NiFe local ordering in segregated Ni3Fe alloys: A simulation study using angular dependent potential

In this work, we analyze the compositional disordering in the Ni3Fe alloy (with surface layers exposed to vacuum) at elevated temperatures in simulations using a Modified Embedded Atom potential with the Angular Dependent terms (ADP). We find that the Fe enrichment primarily occurs in the sub-surface layers of the alloy samples with (1 0 0), (1 1 0), and (1 1 1) orientated surfaces exposed to vacuum at elevated temperatures. Nickel enrichment occurs primarily in the surface layers with the (1 0 0) and (1 1 0) orientations. In the case of (1 1 1) orientated surfaces exposed to vacuum, the surface layer composition (25% Fe, 75% Ni) is not significantly altered even at a relatively high temperature of 1100 K. Our findings are qualitatively in agreement with the experimental and density functional theory (DFT) studies in the literature on the segregation in Ni3Fe alloys. We find that, irrespective of the orientation of the surface layers, Fe segregation (in the bulk of the alloy) predominantly occurs so as to achieve a local ordering similar to that in the NiFe alloy. In particular, the Fe atoms form planar 5-Fe atom clusters in the (1 0 0) planes, which are sandwiched between pure Ni layers with the (1 0 0) orientations. Thus, the central Fe atom of the 5-Fe atom cluster in the (1 0 0) plane has a local environment (in terms of nearest neighbor atoms) similar to that of a Fe atom in the NiFe alloy. We also find that a significant proportion of disordered Ni atoms in the bulk are associated with the Fe segregation in the form of 5-Fe atom clusters (i.e., NiFe local ordering). Furthermore, in the disordered Ni3Fe alloy, the Ni atoms with pure Ni nearest neighbor environment (i.e., with no nearest neighbor Fe atoms) exist primarily in the bulk, while in the surface layers the percentage of such Ni atoms is relatively insignificant.