Binary phase diagram, thermodynamic assessment

As a conclusion to these notes on the assessment and compilation of intermetallic data, we mention a few examples of papers in which criteria to be followed in order to extract from literature data the best version of a phase diagram have been discussed. These are Morral and Gupta (1991) (a figure of merit for predicted phase diagram), Okamoto and Massalski (1991, 1993) (thermodynamically improbable phase diagrams and guidelines for binary phase diagram assessment). See also various comments in several chapters in Cahn (2001). 

The thermodynamic assessment of the binary phase diagram is in good agreement with experimental data [31, 32]. 

The classical compilations of binary phase diagrams, give no information at all on this system. Consequently, it has been supposed that there is a negligible mutual solid solubility of calcium and ruthenium in each other, and a wide miscibility gap in the liquid state, the mutual solubility increasing at high temperature. Thus, the assessed diagram is only qualitative and the solid and liquid solubilities are entirely estimated. No thermodynamic property is available for that system. The system was assessed by Chevalier and Fischer [1996Che]. 

Critical assessment of phase-diagram data employing the thermodynamics of the phase equilibria in the form of phase-diagram modeling provides consistent thermochemical data (including also the liquid solution) for binary metal-boron systems such as V—B Cr—B, Mn—B, Fe —B, Co—B, Ni—Mo—B and Nb—B, W—B, A1 — B Despite these efforts, reinvestigation of binary systems is... 

In earlier times, thermodynamic and phase diagram optimisation were often separate exercises. For example, the compilations of binary diagrams by Hansen and Anderko (1958), Elliott (1965) and Shunk (1969) did not normally take into account thermodynamic information. The compilations of Hultgren et al. (1973) concentrated on thermodynamic measurements, but the phase diagram was also an important factor in their assessment procedure. However, this was mainly for phase identification purposes rather than for making a self-consistent assessment of both phase diagram and thermodynamic measurement. 

It is possible in many cases to predict highly accurate phase equihbria in multi-component systems by extrapolation. Experience has shown extrapolation of assessed (n — 1) data into an nth order system works well for n < 4, at least with metallurgical systems. Thus, the assessment of unary and binary systems is especially critical in the CALPHAD method. A thermodynamic assessment involves the optimization of aU the parameters in the thermodynamic description of a system, so that it reproduces the most accurate experimental phase diagram available. Even with experimental determinations of phase diagrams, one has to sample compositions at sufficiently small intervals to ensure accurate reflection of the phase boundaries. 

Rycerz L, Gong W, Gaune-Escard M (2013) The TbBr3- LiBr binary system experimental thermodynamic investigation and assessment of phase diagram. J Chem Thermodyn... 

---