The phase-change behavior of hydrogen in niobium and in niobium–vanadium alloys

A study was made of the phase-change behavior of hydrogen in niobium and in niobium–vanadium alloys. A lattice-gas model calculation for hydrogen in pure niobium in which no adjustable parameters are used predicts a critical point temperature of 460 K as compared with the experimental value of 443 K. More importantly, the shape of the coexistence curve corresponds quite closely to the shape of the coexistence curve found experimentally. The model is extended to the case where hydrogen is dissolved in the alloys Nb0.94V0.06, Nb0.85V0.15, and Nb0.75V0.25. The resulting phase diagram predicts a depression in the critical point temperature with increasing vanadium concentration in qualitative agreement with experiment. When the model is corrected to include the effect of trapping of hydrogen by vanadium atoms, the results are in quantitative agreement with experiment.