Enhanced Microstructural Stability and Hardness of Multi-component Nanocrystalline Nickel Alloys Processed via Mechanical Alloying

Nanocrystalline alloys are noteworthy for their high strength, and nanocrystalline Ni is being investigated for high-temperature applications. However, nanocrystalline alloys are unstable against coarsening, thereby prone to degradation of strength. In this work, two nanocrystalline alloys, Ni–(0, 11 at pct W)–3 at pct Ta–2 at pct Y, were designed to exhibit microstructural stability and high strength by forming nanoscale precipitates, maintaining stable nanoscale grains to exploit Hall–Petch hardening, and decreasing the stacking fault energy of the Ni-based matrix. Produced via high-energy cryogenic mechanical alloying, both alloys exhibited thermal stability and enhanced mechanical properties due to beneficial impurity yttrium oxide/nitride particles and argon bubbles that pin grain boundaries. Hardness testing and advanced characterization techniques, namely scanning transmission electron microscopy, were used to elucidate microstructure–property relationships. The difference in impurity ceramic phase affected the alloys' relative stability and hardness. The Ni–11W–3Ta–2Y alloy with Y2O3 nanoparticles was even more stable and harder than the Ni–3Ta–2Y alloy with YN particles, maintaining nanoscale grains after annealing at 70 pct homologous temperature for 100 hours and demonstrating hardness enhanced by over 2 GPa above the Hall–Petch contribution. The yttrium oxide/nitride particles, Ni5Y intermetallic phase, and pure artifact W/Ta grains remaining from milling, play a role in the enhanced hardening.