TI2NAL IN L1(2) AL3TI-BASE ALLOYS

A detailed study has been carried out to determine the crystal structu re of a stable Ti-rich second phase seen in Ll(2) Al3Ti-base alloys an d 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 simulatio ns, the phase was identified to be Ti2NAl, which has a hexagonal struc ture (Cr2CAl-type) with lattice parameters a = 2.995 Angstrom and c = 13.61 Angstrom. The formation of the phase does not seem to be depende nt on the Al/Ti ratio but appears to be controlled by the amount of N existing in the Ti starting material. Based on experimental observatio ns of cracks formed in Ll(2) Al3Ti specimens deformed in compression o r fractured in 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 Ll(2 ) matrix.