PHASE-DIAGRAMS OF TRANSITION METAL-HYDROGEN SYSTEMS

Our knowledge on the phase diagram of metal-hydrogen systems has been limited to easily accessible ranges of composition and temperature, and therefore remained very incomplete. In this paper, it is attempted, by extending the range of equilibrium partial pressure of hydrogen to a few GPa, to delineate the general features of the phase diagrams over the whole composition up to temperatures including the melting point (T(m)) of all constituent phases. Examination of existing data on the distribution coefficient of hydrogen in several metals and pressure-composition-temperature relation of the Ti-H system led to the conclusion that T(m) of host metals is lowered drastically by dissolution of hydrogen. This lowering of T(m) is explained as being the consequence of interstitial dissolution of hydrogen both in solid and liquid phases. The general form of the phase diagram is found to be characterized by the following two factors; (1) a peritectic or eutectic reaction between solid solution and hydride (e.g. MH2) phases, (2) the immiscibility of metallic and molecular (hydrogen) phases. Tentative phase diagrams are constructed accordingly for fcc and bcc (hcp) host metals.