Difference in compressive and tensile fracture mechanisms of Zr59Cu20Al10Ni8Ti3 bulk metallic glass
Introduction
Bulk metallic glasses (BMGs) have many potential applications due to their unique properties, for example, superior strength and high hardness, excellent corrosion resistance and high wear resistance [1], [2]. However, the high strength of BMGs is often accompanied by remarkably little plastic deformation and their deformation and fracture mechanisms are quite different from crystalline materials [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. As is well known, there are several plastic deformation modes, such as slipping, shearing, kinking and twining, in crystalline materials [13], and yielding of most single crystals follows Schmid’s law. In general, single crystals often slide along the slip system with the largest Schmid factor. As a result, the yield stress and the angle, θ, between the slip plane and the stress axis can be calculated from the orientation of the single crystal.
In the past three decades, the deformation and fracture behavior of metallic glasses was widely investigated [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. In general, the plastic deformation of metallic glasses is localized in the narrow shear bands, followed by the rapid propagation of these shear bands and sudden fracture. Meanwhile, the following deformation and fracture behavior of metallic glasses was often observed. (1) Under compressive load, metallic glasses deform and fracture along localized shear bands and the fracture angle, θC, between the compressive axis and the shear plane is, in general, smaller than 45° (about 42°) [14], [15], [16], [17]. (2) Under tensile load, however, it is found that the tensile fracture angle, θT, between the tensile axis and the fracture plane is larger than 45°. In most cases, θT is in the range 50–65° with an average value of 56° [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. This indicates that the deformation and fracture of metallic glasses will not occur along the maximum shear stress plane irrespective of whether they are under compressive or tensile load. Donovan [9] has proposed a yield criterion for Pd40Ni40P20 metallic glass under compressive load. He found that the yield behavior of the glass follows a Mohr–Coulomb criterion rather than the von Mises criterion. Since the difference in the fracture angles θC and θT is quite large, however, there is no reasonable explanation for this phenomenon, which should be of special importance for a better understanding of the deformation mechanisms of metallic glasses. In the present work, we attempt to further reveal the basic deformation and fracture mechanisms through compressive and tensile tests of a Zr59Cu20Al10Ni8Ti3 BMG.
Section snippets
Experimental procedure
Master ingots with composition Zr59Cu20Al10Ni8Ti3 were prepared by arc-melting elemental Zr, Cu, Al, Ni and Ti with a purity of 99.9% or better in a Ti-gettered argon atmosphere. For reaching homogeneity, the master alloy ingots were re-melted several times and were subsequently cast into copper molds with different dimensions, i.e. 40 mm×30 mm×1.8 mm for tensile test specimens and 3 mm in diameter and 50 mm in length for the samples used for compressive tests. The amorphous structure of the
Stress–strain curves
Fig. 2(a) shows the compressive stress–strain curves of the metallic glass specimens at strain rates of 4.5×10−5 s−1 and 4.5×10−3 s−1. It can be seen that the metallic glassy samples display an initial elastic deformation behavior with an elastic strain of 1.5%, then begin to yield at about 1.45 GPa, followed by some strain hardening before fracture. The compressive plastic strains for the two specimens are 0.52 and 0.60%, respectively. Obviously, the metallic glass can deform with certain
Discussion
From the above observations, it can be concluded that the fracture processes of the metallic glass under compressive and tensile loading are significantly different. For comparison, two typical fractography morphologies induced by compressive and tensile fracture are shown in Fig. 5(a) and (b) again. First, all the above observations in Fig. 3(b)–(e) and Fig. 5(a) demonstrate that the compressive fracture surfaces only exhibit a vein-like structure with a rather uniform arrangement. This
Conclusions
- 1.
As in most metallic glasses, Zr59Cu20Al10Ni8Ti3 BMG displays a different deformation and fracture behavior under compressive and tensile loading. Under compression, the metallic glass displays some plasticity before fracture. The fracture angle, θC, between the stress axis and the fracture plane is 43°. Under tensile loading, however, the metallic glass always displays brittle fracture without yielding. The tensile fracture angle θT(=54°) between the stress axis and the fracture plane is
Acknowledgements
The authors would like to thank H. Grahl, H. Schulze, R.-H. Reiter and A. Schwab for sample preparation and H.-J. Klauß for assistance with the mechanical tests, and G. He, A. Güth for stimulating discussions. This work was supported by the German Science Foundation (DFG) under grant EC 111/10-1 and by the EU via the RTN-Network on bulk metallic glasses under contract HPRN-CT-2000-00033. One of the authors (Z.F. Zhang) wishes to acknowledge the Alexander von Humboldt (AvH) Foundation for
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