Type of presentation: Oral

MS-9-O-1805 First experimental study of nanoscale plasticity mechanisms in nanocrystalline Pd thin films under hydrogen cycling

Amin-Ahmadi B.1, Idrissi H.1,2, Malet L.3, Delmelle R.2, Godet S.3, Pardoen T.2, Proost J.2, Schryvers D.1
1Electron Microscopy for Materials Science (EMAT), University of Antwerp, Belgium, 2Université catholique de Louvain, Institute of Mechanics, Materials and Civil Engineering, Louvain-la-Neuve, Belgium, 3Université Libre de Bruxelles, Matters and Materials Department, Belgium
behnam.amin-ahmadi@uantwerpen.be


Thin Pd membranes constitute an enabling material in hydrogen permeation and sensing applications. Due to hydriding, the initial volume of the Pd structure expands by about 10% due to the α→β-phase transformation, which induces a large plastic deformation within the material [1,2]. In the present work, nanoscale plasticity mechanisms activated in sputtered nanocrystalline (nc) Pd thin films subjected to hydriding cycles at different hydrogen pressures have been investigated for the first time using advanced TEM. 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 the α-phase transformation and 920 MPa compressive stress for the β-phase transformation (Figure 1a). The automated crystallographic orientation mapping in TEM (ACOM-TEM) before and after hydriding did not reveal significant changes of the grain size and the crystallographic texture, excluding grain boundary mediated processes as dominant hydrogen induced plasticity mechanisms.
Figures 1b and 1c show HRTEM images of ∑3 {111} coherent twin boundaries (TBs) in Pd films before and after hydriding to the β-phase, respectively. In these figures, clear loss of the coherency of these boundaries can be observed. Such a feature is due to the interaction of coherent TBs with lattice dislocations generated during hydriding. This can be confirmed by the local g-maps shown in the same figures and demonstrating a clear increase of dislocation density after hydriding to the β-phase. However, significant changes of dislocation density or the coherency of coherent TBs have not been observed in Pd films hydrated to the α-phase. These results confirm that hydrogen induced plasticity is mainly controlled by dislocation activity at higher hydrogen pressures. Surprisingly, an fcc→9R phase transformation at Σ3 {112} incoherent TBs (Fig. 2) as well as a high density of stacking faults (SFs) (Fig. 2b) have been observed after hydriding to the β-phase indicating a clear effect of hydrogen on the stacking fault energy of Pd. Such observations also suggest that hydrogen atoms remain trapped at the defect cores after dehydriding. Our findings provide precious information for the validation of atomistic simulations on the interaction of hydrogen with extended defects and for better understanding of the effect of hydriding on the macroscopic mechanical properties of nc metallic thin films.

References

1. B. Amin-Ahmadi, H. Idrissi, R. Delmelle, T. Pardoen, J. Proost, D. Schryvers, Appl. Phys. Lett. 071911, 102 (2013).
2. H. Idrissi, B. Amin-Ahmadi, B. Wang, D. Schryvers, Phys. Status Solidi B, 1–14 (2014).


Fig. 1: Figure 1. a) Evolution of the internal stress in nc Pd during hydriding cycle b) HRTEM micrograph of a CTB in the as-deposited Pd film. A local g-map is shown in the upper left. c) HRTEM image showing the loss of coherency of a ∑3{111} CTB after hydriding to the β-phase. The local g-map from the dashed square is shown as upper-right inset.

Fig. 2: Figure 2. a) HRTEM image of 9R band embedded in the Pd matrix after hydriding to the β-phase. b) HRTEM image of hydrated Pd film in the β-phase showing a 9R band at a Σ3 {112} incoherent TB and several SFs indicated by arrowheads. The lower left inset shows the shift in the position of the {111} planes in the SF indicated by dashed lines.