SolutionsHW7

# SolutionsHW7 - HW 7 Problem 9.13 a. To Find: The phases and...

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HW 7 Problem 9.13 a. To Find: The phases and the compositions of these phases, in a binary Cu-Zn alloy, at various temperatures. b. Given: Composition of alloy: 74 wt. % Zn – 26 wt % Cu c. Assumptions: (i) The alloy is cooled/heated very slowly to attain the given temperatures. This ensures thermal equilibrium. The phase diagrams are completely valid only under conditions of thermal equilibrium. Thus, since the equilibrium phase diagrams are used to identify the phases and estimate their compositions, it is of utmost importance to note that: we are identifying the equilibrium phases and estimating the equilibrium compositions of these equilibrium phases ! If the alloy is cooled or heated very rapidly to the temperatures in question, the phases present and their compositions may deviate from what we have predicted using the equilibrium phase diagrams. (ii) There are no impurities or other alloying elements present – their presence could alter the equilibrium phase diagram.

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Fig. 1. Cu-Zn phase diagram d. Solution: This problem can be answered based on Fig. 9.19 in the text, the Cu-Zn phase diagram. (i) At 850 C , a liquid phase (L) is present. Composition of this single-phase region is shown by in Fig.1. C L = 74 wt.% Zn-26 wt.% Cu. It is sufficient to state the composition of any one component, for e.g., in this case, the liquid phase contains 74 wt. % Zn. This is because the alloy is binary and it is understood that the composition of the other component, here Cu, would be (100 – wt.% Zn) = 100 – 74 = 26 wt. % Zn. (ii) At 750 C , and liquid (L) phases are present.
The compositions of the phases may be estimated by drawing a tie line in this 2-phase region at 750 C (blue-line in Fig.1.). The 2-phase region here is flanked by the single phase region on the left and the single-phase liquid (L) region on the right. Thus, the composition at the point of intersection of the (blue) tie-line with the boundary (of the 2-phase region) on the left gives us the composition of the phase at this temperature. This point is shown as in Fig.1. C = 67 wt.% Zn-33 wt.% Cu The composition at the point of intersection of the (blue) tie-line with the boundary (of the 2-phase region) on the right gives us the composition of the liquid (L) phase at this temperature. This point is shown as in Fig.1. C

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## This note was uploaded on 12/08/2011 for the course MSE 2090 taught by Professor Leoindzhiglei during the Fall '10 term at UVA.

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SolutionsHW7 - HW 7 Problem 9.13 a. To Find: The phases and...

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