Study of processing maps of NiTi shape-memory alloy under different instability criteria

Isothermal compression tests of NiTi shape-memory alloy (NiTi SMA) were carried out on a Gleeble-3800 thermal simulating tester in the temperature range 750–1050°C and strain rates of 0.005–10 s−1 with a deformation of 0.7. The mechanical behaviour characteristics and microstructure evolution during the hot compression process are the main items of study. The hot-processing maps under five different instability criteria (Prasad, Gegel, Malas, Murty, and Semiatin instability criteria) were constructed based on hot-compression experimental data and dynamic material model (DMM) theory. Suitable forming processing regions and flow instability regions of the NiTi SMA are predicted by the processing maps in the process of hot deformation. The results indicate that: the prediction for the alloy intersect and supplement each other because of the different theoretical basis of the above instability criteria. The prediction range of the above five instability criteria is verified by combining variation of strain-rate sensitivity exponent, microstructure characterisation and actual hot-deformation results in the process of hot deformation. The result predicted by the Prasad instability criteria is the most accurate for the alloy. Plastic flow bands and adiabatic shear cracks mainly occur in the flow-instability regions, while dynamic recovery (DRV) and dynamic recrystallisation (DRX) mainly occur in the processing regions.