Stress corrosion cracking of rare-earth containing magnesium alloys ZE41, QE22 and Elektron 21 (EV31A) compared with AZ80

Stress corrosion cracking (SCC) of the high-performance rare-earth containing magnesium alloys ZE41, QE22 and Elektron 21 (EV31A) was studied using slow strain rate test (SSRT) method in air, distilled water and 0.5 wt.% NaCl solution. For comparison, the well-known AZ80 alloy was also studied. All alloys were susceptible to SCC in 0.5 wt.% NaCl solution and distilled water to some extent. AZ80 had similar SCC susceptibility in distilled water and 0.5 wt.% NaCl solution. ZE41, QE22 and EV31A had higher susceptibility to SCC in 0.5 wt.% NaCl solution than in distilled water. EV31A had the highest resistance to SCC compared to AZ80, ZE41 and QE22 in both distilled water and 0.5 wt.% NaCl solution. The fractography was consistent with (i) largely transgranular SCC (TGSCC) in distilled water for AZ80, ZE41 and QE22 and also for AZ80 in 0.5 wt.% NaCl solution, and (ii) a significant component of intergranular SCC (IGSCC) in 0.5 wt.% NaCl solution for QE22, ZE41 and EV31A. The TGSCC fracture path in AZ80, ZE41 and QE22 is consistent with a mechanism involving hydrogen. In each case, the IGSCC appeared to be associated with the second-phase particles along grain boundaries. For IGSCC of EV31A and QE22, the fractography was consistent with micro-galvanic acceleration of the corrosion of α-magnesium by the second-phase particles, whereas it appeared that the second-phase particles had corroded itself in the case of ZE41 in 0.5 wt.% NaCl solution. The study suggests that rare-earth elements in magnesium alloys can improve SCC resistance significantly as observed in the case of EV31A. However, the SCC resistance also depends on the other critical alloying elements such as zinc (in ZE41) and silver (in QE22) and the microstructure.