Fracture Toughness Study on Zr-based Bulk Metallic Glasses

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Fracture Toughness Study on Zr-based Bulk Metallic Glasses Jin-Yoo Suh1, Mary Laura Lind1, C. Paul Kim1,2, R. Dale Conner3, and William L Johnson1 1 Materials Science, California Institute of Technology, W.M.Keck Laboratory of Engineering Materials, Mail code 138-78, Pasadena, CA, 91125 2 Liquidmetal Technologies, Rancho Santa Margarita, CA, 92688 3 Department of Manufacturing Systems Engineering and Management, California State University Northridge, Northridge, CA, 91330 ABSTRACT The fracture toughness of Zr-based bulk metallic glasses of various compositions was studied in the as-cast and annealed condition. Properties were characterized using x-ray and differential scanning calorimetry (DSC) and fracture surfaces were examined using electron microscopy (SEM). Quaternary Zr-Ti-Cu-Be alloys consistently had linear elastic fracture toughness values greater than 80 MPa·m1/2, while Vitreloy 1, a Zr-Ti-Cu-Ni-Be alloy, had an average fracture toughness of 48.5 MPa·m1/2 with a large amount of scatter. The addition of iron to Vitreloy 1 reduced the fracture toughness to 25 MPa·m1/2. Fracture surfaces were carefully analyzed using electron microscopy. Some samples had highly jagged patterns at the beginning stage of crack propagation, and the roughness of this jagged pattern correlated well with the measured fracture toughness values. These jagged patterns, the main source of energy dissipation in the sample, were attributed to the formation of shear bands inside the sample. The Zr-Ti-CuBe alloy, having fracture toughness KQ = 85 MPa·m1/2 as cast, was annealed at various time/temperature combinations. When the alloy was annealed 50oC below Tg, the fracture toughness dropped to 6 MPa·m1/2, while DSC and X-ray showed the alloy to still be amorphous. The roughness of the fracture surfaces on relaxed samples also compared well with the relative fracture toughness. INTRODUCTION Since the discovery of Vitreloy 1, the first commercial Bulk Metallic Glass (BMG) having glass forming ability of more than 25mm, numbers of careful fracture toughness measurements on this superior glass former have been carried out, although there is significant scatter in fracture toughness data from those reports [1-4]. As other alloy systems emerge, fracture toughness of other BMG alloys are also evaluated [5-8], but most of them are measured based on crack initiation from finite radius notches. Considering that the notch radius can cause significant differences in fracture toughness [1], it is difficult to compare those fracture toughness data directly with that of Vitreloy 1. This difficulty makes it harder to understand the parameters determining fracture toughness of BMG, although a couple of analyses provide important insight into the fracture toughness of BMGs [6,9]. In this study, fracture toughness (KQ) was measured for a series of Zr-based BMG alloys: Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1), Zr33.5Ti24Cu15Be27.5 (Var1), Zr44Ti11Cu20Be25 (Var2) and Zr44Ti11Cu9.3Ni10.2Be25Fe0.5 (Var3). Fracture toughness values for the systems

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