Creep behavior of pure magnesium and Mg-Al alloys in active environments

Environment-enhanced creep, which we have called "corrosion creep" (CC), was investigated in pure Mg and die-cast AZ91D, AM50 and AS21 alloys in a borate buffer solution and 3.5% NaCl at room temperature. In contrast to the data in the air demonstrating only the first stage of creep process, in corrosive solutions, secondary and tertiary creep due to the plasticization effect of the solution was observed. The lifetime of pure Mg increases by one order of magnitude in the buffer solution in comparison with that in 3.5% NaCl. Elongation-to-fracture in the former was approximately twice as high as that in NaCl, probably, due to hydrogen embrittlement in the latter. In corrosive solutions, the creep life and elongation-to-fracture of Mg alloys decreases with increasing aluminum content from 2.3 to 8.4% in AS21, AM50 and AZ91D alloys. Cracking and final creep-rupture of pure Mg originate in a transcrystalline manner in comparison with intercrystalline fracture of the alloys. Cracks are observed in pure Mg at the primary creep stage, but their depth and amount are relatively small. The effect of environment on the creep behavior of magnesium is connected, mainly, with plasticization of metal assisted by chemical reactions. Also, anodic dissolution of Mg is enhanced by creep stress.