An alloy of 80 atomic % arsenic and 20 atomic % gallium is produced, and the alloy is heated to 1250°C to thoroughly melt the mixture. The alloy is then slowly cooled so that it is always close to equilibrium. Answer the following about the alloy:
(a) At what temperature does a solid first form?
(b) What is the chemical composition of the first solid to form?
(c) Below what temperature is the alloy completely solid?
(d) At room temperature (23°C), what phases are present, what are their chemical compositions, and what is the atom fraction of each phase present?
(a) The 80 atomic % arsenic line intersects the liquidus line at approximately 1080°C, this is the temperature where solid first starts to form upon cooling.
(b) The first solid to form is found by drawing a horizontal line at 1080°C, and this intersects the GaAs compound line at 50 atomic % arsenic. The first solid to form is GaAs with the chemical composition of 50 atomic % arsenic.
(c) Below 810°C this alloy is all solid, it is a mixture of solid beta and the compound GaAs.
(d) At 23°C there is the beta phase that is pure arsenic and the gamma phase has the chemical composition of 50 atomic % arsenic and 50 atomic % gallium. This is a two phase region thus the atom fraction of beta and gamma are given by the lever rule.
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