Quantification and effects of microstructural dispersions on the mechanical behaviour of low carbon martensite

The mechanical properties of martensite have been extensively studied due to its technological significance. It is well-established that quenched martensite exhibits high mechanical strength but low total elongation and is often deemed as a "brittle" microstructure. However, its mechanical behaviour, including low microplasticity yield and strong initial hardening, resembles paradoxically that of multiphase steels. Consequently, recent research has led to a new perspective, proposing that lath martensite should no longer be viewed as a "uniform and homogeneous" phase but rather as a multiphase aggregate. This perspective arises from the recognition of the sequential nature of the martensitic phase transformation, progressing from the Ms temperature to ambient temperature. This progressive transformation results in the initial martensite laths transforming just below Ms in coarse, undeformed austenite, while subsequent laths transform in a highly constrained environment in terms of size, relaxation possibility, or defect densities. This progressive transformation serves as the primary explanation for the observed microstructural heterogeneities in martensitic steels.In this paper, we will demonstrate how the dispersion of these microstructural characteristics (microstructure sizes, density of dislocations) and of internal stresses resulting from the phase transformation explain the peculiar behaviour of martensitic steels through a micromechanical approach. We will detail how these characteristics were measured, determined, or estimated. Special attention will be paid to the description of spatial distributions and their stochastic couplings, which we term "soft-soft" correlation. As a discussion, the adaptation of the framework to address martensite tempering, Dual-Phase steels as well as mechanically induced martensite will be presented.