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 ∼60 μm was subjected to torsion straining to a strain of ∼7 at room temperature, thereby reducing the grain size to ∼34 nm. Similar torsion straining with samples of pure Cu and pure Ni gave grain sizes of ∼170 and ∼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 ∼900 K. By contrast, there was rapid grain growth in the samples of pure Cu and Ni at annealing temperatures in the vicinity of ∼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 ∼650–700 K. The presence of this phase also leads to an apparent negative slope in the standard Hall–Petch relationship.