Question

Briefly explain why, upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases. What is this structure called? What kind of effect does this structure have on the melting point of the alloy and why? (5 points)

Answer 1

Eutective point is the temperature at which a particular eutectic mixture freezes or melts.The point, where the liquidus curves and solidus intersect, is termed the eutectic point. At the eutectic point in this two component system, all three phases, that is Liquid, crystals of A and crystals of B, all exist in equilibrium.

Liquid |(1) (2 (3)

At eutectic point Both alpha and beta crystals start forming, While growing the both cells start growing further but restricted each other from the sides. So the only possible direction of growth is length wise. So the structure forms as alternative alpha and beta crystals . This cintinues untill the alloy cooled down.Since solidification of eutectic alloy occurs at constant temperature, its cooling curve would be the same as that for a pure metal or any congruent melting alloy. The eutectic solidification however is not congruent as there is a difference in composition between the liquid and the individual solid phases.The grains grow by adding alpha to alpha and beta to beta until they encounter another grain (location 3). Further nucleation sites will also continue to form within the liquid parts of the mixture. This solidification happens very rapidly as any given volume of liquid in the melt reaches the eutectic temperature. Remember that a eutectic composition solidifies at a single temperature like a pure element and not over a temperature range.

This layered microstructure is known as lamellar microstructure and the layers are often only of the order of 1 micron across.

Melting temperature of both metals in alloys become same for this combination.