Ball indentation tests for a Zr-based bulk metallic glass
Introduction
Deformation of bulk metallic glasses differs fundamentally from that of crystalline metals because of the absence of long-range crystallographic order. For uniaxial tests, elastic deformation precedes catastrophic shear failure that often occurs within a few percent strain after yielding due to shear localization [1], [2], [3], [4]. The yield stress of bulk metallic glasses shows an asymmetry between tension and compression [3], with the latter value being about 10% higher for Zr52.5Ti5Cu17.9Ni14.6Al10 (denoted as BAA-11 [4]). The asymmetry in the yield stress is accompanied by deviations in the shear plane angle θ measured from the loading axis. This angle typically falls in the range of 50–60° for tension tests and 40–45° for compression tests [3]. These effects have been interpreted as a pressure dependence of the yield stress such that shear deformation is favored by a positive (tensile) component of normal stress acting on the shear plane. This would be consistent with an increase in free volume being associated with the shear mechanism [5]. The shear yield stress asymmetry and shear angle change have been explained using the Mohr Coulomb yield criterion, τc = τ0 − μσn, where τ0 is the intrinsic (zero pressure) shear yield stress, σn is the normal stress component (>0 for tension) on the shear plane and is the pressure sensitivity index [3], [5], [6]. A unique feature for metallic glasses is that μ is different in tension vs. compression. The shear angle values for BAA-11 tested in compression are conflicting. A value of θ = 45° was obtained in Ref. [4], indicating no pressure effect, whereas a value near 40° was reported in Ref. [7]. From both theoretical and application oriented viewpoints, it is important to establish the plastic flow properties of bulk metallic glasses. The purpose of the work reported here was to use an instrumented ball indentation (BI) test method recently developed in our research [8], [9] to determine the deformation response and possible pressure sensitivity of the bulk metallic glass BAA-11 under multiaxial compressive loading conditions.
Section snippets
Experimental procedure
The BAA-11 material was obtained from Oak Ridge National Laboratory in the form of a 6.4 mm diameter rod. X-ray diffraction confirmed that the fully amorphous structure was retained. Disk specimens with a thickness of 2 mm were cut from the rod and used for the tests. The top and bottom specimen surfaces were mechanically polished using successive polishing steps (600, 1200, 2400 and 4000 grit papers and 0.1 μm alumina slurry) to obtain flat, parallel mirror-like surfaces. Fig. 1 shows a schematic
Results and discussion
Fig. 2 shows typical results for a BI test on BAA-11 metallic glass. An intermittent loading, unloading and reloading sequence is used to obtain the load P vs. indentation depth h plot. The initial linear portion of each unloading line for a specified load point P is extrapolated to zero load to obtain the corresponding plastic indentation depth hp. The plastic indentation diameter dp (Fig. 1) is obtained using the relationsν is the
Summary and conclusions
BI tests made on Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass show that the deformation response is perfectly plastic, strain rate insensitive and pressure independent up to equivalent uniaxial plastic strains of 15%. In contrast, other metallic glasses show varying degrees of pressure sensitivity with compressive loading. The structural features that give rise to these differences require further study to identify their role in the shear mechanisms.
Acknowledgements
This work is supported by the National Science Foundation grant DMR-0201474. The authors are indebted to Dr. C.T. Liu (ORNL) for providing the BAA-11 metallic glass used for this investigation.
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