Question

Please explain why micro-segregation and macro-segregation happen during practical solidification.

Answer 1

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, important to a repeated development 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 forming 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 absolute movement of separated liquid and solid during solidification.

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