Date of Award
Doctor of Philosophy (PhD)
Bulk metallic glasses are a relatively novel class of engineering alloys characterized by a "disordered" atomic structure devoid of long-range translational symmetry. Compared to crystalline alloys, the confluence of metallic bonding and amorphous structure imbues bulk metallic glasses with a unique set of properties that makes them particularly attractive for a wide variety of structural applications. Such properties include exceptional yield strengths, high elastic resilience, resistance to corrosion, and in particular, the unparalleled ability among metals to be thermoplastically formed across a wide range of length scales when heated above the glass transition temperature. Formation of metallic glass from a molten liquid depends on whether cooling is sufficiently rapid to bypass crystallization and vitrify into an amorphous solid; for a given alloy composition, the ease with which full vitrification can occur upon cooling from the liquid state is termed the alloy's "glass forming ability." Unfortunately, relatively few excellent glass formers have been reported in the vast, multicomponent composition space in which they reside. The apparent slowness of progress may be attributed largely to the inefficiency of the one-at-a-time experimental approach to discovery and design. In this thesis work, a high-throughput combinatorial methodology was developed to expedite the discovery process of new bulk metallic glasses. Laser deposition was used to fabricate continuously-graded composition libraries of Cu-Zr and Cu-Zr-Ti alloys. By processing the libraries with a range of laser heat input, the best glass formers in each alloy system could be efficiently and systematically deduced. Furthermore, instrumented nanoindentation performed on the libraries enabled rapid evaluation of mechanical property trends.
Despite boasting high strengths, monolithic bulk metallic glasses generally suffer from an intrinsic lack of damage tolerance compared to other high performance alloys. Recent studies indicate that the macroscopic deformation behavior of the material may be controlled by structural heterogeneities, although the exact nature and origin of the heterogeneities remain ambiguous. To further the present knowledge, the heterogeneous microstructure of a zirconium-based bulk metallic glass was investigated with instrumented nanoindentation and dynamic modulus mapping. Significant spatial variations in the mechanical properties measured by both techniques suggests a hierarchical arrangement of structural/mechanical heterogeneities in bulk metallic glasses. Moreover, a previously unobserved elastic microstructure, comprising an interconnected network of elastic features, was revealed by dynamic modulus mapping. Despite the absence of visible contrast when imaged with electron microscopy, the aligned morphology of the elastic features and their sensitivity to thermal processing conditions imply the occurrence of spinodal decomposition in the supercooled liquid prior to glass formation. Finally, based on analysis of load-displacement data from nanoindentation experiments performed throughout the thesis work, a new parameter, the plastic work ratio, was proposed as a figure of merit for quantifying the intrinsic plasticity of monolithic metallic glass alloys.
Katharine M. Flores
Katharine M. Flores, Parag Banerjee, Shankar M. Sastry, Kenneth F. Kelton