Evaluation of Feasibility of Tungsten/Oxide Dispersion Strengthened Steel Bonding with Vanadium Insert

Abstract

A diffusion bonding (DB) technique to reduce thermal expansion coefficient mismatch between tungsten (W) and oxide dispersion strengthened ferritic steel (ODS-FS) was developed by applying a vanadium (V) alloy as an insert material. In order to suppress · phase precipitation at the interface, DB of ODS-FS and V4Cr4Ti was carried out by introducing a Ti insert as a diffusion barrier between V4Cr4Ti and ODS-FS, and examined feasibility of W/V/Ti/ODS-FS joint for application to fusion reactor components by comparing the three-point bending strength and microstructure between the joints with and without a Ti diffusion barrier layer. It is shown that the fracture strength of the joint without a Ti insert was decreased by 25% after aging at 700°C for 100 h, but that with a Ti insert shows no change after the aging treatment up to 1000 h. The result indicates that the introduction of a Ti insert leads to the prevention of the formation of · phase during aging and resultant control of the degradation of the bonding strength.

Evaluation of Tungsten—Steel Solid-State Bonding: Options and the Role of CALPHAD to Screen Diffusion Bonding Interlayers

“…High hardness values in the interfaces are due to solid solution strengthening resulting from the atomic interdiffusion. Oxide dispersion-strengthened (ODS) steel was also considered by Noto [137] to bond with tungsten using a V-4Cr-4Ti interlayer. Ti was used in between ODS steel and V-4Cr-4Ti to inhibit the formation of the σ phase with the exception that low diffusivity of V in Ti would retard the migration of V into the steel side.…”

Development of High Strength Tungsten/Oxide Dispersion Strengthened Ferritic Steel Joints by Innovative Thermal Stress Relaxation Technique Based on Phase-Transformation-Induced Creep Deformation

“…A dif culty in bonding tungsten with ODS-FS is that a large difference in the coef cient of thermal expansion (CTE) between the materials causes a rather large thermal stress at the bonding interface so that the joint was damaged by cracking at tungsten near the interface during cooling after bonding. According to the previous study on the estimation of thermal stress by nite element method (FEM) 8) , a tungsten/ ODSS joint generates an equivalent stress more than 1.5 GPa in tungsten during cooling to ambient temperature after diffusion bonding at 1250 C. In order to reduce the thermal stress, an insertion of material between tungsten and ODS-FS is often considered to be effective because of stress relaxation at the insert material. The insert material should also meet the requirements such as reduced radio-activation, high melting point as ODS-FS and the absence of brittle phase formation by chemical reactions between tungsten and ODS-FS.…”