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)4Cr (D1a), (Co,Ni)3Cr (D022), and (Co,Ni)2Cr (Pt2Mo-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 the CrCoNi MEA 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.
L12-type chemically complex intermetallic alloys (CCIMAs) with the ordered A3B structure have emerged as novel high-temperature materials due to their exceptional strength-ductility synergy and thermal stability. However, quantitative insight into the effects of sublattice chemistry on deformation and fracture mechanisms remains elusive. Here, we systematically elucidate the critical roles of the Co/Ni ratio in the A-site sublattice on the microstructure and mechanical behavior of a model Co76.5-xNixAl13.5Ti3Ta3Nb3.5B0.5 (at. %) CCIMA. All alloys exhibit fully recrystallized L12-dominated grains with a small fraction of borides, indicating that their phase constitution is insensitive to the A-site chemistry. However, the Co/Ni ratio significantly influences the elemental segregation behavior at grain boundaries (GBs) and associated dislocation activities, giving rise to markedly distinct mechanical properties and fracture morphologies. An increased Co/Ni ratio reduces the anti-phase boundary (APB) energy, leading to a reduction in yield strength from ∼839 MPa (26Co-50.5Ni alloy) to ∼680 MPa (46.5Co-30Ni alloy). Concurrently, the deformation substructure evolves from APB-coupled superdislocations to superlattice intrinsic stacking faults (SISFs) and dense SISF networks, which sustains work hardening to larger strains. Moreover, in the CCIMAs with Co/Ni ≥ 1, Co enrichment accompanied by depletion of Ni and L12-forming elements at GBs is associated with the formation of disordered interfacial nanolayers (DINLs) and suppressed intergranular fracture, enabling elongations exceeding ∼35% while maintaining a tensile strength of ∼1.5–1.6 GPa. This study establishes a correlation between A-site sublattice chemistry, planar fault energetics, GB chemistry/structure, and mechanical properties, providing a new paradigm for controllably designing advanced intermetallic alloys for extreme conditions.
