Hydrogen in Bulk Metallic Glasses: Storage Potential and Effects on Structure

Abstract

The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Several new ferrous and no-ferrous compositions tiiat are capable of forming metallic glasses in bulk form have been examined for their potential to reversibly ingest hydrogen and discharge it. Hydrogen charging was done both by gas charging at pressure and temperature and by the electrochemical route. Results from both approaches were in good agreement and showed that the Fe-based and Y-based glasses were not capable of ingesting hydrogen whereas the Zr-based and Cu-based glasses could take in an appreciable amount of hydrogen but were incapable of discharging it. The hydrogen in the latter alloys interfered with the crystallization process upon heating. The residual hydrogen in the material however served as a qualitative probe for determining possible structural variations in the glass from the inside to the outside. Specifically, calorimetric scans of hydrogen-charged specimens extracted from the center of the ingot were visibly different from those obtained from the edge, suggesting that the structure was more relaxed in the central portion of the ingot. Calorimetric scans of the Zr-based glass at various rates from 0.1°C/min to 100°C/min confirmed a strong heating rate dependence of the glass transition temperature and the crystallization response, the effect being more dramatic in the 0.1°C/min-10°C/min regime. Statement of the Problem StudiedNew bulk metallic glass (BMG) compositions are continuously being identified and cast using conventional methods to reasonable sizes (for example > 5 mm diameter).These include alloys in the Fe-based system, Al-based system, Y-based systems, Ni and Cu-based systems, and refractory metals-based system in an on-going research effort at the California Institute of Technology (Professor Bill Johnson) and University of Virginia (Profs. Gary Shiflet and Joe Poon) and sponsored by the Defense Advanced Research Projects Agency (DARPA). Our goal was to examine these new alloy systems for their hydrogen storage potential and to understand the consequence of hydrogen presence on the transformation response of the alloy. Specifically, in the proposed one-year program, we had decided to address the following issues: 1)Identify the maximum solubility of electrochemically charged hydrogen at room temperature and at 70°C at ambient pressure in Fe-based, Al-based, and refractory-metal based amorphous alloys, and how alloy composition affected this solubility limit?2) Understand the ease of hydrogen desorption electrochemically -i.e. can we discharge hydrogen electrochemically from such a glass at near-ambient temperatures and pressures?3) Examine the effect of a surface film on the desorption kinetics. 4)Determine the effect of absorbed hydrogen on the stability of the amorphous material -i.e. how does hydrogen intake affect the glass transition and crystal...