First-principles study of the ideal strength of Fe3C cementite

The ultimate (ideal) mechanical properties of iron carbide Fe3C (cementite) have been calculated using first-principles calculations and the generalized gradient approximation under tensile and shear loading. Our results confirm that cementite is elastically anisotropic, in particular with a low C44 (18 GPa). We also show that cementite is anisotropic from the point of view of the theoretical strength. In tension, the elastic instability is reached at 16% strain and 22 GPa along [100]. Larger elongation (23%) can be reached when the cementite is pulled along [010] or [001] (the ideal tensile stress is then 20 and 32 GPa, respectively). Results are more contrasting in shear. The low C44 value allows very large shear deformation (up to ca. 40%) to be sustained along [010](001) or [001](010) before the cementite structure becomes unstable. The higher ideal shear stress (ISS), 22.4 GPa, is exhibited for [001](100). Other shear loading conditions lead to ultimate strains in the range 14–22% for ideal shear stresses between 12 and 19 GPa.