Chapter 3 - Structural phases: their formation and transitions

Abstract

At some stage of production, the majority of metals and alloys are melted and then allowed to solidify as a casting. The latter may be an intermediate product—such as a large steel ingot suitable for hotworking, or a complex final shape—such as an engine cylinder block of cast iron or a single-crystal gas-turbine blade of superalloy. Solidification conditions determine the structure, homogeneity, and soundness of cast products and the governing scientific principles find application over a wide range of fields. The liquid/solid transition is of great scientific and technological importance. This chapter illustrates some basic principles while also discussing the freezing behavior of a melt of like metal atoms. The melting point (m. p.) is pressure-dependent and specific to the metal. Its value relates to the bond strength of the metal. Thus, the drive to develop strong alloys for service at high temperatures has stimulated research into new and improved ways of casting high-m.p. alloys based upon iron, nickel, or cobalt. The transition from a highly disordered liquid to an ordered solid is accompanied by a lowering of the energy state of the metal and the release of thermal energy (latent heat of solidification).