Effects of Superimposed Pressure on Flow and Fracture of Two Bulk Amorphous Metals
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Effects of Superimposed Pressure on Flow and Fracture of Two Bulk Amorphous Metals Paul Wesseling, Peravudh Lowhahphandu and John J. Lewandowski Case Western Reserve University, Department of Materials Science and Engineering 10900 Euclid Avenue, Cleveland, Ohio 44106, U.S.A. ABSTRACT Tension and compression tests have been performed with high alignment fixtures on two different zirconium-based bulk metallic glass specimens at atmospheric pressure as well as with levels of superimposed hydrostatic pressure up to 700 MPa. The results show essentially no difference in applied flow stress or fracture stress between compression and tension specimens over the range of pressures tested. However, a difference in fracture plane angle between compression and tension specimens was observed, suggesting a normal stress effect, as in granular solids, on the flow and fracture behavior of the bulk metallic glass specimens. The data are compared to various flow and fracture theories over a very wide range of normal stresses. Over the range tested, a Mohr-Coulomb flow theory appears to best describe the data. INTRODUCTION The flow and fracture of crystalline metals have been extensively studied under a wide variety of different stress states. Changing the stress state provides a useful way to investigate the dependence of flow and fracture on the level of triaxiality present. The classic works of Bridgman [1,2] illustrated that flow in most cubic crystalline metals is essentially pressure independent, as reviewed recently [3]. However, the effects of changes in stress state on fracture behavior depend highly on the mechanism(s) of fracture and any dilation which may accompany the fracture process [3]. The purpose of this paper is to report on continuing work [4-6] where the effects of changes in stress state on the flow and fracture of amorphous metals are being determined. EXPERIMENTAL PROCEDURES The materials used in the present investigation were two Zr based bulk metallic glasses, Vitreloy I and Zr-Ni-Cu-Al. The compositions of the two different glasses are given in Table I. The Vitreloy I specimens were tested both in tension and compression. The Zr-Ni-Cu-Al specimens were only tested in compression. Table 1. Composition of metallic glass tested (at%). Ti Ni Al Cu Vitreloy I 12.5 9.4 12.0 Zr-Ni-Cu-Al 9.0 10.0 18.0
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Zr Balance Balance
The Vitreloy I compression specimens were nominally 3.1 mm in diameter and 6.2 mm in length, providing a nominal length-to-diameter aspect ratio of 2 to 1 as recommended by the ASTM E8-E9 standard for testing of high-strength material. The Zr-Ni-Cu-Al specimens were nominally 2.8 mm in diameter and 5.6 mm in length. Initially, compression tests were conducted
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with a high-alignment compression cage under displacement control mode at 10-3 s-1, using a 50 kip MTS servo-hydraulic testing machine. Additional experiments were conducted with a high pressure deformation apparatus with the same high-alignment compression cage. It has been shown that the high alignment fix
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