材料期刊网

The hydrogenation behavior of Ti-6Al-4V, with the starting microstructures of coarse equiaxed alpha and coarse Widmanstatten alpha, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen 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 microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5A1-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed (alpha at 1023 K, just below hydrogen saturated beta (denoted beta (H)) transus temperature, produces a microstructure of alpha, orthohombic martensite (denoted alpha'' (H)) and beta (H). Hydrogenation at 1123 K, above beta (H) transus, results in a microstructure 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 TCP method is below beta (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated beta (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases 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 starting microstructure of coarse Widmanstatten alpha, although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5A1-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe.