The ideal strength of transition metal diborides TMB2 (TM = Ti, Zr, Hf): Plastic anisotropy and the role of prismatic slip

The ideal tensile and shear strengths of TiB2, ZrB2 and HfB2 have been investigated by first-principles stress–strain calculations. Among the studied diborides, TiB2 unexpectedly exhibits the weakest shear strength in the (0 0 0 1) basal plane, comparable to that of ZrB2 and HfB2 in the {101¯0} prismatic planes. Due to the nonlinearity of the stress response at large stains, the plastic anisotropy cannot be derived from elastic constants. Based on the relative stiffness of boron hexagons, a bond length indicator is obtained to characterize the preference for basal or prismatic slip in diborides.