side by side to maintain composition at interface Overall microstructure is

Side by side to maintain composition at interface

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side-by-side to maintain composition at interface Overall microstructure is grain-like eutectic colonies This transformation mechanism is the easiest kinetic path
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Eutectic Diagram: Characteristic Equilibrium Microstructure at x 2 Let L at composition x 2 be cooled slowly Solidification occurs in two steps: L solidifies gradually to α as it cools through L+ α region At T E , the remaining liquid solidifies by the eutectic reaction : L α + β Microstructure resembles that shown (the off-eutectic microstructure) Islands of pro-eutectic α composition at interface Surrounded by eutectic colonies
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Eutectoid Reactions The eutectoid is a eutectic between solid phases: γ α + β γ is a solid The classic example is the Fe-C diagram shown at left Pure Fe: L →δ (bcc) γ (fcc) α (bcc) Eutectic ( cast iron ) L γ +Fe 3 C Eutectoid: γ α +Fe 3 C The most important reaction in steel
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Thermochemical Properties Essential features: Thermodynamics : what material wants to do (forces) Kinetics : what it can do, and how quickly Study – Thermodynamics • Properties Equilibrium phase diagrams – Kinetics Continuous: heat and mass diffusion Structural phase transitions Environmental interactions Wetting and catalysis • Corrosion
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Kinetics Rate of change in response to thermodynamic forces Deviation from local equilibrium continuous change T heat flow temperature changes µ atom flow composition changes Deviation from global equilibrium discontinuous change G ( F) discontinuous change of state phase change
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Flow of Heat Let: T 2 > T 1 (one-dimensional gradient) J Q = heat flow/unit area•unit time (J/m 2 s) Ignore internal sources of heat From the Second Law: Box of unit length, unit cross-section area J Q = k dT dx Fourier s law of heat conduction k = thermal conductivity T 1 T 2 J Q
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Evolution of Temperature Let: T 3 > T 2 >T 1 (one-dimensional gradient) Q/ t = heat added/unit time (J/m 2 s) The net heat added to the center cell is: Q t = J 23 Q J 12 Q [ ] dA J 12 Q = J 23 Q + dJ Q dx dx = dJ Q dx dV = d dx k dT dx dV Q t = E t = C V T t dV T t = k C V 2 T x 2 T 1 T 2 T 3 J Q 12 J Q 23
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J.W. Morris, Jr.
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  • Fall '08
  • Staff
  • Thermodynamics, University of California, Phase transition, J.W. Morris

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