Understanding deformation mechanisms in the grain boundary weakening regime of nanocrystalline Ni–W alloys

Hall–Petch scaling breaks down at grain sizes below ≈ 10 nm at room temperature. Although mechanisms such as diffusion creep, grain rotation, and grain boundary sliding have been suggested, the dominant deformation mechanism in the Hall–Petch breakdown or grain boundary (GB) weakening regime is not well understood. GB weakening regime has also been attributed to enhanced grain boundary segregation or reduced internal stresses upon annealing. Two different approaches are considered using Ni–W alloys in this work: (a) annealing specimens and examining hardness changes along with variations in GB segregation and (b) modifying electrodeposition conditions slightly to get a range of grain sizes below 10 nm and examining them for GB weakening, without annealing. GB weakening was observed in both approaches. The state of segregation remains unchanged upon annealing in the GB weakening regime. The increased activation volume with reduced grain size in as-deposited alloys is attributed to atomic shuffling at GBs.