Question

Please use phase diagram to explain why micro-segregation and macro-segregation happen during practical solidification.

Answer 1

Answer.

Most alloy elements have a lower solubility in the solid than in the liquid phase. During freezing, the solutes are rejected into the liquid phase, leading to a continual enrichment of the liquid and lower solute concentrations in the primary solid. This is cause of micro segregation.

Explanation- Al₂Cu phase- if the distribution of copper in a sample of the casting is studied by microprobe analysis in a SEM, it is finds that the copper content in the aluminum phase varies. In the center of dendrite arms (which corresponds to the first solid) the copper content is low. In the outside of the arms (which corresponds to metal freezing later) the copper content increases.

The cause of macro-segregation is relative movement of segregated liquid and solid during solidification.

Explanation-Macro segregation is the redistribution of solute elements on a larger scale. This is caused by the motion of fluid inside the casting and usually is related to feeding. It is seen in copper-containing alloys.