Some Aspects of the Rapid Solidification of Light Alloys
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SOME ASPECTS OF THE RAPID SOLIDIFICATION OF LIGHT ALLOYS
H. JONES Department of Metallurgy,
University of Sheffield,
Sheffield Sl 3JD, England.
ABSTRACT The engineering light metals, especially aluminium and magnesium, exhibit limitations in alloying behaviour and performance that identify them as particular targets for development via rapid solidification. The present contribution reviews recent work in the author's group aimed at advancing understanding of both alloying and processing, including: (1) parametric mapping as a guide to extension of phase fields (2) prediction and application of observed characterisation of binary and ternary extended solid solutions (3) optimisation and control of conditions during rapid solidification and consolidation,
INTRODUCTION The merits of the engineering light metals, aluminium, magnesium, titanium and beryllium, and their alloys, are reflected by their selection for a range of applications. Limitations to further development include alloying behaviour, especially for Be, Al and Mg which have only 3, 8 and 15 (compared with 50 for Ti) alloying elements, respectively with appreciable (>lat%) equilibrium solid solubilities, and also processability, especially for Be, Ti and Mg (compared to fcc Al) in their more characteristic cph variants. The possibilities of phase field extension, matrix grain and dispersoid particle size refinement and high productivity offered by rapid solidification make it an increasingly attractive processing option for each of these metals and their alloys. There is already a body of knowledge concerning the effect of rapid solidification on aluminium and its alloys [1-3] and further exploration of possibilities for alloys of titanium [4-8] and of magnesium [9,10] has now begun. The present aim is to review recent work in progress, mainly in the author's group at Sheffield, concerned with advancing understanding of both alloying and processing in rapid solidification of light alloys including: (1) parametric mapping as a guide to phase field extent and extension (2) characteristics of binary and ternary extended solid solutions (3) optimization and control of conditions during rapid solidification and consolidation.
PARAMETRIC MAPPING OF PHASE FIELD EXTENT AND EXTENSION The classical work of Hume-Rothery et al.[11] drew attention to the importance of size and chemical factors in determining alloying behaviour including compositional extent of phase fields. Darken and Gurry [12] showed for maximum equilibrium solid solubility in Mg, Al and Ag that more soluble elements cluster round the solvent in a plot of electronegativity against atomic size. An elliptical boundary centred on the solvent in the case of Mg and Al, and with principal radii 0.4 units of electronegativity Hat. Res.Soc. Symp.Proc. Vol. 28 (1904) Published by Elsevier Science Publishing Co., Inc.
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and 0.15 (15 per cent) of the solvent atomic radius was shown to enclose the majority of the more soluble elements (solubility >5at%) and to exclude most of those that were less solu
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