Fracture Mechanisms of Bulk Amorphous Metal under Impact Loading
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Fracture Mechanisms of Bulk Amorphous Metal under Impact Loading Takao Kobayashi and Donald A. Shockey Center for Fracture Physics SRI International 333 Ravenswood Avenue Menlo Park, CA 94025, U.S.A. ABSTRACT Advanced diagnostic instruments and analyses applied to failure surfaces and cross sections of bulk metallic glasses (BMGs) can provide insight into the deformation and failure of these materials and assist in prototyping new materials with improved failure resistance. Confocaloptics scanning laser microscopic analysis of conjugate fracture surface topographs suggests that the formation and stretching of ligaments are likely keys to the high impact toughness of Vitreloy. INTRODUCTION Metallic alloy systems having an amorphous structure exhibit unusually high resistance to fracture and fatigue crack growth. For the composition known as Vitreloy (Zr41.2 Ti13.8 Ni10 Cu12.5 Be22.5), Rosakis and his colleagues [1] and Ritchie et al. [2] measured a fracture toughness of 55 MPa√m, a remarkable value when one considers the limited ability of the material to plastically deform. Furthermore, Rosakis [3] showed that toughness measured under impact loads exceeded quasi-static values by a factor of 4 to 6 — a dramatic increase and a trend (increasing toughness with increasing strain rate) opposite to that normally observed. Even fatigue crack growth behavior in amorphous metals is surprising [2, 4] in light of the low strainto-failure of the material. Flores and Dauskardt [5] sought the reasons for the unusual combination of limited deformation and high failure resistance. They showed that localized shear bands and branching cracks can form at the crack front and that the critical stress intensity at the crack tips of the branched cracks is about 15 MPa√m, similar to the toughness values measured for nonbranched cracks (Lowhaphandu and Lewandowski [6], Flores and Dauskardt [7], and Flores, et al. [8]). They demonstrated that stabilization of plastic flow processes resulted in high fracture toughness by performing experiments on single-edge-notched specimens under tensile and bending loads. No branching occurred in the bend specimen where the slip line field encourages the crack to remain on the normal crack plane. Several authors published results that show fracture toughness is sensitive to compositional variations and processing history, indicating that a measure of control can be exercised by material developers and suggesting that fracture properties of a given composition can be optimized through processing. Ritchie showed that partial crystallization dropped the toughness precipitously from 55 MPa√m to about 1 MPa√m, and Rosakis measured an 80 % difference in dynamic toughness in two nominally similar materials fabricated differently. Most recently, Hays, et al. [9] demonstrated an in-situ ductile phase/BMG composite in which the second phase L12.18.1
dendrites control the spacing of shear bands. By microstructurally precluding catastrophic failure via one dominant shear band, they obtained large increases in the pl
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