Phase field simulation on the super-elasticity, elastocaloric and shape memory effect of geometrically graded nano-polycrystalline NiTi shape memory alloys

Abstract

• A 2D non-isothermal phase field model considering multiple variants which can describe the thermo-mechanical deformation of NiTi SMAs is newly proposed. • The SE, ECE, OWSME and SATWSME of the geometrically graded nano-polycrystalline NiTi SMAs are simulated and predicted by phase field method for the first time. • The controllability of SE and SATWSME can be effectively improved by designing geometrically graded structures. • The design of geometrically graded structures has the potential to regulate ECE. A two-dimensional non-isothermal phase field model considering multiple martensite variants which can describe the thermo-mechanical deformation of polycrystalline NiTi shape memory alloys (SMAs) was newly proposed. The super-elasticity (SE), elastocaloric effect (ECE), one-way shape memory effect (OWSME) and stress-assisted two-way one (SATWSME) of the geometrically graded nano-polycrystalline NiTi SMA systems (for instances, the trapezoid, concave and convex ones) were simulated and predicted by phase field method for the first time. The simulated results show that: Due to the stress gradient caused by the heterogeneous geometry under an external applied load, the martensite transformation (MT) and its reverse in the geometrically graded systems propagate progressively along the direction with stress variation, resulting in an obviously wider transformation stress window or transformation temperature one (with the assistance of external applied load) there than that in the rectangular system (where no heterogeneous geometry is involved), thus, the controllability of the SE and SATWSME can be effectively improved; however, since the sensitivity of martensite reorientation to the external load is much higher than that of stress-induced MT, the stress gradient along the tensile direction of the geometrically graded systems cannot strictly control the gradual propagation of martensite reorientation, so the improvement of the controllability of OWSME is not considerable; moreover, the temperature field evolution is consistent with the progressively propagating MT within the geometrically graded systems in the SE process, and the variation rate of the average temperature is lower than that of the rectangular system, which means that designing the geometrically graded structure components of NiTi SMA systems has the potential to control their ECE. Phase-field simulated microstructure evolution in the processes reflecting super-elasticity (SE) and one-way shape memory effect (OWSME) of rectangular and geometrically graded nano-polycrystalline NiTi SMA systems.