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
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
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
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 |
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
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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