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Unformatted text preview: 3.1 Crystallization from the melt 3.1.1 Freezing of a pure metal 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. For instance, knowledge of the solidification process derived from the study of conventional metal casting is directly relevant to many fusionwelding processes, which may be regarded as 'casting in miniature', and to the fusion- casting of oxide refractories. The liquid/solid transition is obviously of great scientific and technological importance. First, in order to illustrate some basic principles, we will consider the freezing behaviour of a melt of like metal atoms. The thermal history of a slowly cooling metal is depicted in Figure 3.1; the plateau on the curve indicates the melting point (m.p.), which 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 in the energy state of the metal and the release of thermal energy (latent heat of solidification), forming the arrest on the cooling curve shown in Figure 3.1. This order- ing has a marked and immediate effect upon other structure-sensitive properties of the metal; for instance, the volume typically decreases by 1-6%, the electrical Figure 3.1 Cooling curve for a pure metal showing possible undercooling. conductivity rises and the diffusivity, or ability of the atoms to migrate, falls. Solidification is a classic example of a nucleation and growth process. In the general case of freezing within the bulk of pure molten metal, minute crys- talline nuclei form independently at random points. After this homogeneous form of nucleation, contin- ued removal of thermal energy from the system causes these small crystalline regions to grow independently at the expense of the surrounding melt. Throughout the freezing process, there is a tendency for bombard- ment by melt atoms to destroy embryonic crystals; only nuclei which exceed a critical size are able to survive. Rapid cooling of a pure molten metal reduces the time available for nuclei formation and delays the Chapter 3 Structural phases: their formation and transitions Undercooling Evolution of latent heat after undercooling Time T e m p e r a t u r e onset of freezing by a temperature interval of AT....
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- Fall '11