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With reference to the phase diagrams drawn in b and d why are the Gibbs phase lines coming closer from being so further apart?

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closed as off-topic by Mathew Mahindaratne, Tyberius, DrMoishe Pippik, Mithoron, Todd Minehardt Sep 12 at 20:27

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  • $\begingroup$ Those lines are just examples. As the temperature changes, the shape and relative position of the liquid and solid phase free energy curves will change. That is all. $\endgroup$ – Jon Custer Sep 11 at 15:44
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After some thought, it is probably worthwhile to actually write an answer rather than a short, fairly useless comment.

Lets look at the Cu-Ni phase diagram, as calculated using the CALPHAD parameters given in the paper referenced in many of the pictures below (X.J. Liu et al., J. Electronic Materials 37(2) 210-217 (2008)).

Overall: phase diagram

This looks a lot like your pictures above. Remember, red is liquid, blue is the fcc solid. The temperature (1800K) is above the melting point of both elements. The liquid and solid Gibbs free energy curves (top plot) are pretty close together on the Ni side, which makes sense since the Ni just melted. Cu melted some 400K earlier, so it should be expected that the solidus is well above the liquidus line there.

In the middle, the curves look like: 1500K You can see the tie line around about 33 percent Ni. Pure copper has melted, so the liquidus is below the solidus. Pure nickel is still solid, so it is the other way round on the right side of the phase diagram.

Below the melting point of Cu, it looks like: 1300K Now the solid free energy curve is below the liquid free energy curve everywhere - both pure Cu and pure Ni are solids. The liquidus on the copper side is closer to the solidus - the copper is closer to the melt temperature.

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