Homework- Phase Diagrams 700F 660°C For the AI-Cu system at 550C, specify: ID ph
ID: 1767369 • Letter: H
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Homework- Phase Diagrams 700F 660°C For the AI-Cu system at 550C, specify: ID phases present. 2) their chemical composition, and 3) phase fraction for 1. 600 a) 10 wt% Cu. b)20wt% Cu e) 30 wr% Cu 5.65 548°C 33.2 For the Al-Cu system at 550C specify: 1D phases present, 2) their chemical composition, and 3) phase fraction for 2. 0 +2 ?) 4 wt% Cu. b)25 wt% Cu e) 33 wt%Cu 0 10 20 30 40 5060 70 3. For the Al-Cu system at 540C specify: 1) phases present 2) their chem. composition, 3) phase fraction for: 1 00% wt % Copper- a)4wr% Cu, b)25% wt% Cu and c)33 wr% Cu 96 Al-4 Cu 75 Al- 25 Cu 66 Al- 33 Cu 4. Shown on the right are the optical microscope images of microstructures at 25C of the three Al- Cu alloys from the problem 4: 4 wt% Cu; 25 w1% Cu; 33 wt% Cu. Take a gliess at identifying which phases are represented by features of each micrograph. Use an arrow with each of the phase names, ? or ?, to label the phase of the features in each photo. The next topic shows how phase diagrams can be used to describe, explain and predict the internal structure, or microstructure, of an alloy and therefore its properties. On the next page is problem 5, where there are selection banks of microstructures and of a given alloy's phase chemical compositions and phase fractions. To prepare for class fill itn the each rectangular blank with a chem. comp./ fraction number and DRAW the associated m adjoining cirele. This content is in Sec. 9.12 pp. 305-309. 5. cture in theExplanation / Answer
In the Al-rich part, with just 4 % copper (Cu), at 550 oC (932 oF) we just have Cu dissolved in aluminum or the usual a-phase. That means that single Cu atoms are occupying the sites of Al atoms in its fcc lattice. The copper atoms thus are extrinsic impurity atoms of the substitutional kind. You just as well could call then doping atoms because they are intentionally introduced, but nobody does that. This term is more or less reserved for changing electrical properties by introducing impurities. We do call them "alloying element", though.
Our starting material at 550 oC thus is poly-crystalline aluminum with 4 % of copper (Cu) atoms fully dissolved, some dislocations and vacancies, and not much else.
Atlowertemperatures the crystal obviously needs to produce a few large CuAl2 precipitates for achieving nirvana. The chemical compound CuAl2 is also called the theta (Q) phase
When you start at the 150 oC, you now have exactly what you had at 550 oC: 4% copper atomically dissolved in an aluminium polycrystal. The only difference is that at 150 oC the crystal is unhappy about that. Very unhappy. It will now do whatever it can to remedy the situation and to establish the proper nirvana state, which is almost copper-free aluminum plus CuAl2 precipitates. You also can call it a mixture of the a and Q phase; same thing.
A temperature of 150 oC combines an overwhelming need for making precipitates with some sluggishness in doing things. We're thus heeding the "golden rule" from before and control nucleation and kinetics. The crystal has no choice but to start a lot of very small precipitates all over the place.
If there is a sufficient number of defects around, nucleation will now start at those defects, i.e. we have heterogeneous nucleation. Otherwise nucleation will be homogeneous but the details don't matter for our purpose here. We just will produce a large number of nuclei soon after we reached the final 150 oC, and we do not need to care exactly how that happens and exactly how large that number will be.
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