The Status and Potential of Rapid Solidification of Magnesium Alloys
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THE STATUS AND POTENTIAL OF RAPID SOLIDIFICATION OF MAGNESIUM ALLOYS F. HEHMANN AND H. JONES Department of Metallurgy, University of Sheffield, Sheffield Sl 3JD, U.K. ABSTRACT In spite of giving exceptionally low density and excellent machinability at relatively low cost, magnesium alloys have found only restricted application as engineering materials to date because of limitations in formability, corrosion resistance, strength and creep resistance. Although the very earliest work on record to employ rapid solidification as a means to enhance engineering properties was carried out on magnesium-base alloys, subsequent work employing more modern techniques has not, until very recently, been motivated by the need for alloy development. The present study combines a critical survey of relevant published work with an assessment of the potential of magnesium-base alloys for development by the rapid solidification route. 1. INTRODUCTION Major efforts in rapid solidification research over the last decade have been devoted to engineering metals that already find widespread structural application in unalloyed and/or alloyed form i.e. iron, aluminium, nickel and titanium. Arguably the most promising results have been obtained for aluminium, for which restrictions imposed by equilibrium alloying behaviour set definite limits to what can be achieved by conventional ingot metallurgy. Even more serious restrictions in this respect, limit the application of magnesium as a structural material in spite of cost and fusibility similar to aluminium and even lower density (in fact the lowest density of all engineering metals). Some of the limiting characteristics of magnesium and its alloys are: (i) crystallography: cph with consequent limitations on the multiplicity of slip modes which limits deformability when temperature is not elevated and also the capacity for strengthening by alloying or cold working while retaining adequate ductility. Much of the conventional alloy development has been pursued to improve casting alloys for which there is a wide range and finishing of these is aided by the excellent machinability of magnesium and its alloys (the most machinable of all the engineering metals). (ii) chemical reactivity: its extreme position in the electrochemical series combined with its inability to readily form a protective self-healing passive surface film (comparable to alumina on aluminium and its alloys) in corrosive environments make magnesium and its alloys particularly vulnerable to galvanic attack when coupled with more noble metals. This itself provides an important application for magnesium as a competitor to aluminium and zinc for sacrificial protection of steel in corrosive environments. (iii) thermal stability: of properties such as strength and creep resistance, because of limited resistance to coarsening of precipitates resulting from conventional alloying (magnesium seems to be no worse than aluminium in this respect at least). The capability of rapid solidification to induce the constitutional change and microstruc
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