Chapter 10-11 - Phase Transformations and Microstructure...

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1 Phase Transformations and Microstructure Properties (Chapters 10, 11) Martensitic microstructure in CuZnAl
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2 What you should understand by the end of this chapter Why phase transformations are important for controlling microstructure & material properties. The time dependent (kinetic) properties of phase transformations The governing law for the kinetics of solid-state reactions. The most important transformations in steels.
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3 Phase Transformations Examples: Liquid Æ solid alpha, Liquid Æ eutectic Liquid Al-30%wtCu Alpha solid Eutectic
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4 Concentration of phase Amount of phase Arrangement of phases Variety of Alloy Microstructures
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5 Phase Transformations The properties of solid materials are altered by thermomechanical processing . This involves a combination of… … thermal processing - heat treatment . … mechanical processing – deformation … role if to alter material’s microstructure and controls its properties. During a heat treatment… … the nature and amount of a phase changes , moving toward an equilibrium state. These processes are time dependent . Understanding the kinetics of a process is important.
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6 Kinetics of Solid-State Reactions Most reactions involve nucleation and growth. Nucleation is an incubation process. It requires time. Growth is limited by the diffusion process. It also requires time. Avrami Equation ) exp( 1 n kt y = fraction transformed
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7 Forces Controlling Nucleation and Growth During an undercooling process (during which the temperature drops below T E )… … the available reaction energy increases. … the rate of diffusion decreases. These are competing effects. - Δ G TT E D
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8 The reaction rate is a result of both nucleation and the growth of crystals. ¾ Examples: Adapted from Fig. 10.1, Callister 6e . Nucleation and Growth Many nucleation sites but develop very slowly T just below T E T moderately below T E T well below T E Few nucleation sites Coarse lamellae γ γ γ pearlite colony Increased nucleation Finer lamellae Very fine lamellae % Pearlite 0 50 100 Nucleation regime Growth regime log (time) t 50 Nucleation rate increases with | Δ T| Growth rate increases with T
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9 Rate is Temperature Dependent Example: Solidification of a hypothetical Liquid with melting point of 200 o C.
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This note was uploaded on 03/03/2011 for the course MATLS 101 taught by Professor Dr.kish during the Spring '11 term at McMaster University.

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Chapter 10-11 - Phase Transformations and Microstructure...

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