Elucidating the contribution of deformation-induced martensitic transformation and deformation twinning to dynamic strain aging in Fe-Cr-Ni metastable austenitic stainless steel

Abstract

A generally accepted long-range diffusion model based on dynamic strain aging (DSA) cannot explain the occurrence of the serrated flow below 200 °C in austenitic steels and other face-centered cubic (FCC) alloys. In this paper, new mechanisms for the serrated flow as a result of deformation-induced α′-martensitic transformation (DIMT) and deformation twinning (DT) are presented in a Fe-15Cr-9Ni metastable austenitic steel. Herein, uniaxial tensile tests were conducted at temperatures varying from 20 to 100 °C with different strain rates (1 × 10⁻⁶-1 × 10⁻³ s⁻¹). The dominant deformation modes evolved from DIMT to DT as the tensile temperature grew. Serrated flow does not occur during tensile deformation at 100 °C dominated by DT, owing to the slow diffusion of carbon (C) atoms in the austenite. The serrated flow during deformation at 20 °C depends on the interaction between the waiting time of dislocations at α′/γ interfaces and the diffusion time of C atoms from α′-martensite to the interfaces. DIMT can provide an accelerated diffusion path for C atoms, effectively reducing the diffusion time. The occurrence of DSA still requires the uniformly distributed fine α′-martensite at a strain rate lower than 1 × 10⁻⁵ s⁻¹ because the increased fraction of α′/γ interfaces provides abundant obstacles to dislocation motion. Although DIMT is significantly suppressed at 50–75 °C, the DSA effect can still be induced by the synergistic effect of DIMT and DT at a strain rate lower than 2.78 × 10⁻⁴ s⁻¹. DIMT providing obstacles to dislocation motion at α′/γ interfaces is a prerequisite, and then subsequent activation of twins at the α′/γ interface is required. The high-density deformation twins nucleated at α′/γ interfaces lead to an increase in waiting time of dislocations at the interfaces, which contributes to the DSA phenomenon.