Development of cold rolling texture in Ni₃Al (B)

The paper describes the effect of cold deformation on the stability of L1₂ structured hypostoichiometric Ni₇₆Al₂₄(B) alloy. During cold rolling, the order parameter decreases with the degree of deformation. After 85% cold rolling, the material transforms to DO₂₂ structure from its initial L1₂ structure. The lattice parameters are measured as a = 3.567 Å and c = 7.188 Å having a c/a ratio of ∼2.015. After annealing, the metastable structure transforms back to its original L1₂ structure. The DO₂₂ structure can be derived from the L1₂ structure by local rearrangement of the Ni atoms as antisites. The activation energy of the reverse transformation is similar to that for the diffusion of Ni atoms in Ni₃Al.

Thin foils of stoichiometric Ni₃Al with thicknesses ranging from 57 to 315 μm were successfully fabricated by heavily cold-rolling without intermediate annealing. Starting materials were produced by directional solidification using the floating zone method. The total reduction in thickness obtained was as much as 95.5%. This high rolling ductility is considered to be due to the monocrystalline or near monocrystalline form of the starting materials. X-ray pole figures showed the formation of {110} rolling texture. This {110} texture is considered to develop mainly as a result of compressive deformation normal to the rolling plane. The foils recrystallized at temperatures over 1273 K had some tensile ductility (3.0–14.6%) at room temperature in air, in contrast to the usual brittleness of polycrystalline Ni₃Al. Electron back scatter diffraction measurements revealed that low angle and Σ3 coincidence site lattice boundaries, which are considered to be crack-resistant, comprised 41–84% of the total grain boundary area in the recrystallized foils. This large fraction is probably a chief cause of the observed ductility. These results demonstrate that it may be possible to utilize Ni₃Al thin foils as lightweight, high-temperature structural materials, e.g. honeycomb structures.

The recrystallization texture development during annealing of cold rolled Ni₃Al polycrystals and single crystals was investigated. The recrystallization texture in single crystals, regardless whether boron-free or boron-doped, comprised all eight 35° 〈111〉 rotations of the single component deformation texture (close to the brass orientation). No annealing twins were observed in single crystals, in contrast to polycrystals. The recrystallization textures of polycrystals were very weak, but reproducible. Their components can be interpreted as a compromise between ±35° 〈111〉 rotations and second order twinning to the deformed matrix.