Characteristics of the Solvus

Abstract

The solvus is defined as the hydrogen concentration in the dilute phase that is in equilibrium with the hydride phase at a given temperature. It is sometimes also referred to as the terminal solubility of hydrogen or, particularly in the zirconium literature, as the Terminal Solid Solubility (TSS). In a closed system with constant hydrogen content, it represents the temperature during a temperature scan at which the last remaining hydrides have dissolved or have just started to precipitate for a given value of total hydrogen content. The most striking and important characteristic of the solvus in metal-hydrogen systems is the large and reproducible hysteresis between the dissolution and formation stages of the hydride phase. Depending on the direction of the transformation this hysteresis manifests itself in closed systems of constant hydrogen content as a difference in temperature at which the transformation occurs during a temperature scan and in systems open to an external source of hydrogen gas as a difference in the plateau pressures of the gas. Although all first-order phase transformations exhibit hysteresis to some extent because of the finite differences in extensive physical properties between the transforming phases, the greater magnitude of these differences in metal-hydrogen systems appears to be a associated with the disparity between the high mobility of hydrogen relative to that of the metal component. This allows the phase transformation to rapidly proceed at temperatures at which the lack of mobility of the underlying metal lattice could force the crystal into a metastable state.In early experimental determinations of the solvus relationships in hydride forming metals and, particularly in zirconium and its alloys, it was at first not appreciated that there were in fact at least two reproducible, apparently stable, phase relationships, and that neither of these represented a true equilibrium state of the thermodynamic system even though each was highly reproducible. To some extent this lack of understanding (or appreciation of its importance, particularly for DHC) has persisted to this day. As an example, a widely quoted source of the literature M. P. Puls, The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Components, Engineering Materials,