Correlation of coercive force to microstructure in cyclic martensite → austenite transformations in an fe-ni-co alloy

Abstract

Microstructural changes which occur in an Fe-29 Ni-18 Co alloy during cyclic martensite austenite transformations have been monitored by coercive force measurements and transmission electron microscopy. Cycling was performed between liquid nitrogen and temperatures between 400°C (just aboveAs) and 800°C (about 100°C above A_f). For all temperatures belowAf, there was no measurable change in the value ofHc following immersion in liquid nitrogen; changes inHc occurred only in a subsequent high temperature cycle. This stabilization of the reversed austenite was attributed to transformation-induced strains. On the other hand, for temperatures of 700° and 800°C and any given cycle,Hc following liquid nitrogen immersion indicated formation of new martensite. At 800°C, transformation-induced strains were sufficient to promote recrystallization of the reversed austenite. An irrational martensitic habit plane for the nucleation of the reversed austenite was identified.