Atomic-Resolution Characterization of Chemical Ordering in Nanocrystalline Ni-W Alloys

Nanocrystalline materials exhibit beneficial properties because of the high volume fraction of grain boundaries within the microstructure (e.g., Hall-Petch strengthening). While a reduced grain size is planned, additional strengthening strategies, such as chemical ordering, are often neglected when attempting to optimize mechanical properties. This study electroplated a Ni-21 at% W alloy, annealed films for different times at 700 °C, utilized aberration-corrected scanning transmission electron microscopy to characterize chemical ordering, and employed high-throughput nanoindentation to assess influence of chemical ordering on hardness and elastic modulus. The main result was that long-range ordering (i.e., Ni₄W precipitation) occurred as a nucleation and growth process within disordered Ni(W) grains, all of which proceeded short-range ordering. The nanoindentation measurements identified an optimal ‘aging’ time of Ni₄W precipitates to maximize hardness. Overall, this study has taken a direct approach to study chemical ordering in a nanocrystalline system and provides new insights into design of nanocrystalline materials.