Hydrogen-induced lattice migration in Pd–Pt alloys

Initially homogeneous, f.c.c. Pd–Pt alloys are shown to partially separate into Pd-rich and Pd-poor regions as a result of exposure to hydrogen gas at, e.g. 5.5 MPa, and at moderately high temperatures, e.g. 448 K. After this hydrogen heat treatment, HHT, the hydrogen can be removed by evacuation at T≤448 K, leaving a metastable alloy which can then be analyzed for the degree of its phase separation. The presence of the phase separation is demonstrated primarily from changes of diagnostic hydrogen isotherms measured at a lower temperature before and after HHT, but also from Auger depth-profiling analysis, high resolution electron microscopy and small angle neutron scattering (SANS). After heating at the same temperature in the absence of the dissolved hydrogen, no evidence for phase separation is found. The X Pt=0.15, 0.20 and 0.25 alloys, which do not form hydride phases in their homogeneous forms at T≥273 K, develop plateau pressure regions indicative of hydride formation in their diagnostic isotherms (≥273 K) after HHT at 100 MPa (673 K). This is possible because significant phase separation has occurred due to HHT resulting in a phase capable of forming a hydride at 273 K, i.e. X Pt