ch10&11 - Phase Transformations and Microstructure...

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1 Phase Transformations and Microstructure Properties Chapters 10 & 11 ¾ 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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2 Phase Transformations ¾ Examples: ± Liquid Æ solid alpha ± Liquid Æ eutectic Liquid Al-30%wtCu Alpha solid Eutectic
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3 Concentration of phase Amount of phase Arrangement of phases Variety of Alloy Microstructures
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4 Phase Transformations ¾ The properties of solid materials are altered by thermomechanical processing . ¾ This involves a combination of… ± … thermal processing - heat treatment . ± … mechanical processing – deformation ² … role if to alter material’s microstructure and controls its properties . ¾ During a heat treatment… ± … the nature and amount of a phase changes , moving toward an equilibrium state. ± These processes are time dependent . ² Understanding the kinetics of a process is important.
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5 Kinetics of Solid-State Reactions ¾ 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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6 Nucleation and Growth The reaction rate is a result of both nucleation and the growth of crystals. ¾ Examples: Adapted from Fig. 10.1, Callister 6e . 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 | T| Growth rate increases with T
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7 Rate is Temperature Dependent Example: Solidification of a hypothetical Liquid with melting point of 200 o C. Transformation rate: If r is too small the system will never reach equilibrium. = = RT Q A t r exp 1 5 . 0 Thermally-activated process 0 10 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 Time (s) Fraction Transformed (%) Time for 50% trans. 190 o C 10 100 Temperature ( o C) 190 150 o C 150 100 o C 100
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8 Forces Controlling Nucleation and Growth ¾ During an undercooling process (during which the temperature drops below T E )… ± … the available reaction energy increases.
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This note was uploaded on 09/28/2011 for the course 1M03 material 1 taught by Professor Llllll during the Spring '11 term at McMaster University.

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ch10&11 - Phase Transformations and Microstructure...

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