Toward multi-principal component alloy discovery: Assessment of CALPHAD thermodynamic databases for prediction of novel
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Oleg N. Senkov UES, Inc., Dayton, Ohio 45432, USA (Received 22 January 2018; accepted 5 March 2018)
CALPHAD databases have traditionally been developed for investigation of single-principal component alloys. With the advent of batch processing capability, engineering teams have proposed using these models to systematically explore compositional space for multiprincipal element systems. However, the uncertainty of phase equilibria predictions outside of traditional compositional bounds has yet to be evaluated. This study assesses the current capabilities of commercially available CALPHAD databases to predict phase equilibria within ternary phase space as a function of the number of full binary system descriptions contained within the thermodynamic databases, the spatial location in compositional space relative to subsystem descriptions, and the specific database used. A strong correlation was observed between the fraction of subsystem descriptions available for the free energy calculation and the accuracy of phase predictions in undefined ternary space. The accuracy of equilibria predictions degraded with increased compositional extrapolation from defined subsystems.
I. INTRODUCTION
The implementation of computer coupling of phase diagrams and thermochemistry (CALPHAD) techniques early in alloy development efforts provides a critical opportunity to accelerate material transition. Efforts by the CALPHAD community to develop models and thermodynamic databases have modernized the traditional alloy development process. Now, vast compositional spaces can be quickly and economically explored using equilibria simulations as an initial screening step.1–4 Recent development efforts are shifting towards characterizing compositionally complex, multiprincipal element alloys since conventional metallic systems in which a single element dominates the composition are not expected to meet demanding requirements for future applications.5–8 Exploration of the vast composition space benefits from CALPHAD tools to assist in alloy discovery; however, it is unclear how well currently available thermodynamic databases can reliably predict phase equilibria in nontraditional alloys that fall outside the bounds of well-described compositional space. Many detailed resources outline the history, theory, and practice of CALPHAD.9–11 In short, a high-order
Contributing Editor: Michael C. Gao a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.61
polynomial free-energy model is constructed for each relevant phase in a multicomponent system and the system equilibria response is predicted by determining the minimum global Gibbs energy. The calculations employ material descriptions containing thermodynamic properties with model parameters obtained by fitting experimentally measured and/or atomistically derived phase equilibria. CALPHAD relies on a hierarchical scheme to provide equilibria predictions for multicomponent systems. A higher-order system can be modeled via free energy extrapolation from rel
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