Microstructural characterization of rapidly solidified aluminum transition metal alloys

Abstract

Current research in the field of rapid solidification is directed toward the development of new and unusual microstructures with improved engineering properties. The most important advancements in the rapid solidification of aluminum alloys have occured in the aluminum-transition metal alloy systems and in process development. The low diffusivities of the transition metals in aluminum and the high elastic moduli of the intermetallic phases in these systems result in microstructures having combined properties of high strength, thermal stability and high elastic modulus. Since coarse particle sizes have deleterious effects on ductility and toughness, control of solidification and composition must be exercised to limit the volume fraction of such coarse primary phases. Microstructural formation in the materials produced by chill methods (ribbon, fiber and splat) and spray methods (powder) will be limited by heat transfer: conduction and convection respectively. Other important factors affecting microstructural formation are the thermodynamic and kinetic considerations related to undercooling. Interface stability can be discussed in terms of theories of constitutional supercooling and morphological stability (Mullins and Sekerka), and the thermodynamics of non-equilibrium solidification for binary alloys in terms of local equilibrium at the interface. Specific microstructural features of both the as-cast and the consolidated and fabricated melt-spun rib bons and powders will be discussed in relation to their respective solidification sequences and processing conditions. Fracture surfaces will be presented since they are representative of the underlying microstructural features. Mechanical properties and fracture mechanisms are also discussed in terms of these features.