Implications of Ternary Solute Additions to the Granular Stability and Mechanical Behavior of Nanocrystalline Alloys

Stabilized nanocrystalline materials largely focus on the use of a binary alloy, where the solute partitions to the grain boundaries to either reduce the interfacial excess energy and/or cluster to provide a kinetic pinning effect. With this loss of the solute from the matrix interior, the strengthening mechanism is simply governed by grain size. In this work, we address the design concepts and outcomes for creating a ternary order solute addition to a nanocrystalline stabilized alloy using the Ni-Cu-P system as the case study. In prior work, we found that Ni(P) nanocrystalline grains could be stabilized through either a thermodynamic or kinetic pinning mechanism dependent upon the P content and sequential annealing process. Using nanoindentation, we now document a softening behavior with solute depletion to the boundaries under these conditions. Upon adding Cu, the softening effect is reduced and is contributed to solid solution strengthening mechanisms. However, the addition of Cu alters both the stabilizing P content as well as diffusivity of the P within the Ni(Cu) grains. This creates peculiar conditions for sequential annealing sequences needed for nanocrystalline stabilization. The change in diffusivity and the solubility of the two solutes can also promote atomic scale clustering within the grain interior. The outcomes of which reveal a complex series of considerations in designing nanocrystalline stability with tunable strength that relies on more than just grain size.