Effect of Mg ₁₇ Al ₁₂ on brittle and ductile crack extension behaviours in magnesium

Low fracture toughness is one of the factors limiting the widespread applications of magnesium and its alloys. Mg17Al12 is a common precipitation phase in magnesium-aluminum-based alloys, and it favours crack nucleation in the experiment. While its effects on brittle and ductile crack-tip behaviours in magnesium are still lacking. This work examines the effect of Mg17Al12 phase on brittle and ductile crack extension behaviours in magnesium via atomic-scale ‘K-field’ loads simulation. The results show that the Mg17Al12 phase changes the brittle and ductile crack tip behaviours by affecting the distribution of the crack tip stress field. Under model I loading, the stress along the crack extension direction mainly concentrates at the interface of the Mg17Al12 phase and the matrix, which leads to a significant blunting of the crack tip geometry powered by the crack opening stress. For a ductile crack tip, the concentrated interfacial stress is mainly dissipated by dislocation nucleation, slip, void nucleation and growth. For a brittle crack tip, the concentrated interfacial stress is mainly dissipated by twin nucleation and void nucleation, growth, and cleavage along the matrix. This work presents the atomic-scale mechanisms of the effects of the Mg17Al12 phase on brittle and ductile crack tip activities in magnesium, which offer references for crack-tip activity analysis in magnesium-aluminum-based alloys.