Disordered vs. Short-Range-Ordered Complexions: Consequences for Grain-Boundary-Mediated Plasticity in Nanocrystalline Al-Ni Alloys
Disordered grain-boundary (GB) complexions in thermally stable nanocrystalline alloys are commonly assumed to be structurally uniform, yet their amorphous nature makes them susceptible to local short-range ordering (SRO). The influence of local SRO on GB-mediated plasticity mechanisms in such complexions remains poorly understood. This article employs large-scale Monte Carlo and molecular dynamics simulation to address this gap through simulations of nanocrystalline Al-Ni alloys at two Ni concentrations, 2 at.% and 4 at.%. Annealing at 913 K produces thick uniform disordered intergranular film complexions, while annealing at 378 K produces semi-disordered complexions containing FCC-type and BCC-type SRO. These two complexion states produce fundamentally different mechanical responses. Amorphous complexions act as dislocation sinks, promoting homogeneous plasticity through shear transformation zones, but at the cost of intense shear localization and lower strength. SRO complexions generate reduced shear localization and higher strength but also promote heterogeneous stress concentrations across the GB network, regardless of Ni concentration. This contrast reflects a fundamental shift in governing mechanism, from shear-transformation-zone-controlled behavior in disordered complexion alloys to GB-stress-heterogeneity-controlled behavior in SRO complexion alloys. These findings highlight the potential of complexion engineering to tailor the mechanical properties of nanocrystalline materials.