Superelastic properties of nanocrystalline NiTi shape memory alloy produced by thermomechanical processing
(1. Department of Materials Engineering, Malek Ashtar University of Technology, Shahin Shahr, Isfahan, Iran;
2. Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran)
Abstract: Effects of thermomechanical treatment of cold rolling followed by annealing on microstructure and superelastic behavior of the Ni50Ti50 shape memory alloy were studied. Several specimens were produced by copper boat vacuum induction melting. The homogenized specimens were hot rolled and annealed at 900 °C. Thereafter, annealed specimens were subjected to cold rolling with different thickness reductions up to 70%. Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of nanocrystalline and amorphous phases in Ni50Ti50 alloy. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was crystallized and a nanocrystalline structure formed. Results showed that with increasing thickness reduction during cold rolling, the recoverable strain of Ni50Ti50 alloy was increased during superelastic experiments such that the 70% cold rolled-annealed specimen exhibited about 12% of recoverable strain. Moreover, with increasing thickness reduction, the critical stress for stress-induced martensitic transformation was increased. It is noteworthy that in the 70% cold rolled-annealed specimen, the damping capacity was measured to be 28 J/cm3 that is significantly higher than that of commercial NiTi alloys.
Key words: nanocrystalline material; shape memory alloy; superelasticity; thermomechanical processing