Absorption of hydrogen by titanium–cobalt and titanium–nickel intermetallic alloys. Part 1.—Experimental results

Abstract

Pressure–composition–temperature relationships are presented for the titanium–cobalt + hydrogen and the titanium–nickel + hydrogen systems for the ranges 10 kPa to 2 MPa and 325 to 600 K. The TiCo + H system exhibits three distinct two-phase regions, whereas the TiNi + H system contains no two-phase regions in this range of temperatures and pressures. Values for the relative partial molar enthalpy and entropy have been obtained from the equilibrium data.

For TiCo, ΔH^⊖_H2 is –45 kJ (mol H₂)^–1 at infinite dilution. The enthalpy then decreases to a value of –54 kJ (mol H₂)^–1 at the first plateau, and then increases to –48 kJ (mol H₂)^–1 at the second plateau, and –40 kJ (mol H₂)^–1 at the third plateau. The entropy decreases with hydrogen content to a value of –135 J K^–1(mol H₂)^–1 at TiCoH_1.0 and then remains fairly constant as the hydrogen content is increased.

For TiNi, the enthalpy at infinite dilution is –58 kJ (mol H₂)^–1. The enthalpy then decreases to a miniumum of –60 kJ (mol H₂)^–1 at TiNiH_0.9, before increasing rapidly at higher hydrogen contents. The entropy decreases to –140 J K^–1(mol H₂)^–1 at TiNiH_0.9, but then apparently increases at higher hydrogen contents. This ususual entropy effect is attributed to changes in the number of interstitial sites available to hydrogen, leading to an increase in the configurational entropy at higher hydrogen contents.

Data are also presented which describe the temperature-programmed decomposition of the hydrides. The TiCo hydrides decompose in three stages corresponding to the three plateau regions of the isotherms. In contrast, the solubility of hydrogen in the TiNi alloy initially decreases with temperature, but at 700 K a reversal takes place and the solubility of hydrogen increases reaching a maximum at 850 K, before decreasing to zero at 950 K. These solubility effects may be caused by short range ordering of the atoms which surround tetrahedral interstitial sites, resulting in the creation of titanium-rich sites. Parallels are drawn with the martensitic transition which is known to occur in the TiNi intermetallic alloy.