青柳 里果

<title>Abstract</title>The dynamics of hydrogen in metals with mixed grain structure is not well understood at a microscopic scale. One of the biggest issues facing the hydrogen economy is “hydrogen embrittlement” of metal induced by hydrogen entering and diffusing into the material. Hydrogen diffusion in metallic materials is difficult to grasp owing to the non-uniform compositions and structures of metal. Here a time-resolved “operando hydrogen microscope” was used to interpret local diffusion behaviour of hydrogen in the microstructure of a stainless steel with austenite and martensite structures. The martensite/austenite ratios differed in each local region of the sample. The path of hydrogen permeation was inferred from the time evolution of hydrogen permeation in several regions. We proposed a model of hydrogen diffusion in a dual-structure material and verified the validity of the model by simulations that took into account the transfer of hydrogen at the interfaces.

We have improved an electron stimulated desorption (ESD) apparatus to obtain the time evolution of hydrogen permeation for cold-worked stainless steel. Hydrogen permeation through grain structures was visualized by using the operando hydrogen microscope combining ESD and hydrogen supply system. The diffusion coefficients in grains were calculated from time evolution curves of hydrogen permeation. Principal component analysis (PCA) of hydrogen maps was used to classify crystal grains by the degrees of hydrogen diffusion and permeation flux. Grain structures such as the ratio of austenite/martensite, crystallographic orientations and coherent/random grain boundaries were determined by electron backscatter diffraction (EBSD) analysis. The areas with high-speed and high flux permeation of hydrogen were characterized as smaller austenitic grains with grain boundaries. The usefulness of a combined ESD-PCA-EBSD analysis on hydrogen permeation in materials was demonstrated in the present study.