FIX: Fix eqplot for tie-triangles #288
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… composition degrees of freedom
richardotis
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Dec 2, 2020
richardotis
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Dec 2, 2020
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In 2D phase diagrams, 2D three phase regions (and tie-triangles) can only exist in where two molar quantities are axis variables. A binary temperature-composition phase diagram can only have three phases exist on an invariant line that occurs at an infinitesimally small temperature. In a binary system under fixed pressure, temperature and composition conditions, it should be impossible to find three phases in equilibrium, however numerically it is possible to achieve three phase stability within pycalphad's convergence limits under these conditions.
eqplotonly supports T-X binary phase diagrams and X-X ternary isothermal phase diagrams, and implicitly assumes that all three phase equilibria belong to a three phase region in a isothermal phase diagram. If three phases were found to be stable in a binary system and a user tried to plot the phase diagram usingeqplot, they would see an exception becauseeqplottries to find the composition of two independent components when plotting the tie-triangles, but only one independent composition existed. This PR changeseqplot.py:153fromif three_phase_idx[0].size > 0:toif (three_phase_idx[0].size > 0) and (len(indep_comps) == 2):, i.e. tie-triangles will only be plotted if there are two independent composition conditions.I tried to write a test case that exercised the behavior before the PR (raising an except for a three phase equilibrium), but I wasn't able to create a database or use an existing example that reproduced a three phase equilibrium in a binary system.
This PR has some minor code quality cleanups and documentation corrections.