Elucidating Stress-Strain Relations of ZrB12 from First-Principles Studies.

Transition-metal boron-rich compounds exhibit favorable synthesis conditions and mechanical properties that hold great promise for wide-ranging applications. However, the complex bonding networks of these compounds produce diverse structural and mechanical behaviors that require in-depth studies. A notable case is ZrB12, which has been reported to possess high Vickers hardness comparable to those of ReB2 and WB4. Surprisingly, first-principles calculations of stress-strain relations reveal unexpected low indentation strengths of ZrB12 well below those of ReB2 and WB4. Such contrasting results are reconciled by noting that the additional presence of a boron-rich phase of ZrB50 in the experimental synthesis process likely plays a key role in the extrinsic strengthening. These findings uncover mechanisms for the higher measured strength of ZrB12 and offer insights for elucidating extrinsic hardening phenomena that may exist in other transition-metal compounds.