Nucleation and evolution of strain-induced martensitic (b.c.c.) embryos and substructure in stainless steel: A transmission electron microscope study

Abstract

The deformation of type 304 stainless steel produces a preponderance of strain-induced α′ (b.c.c.) martensite, which nucleates as stable embryos at micro-shear band or twin-fault intersections as proposed by Olson and Cohen. The two intersecting micro-shear bands must have a specific defect (fault-displacement) structure, and for stable martensite embryos to form requires a minimal micro-shear band thickness ranging from 50–70 Å. The critical nature of nucleation is influenced by the local temperature and strain. The structure, geometry, and morphology of strain-induced martensite embryos is essentially invariant regardless of the strain rate, strain state or temperature. Larger volume fractions of martensite evolve at large strains (⩾20%) as a result of embryo coalescence to produce a blocky-type morphology. Martensite embryos and coalesced volume elements of α′ are frequently characterized by an irregular, non-homogeneous distribution of smaller b.c.c. regimes which result from the irregular satisfaction of the necessary and specific fault-displacement requirements within a larger intersection volume.