Rate-Dependent Deformation Behavior of Zr-Based Metallic-Glass Coatings Examined by Nanoindentation
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AMORPHOUS metals and alloys are promising structural materials due to their high strength, but the engineering applications are limited by their intrinsic brittleness and low ductility.[1,2] At high temperatures and low strain rates, the deformation of amorphous alloys tends to be homogeneous viscous flow; at low temperatures and high strain rates, the deformation is spatially inhomogeneous, and the plastic strain field is localized into narrow shear bands. The shear bands can catastrophically propagate through a monolithic metallic-glass specimen under unconstrained conditions, so that little plastic strain can be observed prior to the fracture. It is thus anticipated that both external and internal geometric constraints can be introduced to block the shear-band propagation, thus increasing the number and density of shear bands. The shear-band multiplication can significantly improve the ductility and toughness, as demonstrated by the platelike specimens (bending or compression) and the amorphous alloys with embedded second phases.[3–7] In addition, because the shear-banding behavior is a kinetic process that is controlled by stress, strain rate, and temperature, F.X. LIU, Graduate Student, is with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. Y.F. GAO, Assistant Professor, is with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, and the Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Contact e-mail: [email protected] P.K. LIAW, Professor, is with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online January 3, 2008 1862—VOLUME 39A, AUGUST 2008
the amorphous alloys demonstrate an intriguing interplay between the strain-rate dependence and the geometric-constraint effects.[8,9] The work presented in this article investigates the metallic-glass coating, in which the coating-substrate interface can serve as geometric constraints to block the propagation of shear bands. The metallic-glass thin film can be deposited on the substrate by the magnetronsputtering technique, and the resulting coating-substrate system has been found to exhibit the enhanced fatigue behavior in our earlier work.[10–12] From the scanning electron microscopy (SEM) observations, it is believed that the fatigue-crack initiation occurs when the amorphous coating cannot suppress the slip steps caused by dislocations in the substrate material. Consequently, the study of the deformation behavior of the amorphous coating is of critical importance to understanding both the constraint effects on the shear banding behavior and the mechanisms for fatigue-resistance enhancement
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