A Laser Deposition Strategy for the Efficient Identification of Glass-Forming Alloys
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WHEN a molten metallic alloy cools quickly enough to bypass crystallization, the result is a metastable material known as metallic glass (MG). Similar to silicate glasses, MGs are amorphous alloys lacking longrange topological ordering in their atomic structure. However, unlike silicate glasses which are difficult to crystallize upon cooling, metallic glasses typically require high cooling rates to vitrify due to the nondirectionality of the metallic bonds and high diffusivities of the atomic species. Indeed, the first MG alloy was reported in 1960, synthesized with quench rates on the order of 105 to 106 K/s.[1] Since then, several multicomponent bulk metallic glass (BMG) alloys with substantially improved glass-forming ability (GFA) have been discovered, exhibiting critical cooling rates on the order of 1 to 100 K/s, or equivalently, centimeter-scale critical casting thicknesses.[2] The best reported glass former to date, Pd40Cu30Ni10P20, can be cast into fully amorphous rods with a 72-mm-cross section diameter.[3] Despite the steady discovery of new BMGs over the past two decades, relatively few monolithic alloys with casting thicknesses of 1 cm or larger have been identified. This slow progress can be attributed to the present inability to predict GFA within the vast composition space occupied by alloys that feature three or more components. Well-known indicators of GFA, such as the reduced glass transition temperature, are not truly predictive a priori since they require formation of a glass
PETER TSAI, Graduate Research Assistant, and KATHARINE M. FLORES, Professor, are with the Department of Mechanical Engineering and Materials Science, Institute of Materials Science and Engineering, Washington University in St. Louis, One Brookings Drive, Campus Box 1185, St. Louis, MO 63130. Contact e-mail: fl[email protected] Manuscript submitted December 31, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
in order to measure the glass transition temperature (Tg).[4–7] Furthermore, the predictive reliability of Tgbased parameters is inconsistent across different alloy systems.[8,9] In contrast, recent structural models predict glass-forming alloys with limited success by identifying compositions that correspond to efficiently packed configurations of the atomic species.[10–13] However, these models are mainly topological and do not adequately account for chemical contributions to GFA. Without a robust means to predict GFA, investigation of glass formation still relies heavily on experimental trial and error methods. Experimental efforts aimed at exploring glass formation in multicomponent metallic systems conventionally involve casting discrete compositions, followed by application of various diffraction techniques in order to verify amorphous structure in the cast specimen.[14] Although reliable, this one-at-a-time trial and error approach is impractical for interrogating the extensive composition space of multicomponent alloys, motivating the recent developments by several groups toward a combinatorial paradigm for BMG discov
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