Effect of Internal Stress on Sequence of γ–ε–α′ Martensitic Transformations in Austenitic Stainless Steel

Abstract

It is well known that ε-martensite prefers to form before the nucleation of α′-martensite in austenitic steels with lower stacking fault energy, despite its lower phase stability. In the present study, the internal stress development during the formation of ε-martensite as an intermediate phase in the martensitic transformation from γ -austenite to α′-martensite was investigated in 18Cr–8Ni metastable austenitic stainless steel. The c/a value of hexagonal close-packed crystals of plate-shaped ε-martensite linearly increased as the plate thickness increased, proving that the internal stress was gradually stored by the growth of ε-martensite. The subsequent formation of α′-martensite decreased the c/a of ε-martensite, which suggests relaxation of the internal stress. Theoretical calculations based on micromechanics indicated that ε-martensite forms as the intermediate phase in austenitic steels because of (1) minimization of internal stress generation at the nucleation stage owing to its thin plate shape and (2) relaxation of internal stress by the coexistence of ε- and α′-martensite.