Phase transformation behavior of Ti-35Nb-7Al alloy with tempering

Certain β-Ti alloys, Ti-4Fe-7Al and Ti-10Mo-7Al, the compositions of which are characterized by a 7% aluminum addition to the β lower-limit compositions, exhibit an interesting phenomenon upon tempering. For example, these alloys drastically harden upon tempering at 450°C for several seconds, and the sample surface becomes severely uneven. When a strip specimen is bent plastically or elastically into a U-shape and heated to 450°C, the shape spontaneously deforms towards the inside, which is different from the behavior of shape-memory alloys. In this study, Ti-35Nb-7Al alloy was selected as a candidate novel alloy, and the microstructure, the characteristics of age hardening and the shape evolution of a U-shaped specimen upon heating were investigated. The quenched alloy was a single β-phase, and its hardness was considerably higher than that of binary Ti-35Nb alloy. The smooth surface of the quenched specimen became uneven at around 300°C upon heating. Either case of U-shaped specimen, which was plastically or elastically bent, exhibited spontaneous bending towards the inside by heating up. It was found that, with tempering, Ti-35Nb-7Al also exhibits the novel phenomenon, similarly to Ti-4Fe-7Al and Ti-10Mo-7Al. Fine needle-α″and ω- particles were formed in the β matrix after isothermal aging at 450°C for 3 min and gave rise to a remarkable hardening. STEM-EDS analysis revealed that the Nb content decreased in the α″products formed by tempering. Consequently, the β→α″transformation upon tempering on this alloy is accompanied with atomic diffusion. Extra spots at 〈100〉∗, such as those of the B2 structure, were observed in the selected-area diffraction patterns of the TEM foils prepared by common electropolishing techniques. However, a TEM foil prepared with an FIB (focused ion beam) did not exhibit the extra spots. These results suggested that the extra spots were caused by hydrogen absorption in the specimen during electropolishing.