Martensite shear phase reversion-induced nanograined/ultrafine-grained Fe–16Cr–10Ni alloy: The effect of interstitial alloying elements and degree of austenite stability on phase reversion

Abstract

We describe here an electron microscopy study of microstructural evolution associated with martensitic shear phase reversion-induced nanograined/ultrafine-grained (NG/UFG) structure in an experimental Fe–16Cr–10Ni alloy with very low interstitial content. The primary objective is to understand and obtain fundamental insights on the influence of degree of austenite stability (Fe–16Cr–10Ni, 301LN, and 301 have different austenite stability index) and interstitial elements (carbon and nitrogen) in terms of phase reversion process, microstructural evolution during reversion annealing, and temperature–time annealing sequence. A relative comparison of Fe–16Cr–10Ni alloy with 301LN and 301 austenitic stainless steels indicated that phase reversion in Fe–16Cr–10Ni occurred by shear mechanism, which is similar to that observed for 301, but is different from the diffusional mechanism in 301LN steel. While the phase reversion in the experimental Fe–16Cr–10Ni alloy and 301 austenitic stainless steel occurred by shear mechanism, there were fundamental differences between these two alloys. The reversed strain-free austenite grains in Fe–16Cr–10Ni alloy were characterized by nearly same crystallographic orientation, where as in 301 steel there was evidence of break-up of martensite laths during reversion annealing resulting in several regions of misoriented austenite grains in 301 steel. Furthermore, a higher phase reversion annealing temperature range (800–900 °C) was required to obtain a fully NG/UFG structure of grain size 200–600 nm. The difference in the phase reversion and the temperature–time sequence in the three stages is explained in terms of Gibbs free energy change that considers Ni equivalent and Cr equivalent instead of Ni/Cr ratio and secondary precipitation of carbides/nitrides. Also, described is the evolution of deformation structures in NG/UFG Fe–16Cr–10Ni alloy. A number of deformation mechanisms occur during tensile straining involving dislocation glide and twinning.