Mechanical properties of V–4Cr–4Ti alloy after first-wall coating with tungsten

A first-wall coating was fabricated with tungsten on a reference V–4Cr–4Ti alloy (NIFS-HEAT-2, NH2) substrate by a vacuum plasma spray (VPS) process and brazing (BR). The hardness, fracture stress, and elastic modulus of tungsten (W) coating applied by the vacuum plasma spray process (VPS–W) were lower than the tungsten used for brazing (BR–W). The low mass density and defects of VPS–W are thought to be responsible for the degradation of the strength. The NH2 substrate indicated hardening and embrittlement produced by the W coating and some post-coating heat treatment (PCHT). Hardening and embrittlement by a VPS coating can be recovered by removing hydrogen from the NH2 substrate in a vacuum by annealing at 673 K. Oxygen transfer from the W coating to the NH2 substrate was indicated above 1173 K but did not induce embrittlement of the substrate. Hardening by the BR process can be recovered by PCHT at 1273 K, but embrittlement was not improved. The mechanisms of the hardening and embrittlement are discussed based on a microstructural analysis.