A study of the thermodynamics of the crystalline-to-amorphous transformation in Zr-based hydrides by means of thermal an
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I. INTRODUCTION Recently it was discovered that certain crystallinemetallic alloys transform into the amorphous state during hydrogen absorption.1 Subsequently it was found that the amorphous phase could also be produced during the solid-state interdiffusion of elemental composites or thin-film multilayers.2'3 The same thermodynamic and kinetic principles control both of these solid-state amorphization reactions. In order for these reactions to proceed, the final amorphous product must have a lower free energy than the starting crystalline materials and a kinetic barrier must exist to prevent the formation of the equilibrium phase or phase mixture. The mechanisms of the solid-state amorphization reactions are not very well understood. There have been a number of studies of the mechanisms of these reactions utilizing techniques such as x-ray diffraction, electron microscopy, Rutherford backscattering, and resistivity.4 7 The present work is an attempt to study the thermodynamics of the crystal-to-amorphous transformation in some metastable fee Zr-Rh and Zr-Pd alloys during the hydriding process, using differential scanning calorimetry (DSC). The crystallization behavior of amorphous hydrides produced by hydriding asquenched amorphous alloys is compared to that of amorphous hydrides produced by hydriding metastable fee alloys of similar compositions. We also investigate the thermal stability of these alloys as a function of hydrogen-to-metal ratio. II. EXPERIMENTAL Ingots were prepared by inductive melting the appropriate amounts of Zr and Rh or Zr and Pd on a water-cooled silver hearth. Metastable fee or amorphous ribbons were obtained by rapidly quenching from the melt, utilizing the melt-spinning technique.8 Hydriding was done by reacting the alloys with hydrogen gas under a pressure of one atmosphere, at temperatures below 220 °C. The hydrogen-to-metal ratio was determined by massing samples before and after hydriding. The sample J. Mater. Res. 2 (2), Mar/Apr 1987
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structures were characterized using powder x-ray diffraction, which was performed on a vertical Norelco powder diffractometer in reflection mode using Ni-filtered Cu-Ka radiation. The DSC experiments were performed on a PerkinElmer DSC-4 apparatus in a constant heating rate mode. The DSC-4 was interfaced to an Apple HE computer for data collection. Baselines were obtained by running a DSC scan with empty aluminum pans in the sample and reference pan holders. Actual DSC scans of samples always immediately followed baseline scans in order to obtain greater precision. The data were plotted with the data of the baseline scan subtracted from the actual sample scan data. Samples used for DSC studies, typically weighing from 3-10 mg, were hermetically sealed in aluminum pans under inert gas atmosphere.9 III. RESULTS AND DISCUSSION The relative thermal stability of as-quenched amorphous Zr O8O Rh O2O , metastable fee Zr O 8 1 Rh 0 1 9 , and their respective hydrides was examined utilizing DSC. (A typical x-ray diffraction profile
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