Deformation Behavior Of Shape Memory Alloy Research Articles

Size effects under martensitic deformation of shape-memory alloys

The effect of the grain size and film thickness on the parameters of martensitic transitions and deformation behavior of shape-memory alloys is studied in terms of the theory of smeared martensitic transitions. Comparison of theoretical results with experimental data suggests that the transformation kinetics related to the growth mechanism and shrinkage of martensite lamellas when transformation steps (dislocations) move along their boundaries is most sensitive to the size factor.

形状記憶合金に生じる不均一変形挙動の計測

Shape memory alloy (SMA) has been paid attention as smart material revealing shape recovery by heating. Many researchers have investigated its basic properties and applications. However, many problems have existed in usage of SMA such as change of properties (transformation temperature, Young's modulus) by process of fabrication, shape memory treatment, condition of phase and so on. Therefore, it has been pointing in recent years that expressing these phenomena using conventional constitutive relation of SMA has been difficult. In the present study, phenomena needed to construct novel constitutive relation of SMA, such as nucleation and propagation of phase transformation and so on, were measured using digital image correlation method. Here, tensile loading and unloading tests were performed for NiTi alloy plate in two temperature regions where the SMA shows the shape memory effect (26°C) and pseudoelasticity (60°C). The obtained results were as follows; (1) The propagation of the phase transformation during the stress-induced martensitic transformation and austenite transformation progressed from the both ends to the center of the specimen. (2) Staying phenomena of deformation was confirmed at nucleation point of phase transformation. Deformation behavior of SMA can be classified broadly into three categories, such as “Nucleation of phase transformation”, “Process between nucleation and propagation” and “Propagation of phase transformation”. (3) The transformation had a fixed angle along the longitudinal direction of the specimen, propagating from the both ends to the center of the specimen.

Thermomechanical behavior of shape memory alloy under complex loading conditions

Abstract The thermo-mechanical behavior of polycrystalline shape memory alloy (SMA) under multi-axial loading with varying temperature conditions has been studied by experiments. Recently the research has been extended theoretically and a mechanical model of polycrystalline SMA and the corresponding mesoscopic constitutive equations have been developed. The model presented in this paper is constructed on the basis of the crystal plasticity and the deformation mechanism of SMA. The variants in the crystal grains and the orientations of crystal grains in the polycrystal are considered in the proposed model; the constitutive equations are derived on the basis of the proposed model. The volume fraction of the martensite variants in the transformation process and the influence of the stress state on the transformation process are also considered. Some calculated results obtained by the constitutive equations are presented and compared with the experimental results. It is found that the deformation behavior of SMA under complex loading conditions can be well reproduced by the calculation of the constitutive equations.