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Fracture toughness study on bulk metallic glasses and novel joining method using bulk metallic glass solder

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Caltech Theses and Dissertations
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Abstract

The fracture toughness of three new compositional variants of the Zr-Ti-Be-LTM (Late Transition Metal) family of bulk metallic glasses (BMG’s) are studied in the as-cast and annealed condition. Quaternary Zr-Ti-Cu-Be alloys consistently had linear elastic fracture toughness values greater than 80 MPa∙m1/2, while Vitreloy 1, a Zr-Ti-Cu-Ni-Be alloy, had an average fracture toughness of 48.5 MPa∙m1/2 with a large amount of scatter. The addition of iron to Vitreloy 1 reduced the fracture toughness to 25 MPa∙m1/2. The Zr-Ti-Cu-Be alloy, having fracture toughness K_Q = 85 MPa∙m1/2 as cast, was annealed at various time/temperature combinations. When the alloy was annealed 50C below Tg, the fracture toughness dropped to 6 MPa∙m1/2, while DSC and X-ray showed the alloy to still be amorphous. Fracture surfaces were analyzed using scanning electron microscopy. The tougher samples have shown evidence of highly jagged patterns at the beginning stage of crack propagation, and the length scale and roughness of this jagged pattern correlate well with the measured fracture toughness values. These jagged patterns, the main source of energy dissipation in the sample, are attributed to the formation of shear bands inside the sample. This observation provides a strong evidence of significant “plastic zone” screening at the crack tip. Unlike the unstable fracture behavior of monolithic BMG’s, ductile phase containing in-situ BMG composite shows stable crack growth behavior. Application of ductile BMG as a matrix for an in-situ composite with controlled microstructural characteristic length scales maximizes the toughening effect. In order to characterize this highly toughened BMG composite, the elastic-plastic fracture mechanics concept is introduced and the J-parameter is evaluated. A novel thermoplastic bonding concept is demonstrated based on the unique rheological behavior and pattern-replication ability of bulk metallic glass forming liquids. In this approach, the bulk metallic glass is heated above Tg to the “supercooled liquid” region while a small normal force is applied to the joint. This results in liquid reflow, wetting and a strong bond. Complete wetting between copper substrates and a layer of platinum based bulk metallic glass leads to an atomistically intimate void-free interface.

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