Presented below is the aluminium-rich end of the Al-Cu binary phase diagram, whi

Question

Presented below is the aluminium-rich end of the Al-Cu binary phase diagram, which is the basis of many high-strength aluminium alloys. You may assume that all the phase boundaries may be described by straight lines (not necessarily the case in reality 700 660 oC 600 500 5.5 wt% Cu 33 WL Cu 300 200 100 1 wt% Cu 10 20 30 50 AI Composition (wt% Cu) Q2a) For an alloy containing 4wt% Cu i) Calculate the volume fraction and composition of solid under rapid cooling conditions (4) ii) For fast cooling what is the fraction of eutectic present Q2b) Explain why the degree of segregation during solidification is usually less than that predicted by Scheil's equations

Explanation / Answer

2 a) Under rapid cooling of 4wt%Cu, the final product obtained is a non equilibrium supersaturated solid solution. Here the formation of beta atoms are prevented. So there won't be any changes in overall composition.

Here no eutectic phase will be present, for the given composition. as it is having Cu content less than 5.2 wt%.

2 b. scheil equation predict the redistribution ofsolute while solidification. As per the assumption made no diffusion occurred in solid phase, which is not the real case and there is instant diffusion in liquid phase, not at real. These are the reasons behind different segregation in practice and in theory.

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