Superconductivity under pressure in FeSe (T c ∼7.5 K) has been investigated using singlecrystal specimens through the measurements of DC magnetization and electrical resistivity.A characteristic three-step increase in T c has been found under hydrostatic pressure up to ∼34 K above 7 GPa. The structural transition from a tetragonal phase to an orthorhombic phase (T s ∼87 K) is found to disappear at P ∼2.3 GPa, above which T c increases rapidly, suggesting that the superconductivity is enhanced by the tetragonal environment. Under nonhydrostatic pressure, the increase in T c is suppressed and the superconductive volume fraction is considerably reduced above 2 GPa, probably owing to the breaking of the tetragonal lattice symmetry by the uniaxial stress. The intimate correlation between the enhanced (suppressed) superconductivity and the tetragonality (orthorhombicity) in the phase diagram is a common feature of FeSe and other iron-pnictide superconductors.

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For the development of lead-bismuth (Pb-Bi) cooled fast breeder reactors, the corrosion characteristics of various steels in a flowing Pb-Bi were investigated by means of steel corrosion test performed in the flowing Pb-Bi for 1,000 h. It was found that liquid metal corrosion was inhibited by single or multiple oxide layers formed on the steel surfaces. The oxide layers can be classified to an inner layer and an outer one. The existence of the inner oxide layer might be more important for the corrosion resistance since the inner one was compact and stuck to the steel surface while the outer one was easily broken into small pieces with cracks and peeled from the substrate. The effect of Cr content in steels on corrosion resistance was investigated. Stable Cr rich oxide layers were easily formed on the high Cr steels. The weight losses were lower in the steels with higher Cr contents. The content of Cr in the steel was effective to form the thin and compact Cr-rich oxide layer and suppressed the corrosion. In the case of Si rich steel, the weight loss was lower than the trends of weight loss with Cr content.