Time-resolved in-situ dislocation density evolution during martensitic transformation by high-energy-XRD experiments: A study of C content and cooling rate effects

Abstract

Recent studies on the mechanical behavior of martensitic steels show the importance of considering lath martensite as a “polycrystalline aggregate” with a large distribution of the local yield strength. The latter depends on local microstructural features, such as lath sizes, carbon distribution (in solid solution, segregated to defects, in carbides) and density of defects Internal stresses have to be considered as an additional contribution to the yield strength participating to enlarge the distribution of the local flow stresses at the microstructure scale.This study focuses on one of these microstructural features, the dislocation density. Its evolution is followed in situ upon martensitic transformation by High Energy X-Ray Diffraction experiments on a synchrotron beamline. A previously introduced method, based on modified Williamson-Hall (mWH) analysis, is improved in order to take account of the martensite lattice tetragonality when applying the mWH method.The influence of the steel carbon content (0.11, 0.21 and 0.31 wt.% C steels) and of the cooling rate (-10, -50, -100 °C/s and water-quench) on the dislocation density evolution in martensite during the martensitic transformation is established. The effect of the cooling rate on the dislocation density is low between -10 and -100 °C/s, but becomes visible after the water-quench. The higher the C content, the higher the dislocation densities at the end of the transformation. The steels with higher C content also show a wider distribution of the local dislocation density and, therefore, of the local yield strength.