Elsevier

Intermetallics

Volume 14, Issue 12, December 2006, Pages 1411-1416
Intermetallics

Shear punch tests for a bulk metallic glass

https://doi.org/10.1016/j.intermet.2006.01.052Get rights and content

Abstract

A shear punch test technique was used for characterization of the mechanical properties of Zr–5Ti–17.9Cu–14.6Ni–10Al bulk metallic glass. The ultimate shear stress values matched very closely with values derived from uniaxial compression tests reported in the literature. This is consistent with a lack of pressure sensitivity in compression reported for this particular metallic glass. Deformation response was strain rate insensitive up to a critical rate, beyond which softening occurred. The latter was attributed to thermal heating effects.

Introduction

The mechanical properties and failure mechanisms of metallic glasses are controlled by inhomogeneous shear band deformation at low temperatures and homogeneous viscous flow deformation at temperatures above the glass transition [1], [2], [3], [4], [5], [6], [7], [8]. Metallic glasses have high strength, relatively low elastic modulus and are generally strain rate insensitive [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In tension and unconstrained compression, the initiation and propagation of a through-section shear fracture lead to early failure with little plastic strain. The plastic deformation of metallic glasses has been attributed to the nucleation of shear bands that accommodate the applied strain locally. This phenomenon can be observed in the form of load-curve serrations in compression and nanoindentation tests [7], [10], [13], [14], [15], [16], [17], [18]. In contrast to crystalline metals, the plastic flow stress of metallic glasses can be pressure sensitive. This leads to changes in the shear plane angle and an associated asymmetry in the uniaxial tensile vs. compressive flow stresses [2]. The pressure effect is attributed to free volume increase due to shear; the critical stress for shear band propagation is reduced/increased when a tensile/compressive normal stress component acts on the shear plane, as occurs in uniaxial tests [19]. Lowhapandu et al. [20] found inconsistency in this behavior for a Zr–Ti–Cu–Be bulk metallic glass because large shear plane angle changes occurred with no asymmetry in the flow stress. Other investigators have reported flow stress asymmetry in conjunction with changes in the shear plane angle. The intrinsic (zero pressure) plastic flow properties can be determined by tests that generate shear-dominant loading conditions, such as the shear punch test. This motivates the present study on the bulk metallic glass Zr–5Ti–17.9Cu–14.6Ni–10Al (denoted as BAA-11 [21]).

The shear punch test (SPT) technique has been used to obtain estimates of the tensile strength properties for polycrystalline metals [22]. We have recently applied it to strain rate sensitivity effects [23]. The SPT technique produces the equivalent of a stress–strain curve in shear loading. It is similar to the sheet metal blanking process in which shear deformation occurs combined with some degree of compression, tension and bending.

Section snippets

Experimental procedure

A 6.4-mm diameter BAA-11 rod was prepared at the Oak Ridge National Laboratory by a rapid casting technique [21]. An X-ray pattern for the as-received material is shown in Fig. 1. The broad peak at 38° confirms that an amorphous structure has been retained. The tests were done using the setup shown in Fig. 2a. A schematic of the test geometry and the idealized pure shear deformation zone is shown in Fig. 2b. A detailed description of the SPT procedure used here is given in Ref. [22]. For the

Results and discussion

The τ vs. δ/h curves for BAA-11 are shown in Fig. 3. The punch speeds for test conditions #1–6 were 0.424, 2.12, 4.24, 17, 33.9 and 84.7 μm/s, respectively. For clarity, the curves at different speeds are offset from the origin. The slight upward curvature during initial loading is due to punch seating onto the sample. The τ vs. δ/h curves show linear elastic loading behavior and an ultimate (maximum) stress point. Shear failure occurs at this point and the remainder of the curve is due to disk

Summary and conclusions

A shear punch test technique (SPT) was used to characterize the mechanical properties of Zr–5Ti–17.9Cu–14.6Ni–10Al bulk metallic glass. The ultimate shear stress values obtained at lower testing speeds agreed very well with the shear failure stress derived from uniaxial compression tests. This is consistent with the lack of compressive flow stress pressure sensitivity reported for this particular metallic glass. The agreement gives confidence that the SPT technique can be used to measure an

Acknowledgements

This research was supported by the National Science Foundation grant number DMR-0201474. The authors are indebted to Dr. C.T. Liu (ORNL) for providing the BAA-11 sample used for this investigation.

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