The Effects of Fatigue on the Atomic Structure with Cyclic Loading in Zr 50 Cu 40 Al 10 and Zr 60 Cu 30 Al 10 Glasses
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INTRODUCTION
BULK metallic glass (BMG) alloys with good glassforming abilities and slow cooling rates can be potentially suitable in a wide range of engineering applications.[1–5] BMGs have superb physical properties, such as high corrosion and wear resistance, high yield strength, soft magnetic properties, and even superconducting properties.[6–8] Despite their good mechanical properties, their use is limited primarily because BMGs tend to be brittle as they cannot plastically elongate during a uniaxial tensile stress. However, some BMGs can plastically deform under compression, rolling, and bending at ambient temperatures.[9] The deformation process occurs through the formation of localized shear bands.[10–12] The same shear bands may become a site for further plastic flows, resulting in low ductility.[13] Improving their ductility and tolerance to damage is an important step toward allowing BMGs to be used as industrial materials. BMGs are particularly vulnerable to fatigue damage,[14–18] even under low applied stresses below the global yield limits, often with no visible effects, such as the presence of shear bands, until failure occurs. The elastic-to-plastic transition appears suddenly. Under fatigue-loading conditions, a wide range of fatigue endurance limits[19,20] is usually observed. To date, the physical mechanism that leads to catastrophic failure PENG TONG, Research Associate, and DESPINA LOUCA, Professor, are with the Department of Physics, University of Virginia, Charlottesville, VA 22904. Contact e-mail: [email protected] G. WANG, Research Associate, and P.K. LIAW, Professor, are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. E. MAXEY, Beamline Specialist, is with the Argonne National Laboratory, Argonne, IL 60439. Y. YOKOYAMA, Associate Professor, is with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted March 28, 2011 Article published online September 20, 2011 2676—VOLUME 43A, AUGUST 2012
under fatigue-loading conditions, in which localized damage accumulates and eventually leads to failure, is not well understood.[7,14–18] It is presumed that irreversible changes must be taking place under fatigue, but their nature and the mechanism leading to such localized damage have not been identified. To search for evidence of initial changes that may occur in the atomic structure under high-frequency fatigue loading, the local atomic structure of ternary Zr-based bulk metallic glasses subjected to compression-compression cyclic loading tests is investigated via neutron and X-ray diffraction. The response to cyclic fatigue loading is investigated in two compositions, Zr50Cu40Al10 and Zr60Cu30Al10, which differ only by 10 pct in the Cu content.[21] Our results indicate that microstructural changes observed in a small sample volume are most likely the effects from cyclic fatigue during compression, where the same atomic regions may act as nucleation sites for subsequent deformation. The local atomic struc
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