Computational materials science
Which statement about the CALPHAD method is correct?
- AA method that directly solves the electronic Schrödinger equation at each temperature and composition to obtain multicomponent phase diagrams without experimental data or thermodynamic models
- BA method that fits Gibbs energy models of each phase to experimental and other data and calculates phase diagrams from the combination of phases that minimizes the total Gibbs energy
- CA method that merely interpolates and joins the liquidus and solidus lines of experimental binary phase diagrams to suit multicomponent compositions, using no thermodynamic models at all
Show answer and explanation
Answer: B. A method that fits Gibbs energy models of each phase to experimental and other data and calculates phase diagrams from the combination of phases that minimizes the total Gibbs energy
In the CALPHAD (CALculation of PHAse Diagrams) method, the Gibbs energy of each phase, such as liquid, solid solutions and compounds, is expressed as the sum of pure-substance terms, an ideal mixing term and an excess term representing interactions. Its parameters are fitted to experimental data such as phase diagrams, heat capacities and heats of mixing, and to formation energies obtained by first-principles calculations, and are compiled into a thermodynamic database. Finding the phases and compositions that minimize the Gibbs energy of the whole system at a given temperature and composition gives phase diagrams and phase fractions. Solving the electronic equations is first-principles calculation, whose results CALPHAD uses as one of its inputs. Because it uses thermodynamic models rather than interpolating phase diagram lines, it can extrapolate to temperatures and compositions without experiments.