Flynn Walsh1,2,Robert Ritchie1,2,Mark Asta1,2
Lawrence Berkeley National Laboratory1,University of California, Berkeley2
Flynn Walsh1,2,Robert Ritchie1,2,Mark Asta1,2
Lawrence Berkeley National Laboratory1,University of California, Berkeley2
The role of short-range order in fcc medium and high-entropy alloys remains controversial, with theories regarding its presence ranging from ubiquity to insignificance. While heightened configurational entropy may provide additional latitude for specific components to order, the forces driving chemical rearrangement are largely the same as in binary systems. Recent debates concerning high-entropy alloys thus motivate a reexamination of the behavior of certain elements, namely Cr, in fcc lattices, as typified by Ni. As an indirect assessment of ordering, low-temperature measurements of spontaneous magnetization are compared to density-functional theory calculations for a variety of Ni-based alloys. Standard theory is confirmed to quantitatively describe the magnetism of most systems, even across quantum critical transitions where net moment disappears. However, just as in higher entropy materials, the magnetization of random alloys with significant concentrations of Cr (and V) is predicted to be significantly higher than experiment. This discrepancy is resolved by the introduction of nearest neighbor ordering, which excellently reproduces experimental findings. Previously, the observation of quantum critical behavior in these systems has been cited to explain the failure of theory over a wide composition range; instead, it is postulated that such phenomena may originate from the ordering state. Corroborating these results, a growing body of evidence for the presence of short-range order in water-quenched high-entropy alloys is briefly reviewed. The implication that Cr (and similar elements) form a significant degree of order in conventionally processed fcc alloys is then discussed. Additionally, in contrast to prior theory, the only long-range ordered form of Ni-Cr, Ni<sub>2</sub>Cr, is calculated to be strongly antiferromagnetic, plausibly up to the chemical disordering temperature. Exchange interactions double the compound's negative formation energy, suggesting that the role of magnetism in Cr ordering be revisited.