Research paper of the month
January 2015 - On the entropic stabilisation of an Al0.5CrFeCoNiCu high entropy alloy
High Entropy Alloys (HEA) are a relatively new class of metallic materials, which, unlike conventional alloys, do not have a single principal element. Instead, these materials consist of near equiatomic mixtures of five or more elements. Conventional wisdom would suggest that the microstructures of such compositions may contain several intermetallic phases but only simple structured solid solutions were reported by the seminal paper (Yeh et al. Adv. Eng. Mater. 6 (2004) 299). The stability of these solid solutions was postulated to be a result of a high configurational entropy, sufficient to dominate the Gibbs energy of the system, even with respect to the formation enthalpy of intermetallic phases. Since their conception in 2004, over 1300 scientific papers have been published on HEAs, but some results are inconsistent with the original hypothesis of entropic stabilisation. Examples of these inconsistencies include intermetallic phase precipitation and hardness evolution with time at temperature. Therefore, in this paper, we considered the concept of entropic stabilisation using one of the original HEA compositions, Al0.5CrFeCoNiCu. Our results indicated that the Gibbs energy curve for this alloy contains a miscibility gap and we demonstrated that a solid state single phase field does not exist for this composition at any temperature. These conclusions indicate that enthalpic terms dominate the behaviour of this system and clearly show that the original concept of entropic stabilisation does not hold for all systems.
Figure: X-ray diffraction spectra showing evidence of two fcc phases with very similar lattice parameters. These are the equilibrium phases above 1000°C thereby requiring the presence of a miscibility gap in the Gibbs energy curve for this alloy.
N.G. Jones, J.W. Aveson, A. Bhowmik, B.D. Conduit, H. J. Stone, “On the entropic stabilisation of an Al0.5CrFeCoNiCu high entropy alloy” Intermetallics 54 (2014) 148 – 153