This study addresses the investigation of the nature of defects generated during the anodization of aluminum using oxalic acid electrolyte and their influence on the structure and properties of anodic aluminum oxide films (AAO). AAO films, which are obtained by anodization in oxalic acid, and their powdered samples are subjected to thermal annealing at temperatures up to 1050 °C and are subsequently investigated by 27 Al solid-state nuclear magnetic resonance (NMR), electron spin resonance (ESR), powder X-ray diffraction, scanning electron microscopy (SEM), and nanoindentation. By NMR and continuous wave ESR, it is found that the anodization obtained in oxalic acid not only produces amorphous porous alumina, but also gives rise to bulk H defects and unpaired electrons originating from the decomposition of the oxalate ions. Paramagnetic defects are healed by annealing at temperatures of ∼800 °C in air, possibly by an oxidative conversion to CO 2 with the oxygen in air, while the removal of the H defects requires temperatures of at least 1050 °C. The observed hardening of the films and color changes can be explained by changes in structure on an atomic scale and changes in composition: amorphous AAO is converted to a poorly crystalline intermediate phase containing η-Al 2 O 3 at ∼800 °C and subsequently to crystalline α-Al 2 O 3 (corundum) at ∼1050 °C. Moreover, thermal hardening of an AAO passivation layer on top of metallic aluminum with the flame of a H 2 /O 2 burner was demonstrated.

Despite its relevance, fatigue is a phenomenon hardly investigated on the micro- and nano-scale. Recent developments in nanoindentation instrumentation have opened up new opportunities to study the behavior of materials dynamically probed on a small scale. Based on the experimental work on a single crystal copper sample as well as on polycrystalline copper, we show the possibility to sample fatigue information on the nanoscale, which corresponds well with existing literature. Consequently, the method introduced here provides a unique opportunity to explore the fatigue behavior and associated phenomena of surfaces or materials available only in small volumes.

The locally occurring mechanisms of hydrogen embrittlement significantly influence the fatigue behavior of a material, which was shown in previous research on two different AISI 300-series austenitic stainless steels with different austenite stabilities. In this preliminary work, an enhanced fatigue crack growth as well as changes in crack initiation sites and morphology caused by hydrogen were observed. To further analyze the results obtained in this previous research, in the present work the local cyclic deformation behavior of the material volume was analyzed by using cyclic indentation testing. Moreover, these results were correlated to the local dislocation structures obtained with transmission electron microscopy (TEM) in the vicinity of fatigue cracks. The cyclic indentation tests show a decreased cyclic hardening potential as well as an increased dislocation mobility for the conditions precharged with hydrogen, which correlates to the TEM analysis, revealing courser dislocation cells in the vicinity of the fatigue crack tip. Consequently, the presented results indicate that the hydrogen enhanced localized plasticity (HELP) mechanism leads to accelerated crack growth and change in crack morphology for the materials investigated. In summary, the cyclic indentation tests show a high potential for an analysis of the effects of hydrogen on the local cyclic deformation behavior.

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