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Description
Thin metal films exposed to a hydrogen atmosphere are experimentally studied as a hydrogen uptake results in compressive stresses [2]. Compressive stresses are critical for hydride formation. Depending on the atmospheric state, a release of hydrogen can counteract the thermodynamic driving force of hydride formation. However, the thermomechanical state is path-dependent due to the plastic deformation history. In [1] this behavior has been investigated, and an analytical model has been developed accounting for the elasto-plastic behavior of thin niobium hydrogen films. As purely elastically deforming films have been found to suppress hydride formation at ambient temperature, the mechanical state, i.e. the plastic deformation, becomes essential for the thermodynamic behavior of the films.
To deal with this issue, the analytical model, which has been developed in [1], is extended to simulate palladium hydrogen films, that undergo strong plastic deformations upon hydrogen cycling. A cycle of loading and unloading includes an uptake of hydrogen as well as a release of hydrogen which is considered as the main influence on the mechanical stress evolution. During the loading half-cycle, a mechanical stress state including elasto-plastic deformations is obtained. The plastic deformations lead to eigenstresses influencing the following half-cycle of unloading. Considering multiple cycles, the eigenstresses have an influence on both half-cycles.
The simulation results for the mechanical state as well as for the chemical state are compared to experimental results of cycled palladium thin films of different thickness. The comparison reveals that the outlined model suits for the description of the related phenomena in a first attempt.
REFERENCES
[1] A. Dyck, T. Böhlke, A. Pundt, and S. Wagner. Phase transformation in the niobium hydrogensystem: Effects of elasto-plastic deformations on phase stability predicted by a thermody-namic model. 251.
[2] Stefan Wagner, Thilo Kramer, Helmut Uchida, Patrik Dobron, Jakub Cizek, and AstridPundt. Mechanical stress and stress release channels in 10–350 nm palladium hydrogen thinfilms with different micro-structures. 114:116–125. Publisher: Elsevier Ltd.
