Hydrogen-affected Mechanical Behavior and Slip Transmission in Ni-201

Samantha Lawrence, Purdue University

Hydrogen embrittlement of structural materials, such as nickel-based alloys, often is characterized by both enhanced dislocation processes as well as grain boundary decohesion, leading to low-toughness intergranular fracture. In this study, nanoindentation and scanning probe microscopy (SPM) were used to characterize slip transfer across high- and low-energy grain boundaries in nickel (a model system for engineering alloys) before and after hydrogen charging. Gaseous thermal charging results in a hydrogen concentration of about 2,000 appm. Both random grain boundaries (more than 15 degrees misorientation between grains) and sigma-3 recrystallization twins were identified for indentation using electron backscatter diffraction. Nanoindentation induced local deformation along grain boundaries, producing material pile-up and slip steps visible on the surface; thermal hydrogen-charging leads to an increase in the width of slip step spacing and plastic zone length. The likelihood of slip transmission in the presence of hydrogen also appears to be dependent on grain boundary type. Additional indentation within specifically oriented grains indicates that hydrogen charging reduces measured elastic modulus as well as the critical stress required to initiate plastic deformation by about 25 percent. Observed changes in incipient plasticity and mechanical properties suggest that hydrogen may limit dislocation cross-slip, thus necessitating additional nucleation events to accommodate plastic deformation while also decreasing the cohesive strength of interatomic bonds, which may ultimately lead to macroscale intergranular failures. Coupled nanoindentation and SPM investigations provide a unique local method for analyzing the effect of hydrogen on dislocation plasticity as a function of grain boundary type, which will be useful in developing grain boundary engineered materials.

Abstract Author(s): Samantha K. Lawrence, Brian P. Somerday, Neville R. Moody, David F. Bahr