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Katharine FloresAssistant ProfessorPh.D., Stanford University, 2000 Tel. (614) 292-9548 Office: 489 Watts Hall |
Dr. Flores' primary research interest is the mechanical behavior of structural materials. Her work spans from the study of the fatigue behavior of high temperature intermetallics for turbine applications to the characterization of the failure mechanisms of bicycle helmet materials and the development of new techniques to measure the mechanical behavior of structural biomaterials. She has an active research program on bulk metallic glasses, with projects ranging from material characterization to the development of joining and manufacturing techniques in and effort to accelerate the incorporation of these novel materials in mainstream and high performance applications.
Bulk Metallic Glasses
Bulk metallic glasses represent a revolutionary new class of engineering materials with potential applications ranging from automotive and aerospace structures to biomedical devices and sporting goods. They exhibit an impressive array of properties, including extraordinary tensile strengths (~1-2 GPa), and large elastic deflections (~2% elastic strain). Furthermore, the inherent ductility of these alloys results in impressive room temperature fracture toughness values of ~15-20 MPa m1/2, an order of magnitude higher than traditional glasses and high strength ceramics. Beyond opening the door for high strength applications, this combination makes BMGs excellent mechanical energy storage materials (e.g. springs). In fact, the first commercial application of these materials was in golf club heads!
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Computer simulation of the disordered atomic structure of a three-component metallic glass. |
Metallic glasses are a class of metallic alloys in which the crystallization process has been "frustrated" in some way, to the extent that a liquid-like, disordered long range atomic structure exists at temperatures well below the melting temperature of the alloy. Unlike the early metallic glasses which formed only under rapid solidification conditions (>1000's K/s), the highly stable bulk metallic glass alloys form at unusually low critical cooling rates (~1-10 K/s) and require no special processing to obtain their extraordinary mechanical properties. Centimeter scale components may be manufactured using conventional methods, such as extrusion and die casting. Because the glasses do not change phase upon solidification, there is little shrinkage and thus complex structures may be cast very near net shape, reducing the need for expensive machining of the final part. The stability of the BMGs against crystallization in the supercooled liquid state also opens the door for the use of inexpensive processing techniques such as thermoplastic forming, previously only applicable to plastic materials.
The lack of long range atomic order in these alloys dictates their unique mechanical behavior and presents a challenge for understanding the relationships among processing, structure, and properties in BMGs within a consistent physical framework. Defects which facilitate plastic flow in crystalline materials, such as dislocations or grain boundaries, are not present. Instead, flow occurs via a diffusional mechanism. Without microstructural features to direct and distribute the flow, intense shear bands associated with a localized decrease in glass viscosity form and propagate seemingly unimpeded through the material. The potential for catastrophic failure associated with the rapid propagation of shear bands is a concern for the utilization of BMG alloys in structural applications. Strain localization and shear band propagation are particularly problematic under tensile stress states where failure may occur along a single shear plane with very limited, if any, measurable plastic deformation.
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SEM image of shear steps formed by the propagation of highly localized shear bands during rolling of a bulk metallic glass specimen. |
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SEM image of a tensile failure surface produced at a high strain rate. The smeared voids and droplets are indicative of the significant material softening and viscous flow within the shear band. |
In order for BMGs to be used in structural applications, failure due to the formation and propagation of a single dominant shear band must be prevented. Taking advantage of the stability of bulk glass formers against crystallization, numerous BMG matrix composites have been developed utilizing a variety of crystalline reinforcement phases to block shear band propagation. However, very little has been done to optimize the microstructure and mechanical behavior of these composites. Our research focuses on the development and characterization of bulk metallic glasses and their composites, as well as gaining a fundamental understanding of the metallic glass structure and the micromechanisms of plastic flow. We use a variety of techniques in this effort, including mechanical testing, microscopy, calorimetry measurements, and positron annihilation spectroscopy.
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SEM image of a crack tip damage zone in a BMG matrix composite reinforced with a ductile crystalline phase that precipitated in situ during solidification. The damage zone consists primarily of shear bands constrained by the reinforcement phase. |
Dr. Flores received her Bachelor of Science Degree in Mechanical Engineering at Washington University (1995), where she became interested in the relationship between material behavior and mechanical design. She went on to earn her Master of Science Degree (1997) and Ph.D. (2000) in Materials Science and Engineering at Stanford University. Her doctoral thesis was entitled "Fracture and Deformation of a Zirconium-Based Bulk Metallic Glass." She stayed at Stanford as a Postdoctoral Scholar and was the first Director of the Sports Materials Laboratory, where she directed the industry-sponsored research of undergraduate and masters level students. She joined the faculty at OSU in October of 2002.
Her research on bulk metallic glasses has recently been recognized with several invitations for conference presentations, including a presentation at the Third International Conference on Bulk Metallic Glasses in Beijing, China in October 2003. She teaches the undergraduate introductory course on materials science, as well as undergraduate and graduate courses on mechanical behavior and fracture mechanics.
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| Flores Research Group Back row: Matt Levine (undergrad, IWSE), Nick Hutchinson, Kathy Flores, Hongqing Sun Front row: Jeanne Skebo (undergrad, Chem Eng), Ashwini Bharathula, Clarissa Yablinsky |
