Mechanisms of creep deformation in Mg-Sc-based alloys

Binary Mg-Sc alloys show only a very weak age-hardening response due to the low diffusivity of Sc in Mg and exhibit inferior creep resistance compared to WE alloys. The addition of a small amount of Mn (< 1.5 wt pet) improves their creep behavior markedly, decreasing the minimum creep rates by up to about two orders of magnitude at temperatures above 300 degrees C compared to WE alloys. This is due to the precipitation of fine Mn2SC phase basal discs, which are very effective obstacles in controlling creep at temperatures at which cross-slip of basal dislocations and nonbasal slip are the rate controlling mechanisms. The addition of Cc improves the creep resistance even more due to the effect of the grain boundary eutectic. The effect of Mn2Sc discs can still be seen in alloys with a low Sc content (similar to I wt pet) and with the addition of rare earth (RE) elements (Gd, Y, Cc similar to 4 wt pet). Very thin hexagonal plates containing RE and Mn, which lie parallel to the basal plane of the Mg matrix, augment the effect of the Mn2SC precipitates at elevated temperatures (similar to 250 degrees C). The triangular arrangement of prismatic plates of metastable or stable phases of Mg-RE systems controls effectively the motion of basal dislocations during the creep of these alloys at elevated or high temperatures. The combined control of basal slip, cross-slip of basal dislocations, and of nonbasal slip in low Se content alloys ensures minimum creep rates of about one order of magnitude lower than those observed in WE alloys, both at elevated and high temperatures.