Microstructural investigation on strengthening mechanisms of AISI 304L austenitic stainless steel during cryogenic deformation

Abstract

In the present paper, microscopy techniques and mechanical tests were used to investigate in detail the strengthening mechanisms of an AISI 304L austenitic stainless steel during cryogenic deformation. The strain hardening rate–strain response of the alloy indicated three distinct regimes of hardening, being very similar to that previously reported for other low stacking fault energy alloys. The hardening rate initially decreased up to a strain of ∼6% (stage I). Then, a second stage of increasing hardening rate began (stage II). At strains larger than ∼25%, stage III with decreasing hardening rate followed. It was suggested that the formation of ϵ-martensite and α′-martensite together is responsible for the appearance of stage II. The high strain hardening values of the alloy in stage II were related to the increased fraction of α′-martensite and dislocation pile-ups behind the Lomer–Cottrell locks. The appearance of stage III was attributed to the difficulty of α′-martensite nucleation and ease of dislocation cross-slip at higher strains.