Abstract
The lattice structure of the host metal in the hydrogen–hafnium system, whose isothermal pressure–composition curves display an anomalistic pattern among the systems for titanium-group metals, has been investigated by means of X-ray diffraction. An advanced apparatus for in situ measurement of X-ray diffraction at high temperatures (ca. 1100 K) in hydrogen gas has been developed. In the phase diagram, the phase boundaries of the two-phase α+δ region do not correspond to the horizontal part of the isothermal curve: the two-phase α+δ region extends in the domain where the isothermal curves are sloped. The structure of hafnium is scarcely affected by the absorbtion of hydrogen and remains crystalline even in the sloped-plateau region. These results suggest a variation of energy for interstitial sites, which is consistent with results obtained in some theoretical approaches.
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