Mechanical properties and fracture mechanisms of martensitic NiTi shape memory alloy based on various thermomechanical-processing microstructures

Abstract

Mechanical properties and fracture mechanisms of martensitic NiTi shape memory alloy (SMA) were investigated based on various thermomechanical-processing microstructures, where the thermomechanical processing was realized via local canning compression (LCC) and post-deformation annealing at 450 °C. Severe plastic deformation (SPD) occurs when the solution-treated martensitic NiTi SMA is compressed to 50% deformation by LCC, where amorphous structure (AMS) embedded with nano-sized crystalline inclusions (NCIs) and nanocrystalline structures (NCS) involving nanograined structures (NGS) and nanosubgrained structures (NSS) are dominant. Post-deformation annealing at 450 °C leads to the nanocrystallization of AMS, the formation of NSS as well as the growth of NCIs and NGS. The microstructure difference aroused by thermomechanical processing leads to the results that the alloy annealed at 450 °C presents much lower plasticity but higher fracture strength compared with the solution-treated one, while the as-compressed alloy exhibits no plasticity but extremely high fracture strength. In addition, the microstructure difference induced by thermomechanical processing also results in various fracture characteristics. The significant decrease of plasticity in the annealed alloy lies in two aspects, where the first one is that the decrease of reorientation+detwinning zone and plastic zone weakens their shielding effect of preventing the crack growth, and the second one is that crack-path configuration difference leads to larger proportion of fracture mode Ⅰ in the annealed alloy. The river pattern on the brittle fracture surface of the as-compressed alloy is induced by the convergence of cracks which propagate on a serial of parallel interfaces between shear bands and NCS regions.