A constitutive model of nanoporous NiTi shape memory alloys considering tensile-compressive asymmetry, grain size and porosity

Abstract

A constitutive model of the macroscopic behaviors of nanoporous Shape Memory Alloys (SMAs) is developed, and the effects of tensile-compressive asymmetry, grain size, and porosity are all considered because the influences of these factors cannot be ignored. A finite-volume, three-phase micromechanical model that contains the grain boundary phase, grain-core phase, and uniformly distributed pores is first given to represent the grains of nanoporous NiTi SMAs. The secant equivalent bulk modulus and shear modulus of the nanoporous NiTi SMAs were calculated by using the extended Mori-Tanaka method. Due to the high plastic yield limit of the grain boundary phase, the grain boundary phase is regarded as the region where no phase change occurs. And the overall phase transformation can be caused by the grain-core phase. The phase transformation is established according to the J2, J3 theory and the von Mises equivalence effect. The martensite volume fraction is represented in this article as a linear interpolation function of equal effectiveness. Based on the strain-hardening behavior, the equivalent stress of nanoporous NiTi SMAs is represented by a power function consisting of transformation onset stress, hardening modulus, equivalent phase change strain, and hardening index. Then, the constitutive model describing the superelastic behavior can be obtained. By comparing the numerical simulation results with the experimental results, it is verified that the constitutive model proposed in this paper can better describe the superelastic behavior of the nanoporous NiTi SMAs. Finally, through numerical simulations, it was found that tensile-compressive asymmetry, grain size, and porosity have significant effects on the superelastic behavior of nanoporous NiTi SMAs. This study will provide a theoretical basis for the application of nanoporous NiTi SMAs.