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CC9.3.1

Fracture Toughness of Amorphous Metals and Composites J.J. Lewandowski, A.K. Thurston, and P. Lowhaphandu Case Western Reserve University, Department of Materials Science and Engineering Cleveland, Ohio 44106, U.S.A. ABSTRACT The effects of changes in notch radius on the toughness of two different Zr-based bulk metallic glasses have been determined. It is shown that increases in notch radius produce large increases to the toughness, accompanied by extensive shear banding and crack bifurcation. The fracture toughness of twenty (20) fatigue precracked specimens exhibiting planar crack growth were in the range 20.3 ± 6.7 MPa√m for the two Zr-based glasses. Increasing the notch radius to 110 µm produced notch toughness in the range 95.3 ± 8.3 MPa√m for nine (9) tests on Vitreloy I, well in excess of that typically observed in most structural materials. Toughness tests conducted on two fatigue precracked specimens of Vitreloy I at 77 K produced values for fracture toughness that were in the range 17.9 ± 2.7 MPa√m, similar to that obtained at 298 K. The fatigue precracked fracture toughness of metallic glass composites containing large crystalline regions of a body centered cubic Zr-Ti-Nb alloy were in the range 29-42 MPa√m, but the values were temperature dependent over the range 148 K to 500 K. Fracture surfaces were analyzed via Scanning Electron Microscopy (SEM). INTRODUCTION Recent successes in producing bulk amorphous alloys (i.e., metallic glasses) [1-2], have resulted in extensive investigations on the processing techniques/parameters, the decomposition/crystallization process, as well as the effects of nanocrystalline phase formation on the mechanical properties. A few studies have been conducted on the mechanical properties of fully amorphous materials [3-17]. In this study, the effects of changes in notch root radius from fatigue precracked to 250 µm on the fracture toughness of two different Zr-based bulk metallic glasses were determined, in an extension of work reported previously [8,10,12]. In addition, the fatigue precracked fracture toughness was determined at 77 K. Attempts to provide more damage tolerance to amorphous systems have utilized various composite approaches [18]. One approach has been to devitrify the glass during cooling to produce relatively large (e.g. > 100 µm) dendrites of a Zr-Ti-Nb alloy in a metallic glass mixture. The fatigue precracked fracture toughness of such material was determined at 148 K, 298 K, and 500 K and compared to the values obtained on the bulk metallic glass.

CC9.3.2

EXPERIMENTAL PROCEDURE Materials The materials used in this investigation were supplied by Amorphous Technologies International, Inc., Laguna Niguel, California, Liquidmetal Technologies, and Howmet. The composition and thickness of the as-received plates are given in Table I and were analyzed via wet chemistry technique (by Stork Herron Testing Laboratory, Inc., Cleveland, OH). Separate oxygen analyses revealed 1,600 wppm and 1,350 wppm oxygen for the 4 mm and 7 mm Vitreloy I material respect