Sequential γ-ε-α′ transformation preserves high elongation in an ultrafine-grained high Mn steel

Abstract

While grain refinement is a prominent way to improve the yield strength of metastable austenitic steels, it usually leads to a reduction in the uniform elongation as γ-α′ transformation tends to be suppressed. In the current study, it is found that the sequential γ-ε-α′ transformation helps an ultrafine-grained high Mn austenitic steel maintain high uniform elongation while having more than doubled yield strength compared to the coarse-grained counterpart. Although the grain refinement is unfavorable for the γ-ε transformation due to ε variant selection within the refined γ grains, the ε-α′ transformation is not affected as multiple α′ variants can form within one ε grain. The combination of a higher α′ transformation rate and delayed transformation kinetics enables continued strain hardening, leading to high uniform elongation.