The Effect of Solutes on Interface Structure and Microstructural Stability in Binary, Nanocrystalline Alloys

Stabilizing a nanocrystalline material against grain growth is necessary to preserve the nanostructure and associated strength while experiencing conditions experienced during manufacturing and/or service. This work examines the structures and stability provided by solutes in different alloys. First, a platinum-gold alloy thin film is examined during in-situ annealing where observations of a faceted grain boundary structure are made. Atomistic simulations reveal that segregation of gold to that grain boundary stabilize such a structure. Second, a bulk copper-niobium alloy is plastically deformed at elevated temperature. The nanostructure of the alloy is preserved, indicating good stability. The strength of the niobium-doped alloy, however, is more sensitive to temperature than a related copper-tantalum alloy. This sensitivity is found to be a result of the oxidation behavior of the solutes: theoretical calculations of the misfit strengthening provided by these oxide precipitates reveal greater temperature sensitivity than either the niobium or tantalum precipitates. Third, a bulk copper-hafnium alloy is also plastically deformed at elevated temperature, and the nanostructure is also retained. Here the hafnium was processed as a conformal coating over the copper powder prior to consolidation rather than use of an elemental powder-powder mixing. The copper-hafnium alloy has good low-temperature strength, but begins deforming by grain boundary mediated mechanisms at much lower temperatures compared to copper-tantalum and copper-niobium. Again, the precipitates contribute to this behavior, as the hafnium oxidizes to form a monoclinic hafnium dioxide. This oxide provides good resistance to dislocation propagation but does not reinforce grain boundaries due to lack of coherency. Thus, in all three alloys studied, the solute behavior as it seeks to achieve local equilibrium has profound effects on observed structural evolution (and mechanical response).