High-pressure torsion of palladium: Hydrogen-induced softening and plasticity in ultrafine grains and hydrogen-induced hardening and embrittlement in coarse grains

Pure Pd (99.9%) was processed by high-pressure torsion (HPT) to form an ultrafine-grained (UFG) structure with an average grain size of ~220 nm, high hardness of 175 Hv, high tensile strength of 650 MPa and appreciable plasticity of 20%. Tensile tests showed that, unlike coarse-grained sample in which a hydrogen-induced embrittlement and hardening occurred, a hydrogen-induced softening and plasticity occurred in the HPT-processed UFG sample. A hydride phase formed after exposing the samples to hydrogen, while the formation of hydride was facilitated by the HPT processing. The hydride phase was decomposed by holding the samples in the air for prolonged time, but the hydride phase was more stable after HPT processing.