Fractography of Stress Corrosion Cracking of Mg-Al Alloys

Mechanisms for the stress corrosion cracking (SCC) of Mg-Al alloys have been investigated by scanning electron microscopy (SEM) of the fracture surfaces, for the two-phase alloy AZ91 and the single-phase alloys AZ31 and AM30 in distilled water. The mechanism for crack initiation in AZ31 and AM30 involves localized dissolution. The mechanisms for crack propagation in AZ31 and AM30 involve microvoid coalescence and cleavage, respectively. The mechanism for crack initiation in AZ91 is unclear, but may involve the fracture of β particles near the surface. The mechanism for crack propagation at moderate strain rates in AZ91 is similar to that in AZ31, with β particles acting as sources of H for mobile dislocations. The fracture surface for AZ91 tested at the strain rate 3 × 10⁻⁸ s⁻¹ was similar to that for specimens precharged in gaseous H₂. This fracture surface is the result of (1) the nucleation and growth of MgH₂ particles, (2) sudden fracture through the MgH₂ particles at some critical stress, and (3) decomposition of the MgH₂ particles after fracture.