Homogeneous flow of bulk metallic glass composites with a high volume fraction of reinforcement
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Y. Li Singapore–Massachusetts Institute of Technology (MIT) Alliance, National University of Singapore, Singapore 119260; and Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576
C.A. Schuha) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 3 January 2007; accepted 29 January 2007)
We present a systematic study of homogeneous deformation in a La-based bulk metallic glass and two in situ composites based on the same glass. In contrast to prior investigations, which focused on relatively dilute composites, in this work the reinforcement volume percentages were more concentrated at 37% and 52%—near or above the percolation threshold (35–40%). Hot uniaxial compressive testing was conducted over a wide strain rate range from 10−2 to 10−5 s−1 at a temperature near the glass transition. For such concentrated composites, the homogeneous deformation behavior appeared to be dominated by the properties of the reinforcement phase; in the present case the La reinforcements deformed by glide-controlled creep. Post-deformation analysis suggested that bulk metallic glass matrix composites were susceptible to microstructural evolution, which appeared to be enhanced by deformation, in contrast with a stress-free anneal. Consequently, unreinforced bulk metallic glass appeared to be more structurally stable than its composites during deformation near the glass transition.
I. INTRODUCTION
Since the first metal alloy was vitrified by cooling from the melt in 1960,1 amorphous metals have been developed to the point where centimeter-scale billets can be cast in Pd-, La-, Zr-, Mg-, and Fe-based systems.2–7 These bulk metallic glasses (BMGs) have been widely considered as potential structural engineering materials. However, most BMGs display very limited plasticity and can fail catastrophically during mechanical loading once deformation has localized into a shear band.8–12 To improve their toughness and capability for plastic flow, second phases are sometimes added into a BMG matrix to slow or deflect the propagation of shear bands.13–22 Recently, significant progress has been made in the development of an in situ class of bulk metallic glass matrix composites (BMGMCs), in which the reinforcement is precipitated within the glass matrix during cast-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0191 1564 J. Mater. Res., Vol. 22, No. 6, Jun 2007 http://journals.cambridge.org Downloaded: 15 Mar 2015
ing. Several papers have demonstrated methods to control the volume percentage of second phases in such in situ BMGMCs by tailoring the content of minority alloying additions.22–27 The ability to control the second phase distribution in BMGMCs makes it possible to systematically investigate mechanical properties of BMGMCs with different microstructures. For example, Lee et al.22 performed a systematic study of room-temperature mechanical behavior in La
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