Exact average many-body interatomic interaction model for random alloys

Abstract: Understanding the physical origin of deformation mechanisms in random alloys requires an understanding of their average behavior and, equally important, the role of local fluctuations around the average. Material properties of random alloys can be computed using direct simulations on random configurations but some properties are very difficult to compute, for others it is not even fully understood how to compute them using random sampling, in particular, interaction energies between multiple defects. To that end, we develop an atomistic model that does the averaging on the level of interatomic potentials. Then, the problem of averaging via random sampling is bypassed since computing material properties on random configurations reduces to computing material properties on a single crystal, the average alloy. We develop our average model on the class of linear machine-learning interatomic potentials. To that end, using tools from higher-order statistics, we derive an analytic expansion of the average many-body per-atom energy in terms of average tensor products of the feature vectors that scales linearly with the size of an atomic neighborhood. In order to avoid forming higher-order tensors composed of products of feature vectors, we develop an implementation using equivariant tensor network potentials in which the feature vectors are contracted to small-sized tensors before taking the average. We show that our average model predicts the compact screw dislocation core structure in the NbMoTaW medium-entropy alloy, in agreement with density functional theory, as opposed to state-of-the-art average embedded atom method potentials that predict artificial polarized cores. Hence, we anticipate that our model will become useful for understanding mechanistic origins of material properties and for developing predictive models of mechanical properties of random alloys.