Pressure-Temperature-Composition Relationships of the Yttrium-Hydrogen System

Pressure‐temperature‐composition studies of the yttrium‐hydrogen system were made in the temperature range 900°–1350°C and the pressure range 1–760 mm Hg. The existence of a yttrium‐solid‐solution phase and a yttrium‐hydride phase, which approaches in composition, was established. These phases occur over a range of composition and are separated by a two‐phase region at intermediate hydrogen compositions. The calculated heat of reaction of the dihydride phase from the saturated solid solution is , and the entropy of reaction is . Partial molal heats and entropies of solution of hydrogen in the single phases are also presented. The nature of the solubility relationships in the yttrium‐ hydrogen system below 900°C was studied at atmospheric pressure. The existence of a stable trihydride phase was established below approximately 300 °C and 1 atm pressure. The solubility relationships of the yttrium‐hydrogen system were developed from the pressure‐temperature‐composition relationships and from x‐ray diffraction analysis and metallographic examination. X‐ray diffraction structural analysis of yttrium metal, the dihydride and trihydride phases gave the following structures and x‐ray densities: