Mechanical softening of thermoelectric semiconductor Mg₂Si from nanotwinning

Nanotwinning exhibits strengthening effects in many metals, semiconductors, and ceramics. However, we show from ab-initio calculations that nanotwins significantly decrease the strength of thermoelectric semiconductor Mg₂Si. The theoretical shear strength of nanotwinned Mg₂Si is found to be 0.93 GPa, much lower than that (6.88 GPa) of flawless Mg₂Si. Stretching the Mg–Si bond under deformation leads to the structural softening and failure of flawless Mg₂Si. While in nanotwinned Mg₂Si, the Mg–Si bond at the twin boundary (TB) is expanded to accommodate the structural misfit, weakening the TB rigidity and leading to the low ideal shear strength.