Fabrication and thermal stability of a nanocrystalline Ni-Al-Cr alloy: Comparison with pure Cu and Ni

A Ni-Al-Cr alloy with an initial grain size of approximately 60 μm was subjected to torsion straining to a strain of approximately 7 at room temperature, thereby reducing the grain size to approximately 34 nm. Similar torsion straining with samples of pure Cu and pure Ni gave grain sizes of approximately 170 and approximately 130 nm, respectively. Inspection of the Ni-Al-Cr alloy after torsion straining revealed highly strained regions containing dislocations associated with lattice distortions but with an absence of any Ni₃Al ordered phase. The ultrafine grains in the Ni-Al-Cr alloy were extremely stable at high temperatures, and it was possible to retain a grain size of less than 100 nm after annealing at temperatures up to approximately 900 K. By contrast, there was rapid grain growth in the samples of pure Cu and Ni at annealing temperatures in the vicinity of approximately 500 K. The stability of the grains in the Ni-Al-Cr alloy is attributed to the formation of a Ni₃Al-based ordered phase after annealing at approximately 650-700 K. The presence of this phase also leads to an apparent negative slope in the standard Hall-Petch relationship.