Effect of the addition of Ta on microstructure and properties of Ti–Nb alloys

This work is a study of the microstructures and properties of a series of Ti–xNb–yTa alloys (x = 25, 30, 35; y = 0, 2.5, 5, 7.5) (wt. %), solution treated and cooled under the following conditions: furnace cooling (FC), air cooling (AC), oil quenching (OQ) and water quenching (WQ). The results showed the existence of a high density of ω precipitates in FC and AC samples. The microstructures of OQ and WQ alloys containing 25 and 30 wt.% Nb were basically formed by α″ martensite and Ti–35Nb–7.5Ta was the only composition where the β phase was entirely retained. In FC and AC samples, ω phase was most responsible for the high values of elastic modulus and hardness, whereas the OQ and WQ samples showed a continuous decrease in elastic modulus due to the high β retention as the content of alloying elements increased; the WQ sample of composition Ti–35Nb–7.5Ta presented the lowest value of elastic modulus (64 GPa). Hardness tends to decrease with Ta additions. Reduction in area seemed to be independent on Nb and Ta contents but mechanical strength decreased as Nb and Ta percentages increased. Some alloys showed low reduction in area as a consequence of a combination of irregular ω phase distribution and oxygen contamination; these alloys presented the β phase preferably retained at the periphery of specimens, which is attributed to the fall of Ms due to the influence of the cooling rate in Ti-alloys containing Nb and Ta elements, since the higher cooling rate are obtained superficially, and the effect of the oxygen contamination.