純チタンの水素脆化における水素化物および固溶水素の役割

抄録

  The role of solute hydrogen and hydrides in the degradation of the mechanical properties of pure titanium was examined in this study. Commercially pure (99.5%) titanium was electrolytically charged with hydrogen from 50 to 2500 mass ppm and then heated in Ar at 723 K for 3 h to obtain a homogeneous solution of hydrogen. Three types of cooling process after heating were employed to obtain hydrides with different morphologies and solute hydrogen: furnace cooling and water quenching, both to room temperature, and air cooling to 473 K. The microstructures obtained with each process were coarsely precipitated hydrides or finely dispersed hydrides with dissolved hydrogen, and solute hydrogen, respectively. The ratio of the fracture strain of the hydrogen-charged specimens to that of the as-received specimen was used as an index of hydrogen degradation susceptibility (DS). The specimens having coarse hydrides showed a steep increase in DS above 600 mass ppm of absorbed hydrogen due to hydride precipitation and fracture, while the specimens with solute hydrogen showed a milder increase in DS, suggesting that solute hydrogen had a different effect on degradation. The change in DS with the strain rate of the tensile test varied for each type of specimen. The specimens with coarse hydrides showed an increase in DS in the high strain rate region due to preferred fracture of hydrides. On the other hand, DS of the specimens with solute hydrogen increased with a lower strain rate, suggesting interaction between solute hydrogen and mobile dislocations, which is also found in other metallic materials.