MAGNESIUM SILICIDE INTERMETALLIC ALLOYS

Methods of induction melting an ultra-low-density magnesium silicide ( Mg2Si) intermetallic and its alloys and the resulting microstructure a nd microhardness were studied. The highest quality ingots Of Mg2Si all oys were obtained by triple melting in a graphite crucible coated with boron nitride to eliminate reactivity, under overpressure of high-pur ity argon (1.3 x 10(5) Pa), at a temperature close to but not exceedin g 1105-degrees-C +/- 5-degrees-C to avoid excessive evaporation of Mg. After establishing the proper induction-melting conditions, the Mg-Si binary alloys and several Mg2Si alloys macroalloyed with 1 at. pct of Al, Ni, Co, Cu, Ag, Zn, Mn, Cr, and Fe were induction melted and, aft er solidification, investigated by optical microscopy and quantitative X-ray energy dispersive spectroscopy (EDS). Both the Mg-rich and Si-r ich eutectic in the binary alloys exhibited a small but systematic inc rease in the Si content as the overall composition of the binary alloy moved closer toward the Mg2Si line compound. The Vickers microhardnes s (VHN) of the as-solidified Mg-rich and Si-rich eutectics in the Mg-S i binary alloys decreased with increasing Mg (decreasing Si) content i n the eutectic. This behavior persisted even after annealing for 75 ho urs at 0.89 pct of the respective eutectic temperature. The Mg-rich eu tectic in the Mg2Si + Al, Ni, Co, Cu, Ag, and Zn alloys contained sect ions exhibiting a different optical contrast and chemical composition than the rest of the eutectic. Some particles dispersed in the Mg2Si m atrix were found in the Mg2Si + Cr, Mn, and Fe alloys. The EDS results are presented and discussed and compared with the VHN data.