Quasi-static and dynamic constitutive characterization of beryllium bearing bulk metallic glasses

Citation

Bruck, Hugh Alan (1995) Quasi-static and dynamic constitutive characterization of beryllium bearing bulk metallic glasses. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jd42-3s83. https://resolver.caltech.edu/CaltechETD:etd-09112007-130646

Abstract

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Metallic glasses were first discovered by Pol Duwez in 1960 using the fabrication technique of splat quenching. The mechanical behavior of metallic glasses were first characterized in 1969 from tensile tests conducted on thin ribbons. From these tests it was apparent that metallic glasses possessed tensile fracture strengths of approximately [...], which approach theoretical limits. Compressive mechanical data became available in 1974 with the fabrication of small cylindrical rods of [...]. This data indicated that the quasi-static yield behavior of metallic glasses may obey a pressure insensitive von Mises yield criterion. In 1983, Mechanical tests were conducted on [...] in multi-axial stress states which further confirmed the von Mises yield behavior. However, in 1988, mechanical tests performed on [...] indicated that metallic glasses may instead obey a pressure sensitive Mohr-Coulomb criterion.

There is some ambiguity in interpreting the results of mechanical tests performed on metallic glasses. The data from these tests were obtained by testing specimens whose sizes do not guarantee a well-defined stress state. Furthermore, the mechanical behavior of metallic glasses may depend on composition. In order to properly determine the yield behavior of metallic glasses in multi-axial stress states, it was necessary to fabricate specimens with geometries suitable for generating well-defined stress states.

In 1993, a new beryllium bearing bulk metallic glass with the nominal composition of [...] was discovered at Caltech. This metallic glass can be cast as cylindrical rods as large as 16 mm in diameter. Specimens could then be fabricated with geometries that conformed to ASTM testing standards. These specimens were then tested in quasi-static compressive, tensile, and torsional stress states at strain rates of [...] to [...] in order to properly characterize the yield behavior of the metallic glass. From these tests it was determined that the beryllium bearing bulk metallic glass obeys a von Mises yield criterion. In addition it was discovered that the ductility of this metallic glass could be altered by adding Boron and varying the quench rate.

For the first time, the dynamic compressive yield behavior of a metallic glass could be characterized at strain rates of [...] to [...] by using the split Hopkinson pressure bar. High-speed infrared thermal detectors were also used to determine if adiabatic heating occurred during dynamic deformation of the metallic glass. From these tests it appears that the yield stress of the metallic glass is insensitive to strain rate and no adiabatic heating occurs before yielding.

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