Deformation and crystallization of Zr-based amorphous alloys in homogeneous flow regime
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Atul H. Chokshi Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India
Robert D. Conner Department of Manufacturing Systems Engineering and Management, California State University–Northridge, Northridge, California 91330
Guruswami Ravichandran Graduate Aerospace Laboratories, California Institute of Technology, Pasadena, California 91125
William L. Johnson Keck Laboratories of Material Science, California Institute of Technology, Pasadena, California 91125 (Received 11 July 2009; accepted 4 January 2010)
The purpose of this study is to experimentally investigate the interaction of inelastic deformation and microstructural changes of two Zr-based bulk metallic glasses (BMGs): Zr41.25Ti13.75Cu12.5Ni10Be22.5 (commercially designated as Vitreloy 1 or Vit1) and Zr46.75Ti8.25Cu7.5Ni10Be27.5 (Vitreloy 4, Vit4). High-temperature uniaxial compression tests were performed on the two Zr alloys at various strain rates, followed by structural characterization using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Two distinct modes of mechanically induced atomic disordering in the two alloys were observed, with Vit1 featuring clear phase separation and crystallization after deformation as observed with TEM, while Vit4 showing only structural relaxation with no crystallization. The influence of the structural changes on the mechanical behaviors of the two materials was further investigated by jump-in-strain-rate tests, and flow softening was observed in Vit4. A free volume theory was applied to explain the deformation behaviors, and the activation volumes were calculated for both alloys. I. INTRODUCTION 1,2
The inelastic deformation of bulk metallic glass is fundamentally different from that of crystalline solids because of the lack of long-range order in the atomistic structures. It is a complicated process with intrinsic structural instability of the material and the interaction between its thermomechanical behavior and microstructural changes. Based on extensive experimental results for a wide range of metallic glasses including Zr-based alloys,3–5 La-based alloys,6,7 Fe-based alloys,8,9 and Pdbased alloys,10–12 three distinctly different deformation modes can be observed and described using their empirical flow characteristics: shear localization, occurring at room temperature and with high strain rate and featuring limited plasticity accumulated in localized thin shear bands; non-Newtonian flow, occurring at some moderate temperature in the vicinity of glass transition temperature (Tg) of the material and featuring a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0134 J. Mater. Res., Vol. 25, No. 6, Jun 2010
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stress over- and undershooting before steady-state stress is reached; and Newtonian flow, occurring in supercooled liquid regime of the material (above Tg and below crystallization temperature Tx) and featuring steady-state flow once yield starts. The firs
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