Metals mostly occur in nature in form of componuds like oxides. A compound contains differernt atoms which are chemically combined toigether at a fixed ratio. Meanwhile mixture contains different atoms or compunds which have no chemical combinations and at no fixed ratio.Alloys are mixture of base metal and alloy elements.
During solidification, as the tempertature decreases, the solution rate changes. The material composition changes from core to the outside in one crystal. This is so called micro-segregation.