Overcoming strength-ductility trade-off of Mg2Si via Al and O doping: First-principles and experimental investigations

The intrinsic brittleness of Mg₂Si severely deteriorates the mechanical properties of composites and limits their potential application as reinforcing particles. This work puts forward a cost-efficient approach to address this challenge by doping Al/O into Mg₂Si to improve the deformation coordination of Al-Mg₂Si composites. First-principles calculations revealed that Al doping alone reduced the modulus and hardness of Mg₂Si and increased its Poisson's ratio, but Al-doped Mg₂Si still exhibited brittle characteristics. O doping alone or Al/O co-doping further reduced the modulus and hardness of compounds, and realized the transformation of Mg₂Si from brittle to plastic. Additionally, the anisotropy of doped Mg₂Si was slightly increased, but Al/O co-doping suppressed the anisotropy to a certain extent compared to doping alone. The underlying mechanism is that Al doping enhanced the metallicity of the compounds, and Al/O co-doping significantly weakened the strength of Mg-Si and Si-Si covalent bonds, and formed new O-Mg and O-Si ionic bonds. Subsequently, nano-indentation experiments were carried out to test the Young's modulus and hardness of samples with different doping elements. The results, in descending order, were as follows: Al₈Si₄, Al₈Si₄Al, Al₈Si₄O, Al₈Si₄AlO, which aligned consistently with the theoretical calculations. The Young's modulus and hardness of Mg₈Si₄AlO were 72.9 GPa and 198 HV, respectively, which were approximately 48.4 % and 32.4 % lower than those of undoped Mg₂Si. This work provides valuable insights and guidance for designing novel high-strength and high-toughness composites by doping modified reinforced particles.