Fabrication using electron beam melting of a V-4Cr-4Ti alloy and its thermo-mechanical strengthening study

A 30 kg V-4Cr-4Ti ingot was fabricated by double electron beam melting for a structural materials application study for fusion reactors. The fabricated alloy was forged, hot rolled, and cold rolled to sheets of more than 90% deformation. Preliminary thermo-mechanical strengthening studies were completed, such as SACWA (solution annealing, cold working and aging), SAACW (solution annealing, aging and cold working), and SACWACWA (solution annealing, cold working, aging, cold working and aging), which were evaluated by hardness and tensile tests. Compared with traditional SAA (solution annealing and aging), multiplex strengthening is prominent via the imposed thermo-mechanical treatments, especially SACWACWA. SEM (scanning electron microscope) observation of the fracture surfaces was made after tensile testing; and all show ductile dimple fracture characteristics. In all conditions, there are coarse Ti-oxycarbonitride precipitates of about 200 nm in the grains or on grain boundaries. In the SA (solution annealing) and SAA conditions, there are just a few of the coarse precipitates on the boundaries, but in SAACW, the precipitates on the boundaries have a higher density and nearly contiguous morphology. This may be harmful for mechanical properties, especially in high-temperature environments, as these large precipitates may coarsen. There are also just a few coarse precipitates in the SACWA and SACWACWA conditions, while the density of nanometer-sized precipitates increased greatly, especially with SACWACWA. Cold working before aging manifests a better strengthening effect. This is possibly due to the dislocations, created in the cold working being pinned by precipitates formed in the succeeding aging process.