Properties and structural characteristics of Ti-Nb-Al alloys

The use of Al as an α stabilizer in different Ti-Nb alloys has been investigated for its effect on some properties and changes in structural characteristics. Quenched alloys with 10-40% Nb and 2-15% Al were analyzed in terms of their phase transformations, as well as elastic modulus, density and internal friction. It was found that the rhombic distortion introduced in the α′-HCP phase transforms into another martensitic α″ structure. Above 5% Al, metastable β-BCC is formed and eventually predominates. The appearance of another metastable phase, ω, was verified in alloys between 2 and 5% Al with less than 35% Nb. These structure transformations are a consequence of the obstructing effect of Al on the redistribution of Ti and Nb. The elastic modulus increases with Al and decreases with Nb percentage due to different developed structures, especially at lower Al content. Small fluctuations in the generally increasing variation of the density with Nb, for Al percentages, were attributed to the phase transformations. The internal friction was relatively large for the martensitic, α′ or α″, structure but decreases significantly with the appearance of β phase.