Microstructure and mechanical properties of cold-rolled Ti50Ni47Fe3 shape memory alloy

Mechanical and shape memory properties of a Ti50Ni47Fe3 alloy annealed at 450-750 °C for 1 h after a cold-rolled reduction of 25% were investigated by phase transformation analysis and microstructure characterization using tensile tests, Vickers hardness tests, electrical resistivity-temperature tests, SEM and TEM. From the results of the tensile, it can be inferred that the fracture stress and yield stress decreased and the fracture elongation increased as the annealing temperature increased for the rolled Ti50Ni47Fe3 alloy. They reached stead values when the temperature was above 650 °C. The change in Vickers hardness corresponded to the change in the fracture stress and yield stress. The electrical resistivity-temperature curves suggest that a two- stage martensitic transformation (B2-R-B19') occurred during cooling and heating. The transformation temperatures decreased to lower temperatures when the annealing temperature was increased and maintained the same after the annealing temperature reached 650 °C. TEM revealed the distinct processes occurring at elevated temperatures: recovery, polygonization, and recrystallization.