Metastable polycrystalline A1-xBx (A = Zr, Hf, B = Pd, Rh, 0.15 ≤ x ≤ 0.25) alloys having a fcc structure are reacted with hydrogen gas at temperatures ranging from 25°C to 250°C. It is demonstrated for the first time that an amorphous phase can be formed during such a solid state reaction when the temperature lies below 220°C. Such a reaction is possible only if the following requirements are satisfied: The existence of a thermodynamic driving force (i.e., the amorphous phase must have a lower free energy than the free hydrogen and the crystalline phase from which it forms) and the existence of a kinetic constraint (i.e., the formation of thermodynamically preferred equilibrium phases or phase mixtures must be kinetically suppressed).
X-ray diffraction and TEM studies show that the amorphous phase grows at the expense of the crystalline phase during hydrogen absorption by these metastable fcc alloys. The formation of the amorphous hydride phase is observed by TEM to begin at grain boundaries of the polycrystalline alloys much in the same manner that "melting" nucleates at grain boundaries. X-ray analysis indicates that the Zr-Zr distance increases as hydrogen is absorbed, suggesting that hydrogen atoms prefer to stay in tetrahedral sites surrounded by four Zr atoms. This provides evidence as to why the amorphous hydride phase is more stable than the fcc hydride phase. The thermal behavior of amorphous hydrides obtained by hydriding metallic glasses and that obtained by hydriding metastable crystalline alloys are compared and found to be similar. The hydrogen distribution and surface effects are investigated using hydrogen depth profiling, SEM and Rutherford backscattering. Hydrogen permeation through the sample surface has been found to be the rate limiting step in the hydriding reaction.
Based on the present experiments and an analysis of the relevant free energy curves, we discuss the thermodynamic and kinetic aspects of this phase transformation to explain why an amorphous phase is formed. The mechanism for this can be viewed as melting in the solid state. A simple "chemical frustration" model is proposed to explain the kinetics of amorphization via hydriding.
