Influence of ternary solute additions on nanocrystalline stability and mechanical behavior

The addition of a ternary solute to reduce the accompanying loss in hardness in a Ni-Cu-P alloy is reported. Here, P serves as the stabilizing addition by its partitioning to the grain boundaries (GBs). In a Ni-40Cu-0.6P (at.%) alloy, nanocrystalline (NC) stability is achieved thermodynamically, i.e. , the P solute rapidly diffuses from the matrix to wet the GBs reducing the excess interfacial free energy. In a Cu-19.4Ni-0.6P and a Cu-19.8Ni-0.2P alloy, the P precipitates nanoscale Ni ₃ P in the GBs but such kinetic pinning precipitates where less effective in providing NC stabilization as both the phosphate phase and grains coarsened. For a Ni-40Cu-0.3P alloy, the Ni ₃P phase also precipitated in the GBs but did not coarsen. Rather, as the GBs broke free from these phosphate precipites they became incorporatedwithin the grains’ interior as evidenced by in situ TEM annealing characterization studies. Nanoindentation confirmed that the addition of the tenary element was effective in mitigating the loss of strength as the stabilizing solute partitions to the GBs as compared to a binary stabilized NC alloy using the same stabilization solute. The collective outcomes of this work concretely demonstrate the need to achieve a sufficient concentration of the stabilizing solute to the GBs and their associated mechanisms by which NC stability is achieved; the influence of a tenary element on solute partitioning diffusivity; and the mechanical tunability offered through ternary alloying in NC alloys.