Fatigue Softening of (Zr 58 Ni 13.6 Cu 18 Al 10.4 ) 99 Nb 1 Bulk Metallic Glass
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INTRODUCTION
DEFORMATION modes of bulk metallic glasses (BMGs) include homogeneous and inhomogeneous deformation. At high temperatures and low strain rates, BMGs tend to deform homogeneously. However, at low temperatures and high strain rates, BMGs tend to exhibit an inhomogeneous deformation.[1–5] In the case of the inhomogeneous deformation, the plastic flow concentrates in shear bands with the decrease of the viscosity and the increase of the free volume, and, thus, the plastic deformation shows the strain softening.[6,7] Generally, under static loads, shear bands form when the stress is higher than the yield strength.[8] However, shear bands were also observed under different fatigue experiments with the maximum stress much lower than the yield strength, which was considered as a characteristic of the fatigue softening.[9,10] The strain softening of BMGs has been widely investigated under a staticcompression stress and has also been proved directly from the decreasing stress in the plastic region.[11] However, to the authors’ knowledge, few results have been reported and proved on fatigue softening, which is a subject worthy of effort, since the formation of the shear bands and the fatigue softening are closely related to the fracture of BMGs during the fatigue process. DONGCHUN QIAO, Research Assistant, formerly with the Department of Material Science and Engineering, The University of Tennessee, is with the Applied Science Laboratory/Institute for Shock Physics, Washington State University, Spokane, WA 99210. Contact e-mail: [email protected] WENHUI JIANG and CANG FAN, Research Associates, and PETER K. LIAW, Professor, are with the Department of Material Science and Engineering, The University of Tennessee, Knoxville, TN 37996. LU HUANG, Research Assistant, is with the Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, P.R. China. This article is based on a presentation given in the symposium ‘‘Bulk Metallic Glasses VI,’’ which occurred during the TMS Annual Meeting, February 15–19, 2009, in San Francisco, CA, under the auspices of TMS, the TMS Structural Materials Division, TMS/ASM: Mechanical Behavior of Materials Committee. Article published online April 13, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
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EXPERIMENTAL PROCEDURES
The starting materials used in preparing the alloys were high-purity metals: Zr (99.95 pct), Al (99.999 pct), Ni (99.995 pct), Cu (99.999 pct), and Nb (99.995 pct) in wt pct. In order to obtain the chemical homogeneous ingot, a two-step processing method was used for the preparation of master alloys. First, the binary solidsolution ingots of Zr-Nb were fabricated due to the high melting temperatures of Nb and Zr. Second, Zr-Nb ingots were arc melted with the remaining elements to form master-alloy ingots. For each step, the ingot was melted and flipped 5 times for a total of about 8 minutes. Then, the BMG of the nominal composition of (Zr58Ni1
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