Using architectured materials to control localized shear fracture

A medium carbon steel was partially decarburized to create a compositionally graded material consisting of a surface layer with 0.1 wt% C and a core with 0.4 wt% C. The graded material was heat-treated to produce a martensitic microstructure which was subsequently cold-rolled in the as-quenched state. Localized shear bands developed within the core region during the rolling process. The shear bands did not, however, penetrate the outer decarburized layer. In this way, it was possible to roll 0.4 wt%C martensite, in the as-quenched state, to equivalent strains in excess of 2. The microstructure evolution of the cold-rolled martensite was followed using optical microscopy, Electron Back Scatter Diffraction (EBSD), Transmission Electron Microscopy (TEM) and nano-indentation. The development of shear bands is interpreted in terms of the collapse of martensite blocks during deformation, leading to geometric softening and shear localization within the high carbon core.