Development of Mg–Sc–Mn alloys

Scandium is a potential alloying element for improving the high temperature properties (above 300°C) of magnesium alloys. Scandium (Tm=1541°C) increases the melting point of the magnesium solid solution, diffuses slowly in magnesium and has a lower density than other potential alloying additions (3 g cm−3). Based on thermodynamic equilibrium calculations the alloys MgSc6Mn1 and MgSc15Mn1 were prepared using the squeeze casting technique. The alloys showed a strong annealing response due to the formation of Mn2Sc precipitates. The low diffusivity of the alloying elements retards the overageing process. The existence of the Mn2Sc precipitates was confirmed by X-ray diffraction and energy dispersive X-ray microanalysis. The ultimate tensile strength of the new alloys was somewhat lower than the corresponding values for the high creep resistant conventional alloy WE43 (Mg–4.0wt.% Y–1.0wt.% Heavy Rare Earths–2.25wt.% Nd–0.5wt.% Zr) whereas the tensile yield strength was comparable to WE43 at higher Sc content. The alloys showed a low elongation to fracture due to a strongly localised plastic deformation. Creep tests of the material in the as cast condition resulted in secondary creep rates which were comparable to WE43 at higher stresses but significantly lower at lower stresses. A T5 heat treatment of the new alloys led to creep rates which were up to two orders of magnitude lower than for WE43. Hence it is possible to increase the operating temperature by 50°C for the new alloys.