An experimental and modeling investigation of tensile creep resistance of a stable nanocrystalline alloy

Nanocrystalline materials possess excellent room temperature properties, such as high strength, wear resistance, and toughness as compared to their coarse-grained counterparts. However, due to excess free energy, nanocrystalline microstructures are unstable at higher temperatures. Significant grain growth is observed already at moderately low temperatures, limiting their broader applicability. Here, we present a design approach that leads to a significant improvement in the high temperature tensile creep resistance (up to 0.64 of the melting temperature Tm) of a nanocrystalline Cu-Ta alloy. The design approach involves alloying of pure elements for engineering nanometer sized solute clusters within the solvent grains as well as along the grain boundaries. Using a chemically optimized nanocrystalline Cu-3at.%Ta alloy as a model material system, we demonstrate that the addition of Ta nanoclusters inhibits the migration of the planar defects at higher temperatures and reduces the dislocation motion, leading to extraordinary high temperature properties. For instance, the NC Cu-3Ta alloy tested under tensile creep conditions up to the temperature of 873 K (0.64Tm) displays highly unusual behavior, including the absence of any appreciable steady-state creep deformation which is normally observed in almost all materials. This approach can be readily scaled-up for bulk manufacturing of creep resistant nanocrystalline parts. Moreover, this design strategy can be transferred to other multicomponent systems such as Ni-based alloys for making nanocrystalline materials with tailored properties.