Why Does an Al-17Si Alloy Solidify Over a Range of Temperatures?

[pukb]

Why does an alloy solidify over a range of temperature unlike a pure metal.
Consider Al-17Si alloy.
Growth of primary Si after undercooling does not take place at liquidus temperature but temperature keeps on decreasing until it reaches eutectic arrest temperature. I am not able to understand why p-Si being one component phase has a different cooling curve compared to its pure alloy( pure Si).

 

[uby]

It's not clear what you're referring to. Are you talking about equilibrium cooling of a hypo-eutectic composition, where the equilibrium phase content varies with temperature?