Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass
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I. INTRODUCTION FIRST developed some 40 years ago,[1] amorphous metallic alloys have long represented an intriguing class of potential structural materials. The lack of any long-range order and the subsequent absence of microstructure has led to a range of interesting properties. These include neartheoretical strength, large elastic deflections, high hardness, excellent wear properties, and good potential for forming and shaping. Due to the very high cooling rates (.105 K/s) necessary to prevent crystallization, however, all prior attempts to characterize the mechanical properties have been confined to very thin ribbons or wires (,10 to 100 mm). Indeed, past studies have focused almost exclusively on constitutive properties, as the restrictive nature of the ribbons made the measurement of fracture and fatigue properties very difficult. Early studies established that, unlike oxide glasses, metallic glasses can be quite ductile.[2–5] Flow in metallic glass is often inhomogeneous, particularly at high stresses and low temperatures, localizing into slip bands along planes of maximum shear. Although the precise flow mechanisms are unclear, bubble-raft and computational studies suggest that they are associated with localized atomic-shear rearrangements correlated to regions of either excess free volume[4,6,7] or extreme shear-stress concentration.[8] Such flow mechanisms, however, have never been verified experimentally due to both a lack of data and the difficulty in characterizing the internal state at the atomic level. C.J. GILBERT, Postdoctoral Research Associate, V. SCHROEDER, Graduate Student Research Assistant, and R.O. RITCHIE, Professor, are with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760. Manuscript submitted March 25, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
Even less work has been completed on fracture toughness and fatigue-crack propagation in amorphous metals, aside from early limited studies on thin ribbons.[7,9–19] Moreover, since traditional notions of microstructure, crystal defects, and dislocation plasticity (which govern our understanding of the behavior of crystalline alloys) do not apply, the mechanisms and microstructural parameters which govern fracture toughness and fatigue-crack propagation in metallic glasses are essentially unknown. The recent development of bulk metallic glass permits, for the first time, detailed measurement of fatigue and fracture characteristics, as the severe specimen-geometry limitations associated with rapid quenching no longer apply. In recent years, several families of multicomponent metallic alloys have been developed which exhibit exceptional glass-forming ability. These include, for example, Mg-based alloys like Mg-Cu-Y,[20] some recently discovered Fe-based alloys,[21] and the Zr-Ti-Ni-Cu, Zr-Ti-Ni-Cu-Be, and Zr-Ti-Ni-Cu-Al alloys.[22,23] All exhibit very high resistance to crystallization in the undercooled liquid state, so that relatively low cooling rates result in a fully amorphous s
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