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

A generally accepted long-range diffusion model based on dynamic strain aging (DSA) cannot explain the occurrence of the serrated flow below 200 degrees 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 alpha '-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 degrees C with different strain rates (1 x 10- 6-1 x 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 degrees C dominated by DT, owing to the slow diffusion of carbon (C) atoms in the austenite. The serrated flow during deformation at 20 degrees C depends on the interaction between the waiting time of dislocations at alpha '/gamma interfaces and the diffusion time of C atoms from alpha '-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 alpha '-martensite at a strain rate lower than 1 x 10-5 s- 1 because the increased fraction of alpha '/gamma interfaces provides abundant obstacles to dislocation motion. Although DIMT is significantly suppressed at 50-75 degrees C, the DSA effect can still be induced by the synergistic effect of DIMT and DT at a strain rate lower than 2.78 x 10-4 s- 1. DIMT providing obstacles to dislocation motion at alpha '/gamma interfaces is a prerequisite, and then subsequent activation of twins at the alpha '/gamma interface is required. The high-density deformation twins nucleated at alpha '/gamma interfaces lead to an increase in waiting time of dislocations at the interfaces, which contributes to the DSA phenomenon.