Effect of microstructure on the hardening and softening behaviors of polycrystalline shape memory alloys - Part I: Micromechanics constitutive modeling

The effects of microstructure and its evolution on the macroscopic superela stic stress-strain response of polycrystalline Shape Memory Alloy (SMA) are studied by a microstructure-based constitutive model developed in this pap er. The model is established on the following basis: (1) the transformation conditions of the unconstrained single crystal SMA microdomain (to be dist inguished from the bulk single crystal), which serve as the local criterion for the derivation of overall transformation yield conditions of the polyc rystal; (2) the micro- to macro- transition scheme by which the connection between the polycrystal aggregates and the single crystal microdomain is es tablished and the macroscopic transformation conditions of the polycrystal SMA are derived; (3) the quantitative incorporation of three microstructure factors ( i.e., nucleation, growth and orientation distribution of martens ite) into the modeling. These microstructural factors are intrinsic of spec ific polycrystal SMA systems and the role of each factor in the macroscopic constitutive response is quantitatively modeled. It is demonstrated that t he interplay of these factors will result in different macroscopic transfor mation kinematics and kinetics which are responsible for the observed macro scopic stress-strain hardening or softening response, the latter will lead to the localization and propagation of transformation bands in TiNi SMA.