Magnesium stress corrosion cracking

The significant positive green environment influence of magnesium alloy usage in transport could be compromised by catastrophic fast fracture caused by stress corrosion cracking(SCC). Transgranular stress corrosion cracking(TGSCC) of AZ91 was evaluated using the linearly increasing stress test(LIST) and the constant extension rate test(CERT). The TGSCC threshold stress was 55-75 MPa in distilled water and in 5 g/L NaCl. The TGSCC velocity was 7×10-10-5×10-9 m/s. A delayed hydride-cracking(DHC) model for TGSCC was implemented using a finite element script in MATLAB and the model predictions were compared with experiment. A key outcome is that,during steady state TGSCC propagation,a high dynamic hydrogen concentration is expected to build up behind the crack tip. A number of recommendations are given for preventing SCC of Mg alloys in service. One of the most important recommendations might be that the total stress in service(i.e. the stress from the service loading + the fabrication stress + the residual stress) should be below a threshold level,which,in the absence of other data,could be(conservatively) estimated to be about 50% of the tensile yield strength.