Anisotropic distribution of the micro residual stresses in lath martensite revealed by FIB ring-core milling technique

Lath martensite structures in medium-carbon steels incorporate a significant amount of residual stresses that are mostly induced by the martensitic transformation process. Although former studies could identify these stresses using diffraction techniques, it was not possible to correlate the micro-scale distribution of the stress fields with respect to the morphological and the crystallographic parameters of the martensitic structure. In this study, we employ the micro-scale focused ion beam (FIB) ring-core milling technique for the measurement of local residual strain and stress distributions in fully martensitic microstructures. The aim is to study the residual stresses occurring within individual lath martensite crystals, and within areas of lath martensite which incorporate a parent austenite grain boundary. The relaxation strains obtained by the micrometer-sized ring-core milling, which correspond to the residual stresses prior to milling, are shown to exhibit an anisotropic distribution for each martensite variant. High extension relaxation strains (i.e. compressive stresses) prevail in the direction of the transformation-induced crystal shape deformation direction. Contraction strains (i.e. tensile residual stresses) are measured normal to the extension strains. In an area containing a prior austenite grain boundary, the residual stresses appeared – altogether – lower than in single crystal martensite laths. The significant residual tensile stresses identified in the martensite structures may support the formation of martensite micro-cracks, either in the as-quenched state or during deformation.