Phase Transformation Behavior of Ti-35Nb-7Al Alloy with Tempering

Certain beta-Ti alloys, Ti-4Fe-7Al and Ti-10Mo-7Al, the compositions of which are characterized by a 7% aluminum addition to the beta lower. limit compositions, exhibit an interesting phenomenon upon tempering. For example, these alloys drastically harden upon tempering at 450 degrees 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 degrees 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 beta-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 degrees 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-alpha '' and omega-particles were formed in the beta matrix after isothermal aging at 450 degrees C for 3 min and gave rise to a remarkable hardening. STEM. EDS analysis revealed that the Nb content decreased in the alpha '' products formed by tempering. Consequently, the beta ->alpha '' 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.