Figure 1 shows the Mg-Pb phase diagram. Sketch the microstructure that would be obser for conditions of slow cooling for: (i) 20wt% Pb at the following temperatures: 700
°
C,600
°
C,400
°
C and 200
°
C. (ii) 67wt% Pb at the following temperatures: 600
°
C,300
°
C. (iii) 90wt% Pb at the following temperatures: 600
°
C,400
°
C, and 200
°
C. Label all phases and indicates their approximate composition. (ii) Composition (at\% Pb ) Figure 1: Mg-Pb phase diagram
The Correct Answer and Explanation is :
It seems like I can’t directly access the image you’re referring to. However, I can still explain how to sketch the microstructure for each of the cooling conditions based on the information you provided, using the Mg-Pb phase diagram.
Phase Diagram Concepts:
- The Mg-Pb phase diagram consists of regions where various phases like α-Mg (solid solution of Mg), β-Pb (solid solution of Pb), and eutectic mixtures exist.
- As you cool an alloy, different phases crystallize at different temperatures depending on the composition of the alloy.
(i) For 20 wt% Pb:
- 700°C: At this temperature, the alloy is in the liquid phase. There is still a high proportion of liquid, with very little solid. The structure would mainly be liquid with some solid α-Mg crystals.
- 600°C: As cooling continues, the liquid phase starts to crystallize. You’ll find a mix of liquid and solid phases. The solid phase is likely to be α-Mg, while the liquid phase would still be present, now richer in Pb.
- 400°C: By this temperature, the solid phase is much more predominant. A mixture of solid α-Mg and the eutectic phase, which consists of fine structures of α-Mg and Pb-rich β-Pb, will appear.
- 200°C: At this temperature, most of the alloy would be solidified, and the structure would consist mostly of α-Mg with some small regions of Pb-rich β-Pb.
(ii) For 67 wt% Pb:
- 600°C: At this composition and temperature, the eutectic mixture forms. The solid phase consists mainly of β-Pb with some α-Mg. The structure would be a fine-grained eutectic of α-Mg and β-Pb.
- 300°C: Here, the phase structure remains eutectic, with mostly β-Pb and some residual α-Mg solidifying as temperature drops.
(iii) For 90 wt% Pb:
- 600°C: The alloy would mostly consist of β-Pb with little α-Mg. The eutectic mixture would be observed in small proportions.
- 400°C: More β-Pb solidifies, and you’ll find a solid structure primarily composed of β-Pb with a few regions of eutectic.
- 200°C: At this lower temperature, nearly all of the alloy would be solidified into β-Pb, with only a small amount of α-Mg remaining if there was any eutectic formation.
Explanation:
In general, for alloys with low Pb content, you have more α-Mg solidifying, while higher Pb contents favor the formation of β-Pb, and eutectic structures become more prominent as the Pb concentration increases. The cooling process is slow enough that the phases have time to form as per the equilibrium phase diagram.
Let me know if you’d like further clarification or help sketching the microstructure!