Effects of prior plastic deformation on the martensitic transformation in oriented single crystals of Au-47.5 at.% Cd

Abstract

Oriented single crystal samples ofβ phase Au-47.5 at.% Cd were progressively compressed along [110], [111] and [112] axes, causing each sample to deform with a different {110} 〈001〉 slip multiplicity and, therefore, develop a different defect substructure. The effects of the existing substructure upon subsequent martensite and reverse transformation were studied as a function of deformation for each sample, using a resistance technique.

It was observed that for small amounts of reduction in thickness (≤ 5%), the greater the work-hardening rate associated with deformation, the greater the enhancement of the martensite nucleation (increase inM _s temperature). It is suggested that {110} 〈1¯10〉 slip band intersections, produced during prior deformation, are responsible for this enhancement. For larger amounts of reduction in thickness (>5%), for the [110] and [111] samples, martensite nucleation becomes less enhanced, and finally inhibited. Deformation of the [112] sample has little effect upon martensite nucleation behaviour for any amount of reduction in thickness, because it undergoes almost single slip and presumably, few slip band intersections are formed. There is no apparent correlation between prior deformation andA _s temperature.

The greater the work-hardening rate associated with deformation, the greater the increase in difficulty of propagation of both the martensite and the austenite as a function of reduction in thickness. Defect pinning of the transformation interface is suggested as a possible mechanism to explain this observation. Also, practically the same increase in difficulty of propagation is observed for the A → M and M → A transformations for any sample, suggesting that the defects responsible for pinning are not erased after the two cycles that the sample is subjected to during testing.