Evolution of lattice defects, disordered/ordered phase transformations and mechanical properties in Ni-Al-Ti intermetallics by high-pressure torsion

Powder mixtures of Ni-25 mol.% Al-25 mol.% Ti were subjected to severe plastic deformation using high-pressure torsion (HPT) to examine the formation of ternary ordered intermetallics. In consistency with the Al-Ni and Ti-Al systems, in which the in situ formation of binary ordered intermetallics was achieved during HPT, a partially-ordered nanostructured B2-Ni(Al,Ti) phase with ∼11 nm grain size and high dislocation density, >10¹⁶ m ², was formed in the Ni-Al-Ti system. The hardness-strain behavior of the Ni-Al-Ti mixture was similar to pure aluminum having a hardness maximum followed by a strain softening at large strains. The B2 phase transformed to a fully-ordered Ni₂AlTi phase with L2₁ structure after annealing at 873 K with an activation energy of 270 kJ/mol. Atomic-scale elemental mapping using scanning transmission electron microscopy confirmed the occurrence of ordering after annealing as well as partial twining. Micropillar compression tests showed that both yield stress and plasticity increased after annealing, and high strength and high ductility with values as 3.6 GPa and 7%, respectively, were achieved.