Effect of gaseous hydrogen on embrittlement of PBF-LB alloy 718: Influence of build orientation and microstructure on mechanical properties

This study reports the effects of build orientation and precipitates on the hydrogen embrittlement susceptibility of powder bed fusion – laser beam (PBF-LB) alloy 718 for aerospace applications. The material is subjected to hot isostatic pressing (HIP) then to a modified aerospace heat treatment to minimize delta (δ) phase. The horizontal and vertical specimens are tensile tested at room temperature in air and in a 150 bar gaseous hydrogen environment. In air, horizontal and vertical specimens show similar mechanical properties. In hydrogen, anisotropy was apparent with elongation at fracture of about 6% for horizontal and 17% for vertical specimens. The specimens tested in hydrogen also show a lower reduction of area than those tested in air. The microstructure shows a localized plasticity along grain boundaries, leading to preferential decohesion at precipitates. These findings are consistent with hydrogen embrittlement (HE) mechanisms, like hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE).