内部水素による析出強化型Fe基超合金A286の延性低下メカニズム

抄録

The hydrogen embrittlement mechanism of a precipitation-hardened iron-based superalloy A286 was investigated by slow strain rate tensile (SSRT) test in combination with the analyses of surface slip step patterns and internal deformation substructures via optical microscopy and scanning electron microscopy (SEM) techniques. Hydrogen was introduced into the material by exposing the specimens to high-pressure hydrogen gas environment at elevated temperature prior to the tensile testing. The material exhibited substantial loss of reduction in area by the hydrogen charging, and accordingly the fracture surface morphology was transitioned from ductile microvoids coalescence to brittle-like faceted features stemmed from intergranular fracture. In the course of plastic deformation, solute hydrogen increased the number of active slip systems thereby facilitated the misorientation development within individual grains, i.e. grain subdivision. In addition, solute hydrogen enhanced the emergence of deformation twins. These dual effects might respectively increase the frequency of dislocations tangling and retarded the onset of plastic instability, leading to the higher strain hardening rate and larger uniform elongation than non-charged specimen. However, the impingement of deformation twin bundles onto grain boundaries triggered severe local stress/strain concentration and made such internal boundaries as the potential nucleation sites of hydrogen-induced microcracks. Even though the uniform elongation was evidently improved by solute hydrogen, the coalescence of grain boundary microcracks encountered the premature fracture immediately after the specimen started necking.