Hydrogen storage characteristics of mechanically alloyed amorphous metals
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L. Schultz Siemens AG, Research Laboratories, D-8520 Erlangen, Federal Republic ofGermany (Received 7 March 1988; accepted 6 May 1988) The hydrogen storage properties of a series of mechanically alloyed (MA) amorphous N i ^ Zr x alloys are studied, using both gas phase and electrochemical techniques, and are compared to H storage of rapidly quenched (RQ) amorphous N^ _ x Zr x . In the MA alloys, hydrogen resides in the Ni 4 _ „ Zr n (n = 4,3,2) tetrahedral interstitial sites, with a maximum hydrogen-to-metal ratio of 1.9 ( 4 )xn( 1 — x)A~n. These features are identical to those of the RQ alloys and indicate that the topological and chemical order of the MA and RQ materials are essentially the same. However, the typical binding energy of hydrogen in a Ni 4 _ „ Zvn site, En, is shifted in the MA alloys relative to the RQ alloys and the distribution of binding energies centered on En is significantly broader in the MA samples. Thus, the MA and RQ alloys are not identical and sample annealing does not alter this subtle distinction. The sensitivity of H storage to the presence of chemical order in binary alloys are analyzed theoretically and the data are found to be most consistent with little or no chemical order (random alloys).
I. INTRODUCTION In the past few years, a wide range of early transition metal-late transition metal (ETM-LTM) amorphous metal alloys have been prepared by mechanical alloying techniques. The alloys Ni-Zr, 1 " 4 Co-Zr, 1 FeZr,1-5 Cu-Zr, 1 Ni-Nb, 6 Ni-Ti, 7 Co-Ti, 7 and Fe-Ti 7 have been fabricated by ball milling crystalline elemental powders for 30-60 h and subsequent x-ray diffraction measurements have indicated the formation of an amorphous phase. Critical questions that have been addressed by a number of workers in this area are whether these powders are actually amorphous (as opposed to consisting of extremely small, highly strained crystallites) and, if so, how does the structure of a mechanically alloyed (MA) powder compare to an alloy of the same composition prepared by rapid quenching. Bruening and co-workers investigated these issues by comparing x-ray diffraction, differential scanning calorimetry (DSC), and superconducting transition temperature measurements on mechanically alloyed and rapidly quenched (RQ) Ni-Zr alloys.2 These workers observed small differences in crystallization temperature Tx and superconducting transition temperature Tc between the two sets of alloys, but argued that these differences could be accounted for by considering oxygen impurities in the MA sample and the possibility that the two preparation methods produce samples with different degrees of thermal relaxation. Bruening et al. concluded that mechanically prepared Ni-Zr alloys are amorphous and have a structure similar to that of RQ Ni-Zr glasses. These conclusions are in general agree872
J. Mater. Res. 3 (5), Sep/Oct 1988
ment with those of a recent superconducting transition temperature study by Schultz et al. on the same alloy system.4 For samples with greater than 30% Ni, these workers observed a l
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