STRUCTURE OF MECHANICALLY ALLOYED TI-AL-NB POWDERS

Ti-Al-Nb ternary powder mixtures containing 24Al-11Nb, 25Al-25Nb, 37.5 Al-12.5Nb, and 28.5Al-23.9Nb (at. pet) were mechanically alloyed in a SPEX 8000 mixer mill using a ball-to-powder weight ratio of 10:1. The structural evolution in these alloys was investigated by X-ray diffrac tion and transmission electron microscopy techniques. A solid solution of Al and Nb in Ti was formed at an early stage of milling, followed by the B2/body-centered cubic (bcc) and amorphous phases at longer mil ling times. The stability of these phases and their transformation to other phases have been investigated by heat treating these powders at different temperatures. The B2/bcc phase transformed into an orthorhom bic (O-Ti2AlNb) or a mixture of the orthorhombic (O) and hexagonal clo se-packed (alpha(2)-Ti3Al) phases, the proportion of phases being depe ndent on the powder composition. Milling beyond the amorphous phase fo rmation resulted in the formation of an fee phase in all the powders, which appears to be TiN, formed as a result of contamination of the po wder.