Kine cs of Phase Transforma ons The phase diagram tells you where a system wants to go in terms of composi on and microstructure but it tells you nothing about how long it will take to get there We need to learn about how rapidly phase transforma ons occur because we can make microstructures and composi ons not predicted by the phase diagram by a combina on of heat treatments and varying cooling rates We will rst examine di usive phase transforma ons that occur but the movement of atoms at elevated temperature di usion Most phase transforma ons in materials are di usive 1 Precipita on Strengthening 2 Check out the reading in Chap 11 9 on precipita on hardening Al rich Al Cu alloys are good candidates for precipita on hardening strengthening The Al rich sec on of the Al Cu phase diagram 3 Consider an alloy of Al with 3 wt Cu At 550 C we have a single phase but upon cooling to 400 C we are in the two phase region resul ng in the forma on of a small weight frac on of par cles the precipitate 4 AVer cooling from 550 to 400 C the forma on of the par cles takes me y is the frac on of transforma on where y 0 means that par cle forma on has not yet begun and y 1 means that the maximum weight frac on of par cles as calculated by the lever rule has been reached 5 Let s look at the par cle evolu on as a func on of me During the nuclea on period no second phase par cles are observed even though we are in a two phase region according to the phase diagram y 6 AVer some me in the two phase region has passed par cle nuclea on and growth occurs by the di usion of atoms from the matrix into the growing par cles Note that being in the two phase region allows the maintenance of sharp concentra on gradients that would disappear in a single phase region y C 7 Once y 1 the net movement of atoms from the matrix to the precipitates is complete y C 8 The me frame over which the frac on transformed goes from 0 to 1 depends on the temperature in the region 9 For each curve we begin in the single phase region e g at 550 C then cool rapidly to a temperature in the two phase region No ce any trends 10 The temperature at which the phase precipitates also e ects the resul ng microstructure 450 C 11
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