477727 Planar-Flow Processing of Glassy Metals: Ductility-Thickness Correlation

Monday, November 14, 2016
Grand Ballroom B (Hilton San Francisco Union Square)
Andrew Lepore and Joseph Hassler, Chemical Engineering, Cornell, Ithaca, NY

Planar-flow processing of glassy metals: ductility-thickness correlation

Molten metal alloys freeze to amorphous atomic structures provided they are cooled sufficiently rapidly. Glassy or amorphous metals are important in today’s world since they enable energy conversion devices that are ultra-efficient. When these metals are produced by planar flow spin casting, an added benefit is that production has low energy and capital costs. Planar-flow spin casting can achieve high enough cooling rates to yield glassy metals by casting thin enough ribbon. Insufficient cooling leads to sub-regions with nano- or micro-crystalline structures within the amorphous background, however. In this study, we report a correlation of the brittle/ductile nature of the ribbon with ribbon thickness and processing conditions. The relationship to amorphous/non-amorphous atomic structure is also reported. Connecting macroscopic mechanical properties to atomistic structure through processing conditions may yield process improvements, product enhancements and materials for new applications. 

During planar-flow spin casting, an applied overpressure forces molten metal through a nozzle into a thin gap between the nozzle and a rotating wheel, forming a liquid bridge referred to as the puddle region. The wheel acts as a heat sink, cooling the molten metal at rates approaching 106 K/s while maintaining production rates on the order of tens of meters per second. This provides the potential for unparalleled nanostructure control, with the ability to incorporate fine features such as crystals into an otherwise amorphous ribbon, influencing both the physical and electromagnetic properties of these glassy metals. Through varying the overpressure, wheel speed, and gap height, casts successfully produced ribbons with either completely glassy or partially crystalline structures. These ribbons show positive correlations between ribbon thickness, temperature, and embrittlement. Such findings suggest the existence of a critical boundary where the onset of embrittlement occurs at a ribbon thickness of 50-70 microns and a ribbon surface temperature of 300-360°C after leaving the puddle region. Intuition may suggest fewer crystalline features at the wheel side of the ribbon, but experiments show that this may not necessarily be true. A potential explanation is a “secondary quench” in which a longer contact duration between the ribbon and the wheel causes a prolonged cooling of the ribbon. Further study into the correlation between macroscale qualities: thickness, ribbon temperature, and location of ribbon departure from the wheel, and microscale properties, such as ductility and crystallinity will yield a higher level of control over the planar-flow spin casting process.

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