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Hypoeutectic and Hypereutectic 2 L+α^ L+β α + β 200 C, wt% Sn 0 20 60 80 100 300 100 L α β TE 40 (Pb-Sn System) Fig. 9.8, Callister & Rethwisch 9e. [Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 3, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International, Materials Park, OH.] 160 μm eutectic micro-constituent Fig. 9.14, Callister & Rethwisch 9e. hypereutectic: (illustration only) β β β β β β Adapted from Fig. 9.17, Callister & Rethwisch 9e. (Illustration only) (Figs. 9.14 and 9.17 from Metals Handbook, 9th ed., Vol. 9, Metallography and Microstructures, 1985. Reproduced by permission of ASM International, Materials Park, OH.) 175 μm α α α α α α hypoeutectic: C 0 = 50 wt% Sn Fig. 9.17, Callister & Rethwisch 9e. T(°C)
eutectic eutectic: C 0 = 61.9 wt% Sn hypoeutectic hypereutectic primary phase eutectic phases
Intermediate Phases 4 Al FCC Mg HCP no complete solubility http://al-chemist.info/sites/default/files/Al-Mg_PhaseDiagram.JPG
Intermetallic Compound 5 Mg 2 Pb Temperature / º C Fig. 9.20, Callister & Rethwisch 9e. [Adapted from Phase Diagrams of Binary Magnesium Alloys, A. A. Nayeb-Hashemi and J. B. Clark (Editors), 1988. Reprinted by permission of ASM International, Materials Park, OH.] Composition / wt% Pb Line on the diagram because of stoichiometry (i.e. composition of a compound is a fixed value).
6 Sneek Peek: Nonequilibrium Cooling Phase Transformations are governed by Thermodynamics Equilibrium is lowest energy state Kinetics time-dependence of the reaction Readjustment of the composition in the phases through atomic movement Is the reaction possible? (Necessary condition) Often cooling rates too rapid to reach equilibrium state
How do atoms move in solids? 8 What are the “holes” in materials? Vacancies Interstitials Substitutional solid solution Interstitial solid solution Vacancy diffusion Interstitial diffusion
9 Notes on Vacancy Diffusion Where do vacancies come from? How does diffusion work over time? vacancies originate at internal and external boundaries (grain boundaries, surface)
Phase Diagrams
