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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 (Ͻ1.5 wt pct) improves their creep behavior markedly, decreasing the minimum creep rates by up to about two orders of magnitude at temperatures above 300 °C compared to WE alloys. This is due to the precipitation of fine Mn 2 Sc 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 Ce improves the creep resistance even more due to the effect of the grain boundary eutectic. The effect of Mn 2 Sc discs can still be seen in alloys with a low Sc content (ϳ1 wt pct) and with the addition of rare earth (RE) elements (Gd, Y, Ce ϳ4 wt pct). Very thin hexagonal plates containing RE and Mn, which lie parallel to the basal plane of the Mg matrix, augment the effect of the Mn 2 Sc precipitates at elevated temperatures (ϳ250 °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 Sc 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.

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The effect of Mn addition on the microstructure, thermal and mechanical properties in as-cast and cold-rolled Al–Sc–Zr alloys was studied. Electrical resistometry, differential scanning calorimetry and microhardness measurements were used. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction of specimens quenched from temperatures of pronounced changes in resistivity helped to identify the microstructural processes responsible for resistivity changes. The distinct microhardness increase observed after annealing above ∼320°C is caused by precipitation of the Al3Sc particles. The cold-rolling prior to a heat treatment has no substantial effect on temperature position of the Al3Sc-phase precipitation. The formation of Al6Mn and/or Al6(Mn,Fe) particles is responsible for the intensive resistivity decrease of the cold-rolled materials compared to the as-cast materials. Precipitation of these particles has an insignificant effect on microhardness. The apparent activation energy for the precipitation of the Al3Sc particles was determined.