Predicting the formation and structure of martensite in steels

Abstract

Martensitic transformations have been the subject of extensive experimental studies over the past decades due to their technological importance in high-strength applications. However, existing theories describing the microstructure and its influence on mechanical properties are material-specific and are often empirically based. This work introduces recent efforts by the author and collaborators in understanding and predicting the microstructure and strength of martensite based on its mechanisms of formation. The hierarchical structure of lath martensite in low-C steels is described in terms of the transformation (Bain) strain, lattice distortions produced by C atoms and the number of crystallographic variants of the transformation. The results are linked with a thermodynamic description for the martensite start and finish temperatures across different metals, not only steels, where the transformation strain is shown to be the main factor controlling the driving force for martensite formation. The twinned structure of high-C steels is described by quantifying local C enrichment at twin boundaries, where it is rationalised that twins form to accommodate the large strains of higher C additions. Descriptions of the lath-dislocation, twin and grain boundary density in low- and high-C steels allow us to postulate a unified description of the hardness in martensitic steels for a wide range of compositions. The influence of retained austenite in the maximum strength of martensitic steels is also discussed.