Question

No. 12

20 80 60 80 100 Composition s 12. Cite the phases that are present and the phase compositions for the following alloys (a) 90 wt% Zn-10 wt% Cu at 400°C (750°F) (b) 30 wt% Pb-70 wt% Mg at 425°C (7950 ) (c) 2.12 kg Zn and 1.88 kg Cu at 500°c (930°F) (d) 37 lbn Pb and 6.5 Ibm Mg at 400°C (750 F)

Answer 1

(a) That portion of the Cu-Zn phase diagram (Figure 9.19) that pertains to this problem is shown below, the point labeled "A" represents the 90 wt% Zn-10 wt% Cu composition at 400°C. 800 1400 Liquid 1200 600 1000 800 400 600 200 400 80 87 97 100 Composition (wt% Zn) As may be noted, point A lies within the ε and η phase field. A tie line has been constructed at 400°C; its intersection with the ε-e η phase boundary is at 87 wt% Zn, which corresponds to the composition of the ε phase Similarly, the tie-line intersection with the ε η-η phase boundary occurs at 97 wt% Zn, which is the composition of the η phase. Thus, the phase compositions are as follows C,-87 wt% Zn-13 wt% Cu 97 wt% Zn-3 wt% Cu

( For an alloy composed of 37 lbPb and 6.5 lb Mg and at 400°C, we must first determine the Pb and Mg concentrations, as 37 lb 100-85 wt% Pb 37 lbm +6.5 lbm 6.5 lb Mg 37 lbm 6.5 lbm That portion of the Mg-Pb phase diagram (Figure 9.20) that pertains to this problem is shown below; the point labeled "F"represents the 85 wt% Pb-15 wt% Mg composition at 400°C. 1000 800 400 600 300 200 B 400 100 Mg2Pb 200 (81) 93 100 Composition (wt% Pb) As may be noted, point F lies within the L + Mg2Pb phase field. A tie line has been constructed at 400°C it intersects the vertical line at 81 wt% Pb, which corresponds to the composition of Mg-Pb. Furthermore, the tie line intersection with the L + Mg-PDL phase boundary is at 93 wt% Pb, which is the composition of the liquid phase. Thus, the phase compositions are as follows -81 wt% Pb-19 wt% Mg Mg2Pb CL-93 wt% Pb-7 wt% Mg