) with a nominal composition of Ti–50.0 at.% Ni. The as-received material was in a cold-rolled state in plate form. Straight specimens of 85×5.05×1.37 mm3 in dimension for mechanical testing were spark cut from the plate along the rolling direction, followed by an anneal at 938 K for 1.8 ks.
Tensile deformation was carried out using an Instron 1196 testing machine in air at room temperature, which was 20 K below the As
Results
The transformation behaviour of the as-annealed material is shown in Fig. 1. Curve (a) was measured by DSC. Transformation temperatures were determined from the DSC measurement to be Ms=304 K, Mf=294 K, As=315 K and Af=330 K. The transformation heat was measured by the DSC to be 27.3 J/g for the forward martensitic transformation on cooling and 28.1 J/g for the reverse transformation on heating. Curve (b) shows a TMA measurement of the transformations. The thermal expansion coefficient was measured
Deformation behaviour of martensite
It is generally perceived that the deformation of a thermally induced martensite in near-equiatomic polycrystalline NiTi alloys proceeds in four different stages corresponding to different deformation mechanisms, as indicated in Fig. 2(a): the initial elastic deformation of the self-accommodating multiple-variant martensite (stage I), the reorientation of the martensite over the stress plateau (stage II), the elastic deformation of the reoriented martensite following the reorientation
Conclusions
- 1.
The martensite reorientation deformation of the self-accommodating martensite in tension in polycrystalline near-equiatomic NiTi proceeds in three stages: stage I that consists of elastic deformation and homogeneous reorientation, stage II by the propagation of localized reorientation bands, and stage III that extends well into the range of plastic deformation.
- 2.
Deformation by martensite reorientation causes a thermal stabilization effect on the reoriented martensite. This stabilization effect is
Acknowledgements
Yinong Liu wishes to thank the Research Council of the Katholieke Universiteit Leuven for a Research Council Fellowship that financed his research work at the Katholieke Universiteit Leuven during the period from September to December 1997. The authors acknowledge the financial support of FKFO-project 2.0093.93.
References (22)
- et al.
Scripta metall.
(1974) - et al.
Acta metall. mater.
(1990) - et al.
Acta metall. mater.
(1990) - et al.
Acta metall. mater.
(1992) - et al.
Acta mater.
(1998) - et al.
Acta metall. mater.
(1996) - et al.
Acta mater.
(1997) - et al.
Acta mater.
(1998) - et al.
Scripta metall.
(1981) - et al.
Acta metall. mater.
(1994)