Supersaturation and hydride formation in the dilute phase of Pd–H and Pd–Mn–H alloys

Annealed Pd exhibits supersaturation of the dilute phase before the hydride phase forms and this is characterized here by measurements of both |ΔHH| and equilibrium pH2pH2. Dilute phase |ΔHH| and pH2pH2 values were determined for annealed Pd for both absorption and desorption of H2 and the |ΔHH| were the same for both and the dilute phase H2 isotherms were also the same, i.e., absorption/desorption was reversible. Pd, which has a high dislocation density, however, does not exhibit supersaturation, and its |ΔHH| values are a continuous function of H/Pd in contrast to annealed Pd where there is a discontinuity at the phase boundary. In addition, absorption/desorption of H2 is irreversible for this “cycled” Pd. The same phenomena were examined for several substitutional fcc Pd–Mn alloys; supersaturation was also observed for the three alloys: XMn = 0.01, 0.05 and 0.075.