Interpretation of Fracture Toughness and R -Curve Behavior by Direct Observation of Microfracture Process in Ti-Based De
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ADVANCES in bulk amorphous alloys have been remarkably made by developing amorphous alloys with high glass-forming ability.[1–6] However, there are problems that remain to be solved, typical one of which is brittle fracture. When amorphous alloys are externally loaded, shear bands play an important role in initiating the fracture, leading to abrupt shear fracture.[7,8] In order to overcome this problem of brittle fracture, the developmental activities on composite-type alloys by homogeneously distributing ductile crystalline particles in an amorphous matrix have been performed.[9–11] In the recently developed Zr- and Ti-based amorphous alloys, where ductile dendrites are formed in situ from the amorphous alloy melt, the ductility is greatly enhanced by forming a number of deformation bands at dendrites.[10–15] According to Hofmann et al.,[12] the tensile strength and ductility were increased to 1510 MPa and 9.5 pct, respectively, by increasing the volume fraction of dendrite
CHANGWOO JEON, Postdoctoral Research Associate, and CHOONGNYUN PAUL KIM, Research Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. HYOUNG SEOP KIM and SUNGHAK LEE, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with the Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] Manuscript submitted August 15, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
to 47 pct in Zr-based amorphous alloys containing more Ti content (31 to 34 at. pct) than that in a conventional Zr-based amorphous alloy containing dendrites (i.e., an ‘LM2’ alloy (commercial brand name of the Liquidmetal Technologies, Lake Forest, CA)). In some Ti-based amorphous alloys, whose volume fraction of dendrite was higher than 70 pct, the tensile ductility reached 10 pct.[13] In order to further enhance the tensile ductility, together with crack initiation and growth toughness, the size and volume fraction of dendrite should be optimized. Systematic understanding of the correlation between microstructure and fracture toughness is also required for the accurate evaluation of fracture toughness, and microfracture mechanisms should be verified in relation with microstructure. A simple way for examining microfracture mechanisms is an observation of fractured surfaces, which can lead to effectively explaining the crack initiation and growth toughness. Studies on detailed fracture mechanisms such as direct observation of crack initiation and propagation processes[16,17] or deformed microstructures beneath the fractured surfaces[18,19] have also been conducted. In spite of these efforts, many difficulties still remain to be addressed to correlate microscopic fracture mechanisms with macroscopic fracture toughness. In this study, crack initiation toughness of Ti-based amorphous alloys was measured, and mechanisms related with improvement of crack ini
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