THERMODYNAMIC CHARACTERIZATION OF THE ZR-MN-H SYSTEM .2. REACTION OF H-2 WITH 2-PHASE ALLOYS

The reactions of H-2(g) with the two-phase alloys (a) alpha-Zr + ZrMn2 + x and (b) alpha-Mn + ZrMn2 + x have been investigated. The homogeneity range of the Laves phase ZrMn2 + x intermetallic compound has been determined from the dependence of its hydrogen absorption behavior upon x and found to lie within the limits x = -0.3 +/- 0.1 and 1.0 +/- 0.1. Normally H-2(g) does not react with alpha-Zr below about 773 K, but it has been found that when it exists within the two-phase mixture, it reacts readily with hydrogen at room temperature. For alloys in the zirconium-rich two-phase field the hydrogen first reacts to form ZrH approximately 2 and then it reacts with the ZrMn2 + x phase. The former reaction leads to equilibrium pressures which are too small to measure, but the latter leads to measurable pressures. Results for alloys within the manganese-rich two-phase field, x greater-than-or-equal-to 1.0, are also reported. Calorimetric determinations of the enthalpies of reaction with hydrogen for several zirconium-rich alloys within the two-phase fields are reported for both the zirconium-rich and ZrMn2 + x phases.