Active Plasticity Mechanisms during Hydriding of Nanocrystalline Pd Thin Films

Plasticity mechanisms were studied using advanced transmission electron microscopy (TEM) techniques in nanocrystalline sputtered Palladium (Pd) thin films with nanoscale twins subjected to hydriding cycle. Plastic deformation occurs in the Pd film due to the increase of the internal stress stored within the films during hydriding transformation into the βべーた phase. The initial volume of the Pd structure expands by about 10% due to this reversible phase transformation. Various TEM techniques were used in order to unravel the fundamental defect mechanism(s) activated during hydriding. Ex-situ TEM observations of the hydrated Pd films did not show any significant changes of the crystallographic texture and twin boundary (TB) density in comparison with as-sputtered Pd films. Therefore, grain boundary sliding, grain rotation and deformation twinning can be excluded as possible mechanisms dominating the plastic deformation. However, grain growth may play a role since the grain size slightly increased after hydriding. Lose of coherency of Σしぐま3 {111} coherent TBs due to the interaction of these interfaces with lattice gliding dislocations as well as an increase of dislocation density after hydriding both confirm that dislocation activity is the most dominant plasticity mechanism in hydrated Pd films. Further examination of hydrated Pd films using high resolution TEM shows the formation of sessile Lomer-Cottrell locks due the interaction between dislocations lying in two inter-crossing {111} planes. This brings about strain hardening under H loading. Surprisingly, stacking faults were also observed after hydriding the Pd films to the βべーた phase which can be attributed to a decrease of stacking fault energy of Pd due to hydriding and which yields the formation of a 9R structure.