Uncovering the unexpected changes of creep properties in AZ-series Mg alloys

Abstract

The creep behaviors of solution-treated pure Mg and AZ-series alloys including AZ31, AZ61 and AZ91 were systematically unveiled at 453 K along the rolling direction. The most surprising finding was that the creep resistance could not be monotonically improved with increasing Al contents. The strongest creep resistance was achieved in AZ61 sample, regardless of the highest yield stress obtained in AZ91 sample. The specific order of creep resistance, from strong to weak, was AZ61, AZ91, AZ31 and pure Mg samples. The reasons for the unexpected changes of creep properties between AZ61 and AZ91 samples were explained in detail. Although the solution strengthening effect was stronger in AZ91 sample before creep, the higher volume fractions of dynamic precipitates formed under creep loading led to the sharper solute depletion and the faster reduction of solution strengthening. Meanwhile, the fat block-like precipitates provided narrower spaces for dislocation movement in AZ91 sample, leading to the lower dislocation mobility and the more serious stress concentration. Consequently, the dynamic precipitates could not compensate the weakened solution strengthening and failed in inducing effective precipitation strengthening for the creep resistance of AZ91 sample. Furthermore, a peak-aged AZ91 sample (AA-AZ91) containing continuous precipitates (CPs) was prepared as a comparison. Although the CPs in AA-AZ91 sample had a more positive effect on the creep resistance than the discontinuous precipitates (DPs) in AZ91 sample, the CPs with weak thermal stability quickly evolved into DPs under creep loading. The creep resistance of AA-AZ91 sample was still weaker than the creep resistance of AZ61 sample. Therefore, the optimal creep resistance was related to the coordination of solution strengthening and precipitation hardening. This work provides a new guideline for optimizing the creep resistance of AZ-series and other Mg alloys with the major element of Al.