Microstructure evolution during superelastic cycles and related elastocaloric effect in N-doped Ti-based shape memory alloys

Abstract

The superelasticity and elastocaloric effect (eCE) in N-free Ti-Nb-Zr-Ta alloy and 0.6N(at.%)-doped Ti-Nb-Zr-Ta alloy were comparatively studied. It was found that nitrogen doping played roles in elevating β→α transition temperature, refining grain sizes, homogenizing microstructure and altering dominant texture index. The N-free Ti-Nb-Zr-Ta alloy exhibited a temperature change of +6.7/−6.5 K during loading/unloading processes in the first superelastic cycle, but gradually decreased to +5.7/−5.2 K in 200^th cycle owing to the accumulation of newly codirectional dislocation lines and the following single-system dislocation slip during cyclic tests. By contrast, the N-doped alloy showed a lower initial temperature change of +3.7/−3.1 K but increased to +4.6/−4.1 K in 200^th cycle due to the extra caloric effect generated from nanoscale O' phase to α'' phase which experienced reorientation to favorable variants in early cycles. Residual α'' phase laths derived from stress-induced martensitic transformation (SIMT) appeared in both alloys after tensile cycles. The phase interface between β and α’’ phase was determined to behave a terraced shape, a type of interface compromising the reversible martensitic transformation (MT) and stabilization of martensite phase. The amount of nanodomains (O' phase) in regions situated at a distance from martensite significantly increased after cycles in both alloys, which accounted for the quickly reached stable superelastic deformation and much narrower hysteresis after the first cycle. Therefore, in light of the reproducibility and reversibility of elastocaloric performance in practical application, N-doped β-Ti shape memory alloys (SMAs) are promising candidate materials.