Collaborative Research: Closing the Bulk Metallic Glass Data Gap in the Supercooled Liquid Region

The University Corporation, Northridge, Northridge CA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Metallic glasses are high-strength metallic alloys that have a random, non-crystalline structure. Like all glasses, they behave like a frozen liquid with flow characteristics determined by its viscosity, or resistance to flow, which varies with temperature. Understanding how flow varies with temperature is a key component both to unlocking the physics behind the underlying structure of metallic glasses, and knowing how its properties influence, and are influenced by, processing. There are three different behaviors: At low temperature the glass behaves as a solid; at moderate temperature, in which the glass flows like a thick liquid, and at high temperature the glass is fully melted. Viscosity measurement of the low- and high-temperature regimes is readily accessible through existing methods, but to date no methods are available to measure viscosity in the thick liquid regime. The focus of this research is to employ newly developed methods to measure viscosity of the thick liquid by rapidly heating the glass while measuring their elastic response. The data collected will be used to identify the fundamental parameters that control flow, develop and refine appropriate models for describing flow behavior, and improve processing techniques need for creating viable commercial products. This collaborative effort will be supported by faculty and graduate students at California State University, Northridge, and California Institute of Technology. Graduate students will mentor underrepresented summer high school and college interns, expanding their representation in the professional community and exciting them to pursue STEM careers. TECHNICAL DESCRIPTION: This proposal describes interdisciplinary and collaborative research addressing a conspicuous data gap in the rheological properties of bulk metallic glass alloys (BMGs). BMGs, with their outstanding hardness, toughness, strength and processability, show promise to combine the strengths of metals with the ease of plastic processing, and the potential for energy efficient, environmentally clean manufacturing. The gap in the rheological data exists in the supercooled liquid (SCL) region, between the glass transition and the crystallization temperatures. This data is pivotal in advancing the theory of glassy liquids, and directly impacts processability. We propose to measure the viscosity, and other thermodynamic properties, of a series of metallic glass compositions in this range using rapid temperature excursions driven by ohmic heating (rapid discharge forming, RDF). High-speed time-dependent deformation measurements of a sample under load will be used to determine viscosity variation with temperature. In separate experiments, the ultrasonic pulse-echo technique, coupled with high-speed infrared pyrometry, will be used to measure the elastic shear wave velocity variation with temperature. Data will be incorporated in existing glass rheology models, and new models will be developed as required. Application of the knowledge gained through these studies will be applied to the proof-of-concept processing of metallic glasses, making net-shape objects through thermoplastic forming, and further understanding of the microstructure/processing/properties relationship in glasses and glassy metals.