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 Vc 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.

Corrosion and stress corrosion cracking of ultra-high-purity Mg5Zn

“…In contrast, transgranular stress corrosion cracking (TGSCC) can occur in distilled water, indicating that no deleterious ions are needed [18,19,20]. TGSCC involves hydrogen [21]. SCC susceptibility was reported to increase with Zn content [22], although this was disputed by Fairman and Bary [23].…”

Stress Corrosion Cracking Behavior of Fine-Grained AZ61 Magnesium Alloys Processed by Equal-Channel Angular Pressing

“…The fracture surfaces of the as-cast sample that failed in air and in distilled water are shown in Figure 6a,b, respectively. Figure 6a reveals mixed mode with transgranular cleavage and dimpled features, as well as intergranular fracture, mainly initiated by preferential anodic dissolution of the matrix adjacent to β-Mg17Al12 [1], whereas the same sample tested in distilled water showed a predominantly transgranular feature with parallel facets, attributed to hydrogen-assisted embrittlement for the AZ series alloys [5,19,20]. The primary conclusion is that the as-cast AZ61 Mg alloy behaved fairly susceptible to SCC in distilled water.…”

“…Furthermore, it has been demonstrated [5] that hydrogen is crucial to the mechanism of the SCC of magnesium alloys. Based on these, many investigations have been carried out to obtain less susceptibility to SCC of Mg alloys.…”

Environmentally Assisted Cracking of Magnesium Alloys

“…In order to rationalise these and other experimental findings, a delayed hydride cracking (DHC) model for transgranular stress corrosion cracking (TGSCC) of Mg alloys was presented in one of our earlier publications [8], Another modelling approach which is followed is based on a mesoscale model in which the material under test is represented by a bundle of parallel bars or fibres. Details of this model can be found in the literature [6,10,[16][17][18][19].…”

“…In various studies by the authors group experimental data which had been obtained from fracture mechanics tests on smooth and pre-cracked specimens were analysed using a number of modelling approaches for crack initiation and growth caused by hydrogen embrittlement [6][7][8][9][10]. The models proved that they had the potential to simulate the effects of hydrogen embrittlement in the material under investigation.…”