Evaluation of the delayed hydride cracking mechanism for transgranular stress corrosion cracking of magnesium alloys

Abstract

This paper evaluates the important elements of delayed hydride cracking (DHC) for transgranular stress corrosion cracking (TGSCC) of Mg alloys. A DHC model was formulated with the following components: (i) transient H diffusion towards the crack tip driven by stress and H concentration gradients; (ii) hydride precipitation when the H solvus is exceeded; and (iii) crack propagation through the extent of the hydride when it reaches a critical size of ∼0.8 μm. The stress corrosion crack velocity, V_c, was calculated from the time for the hydride to reach the critical size. The model was implemented using a finite element script developed in MATLAB. The input parameters were chosen, based on the information available, to determine the highest possible value for V_c. Values for V_c of ∼10⁻⁷ m/s were predicted by this DHC model. These predictions are consistent with measured values for V_c for Mg alloys in distilled water but cannot explain values for V_c of ∼10⁻⁴ m/s measured in other aqueous environments. Insights for understanding Mg TGSCC are drawn. A key outcome is that the assumed initial condition for the DHC models is unlikely to be correct. During steady state stress corrosion crack propagation of Mg in aqueous solutions, a high dynamic hydrogen concentration would be expected to build up immediately behind the crack tip. Stress corrosion crack velocities ∼10⁻⁴ m/s, typical for Mg alloys in aqueous solutions, might be predicted using a DHC model for Mg based on the time to reach a critical hydride size in steady state, with a significant residual hydrogen concentration from the previous crack advance step.