Development of Mg-Sc-Mn alloys

Scandium is a potential alloying element for improving the high temperature properties (above 300 degrees C) of magnesium alloys. Scandium (T-m = 1541 degrees C) increases the melting point of the magnesium solid solution, di ffuses slowly in magnesium and has a lower density than other potential all oying additions (3 g cm(-3)). Based on thermodynamic equilibrium calculatio ns the alloys MgSc6Mn1 and MgSc15Mn1 were prepared using the squeeze castin g technique. The alloys showed a strong annealing response due to the forma tion of Mn,Se precipitates. The low diffusivity of the alloying elements re tards the overageing process. The existence of the Mn,Sc precipitates was c onfirmed by X-ray diffraction and energy dispersive X-ray microanalysis. Th e ultimate tensile strength of the new alloys was somewhat lower than the c orresponding values for the high creep resistant conventional alloy WE43 (M g-4.0wt.% Y-1.0wt.% Heavy Rare Earths-2.25wt% Nd-0.5wt.% Zr) whereas the te nsile yield strength was comparable to WE43 at higher Sc content. The alloy s showed a low elongation to fracture due to a strongly localised plastic d eformation. Creep tests of the material in the as cast condition resulted i n secondary creep rates which were comparable to WE43 at higher stresses bu t significantly lower at lower stresses. A T5 heat treatment of the new all oys 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 degrees C for the new alloys. (C) 1999 Elsevier Science S.A. All rights res erved.