Question

1. a) Make sketches to differentiate between microstructures in castings generated by homogeneous and by heterogeneous nucleation. What sort of structures provide the best mechanical properties?

b) For a simple binary alloy exhibiting complete solid solubility, describe how a typical composition will solidify from the moltern state. What is coring and what would encourage such an alloy to be cored? Can coring happen in alloys exhibiting an eutectic reaction?

Answer 1

In equilibrium cooling, sufficient time for redistribution of composition from liquid to solid is got at every temperature.

But in non equilibrium cooling, on cooling down from liquidus line, time won't get redistribution of composition between liquid phase and solid phase. Alpha is still at 46Ni%. Similarly on cooling down this alpha formed becomes a core and new alpha us formed surrounding it. consequently on cooling down further coring continues. Hence in a non equilibrium cooling of a binary isomorphous alloy coring occures. It is creating a high melting point temperature element inside the core. But in eutectic system on cooling as proeutectic phase may form, chances of coring is not there.