Characterization of dispersed intermetallic phases in rapidly quenched Al-Ti-Ce alloys
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INTRODUCTION
THE techniques of rapid solidification processing offer considerable potential both for the production of existing alloys currently difficult to process by conventional methods and for the development of novel alloys with improved physical and mechanical properties. One of the most successful applications of rapid solidification processing to date has been in the introduction of a new class of aluminum-based alloys for elevated temperature applications. Design of these alloys typically involves the production of microstructures comprising a large volume fraction of thermally stable intermetallic phases distributed uniformly in an aluminum matrix. The alloying elements favored in such developments are those miscible with aluminum in the liquid state and having low solubilities and diffusivities in the solid state, since these will contribute to low coarsening rates of the strengthening intermetallic dispersoids. For a similar reason, the preferred intermetallic phases are those which are intrinsically stable at elevated temperatures and which possess low interfacial energy in an aluminum matrix. The most successful group of alloys developed using this approach has been that based on the A1-Fe system, with ternary and often quaternary additions, tll However, in these alloys, the dispersed intermetallic phases mostly form directly from the melt during rapid solidification 121 and are relatively coarse in scale (50 to 100 nm) compared to strengthening solid-state precipitates (2 to 10 nm) in conventional high-strength alloys. The alloys themselves are of relatively high density, since to achieve the required volume fractions of dispersed intermetallic phases requires large concentrations (8 to 12 wt pct) of J.F. NIE, Graduate Student, and B.C. MUDDLE, Reader in Materials Engineering, are with the Department of Materials Engineering, Monash University, Clayton, Victoria, 3168, Australia. S. SRIDHARA, formerly Research Fellow, Department of Materials Engineering, Monash University, is with the AMCAST Corp., Cedarburg, WI 53012. Manuscript submitted November 11, 1991. METALLURGICAL TRANSACTIONS A
what are commonly higher density solute elements (Fe, Mo, V, Zr, Cr, and Ce). There thus remains substantial scope for the design and development of improved alloys, particularly alloys of lower density and refined, thermally stable microstructure. Of the alternative alloy systems possible, the A1-Ti system appears to be one of the most promising, t3j The titanium is low density and has low solid solubility and diffusivity in aluminum. I4'5~In dilute binary A1-Ti alloys subjected to rapid solidification, formation of the equilibrium intermetallic phase A13Ti (body-centered tetragonal, DO22) is generally suppressed and replaced by formation of a metastable ordered cubic phase (L12), which appears to be substoichiometric with respect to titanium (approximately A14Ti). I61 The metastable intermetallic particles share an identity orientation relationship and a low lattice misfit with the aluminum matrix phase and w
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