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DUCTION

DURING the past decades, remarkable advances in bulk amorphous alloys have been made by developing amorphous alloys such as La-Ni-Al,[1] Zr-Al-Cu-Ni,[2] Zr-Ti-Cu-Ni-Be,[3,4] and Ti-Zr-Ni-Cu-Be[5] having high glass forming ability. Among them, Zr-based amorphous alloys show excellent glass forming ability (critical cooling rate: about 1 K/s), hardness, strength, stiffness, and corrosion resistance,[6,7] and thus are accepted as leading-edge materials because of their potentialities in structural applications. 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. In addition, there are problems to be addressed, a typical one being brittle fracture. When amorphous alloys are subjected to externally applied loads, shear bands work as a major initiator of deformation and fracture, thereby leading to abrupt fracture in a shear type.[8–10] In order to improve the fracture properties of amorphous alloys, active studies to fabricate composite-type

CHANGWOO JEON, Research Assistant, and CHOONGNYUN PAUL KIM, Research Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SUNGHAK LEE, Professor, is with the Center for Advanced Materials, Pohang University of Science and Technology, and jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted February 1, 2012. Article published online June 12, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

alloys by distributing ductile crystalline particles in an amorphous matrix have been conducted.[11–14] In recently developed Zr- and Ti-based amorphous alloys where ductile dendrites are formed in situ from the amorphous matrix, the tensile ductility is greatly improved by forming multiple shear bands in the amorphous matrix and deformation bands at dendrites.[15–17] Szuecs et al.[18] reported that a Zr-based amorphous alloy containing ductile dendrites of crystalline b phases (structure: bcc) showed the tensile strength of 1470 MPa and ductility of 3 pct. This improvement of tensile ductility was caused by the formation of more shear bands than those formed in a monolithic Zr-based amorphous alloy. In order to further improve the tensile ductility and fracture toughness, the size and volume fraction of dendrites need to be optimized, while trends of increased ductility and toughness with increasing volume fraction of dendrites are generally accepted. In addition, for the accurate evaluation of fracture toughness, systematic understanding of the correlation between microstructure and fracture toughness is required, and elucidation of microfracture mechanisms in relation with microstructure is essentially needed. A simple way widely used to investigate microfracture mechanisms is a phenomenological observation of fractured su