Synthesis and Processing

Abstract

Rapid Solidification (RS) involves propagation of a solidification front at high velocity. This is most readily achieved by suitable treatment of a volume of melt. Suitable treatments include: (i) dividing it up into a multitude of small droplets (atomisation, emulsification or spray-forming) so that most of them can under-cool deeply prior to solidification; (ii) stabilising a meltstream of small cross section in contact with an effective heat sink (melt-spinning or thin-section continuous casting); (iii) rapid melting of a thin layer of material in good contact with an extensive heat sink, which may be the same or related material (electron or laser beam surface pulse or traverse melting). In each case rapid solidification results from rapid extraction of the heat of transformation either directly by the external heat sink and/or internally by the undercooled melt (in which case the system rapidly reheats, i. e., recalesces during solidification). The large undercoolings developed amount to large departures from equilibrium leading to formation of extended solid solutions and new non-equilibrium phases (crystalline, quasicrystalline or glassy) while the short freezing times give rise to sizerefined and compositionally rather uniform microstructures as well as relatively high rates of throughput of material. The products of RS range from powder or flake particulate, through thin discontinuous or continuous ribbon or filament to thick spray deposits containing some trapped porosity. These products can sometimes be applied directly as in the cases of finely divided light metal particulate used as the basis for space shuttle and satellite launch rocket fuel and signalling flares, and planar-flow-cast strip used in certain magnetic applications or for braze assembly of engine components. For most applications, however, they must be suitably incorporated or consolidated into full size, fully dense sections or components. This may involve processes such as polymer bonding or liquid metal infiltration but most commonly involves powder metallurgy techniques such as die or isostatic pressing and/or hot working.