Formation Of Core-Shell Type Structure In Duplex Martensitic Steel

Microstructures of both as-quenched and intercritical-annealed medium-Mn steel were investi-gated by transmission electron microscopy to acquire a better understanding of gamma-epsilon and gamma-alpha' martensitic transformations. The orientation relationships among each phases were determined as ((1) over bar 10 )(alpha') //(0001)epsilon, [111](alpha')//[11 (1) over bar0](epsilon) for the case of as-quenched sample, and ((1) over bar 10)(alpha')//((1) over bar 11)(gamma)// (0001)(epsilon), [111](alpha') //[110](gamma) //[11 (2) over bar0](epsilon) for the case of intercritical-annealed sample, respectively. In addition, the presences of core-shell type microstructures with the gradient of Mn concentration were confirmed with -martensite being surrounded by retained gamma. Mn concentration of each phase was found dependent on the growth of reversed gamma controlled by Mn diffusion during intercritical annealing. It was strongly suggested that the gradient of Mn concentration occurred during intercritical annealing affected the phase stability, which resulted in the formation of epsilon/gamma core-shell microstructures. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.