FRACTURE CHARACTERISTICS OF TI-6AL-4V AND TI-5AL-2.5FE WITH REFINED MICROSTRUCTURE USING HYDROGEN

The hydrogenation behavior of Ti-6Al-4V, with the starting microstruct ures of coarse equiaxed alpha and coarse Widmanstatten alpha, respecti vely, was investigated under a hydrogen pressure of 0.1 MPa at tempera tures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrog en flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the micros tructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5A1-2.5Fe subjected to the various TCP were then inv estigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed (alpha at 1023 K, just below hydrogen saturated be ta (denoted beta (H)) transus temperature, produces a microstructure o f alpha, orthohombic martensite (denoted alpha'' (H)) and beta (H). Hy drogenation at 1123 K, above beta (H) transus, results in a microstruc ture of alpha'' (H) and beta (H). Microstructure refinement during TCP results mainly from decomposition of (alpha'' (H) and beta (H) into a fine mixture of alpha + beta during dehydrogenation. An alternative T CP method is below beta (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated beta (H) transus te mperature, air cooling to room temperature, and dehydrogenation at a l ower temperature, which is found to improve mechanical properties sign ificantly over a conventional TCP treatment. Compared with the untreat ed material, the BTH treatment increases the yield strength and increa ses the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed alpha or with a little decrease in the tensile elongation in the start ing microstructure of coarse Widmanstatten alpha, although the convent ional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventiona l TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance bet ween strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-t reated Ti-5A1-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in f atigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe.