Quantification of the atomic-level structures and motifs of short-range order in Cr-Co-Ni alloys

Despite intense research on chemical short-range order (SRO) in the CrCoNi medium-entropy alloy (MEA), quantification of its atomic structures and motifs has remained elusive. Here we demonstrate an approach to quantify SRO in two Cr-Co-Ni MEAs that utilizes advanced scanning transmission electron microscopy in combination with density functional theory-based artificial neural networks-accelerated kinetic Monte Carlo chemical ordering simulations. We find that both MEAs exhibit SRO of the so-called {0 1/2 1}-type comprising subunit cells of (Co,Ni) 4 Cr ( D 1 a ), (Co,Ni) 3 Cr ( D 0 22 ), and (Co,Ni) 2 Cr (Pt 2 Mo-type) motif structures, distributed homogeneously. Furthermore, the maximum degree of SRO attainable in laboratory timescales was determined by comparing with images calculated using atomic coordinates generated by Monte Carlo simulations. Our finding that SRO in CrCoNi forms homogeneously rather than heterogeneously with well-defined domain sizes explains the lack of dependence of its yield strength on the degree of SRO. The methodology presented here is broadly applicable to other complex metallic systems, especially those with similar atomic scattering factors.