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Unformatted text preview: ENGR 240 November 12, 2007 Diffusion o d = rate of atoms from liquid that attach to solid nuclei = c2exp(-Qd/kT) as T decreases d decreases o n-dot = nucleation rate = C[exp(-G*/kT)exp(-Qd/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 Thomo could be several hundred degrees Thetero 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 Rdot Temperature-Dependent Microstructure Ndot o T near Tm Ndot is low, Gdot is high Few nuclei that grow rapidly Gdot Coarse grains few relatively large phase particles Higher ductility o Low T high Ndot, low Gdot Fine grains many small particles High hardness, high strength Kinetics of solid state transformations o How phase transformations occurs over time at constant T o Avrami Equation y = 1 exp(-ktn) y is the fraction that transformed growth k and n are time-independent constants for a particular transformation o Rate of Transfromation Nuc process Taken to be the reciprocal of time required for transformation to be halfway complete ...
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- Fall '07