Intergranular blistering in tungsten: a survey of grain-boundary-freedom dependence using tungsten with inclined microstructure

Abstract The dependence of hydrogen-induced intergranular blistering on the grain boundary (GB) was investigated in chemical vapor deposition tungsten (CVD-W). Three specific inclined CVD-W samples were prepared by cutting across the surface at specific angles. Microstructure analysis, conducted via electron backscatter diffraction, confirmed that the inclined cutting does not alter the relationship between neighboring grains in the sample, but changes the surface grain orientations in the crystallographic coordinate system with the normal surface along the Z -axis. Following D plasma exposure at a fluence of ∼2.0 × 10 25 m −2 , a flux of ∼1.0 × 10 21 m −2 s −1 , and a sample temperature of ∼500 K, intergranular blistering is predominant in all four samples. The most pronounced intergranular blistering was observed in the sample with a minimum GB inclination angle of 60°. A comprehensive analysis was conducted by effectively decoupling the contributions from the macroscopic degrees of freedom (MDFs) and external degrees of freedom (EDFs) of a GB. No clear dependence of blistering was observed in GB MDFs that describe the misorientation across a GB plane between two grains. It is proposed that intergranular blister formation is primarily governed by the EDFs of the GB, including grain orientation and the dihedral angle between the GB plane and the surface. Grains with orientations distributed near the center of the inverse pole figure map were found to exhibit a higher susceptibility to blistering, which contrasts with the commonly reported propensity of grains with orientation. The discussion indicates that the influence of grain orientation on blistering in this study is linked to the D concentration, which affects blister nucleation. Besides, blisters in the inclined samples are present on the side of GBs with acute dihedral angles, with cavity locating on the GBs with triple junctions or large curvature variations. Microstructural features identified in this work as vulnerable to blistering should therefore be avoided to mitigate blistering either during material fabrication or during in situ repair of first-wall or divertor materials.