Transformation and Treatments

Transformation and - Transformation and Treatment Chapter 11 In the last chapter We looked at some fairly simple 2 component phase diagrams in some

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Transformation and Treatment Chapter 11
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In the last chapter… We looked at some fairly simple 2 component phase diagrams in some detail We explored more complicated phase diagrams We did not examine how the phase change from one solid phase to another or others occurs
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Solid State Reactions The change from one solid to another has a lot in common with the solidification process It does not happen instantly Need nucleation Need time for growth
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Recall For solidification G = 4/3 π r 3 G v +4 π r 2 σ Volume free energy + surface energy For one solid phase changing to another G = 4/3 π r 3 G v +4 π r 2 σ + 4/3 π r 3 ε Volume energy + surface energy + strain energy Because the new solid does not take up the same volume as the old solid
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Nucleation Nucleation usually occurs at grain boundaries Unlike solidification, it isn’t too hard to get a nucleus going However, the nucleation rate increases as the temperature goes down
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Growth The nucleus grows as material diffuses to the site Diffusion is a function of temperature If you cool the material off immediately, it is hard for diffusion to occur Supersaturated non-equilibrium structures can occur
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Kinetics Nucleation and growth determine how fast the transformation will occur. Avrami relationship f=1-exp(-ct n ) f is the fraction converted t is time c and n are constants for a given temperature
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Avrami Plot Fraction Converted Time (sec) Conversion is 50% Complete τ is the time required for 50% conversion | Incubation Time |
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Growth Rate Often expressed as 1/ τ Τ he growth rate is a function of temperature Often, the higher the temperature, the faster the solid transforms Why? Diffusion dominates in many systems Not always true though – for example. .
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Effect of Temperature on Phase Transformation Growth Rate Nucleation Rate Overall Transformation Rate Temperature Rate Equilibrium transformation temperature
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Effect of Temperature on Phase Transformation T e m p r a t u Time Time for 50% Transformation Minimum Time required for Transformation
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C-curve Typical of many metals, ceramics, glasses and polymers Ex. Iron changes phase this way
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This note was uploaded on 01/22/2012 for the course ME 2733 taught by Professor Meng during the Fall '10 term at LSU.

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Transformation and - Transformation and Treatment Chapter 11 In the last chapter We looked at some fairly simple 2 component phase diagrams in some

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