Chapter 10 Phase Transformations in Metals: development of microstructure and alteration of mechanical properties
1. Phase Transformations: a change in the number and/or character of the phases that constitute the microstructure of an alloy *phase: a homogenous portion of a system that has a uniform physical and chemical make up. *Stages of phase transformation
1. Nucleation: the formation of very small particals, called nuclei, of the new phase, which are capable of growing. 2. Growth: The stage in which nuclei increase in size. This stage officially begins once an embryo has exceeded r* and become a stable nucleous. It follows that some of the parent phase volume disappears. * Transformation reaches completion if growth is allowed to proceed until the equilibrium fraction is attained. Two types of Nucleation
1. Homogenous: nuclei of the new phase form uniformly throughout the parent phase. 2. Heterogeneous: nuclei form preferentially at structural inhomogeneities, such as container surfaces, grain boundries, insoluable impurities, dislocations, ect. Homogenous nucleation: solidification of a pure material, assume nuceli of solid phase form in the interior of the liquid phase. There are two contributions to the total free energy change ΔG that accompany a solidification transformation 1. The volume free energy ΔGv – which is the difference in free energy between the solid and liquid phases. ΔGv will be negative if the temperature is below the equilibrium solidification temperature. The magnitude of its contribution is the product of ΔGv and the volume of the spearical nucleous (4/3 πr^3 ) 2. Surface free energy γ: energy comes from the formation of the solid-liquid phase boundary during the solidification transformation. γ is postitive; the magnitude of this contribution is the product of γ and the surface area of the nucleus (4πr2) * the total free energy change ΔG is equal to the sum of these two contributions: ∆G=4/3 πr^3 ∆G_v+4πr^2 γ * In a physical...
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