X-Ray diffraction study on the hydrogen–hafnium system at high temperatures

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.