HRTEM investigation of dislocation/hydrogen interaction mechanisms in hydrided nanocrystalline palladium films

Abstract

Thin Pd membranes constitute an enabling material in hydrogen permeation and sensing applications. During hydriding of Pd, as long as the H/Pd (atomic ratio) stays below α SSmax ≈0.02, the α‐Pd with face centered cubic (fcc) lattice will expand from 3.889 Å to 3.895 Å. When the ratio reaches 0.02 a β‐phase, again fcc based, having a lattice constant near 4.025 Å appears. The initial volume of the Pd structure thus expands by about 10% due to α→β phase transformation which induces a large plastic deformation within the material. In the present study, we have performed detailed HRTEM characterizations of defect/hydrogen interactions on nc Pd thin films hydrided at low and high pressures for α‐phase and β‐phase transformations, respectively. The in‐situ measurement of the evolution of the internal stress during hydriding of the nc Pd films shows that this internal stress increases rapidly in the compressive direction, and gradually reaches a constant value of 120 MPa tensile stress for α‐phase transformation and 920 MPa compressive stress for β‐phase transformation which affect the microstructure of the Pd film. Figures 1a and 1b show HRTEM images of ∑3 {111} coherent twin boundaries (TBs) in Pd films before and after hydriding to α‐phase, respectively. In contrast with Pd films hydrated to β‐phase (see below), intrinsic or extrinsic stacking faults (SFs), dissociation of incoherent TB to form 9R and distortion of CTBs have not been observed in Pd films after hydriding to α‐ phase (Figure 1c). Surprisingly, an fcc→9R phase transformation at Σ3 {112} incoherent TBs as well as a high density of SFs (Figure 2a) have been observed after hydriding to β‐phase indicating a clear effect of hydrogen on the stacking fault energy (SFE) of Pd. Ab‐initio calculations of the effect of hydrogen on the intrinsic stable and unstable SFEs of Pd confirm the experimental observations. The experimental results confirm that hydrogen induced plasticity is mainly controlled by dislocation activity at higher hydrogen pressures. Shear type faulted loops rarely reported in nc materials were also observed within the Pd grains after hydriding to β‐phase (Figure 2b). In order to investigate the stability of this shear type loops, different internal stress fields originating from the neighboring dislocation (dislocation “d3”) and surface effects (image forces) have been computed using a Finite Element method (Figure 2c). Such calculations confirm that high attractive forces exist between the dislocation “d2” and “d3” forming the dipole. On the other hand, although the Peach Koehler force on the dislocation “d1” tends to extend the SF, the force magnitude is much smaller than the force induced by the fault on the partial segments. Therefore, an extra shear stress of +385MPa (τ dis. ) acting on the glide plane of the dislocation “d1” is required in order to counter balance the attractive force of the SF which thus explains the stability of this dislocation in the TEM thin foil after dehydriding. This shear stress can not be compensated by the negligible image force in such a thin foil. Moreover, no residual hydrides were detected using high resolution EELS. Therefore, the stability of glissile intrinsic SF loops in nc Pd films after dehydriding can thus be attributed to the presence of large internal stress heterogeneities typical of nc materials. Since the 9R phase and SFs are unexpected at high SFE Pd and considered as unstable phases, the stability of these defects was also investigated using in‐situ HRTEM heating experiments at different temperatures and the critical temperature for removing these unstable SFs in the hydrided Pd film was determined.