Modeling the two-way shape memory and elastocaloric effects of bamboo-grained oligocrystalline shape memory alloy microwire

Recent experimental studies reported that the bamboo-grained oligocrystalline shape memory alloy (SMA) microwire showed excellent two-way shape memory effect (TWSME) and elastocaloric effect (eCE) due to the reduced constraints from grain boundary. In this paper, a theoretical model is established to predict the TWSME and eCE of such a kind of SMAs. At single crystal scale, an anisotropic thermomechanically coupled constitutive model is proposed to describe the thermo-elastic martensitic transformation based on the crystal plasticity theory. The driving force of martensite transformation, the internal heat release/absorption, and the evolutions of internal variables and temperature are deduced within the framework of irreversible thermodynamics. To calculate the temperature, stress and strain fields in the bamboo-grained oligocrystalline SMA microwire and obtain the overall thermo-mechanical response of the microwire, the proposed constitutive model is implemented into the finite element program ABAQUS/Explicit by writing a user-defined material subroutine. Meanwhile, to reduce the computational cost faced in the fully coupled thermo-mechanical analysis by using the finite element method, a scale transition rule from single crystal scale to macroscopic oligocrystalline one is constructed based on a new proposed sub-region integral method by considering the special geometric characteristics of the microwire. The capability of proposed model to describe the TWSME and eCE of bamboo-grained oligocrystalline SMA microwire is validated by comparing the predictions with the experimental data. In addition, the effect of grain orientation is discussed. The proposed model can provide a theoretical guidance for the optimal design of grain microstructures to obtain the SMAs with excellent TWSME and eCE. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.