In-situ microfracture observation of strip-cast Zr-Ti-Cu-Ni-Be bulk metallic glass alloys
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I. INTRODUCTION
DURING the past several years, remarkable advances in amorphous alloys where atoms are arranged, such as in liquid, have been made because of the development of bulk metallic glass (BMG) alloys such as La-Ni-Al,[1] Zr-AlCu-Ni,[2,3] Zr-Ti-Cu-Ni-Be,[4] and Zr-Ti-Cu-Ni[5] having substantially high glass forming ability. Among them, Zr-base BMG alloys show superb glass forming ability (critical cooling rate, about 1 K/s), excellent hardness, strength, stiffness, and corrosion resistance,[6,7] and thus have been applied to components of sporting goods and electrical products. However, the size and shape of the fabricated products are quite limited, and the manufacturing cost is high because the vacuum environment is required for die casting, thereby preventing a wide range of their application to other industrial areas. Considering that most structural materials are of plate or sheet type, continuous strip casting of these BMG alloys would prove an economic way to reduce manufacturing cost and to expand applicability.[8] This strip casting is also appropriate for the fabrication of strips of BMG alloys having slow critical cooling rate because its cooling rate is relatively fast (102 C/s to 103 C/s). These BMG alloys have excellent properties required for structural materials. In particular, Zr-Ti-Cu-Ni-Be alloys show a tensile strength of 1.9 GPa and an elastic strain limit of 2 pct under compressive or tensile loading conditions.[9,10] Their plane strain fracture toughness was reported to be 16 to 55 MPa 1m,[10,11,12] depending on purity and residual stress of the tested alloys. However, they fail catastrophically on highly localized shear bands, thereby leading to very low JUNG G. LEE, Research Assistant, and DONG-GEUN LEE, Postdoctoral Research Associate, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. SUNGHAK LEE and NACK J. KIM, Professors, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, are jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: shlee@ postech.ac.kr Manuscript submitted February 12, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
ductility and fracture toughness.[13] Thus, current research has focused on how plastic flow can be stabilized and fracture resistance can be increased in the BMG alloys. For example, very high fracture toughness values can be achieved by the development of crack branching zones inside the plastic zone ahead of the main propagating crack tip.[14] In addition, intensive studies have been followed to fabricate composites by dispersing crystalline particles in an amorphous matrix. Methods for fabricating these composites include producing a dispersion of nanocrystallines through partial devitrification of BMG alloys,[15] adding crystalline particles to the melt prior to casting,[16] casting a glass forming alloy around reinforcing particles or fibers,[17] or precipit
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