The galvanostatic anodizing behavior of additive manufactured (AM) Al-Si10-Mg alloy was studied in H2SO4 electrolyte. The analysis of the voltage vs time response was complemented with a systematic characterization of the anodic oxide layer using a variety of techniques. In addition, a cast alloy of approximately the same chemical composition as that of the AM specimens was used as a reference in this study. Significant differences were found in the voltage-time characteristics of the samples analyzed. Besides, an anisotropic anodizing behavior was observed in the additive manufactured specimens. Due to the fine silicon microstructure present in the additive manufactured samples, the anodic oxide growth was much more obstructed than for the cast alloy. Nevertheless, even though the oxide layer was generally thinner in the AM samples for the same conditions and anodizing time, a much more continuous and uniform oxide layer was found in the additive manufactured specimens compared to the cast alloy. The porous structure was found to be greatly affected by the fine distribution of the silicon phase in the AM parts.
The high-rate sensitivity of nanostructured metallic materials demonstrated in the recent literature is related to the predominance of thermally activated deformation mechanisms favoured by a large density of internal interfaces. Here we report time-resolved high-resolution electron transmission microscopy creep tests on thin nanograined films using on-chip nanomechanical testing. Tests are performed on palladium, which exhibited unexpectedly large creep rates at room temperature. Despite the small 30-nm grain size, relaxation is found to be mediated by dislocation mechanisms. The dislocations interact with the growth nanotwins present in the grains, leading to a loss of coherency of twin boundaries. The density of stored dislocations first increases with applied deformation, and then decreases with time to drive additional deformation while no grain boundary mechanism is observed. This fast relaxation constitutes a key issue in the development of various micro- and nanotechnologies such as palladium membranes for hydrogen applications.
International audienceThe nanoscale plasticity mechanisms activated during hydriding cycles in sputtered nanocrystalline Pd films have been investigated ex-situ using advanced transmission electron microscopy techniques. The internal stress developing within the films during hydriding has been monitored in-situ. Results showed that in Pd films hydrided to beta-phase, local plasticity was mainly controlled by dislocation activity in spite of the small grain size. Changes of the grain size distribution and the crystallographic texture have not been observed. In contrast, significant microstructural changes were not observed in Pd films hydrided to alpha-phase. Moreover, the effect of hydrogen loading on the nature and density of dislocations has been investigated using aberration-corrected TEM. Surprisingly, a high density of shear type stacking faults has been observed after dehydriding, indicating a significant effect of hydrogen on the nucleation energy barriers of Shockley partial dislocations. Ab-initio calculations of the effect of hydrogen on the intrinsic stable and unstable stacking fault energies of palladium confirm the experimental observations. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Ocean acidification (OA) and its subsequent changes in seawater carbonate chemistry are threatening the survival of calcifying organisms. Due to their use of calcium carbonate to build their shells, marine molluscs are particularly vulnerable. This study investigated the effect of CO2-induced OA on adult European abalone (Haliotis tuberculata) using a multi-parameter approach. Biological (survival, growth), physiological (pHT of haemolymph, phagocytosis, metabolism, gene expression), and structural responses (shell strength, nano-indentation measurements, Scanning electron microscopy imaging of microstructure) were evaluated throughout a 5-month exposure to ambient (8.0) and low (7.7) pH conditions. During the first 2 months, the haemolymph pH was reduced, indicating that abalone do not compensate for the pH decrease of their internal fluid. Overall metabolism and immune status were not affected, suggesting that abalone maintain their vital functions when facing OA. However, after 4 months of exposure, adverse effects on shell growth, calcification, microstructure, and resistance were highlighted, whereas the haemolymph pH was compensated. Significant reduction in shell mechanical properties was revealed at pH 7.7, suggesting that OA altered the biomineral architecture leading to a more fragile shell. It is concluded that under lower pH, abalone metabolism is maintained at a cost to growth and shell integrity. This may impact both abalone ecology and aquaculture.
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