How Solute-Contaminant Structures Alter Nanocrystalline Stability and Strength

While nanocrystalline copper-tantalum (Cu-Ta) has documented thermomechanical stability and strength, there have been far fewer investigations regarding the use of other Group VB transition metal solute elements, e.g., vanadium and niobium (Nb). This work explores the stabilizing and strengthening effect of Nb on nanocrystalline Cu. While nanocrystalline Cu-Nb alloys have comparable strength to Cu-Ta at ambient temperature, mechanical tests reveal Cu-Nb alloys to be only half as strong as Cu-Ta alloys when at temperature. To explain this loss of strength, we use transmission electron microscopy and atom probe tomography to elucidate the role that solute choice has on microstructure and strengthening mechanisms. Divergent solute-contaminant interactions, mainly involving oxygen (O), are the major differences uncovered between the two alloys. In the case of Ta, a pentoxide (i.e., Ta₂O₅) is the only equilibrium configuration, while Nb can form multiple stable oxides including a monoxide (NbO) and a dioxide (NbO₂) in addition to a pentoxide (Nb₂O₅). These different oxides create different extents of possible coherency with the matrix. The consequences of these different oxide structures on nanocrystalline stability and strength in Cu-based alloys are discussed, with implications on solute selection for the broader nanocrystalline community.