Improved elastocaloric effect of NiTi shape memory alloys via microstructure engineering: A phase field simulation.

• A grain size dependent and thermo-mechanically coupled phase field model is newly proposed. • The effects of grain size, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the elastocaloric effect of NiTi SMAs are revealed. • The COP mat of the optimal microstructure engineering scheme is up to 35.4, which is about 200% higher than that of uniform coarse-grained NiTi SMAs. • The microstructure engineering scheme with geometric gradient shows the lowest input stress of 195 MPa. By newly introducing a grain boundary energy, a grain size (GS) dependent and thermo-mechanically coupled phase field model was proposed to investigate the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs), and discuss the effects of GS, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the eCE through simulations. The simulated results show that with decreasing the GS, the gradual reduction of the degree of martensite transformation (MT) and the variation of transformation mode in the nano-polycrystalline NiTi SMA system lead to the decreasing absorption of transformation latent heat, and the gradual weakness and even disappearance of local instability during MT decreases the hysteresis, resulting in the increase of the coefficient of performance (COP mat); the deformation dependence of COP mat and the grain orientation dependence of transformation instability lead to a dependence of COP mat on texture, which can be enhanced with decreasing the GS. It is further found that the microstructure engineering (from the perspectives of GS, texture, gradient-nanograin and bimodal grain structures) can significantly improve the COP mat , and the introduction of geometric gradient can effectively reduce the MT start stress. Several microstructure and geometric schemes with considerable adiabatic temperature change (Δ T ad) and high COP mat as well as relatively low MT start stress are reported, which can provide an important theoretical guidance for improving the eCE of NiTi SMAs and exploiting excellent SMA-based elastocaloric materials. The proposed phase field model and the characterization method for texture can be used as a reference to simulate the eCE and anisotropic MT of other SMAs. I. Microstructure engineering schemes: polycrystalline systems with different gradient-nanograin structures and bimodal grain structure; II. Simulated microstructure evolution; III. Improved elastocaloric effect [Display omitted] [ABSTRACT FROM AUTHOR]