Effects of transformation-induced defects in cobalt and Ni 48Ti 52

Abstract

The transformation behavior of pure cobalt and Ni 48Ti 52 were investigated by using a calorimetric technique in order to reveal the influence of transformation cycles. These transformations show a hysteresis with a small thermal width. The FCC–HCP transformation temperature of pure cobalt, during cooling, decreases considerably with increasing transformation cycles at the initial stages and increases gradually for transformation cycles in excess of about 50. The transformation temperature during the direct and the reverse transformations of Ni 48Ti 52 decreases with increasing thermal cycles. These behaviors on the transformation cycles are attributable to the formation of transformation-induced defects. It is thought that the amount and the microstructure of defects play an important role in the shifts of the transformation temperatures, and contribute to the non-chemical free energy and the stability of each phase.