Thermodynamics of the Zr-H System

The consistencies among the reported univariant hydrogen pressures in the three two‐phase regions of the Zr‐H system, aZr/δZrH2‐xαZr/βZr, and βRZr/δZrH2–x, are evaluated with the thermodynamic constraint derived by Speiser.16 Hydrogen dissolution in αZr is evaluated by checking the consistencies among the Sieverts’law constant, the terminal solubility, and the corresponding two‐phase univariant pressures involving this phase. A modified Sieverts’law is proposed for the complicated behavior of hydrogen dissolution in βZr. An analytical compositiontemperature‐pressure relationship for δZrH2–x is established by using the quasi‐chemical model derived by Arita et al. The thermodynamic constraint of the free energy of formation of ZrH2 shows that the value measured by Fredricken et al. is the most reliable. In addition, a more accurate free energy of formation of zirconium hydride at the upper phase boundary of αZr/δZrH2–x two‐phase region is suggested.