HYDROGEN DIFFUSION IN NANOCRYSTALLINE, MESOSCOPIC, AND MICROCRYSTALLINE HETEROGENEOUS ALLOYS

The nano-range and long-range diffusion processes of hydrogen in amorp hous, nanocrystalline, microcrystalline, and heterogeneous transition- metal alloys were studied by magnetic after-effect and thermal desorpt ion spectrometry measurements respectively. Microstructural and thermo dynamic parameters, such as the grain size or solubility in interfacia l and matrix phases, show a strong influence on hydrogen diffusion. In amorphous and nanocrystalline Fe90Zr10 and Fe60Co30Zr10 alloys the me an activation enthalpy of hydrogen diffusion decreases with increasing hydrogen concentration. This behaviour, well known from amorphous all oys, can be assigned to the topologically disordered interfacial phase possessing a high hydrogen solubility. In contrast, heterogeneous Co9 0Zr10 alloys show no dependence of the activation enthalpy on hydrogen concentration. Additionally, the desorption kinetics observed over a wide range of grain sizes indicate that hydrogen is mainly dissolved w ithin the crystalline grains and diffuses very fast in the interfacial phase. In a theoretical analysis the hydrogen diffusion in heterogene ous systems was treated by Monte Carlo simulations. The model system c onsisted of cubic grains separated by an interfacial layer in three di mensions. The effective diffusion coefficient as a function of the vol ume fraction of grains was calculated for different potential barriers between grains and the interfacial phase. The experimental and theore tical results are compared and their implications for material optimiz ation are discussed.