On the reduction and effect of non-metallic impurities in mechanically alloyed nanocrystalline Ni-W alloys

Non-metallic contamination is a practically unavoidable byproduct of commercial synthesis processes; however, non-metallic impurities are often overlooked when considering material design and performance of nanostructured materials. Importantly, there is disputing evidence as to whether non-metallic contamination stabilize or destabilize nanocrystalline materials against grain growth. Furthermore, it is unclear if non-metallic contamination has a significant impact on hardness of nanocrystalline systems. In this work, two nanocrystalline Ni-28at%W alloys with different impurity concentrations were produced via mechanical alloying to directly evaluate the effect of contamination on thermal stability and hardness of nanostructured materials. The alloys were isothermally annealed at several temperatures, and the microstructures were compared by applying atom probe tomography and aberration-corrected scanning transmission electron microscopy. It was determined that impurity concentrations can be substantially reduced by using specific milling media and pre-milling reduction processes. Furthermore, the clean and contaminated alloys exhibited similar thermal stabilities against grain growth, but the clean alloy displayed a 100% improvement in hardness despite a similar grain size and distribution of second phases. Impurity phases including CrOx, Ni6W6C, and trapped Ar pores were also identified and likely contributed to the thermal stability and mechanical properties observed in this study. Overall, this work suggests that impurities my not always be detrimental to thermomechanical behavior of nanocrystalline systems.