Computational materials science
The CALPHAD method is used to predict the precipitating phases and their fractions in a nickel-based superalloy or steel containing close to ten elements. Which combination of the underlying idea and a caution is correct?
- AExperimental data for every composition of the multicomponent system are stored in the database, so no extrapolation is made and the results always agree with the actual microstructure
- BThe Gibbs energy of a multicomponent system is determined by the pure-substance values alone, so no assessment of binary or ternary systems is needed
- CThe Gibbs energy of the multicomponent system is extrapolated by combining parameters assessed mainly for binary and ternary systems. What is obtained is the equilibrium state, and accuracy drops for compositions outside the assessed range
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Answer: C. The Gibbs energy of the multicomponent system is extrapolated by combining parameters assessed mainly for binary and ternary systems. What is obtained is the equilibrium state, and accuracy drops for compositions outside the assessed range
Experimenting at every composition of a multicomponent system is impossible, but with Gibbs energy models, parameters determined for binary and ternary systems can be combined to extrapolate to the multicomponent system. Interactions of quaternary and higher order are often neglected as small, and on this basis the liquidus temperatures of practical alloys, the kinds and fractions of precipitating phases, solubility limits and so on can be predicted, greatly reducing trial and error in alloy design. However, what is obtained is the thermodynamic equilibrium state; it does not tell whether the actual microstructure reaches equilibrium or how fast transformations proceed. To handle rates, data on diffusion mobilities and calculations of precipitation kinetics are combined. Pure-substance values alone cannot represent the interactions between components.