ENGR 240 - 11.12.07

ENGR 240 - 11.12.07 - ENGR 240 Diffusion o d = rate of...

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ENGR 240 – November 12, 2007 Diffusion o ν d = rate of atoms from liquid that attach to solid nuclei = c 2 exp(-Q d /kT) as T decreases ν d decreases o n-dot = nucleation rate = C[exp(-ΔG*/kT)exp(-Q d /kT)] o during cooling of liquid, significant nucleation rate occurs below melting temperature (equilibrium solidification temperature called supercooling or undercooling o you need a greater degree of undercooling for homogeneous nucleation because it is more difficult to form ΔT homo could be several hundred degrees ΔT hetero a couple degrees Growth o Occurs by diffusion of atoms from parent phase to product phase across the interface G-dot = C*exp(-Q/kT) G is growth rate Q is activation energy o Diffusion increases as Temperature increases Temperature-Dependent Microstructure
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