A Mg-8%A1 liquid alloy (close to the composition of a commercial alloy, AZ80) is

Question

A Mg-8%A1 liquid alloy (close to the composition of a commercial alloy, AZ80) is to solidify assuming no solid state diffusion and complete liquid diffusion. The simplified phase diagram for the Mg rich side is given below, taking the liquidus and solidus as straight lines. Determine how much beta phase will be present immediately after the solidification. As shown in the figure, data for the alpha + liquid region are: melting point of pure Mg is 650 degree C, maximum solubility of A1 in alpha - Mg (at 437 degree C) is 12.5%, and eutectic (alpha + beta) composition (at 437 degree C) is 33.0%A1 and the composition of eutectic beta is 42.5%A1.

Explanation / Answer

The process of solidification at 437C can be represented as L = + . This involves equilibrium between 3 phases. Such a system is known as eutectic. The phase diagram has been given in the question.

As per diagram phases are given by lever rule

% at just above peritectic temp = [(50-42.5)/( 50-12.5)]x100 = 0.2x100 = 20%

Just below peritectic temp: % = [(50-42.5)/(50-33)]x100= 0.44x100=44%

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