A microstructural study of a Ni₂AlTi–Ni(Al, Ti)–Ni₃(Al, Ti) three-phase alloy

Abstract

Early studies showed that the two-phase ordered alloy of semi-coherent β–Ni₂AlTi (L2₁) and β–Ni(Al, Ti) (B2) exhibits excellent elevated-temperature creep strength, and the precipitation of the “rod-like” γ’–Ni₃(Al, Ti) (L1₂) from either the β or the β’ phase improves the room-temperature ductility of the phases concerned. In the present investigation an attempt is being made to combine the above microstructural features in β’–β–γ’ three-phase alloys and for this purpose the composition Ni₆₃Al₂₂Ti₁₅, near the β’–γ’ edge of the three-phase region in the recently estimated Ni–Al–Ti isotherm at 900 °C, has been selected for detailed study. The expected precipitation of both the β and the γ’ phases occurs in the dendritically solidified β’ phase after a 1100 °C/3 h homogenization and a 900 °C/115 h anneal, although the original interdendritic γ’ phase remains. The morphology of the two types of precipitates and their orientation relationships with the β’ parent phase have been examined using transmission electron microscopy and diffraction, and the experimentally obtained data compared with those predicted by Khachaturyan’s elastic strain energy theory. The β precipitates are nearly cuboidal in shape and are bounded by interface dislocations of a_β〈100〉 edge type. For the β precipitates, both morphology and orientation relation agree with those predicted by the theory. The γ’ precipitates were found to obey the Nishiyama–Wassermann orientation relationship with the parent phase. These precipitates are about 0.5 μm thick and elongated along their 〈211〉 directions, and in all cases consist of two twin-related variants, giving a sword-like morphology. The {i11} twin planes, parallel to the {1i0} of the parent phase, have been identified as the habits of the precipitation. The theory, however, predicts a habit of {0.732, 0, 0.681}_γ’ type and a Baker–Nutting orientation relationship. This discrepancy has been attributed to the inapplicability of some assumptions made in the theory: equal elastic moduli between parent and product phases and a tetragonal transformation strain based on Bain’s model of the bcc → fcc transformation. The presence of diffuse streaks in the diffraction patterns of the parent phase, which can be correlated with the 〈110〉〈1i0〉 shear waves, suggests high elastic anisotropy and lends credit to Zener’s model. Crystallographic consideration shows that this model is feasible for the L2₁ → L1₂ transformation and explains the observed morphological features of the γ’ precipitates. Some earlier studies are also discussed.