2. Explain the microstructural changes that take place when an alloy having a co

Question

2. Explain the microstructural changes that take place when an alloy having a composition of 15wt% Sn-85 wt% Pb is cooled slowly from 330°C to 130°C. (10 marks) Composition (at% Sn) 20 40 60 80 100 27 C 600 300 232?-1500 200 400 183 C 61.9 97.8 300 100 100 100 Sn) Figure 9.8 The lead-tin phase diagram. [Adapted from Binary Alloy Phase Diagrams 2nd edition, Vol. 3, T. B. Massalski (Editor-in-Chief), 190. Reprinted by permission of 80 Pb) Composition (wt% Sn) ASM International, Materials Park, OH.

Explanation / Answer

Draw a vertical line at 15 wt% sn, and a horizontal line at 330°C, one can observe liquid phase of the composition. On cooling from this temperature, first alpha + liquid phase will be observed and at around 220-230°C, it changes to alpha phase. Subsequent cooling to 180°C the microstructure changes from alpha to alpha + beta. And this phase remains until cooling up-to 130°C.

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