Co-based ternary bulk metallic glasses with ultrahigh strength and plasticity
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A family of ultrahigh strength Co-based bulk metallic glasses (BMGs) with critical diameters up to 2 mm is synthesized in Co65–xTaxB35 (at.%, x 5 5–10) alloys by copper mold casting. The improved glass-forming ability associated with near eutectic compositions is attributed to the appropriate addition of Ta. The glassy alloys exhibit high glass transition temperature of 930–975 K, ultrahigh compressive strength of 5.6–6.0 GPa, high specific strength of 639–654 Nm/g, Vickers hardness of 15–16 GPa, and distinct plastic strain of 0.5–1.5%. The strength and the specific strength are the highest values reported for bulk metallic materials known so far. Several universal criteria correlated with the thermal properties, elastic constants, and mechanical properties were validated in the Co-based BMG system. These Co–Ta–B BMGs combining with superior mechanical properties, high thermal stability, and simple elemental composition are significant for scientific research as modeling materials and industrial application as advanced structural materials.
I. INTRODUCTION
Compared with crystalline alloy materials, bulk metallic glasses (BMGs) exhibit higher strength and specific strength due to their disordered structures and structural uniformity.1–3 Intensive efforts including the optimization of alloy composition, enhancement of glass-forming ability (GFA), and improvement of mechanical properties have been made due to their great potential application as advanced structural materials.3–5 How to design desirable BMGs with superhigh mechanical strength has brought up extensive attention. It has been clarified that there exists universal linear relationships between strength and Young’s modulus (E) or shear modulus (G) for metallic glasses.6–8 Generally, elastic constants (M) of a designed glassy alloy are related to those of the constituent elements. It can be roughly calculated with an equation of M 1 5+ fi Mi1 , where Mi and fi denote the corresponding elastic constant and atomic concentration of the constituent elements, respectively.9,10 On the other hand, metallic glasses containing small-atomic size metalloid exhibit higher elastic moduli due to the formation of local solute-centered coordination polyhedra, which are the manifestation of intensive chemical short-range order caused by the strong covalent metal–metalloid bonds.11 Moreover, the elastic moduli and mechanical properties of metallic glasses can be improved by appropriately tuning the fraction of
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2011.187 2072
J. Mater. Res., Vol. 26, No. 16, Aug 28, 2011
http://journals.cambridge.org
Downloaded: 12 Oct 2015
metalloid elements.12 Therefore, it is advantageous to design a high-strength metallic glass consisting of metallic elements with high elastic moduli and metalloid elements with appropriate chemical interaction. Co-based BMG alloys with high GFA are among the best candidates to develop new structural metallic materials with ultrahigh st
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