Origin of the Simultaneous Improvement of Strength and Plasticity in Ti-based Bulk Metallic Glass Matrix Composites
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Jae-Chul Lee Division of Materials Science and Engineering, Korea University, Seoul 136-701, Korea
Do Hyang Kim Center for Non-crystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea (Received 25 February 2005; accepted 19 May 2005)
W-rich particle-reinforced Ti-based bulk metallic glass (BMG) matrix composites with a compressive strength approaching 3 GPa and a fracture strain of approximately 12% were developed. In contrast to most existing BMG matrix composites, in which the improved ductility was obtained only at the expense of the strength, the composites developed in this study exhibited a significant enhancement in their strength, as well as an improvement in the plasticity. This improvement in the plasticity was attributed to the blocking and circumscription of the shear band propagation, leading to the formation of a large number of shear bands. Using a classical elasticity theory of inclusions, the improvement of the strength was interpreted as resulting from the generation of tensile residual stresses in the matrix due to the difference in the coefficient of thermal expansion between the W-rich particles and the BMG matrix.
I. INTRODUCTION
Compared with the crystalline materials, due to inherently different atomic structures of the amorphous phase, bulk metallic glasses (BMGs) exhibit exotic properties, such as high tensile strength, high corrosion resistance, and good soft magnetic properties, etc. Extensive studies on the mechanical properties of these materials have shown that, although BMGs exhibit superplastic behavior in the supercooled liquid region, the absence of an adequate amount of plasticity at low temperature inhibits their widespread application as structural components. At temperatures below the glass transition temperature, BMGs exhibit catastrophic failure, due to localized deformation on the dominant shear band.1 One way to reduce the brittleness of BMGs is to produce a composite microstructure, consisting of a crystalline phase embedded in an amorphous phase. The role of the crystalline phase dispersed uniformly in the amorphous matrix is to act as an obstacle to sudden propagation of the shear band, leading to the formation of a large number of shear bands throughout the specimen and thereby enhancing plasticity.2–4 These amorphous matrix composites can be a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0315 2474
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 11 Mar 2015
obtained in various ways, including partial devitrification of as-cast metallic glasses, the mechanical alloying of elemental powders blended with insoluble particles, and the precipitation of a dendritic crystalline phase from the melt during casting.4–8 Recent publications have described a successful increase of plastic deformation, but one that was generally accompanied by a decrease in the yield stress and/or ultimate strength.7,8 BMGs have been produced in a number of alloy sys
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