The effect of thermomechanical processing on the microstructure and mechanical properties of the nanocrystalline TiNiCo shape memory alloy

The effect of thermomechanical processing comprising cold rolling followed by annealing on the microstructural evolution and mechanical behavior of the Ti50Ni48Co2 shape memory alloy was investigated. The annealed specimens were subjected to cold rolling at room temperature with various thickness reductions up to 70%. Transmission electron microscopy revealed that the initial deformation mechanism of Ti50Ni48Co2 alloy during cold rolling was stress-induced martensitic transformation followed by plastic deformation of martensite via dislocation slip and subsequent martensite to austenite transformation via the reverse transformation after unloading. Microstructural investigations showed that by increasing the cold deformation, a high density of dislocations is accumulated, leading gradually to nanocrystallization and amorphization. After annealing at 400 degree C for 1h, the amorphous phase formed in the cold rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. Results showed that the stress-strain curves of the cold rolled specimens exhibited plastic deformation of austenite without the stress plateau region. However, the stress plateau appeared in the stress-strain curves of the annealed specimens, whose stress level and length were increased with increasing thickness reduction.