Microstructure Changes

The phase diagram for the Al – Si system (the basis of aluminium casting alloys) is shown in Figure 1. Explain what is happening when the alloy Al – 30 wt % of Si is cooled from 900 ? to room temperature. You need to consider phasetransformation, microstructure changes, temperatures, and composition.(6 marks)Figure 1: Phase Diagram of Al – Si binary system(ii) You have been asked to calculate the critical nucleus radius, r* and the activation free energy ?????? for the solidification of pure nickel if nucleation is homogenous. The latent heat of fusion and surface energy of nickel are given as -2.53 x 109 J/m3 and 0.250 J/m2 respectively. The supercooling curve value for nickel is 319 ?. The melting temperature of nickel is 1455 ?.(4 marks)

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