Ti₂NAI in L1₂ AI₃Ti- base alloys

Abstract

A detailed study has been carried out to determine the crystal structure of a stable Ti-rich second phase seen in Ll₂ Al₃Ti-base alloys and the effect of the phase on the mechanical properties. As determined by energy-dispersive X-ray analysis in a scanning electron microscope (SEM), the phase contains not only Ti and Al in a 2:1 ratio, but also N. Using the X-ray powder diffraction technique and computer simulations, the phase was identified to be Ti₂NAl, which has a hexagonal structure (Cr₂CAl-type) with lattice parametersa = 2.995 Å andc = 13.61 Å. The formation of the phase does not seem to be dependent on the Al/Ti ratio but appears to be controlled by the amount of N existing in the Ti starting material. Based on experimental observations of cracks formed in Ll₂ Al₃Ti specimens deformed in compression or fracturedin situ in an Auger microscope, it was found that a small volume fraction of the Ti₂NAl precipitates plays an important role in the early stages of brittle failure: cracks often initiate within the second phase or at the interface between the second phase and the Ll₂ matrix.