Enhanced ideal strength of thermoelectric half-Heusler TiNiSn by sub-structure engineering

TiNiSn based half-Heusler (HH) compounds exhibit excellent thermoelectric (TE) performance. In realistic thermoelectric applications, high strength, high toughness TiNiSn based TE devices are required. To illustrate the failure mechanism of TiNiSn, we applied density functional theory to investigate the response along various tensile and shear deformations. We find that shearing along the (111)/〈110〉 slip system has the lowest ideal shear strength of 10.52 GPa, indicating that it is the most plausible slip system under pressure. The Ni-Sn covalent bond is more rigid than the Ni-Ti and Ti-Sn ionic bonds. The TiSn framework resists external deformation until the maximum shear stress. The softening of the Ti-Sn ionic bond leads to the decreased rigidity of the TiSn framework in TiNiSn, resulting in reversible plastic deformation before failure. Further shear deformation leads to the breakage of the Ti-Sn bond, hence resulting in the collapse of the TiSn framework and structural failure of TiNiSn. To improve the ideal strength, we suggest a sub-structure engineering approach leading to improved rigidity of the TiSn framework. Here, we find that the substitution of Ti by Hf and Zr can enhance the ideal shear strength to 12.17 GPa in Hf_0.5Zr_0.5NiSn, which is attributed to a more rigid XSn (X = Hf and Zr) framework compared to TiSn.