The formation of Hcp and Bcc phases in austenitic iron alloys

The formation of the hcp (∈) and bcc (α) structures in pure iron under high pressure conditions, as well as the morphological and crystallographic aspects of martensitic transformation to these structures at atmospheric pressure in iron alloys, are reviewed., It is concluded that the unique features of α- or lath martensite formation are not dependent upon the presence of the ∈ phase. Application of the phenomenological theory of martensitic transformation has not successfully rationalized the crystallography of lath-martensite. The criterion for ∈- and α-phase formation is established using the regular solution approximation and appropriate lattice stability parameters. In particular, the ∈ phase can be formed in Fe?Ni?Cr compositions through stress-induced transformation attending α-martensite formation. Further consideration suggests that the ∈→α transformation is not expected at atmospheric pressure at temperatures below approximately 500°K in the alloys considered. Thus, two martensitic transformations, γ→∈ and γ→α, can occur jointly in certain alloys.