Microstructural enhancement of magnesium alloys for elevated temperature applications using novel processing techniques

Mg-base alloys show excellent potential for property improvement using novel processing techniques and alloying elements. These methods may be used to improve the elevated temperature behavior of Mg alloys. Additions of various amounts of a rare earth element, Gd, into Mg produced an alloy that with careful heat treatment had improved mechanical properties when compared to conventional Mg alloys. The precipitation process in these alloys was studied and a previously unreported sequence of precipitation was uncovered. Rapid solidification processing was used to produce a uniform dispersion of Mg{dollar}sb2{dollar}Si particles within the Mg matrix. The effects of rapid solidification on the microstructure of several different alloy compositions was investigated. Three different microstructures were observed in these alloys. Using direct observations from the melt-spun ribbon, and experience from the study of Al-transition metal systems, a metastable phase diagram was developed which could be used to determine which of the three microstructures would be present in the ribbon after solidification at a given degree of undercooling. Laser surface melting was conducted on several alloys to investigate the effects of kinetic undercooling on the microstructure of the alloys. The microstructures produced were similar to those seen in the melt-spun ribbon. The experience gained working with the binary Mg-Si alloys was used to choose interesting compositions of Mg-Li-Si alloys for study. It was found that the amount of Si which could be effectively dispersed in these alloys was less than could be dispersed in the binary alloys. The silicide which formed on solidification proved to be an effective barrier to grain growth in the rapidly solidified material. Centrifugally atomized powder was consolidated by dynamical compaction and hot isostatic pressing. The resulting material was characterized both microstructurally and mechanically.