Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys

Abstract

High configurational entropies have been hypothesized to stabilize solid solutions in equiatomic, multi-element alloys which have attracted much attention recently as "high-entropy" alloys with potentially interesting properties. To evaluate the usefulness of configurational entropy as a predictor of single-phase (solid solution) stability, we prepared five new equiatomic, quinary alloys by replacing individual elements one at a time in a CoCrFeMnNi alloy that was previously shown to be single-phase [1]. An implicit assumption here is that, if any one element is replaced by another, while keeping the total number of elements constant, the configurational entropy of the alloy is unchanged; therefore, the new alloys should also be single-phase. Additionally, the substitute elements that we chose, Ti for Co, Mo or V for Cr, V for Fe, and Cu for Ni, had the same room temperature crystal structure and comparable size/electronegativity as the elements being replaced to maximize solid solubility consistent with the Hume-Rothery rules. For comparison, the base CoCrFeMnNi alloy was also prepared. After three-day anneals at elevated temperatures, multiple phases were observed in all but the base CoCrFeMnNi alloy, suggesting that, by itself, configurational entropy is generally not able to override the competing driving forces that also govern phase stability. Thermodynamic analyses were carried out for each of the constituent binaries in the investigated alloys (Co-Cr, Fe-Ni, Mo-Mn, etc.). Our experimental results combined with the thermodynamic analyses suggest that, in general, enthalpy and non-configurational entropy have greater influences on phase stability in equiatomic, multi-component alloys. Only when the alloy microstructure is a single-phase, approximately ideal solid solution does the contribution of configurational entropy to the total Gibbs free energy become dominant. Thus, high configurational entropy provides a way to rationalize, after the fact, why a solid solution forms (if it forms), but it is not a useful a priori predictor of which of the so-called high-entropy alloys will form thermodynamically stable single-phase solid solutions.

High-Entropy Alloys: A Critical Review

“…Similar phenomena have also been observed in other high-temperature annealing experiments. [15][16][17] The second myth is that only simple solid solution phases (BCC/FCC) are the desired phases for practical applications of HEAs. This is also not true.…”

Hybrid Monte Carlo/Molecular Dynamics Simulation of a Refractory Metal High Entropy Alloy

“…If every lattice site were to be occupied with equal probability by each of the four elements, a configurational entropy of k B ln 4 ∼ 1.2 × 10 −4 eV/K would result. However, atomic size, enthalpy of mixing and additional considerations surely affect stability also [3][4][5][6][7]. Indeed, decomposition into multiple phases has been observed experimentally in certain high entropy alloy families [8][9][10].…”

A Novel Low-Density, High-Hardness, High-entropy Alloy with Close-packed Single-phase Nanocrystalline Structures

“…[3] Otto et al recently published a systematic investigation on the effect of entropy on phase stability of HEAs. [4] The authors demonstrated that high configurational entropy is not the only sufficient criterion to predict whether an equiatomic multi-component alloy will form as a single-phase solid solution. In their study, they found only NiFeCrCoMn to be a single-phase fcc solid solution and HEA.…”

“…This led the system to act less like an ideal solution and resulted in the formation of a multi-phase alloy with a reduced overall entropy. [4] There has also been interest in finding descriptors to predict the likely crystallographic lattice (fcc or bcc) that the solid solution will form upon. One descriptor that has been reasonably successful in predicting the lattice structure is the average valence concentration (or average periodic table group number) (VEC).…”