Thermodynamic modelling to predict phase stability in BCC + B2 Al–Ti–Co–Ni–Fe–Cr high entropy alloys

• Published in: Materials chemistry and physics 2022-01, Vol.276, p.125395, Article 125395
• Main Authors: Conway, Patrick L.J., Golay, David, Bassman, Lori, Ferry, Michael, Laws, Kevin J.
• Format: Article
• Language: English
• Subjects: Aluminum; Aluminum alloys; B2 crystal; B2 crystal structure; BCC crystal Structure; CALPHAD; Casting alloys; Chromium; Chromium alloys; Cobalt; Cobalt alloys; Crystal structure; Crystals structures; Enthalpy; Entropy; Heat treatment; High entropy alloys; Intermetallic compounds; Iron; Iterative methods; Modelling; Nickel; Phase stability; Simple; Stability analysis; Temperature dependence; Thermodynamic modelling; Thermodynamic models; Thermodynamic stability; Titanium; Titanium alloys; Two phase
• ISSN: 0254-0584; 1879-3312; 1879-3312
• DOI: 10.1016/j.matchemphys.2021.125395

This paper examines the potential of thermodynamic modelling as a simple and inexpensive means for assessing phase stability in a series of non-equiatomic high entropy alloys and compares with CALPHAD calculations to demonstrate an appropriate level of simplifying assumptions. The modelling was motivated by alloys from the Al–Ti–Co–Ni–Fe–Cr system, which were produced by iteratively following the natural compositional segregation of the two-phase BCC + B2 microstructure present in a Al2TiCoNiFeCr alloy after casting and heat treatment. This produced a range of multicomponent B2-type alloys with different volume fractions of a BCC secondary phase. The solubility limits and traditional empirical thermodynamic driving forces for phase stability were investigated to explain the formation of the two phases. Limitations of prior semi-empirical models are highlighted, with advancements demonstrated by accounting for contributions from the effect of ordering on configurational entropy, the difference in enthalpy from intermetallic compounds, and thermal influences on both entropy and enthalpy. The new models are compared against the current leading thermodynamic modelling approach, CALPHAD, with excellent correlation. This work outlines a methodology to predict and design phase constitution in future high-performance BCC + B2 alloys and, more generally, it demonstrates the value of models with temperature-dependent thermodynamic quantities for exploring new, complex compositional regions. •Several novel body-centred cubic + B2 Al–Ti–Co–Cr–Fe–Ni high entropy alloys have been developed.•The effects of compositional variation on phase equilibria, phase composition and volume fraction were studied.•A novel method to design or predict phase constitution in body centred cubic + B2 high entropy alloys is developed.•Thermal influences and ordering tendencies on entropy and enthalpy are incorporated to determine the Gibbs free energy.•Calculated Gibbs free energies correlate exceptionally well with CALPHAD calculations in multicomponent systems.