Dependence of Ductility on Free Volume in a Cu‐Zr‐Based Metallic Glass

Abstract

Due to the absence of long-range order, bulk metallic glasses (BMGs) exhibit unique properties, such as high strength, high hardness, large elastic limit, good soft magnetic properties, and high corrosion resistance. [1,2] However, there is a fatal weakness that prevents wide application of BMGs: that is almost zero plasticity under tension and limited plasticity under compression. [3][4][5] Therefore, improving the plasticity of metallic glass is currently the subject of extensive study. Recent research has shown that impressive improvements in plasticity can be achieved for some Zr-, [6] Pd-, [7] Pt-, [8] Cu-Zr-. [9][10][11][12][13] Ti-based bulk metallic glasses (BMGs) under compression, in which the enhanced plasticity was attributed to the presence of small nanocrystals, [9][10][11] stress-induced nanocrystallization, [12,13] and a high Poisson ratio. [6][7][8] However, the mechanism is still not fully understood. The excellent plasticity of a Ti-based BMG sample with 1 mm diameter [14] and a Cu-based BMG [15] under compression were attributed to an increase in free volume introduced by high cooling rate and minor alloying, respectively. Moreover, many earlier investigations have shown that annealing imparts severe brittleness to the BMGs. [16][17][18][19][20][21] This is attributed to various factors including reduction in free volume, [16][17][18][19] precipitation of crystalline phases [20] and phase separation. [21] Of these mechanisms, the most prominent is the reduction in free volume that occurs during structural relaxation of metallic glasses, whereby free volume gets redistributed and annihilated, resulting in the loss of ductility. [16] There have also been several reports that the ductility of thermally embrittled amorphous ribbon samples could be recovered by thermal treatment [22][23][24] or neutron-irradiation. [25] Furthermore, Nagel et al. demonstrated that the annihilated free volume can be restored by heat treatment above the glass transition temperature (T g ). [26] Thus, detailed understanding of the effect of the free volume on the mechanical properties of BMGs, in particular ductility is still important from the applications point of view. In this paper, Cu 46 Zr 47 Al 7 BMG alloys with different free volume states were obtained through thermal treatments such as annealing and quenching. We demonstrate that the ductility of BMGs is closely related to the free volume and the ductility of thermally embrittled BMGs can be restored by thermal treatments.A Cu 46 Zr 47 Al 7 alloy (in at.%) was used in the current investigation. Plate with dimensions of 50 mm  20 mm  1.5 mm was prepared by arc melting, and the as-cast sample had the fully glassy structure. [27] The glass transition temperature T g and the onset temperature T x of the amorphous alloy have been reported to be about 696 and 769 K, respectively, at a heating rate of 0.33 k s À1 . [28] Different free volume states were achieved by different thermal treatment processes. The as-cast samples were sealed in an evacuated quartz capsul...

Ambient temperature embrittlement of a Zr-based bulk metallic glass

“…The smooth region in the tensile fracture surface is corresponding to the critical shear displacement for crack nucleation occurred in shear bands [29][30][31]. This shear displacement is, to some extent, proportional to the plastic deflection [32]. The variations of measured critical shear displacement with the test temperature were also added into Fig.…”

The Coupling Effect of Small Nanocrystals and Free Volume on the Ductility of Cu46Zr47Al7 Bulk Metallic Glass Alloy

“…Similarly, the cast, annealed, and quenched composite samples were named as sample B1, B2, and B3, respectively. It should be noted that, some experimental results of Cu-Zr-Al BMG in our previous paper [18] were given again at the goal of comparing with the present results of Cu-Zr-Al BMG matrix composites. Figure 1 shows the XRD patterns of the prepared Cu 46 Zr 47 Al 7 alloys.…”

“…In the early work of Inoue et al [23] and our previous work, [18] it has been suggested that the surface appearance consists of a smooth region and a cores and vein-like pattern region. The former was caused by shear sliding and the latter was caused by subsequent catastrophic failure.…”

“…It means that the annealing and quenching processes have not changed their fully glassy structure and A1, A2, and A3 are all fully amorphous glasses. [18] For the quenched sample B3 from 2.0 mm plate, there are small nanocrystals with size of about 5 nm in amorphous matrix as shown in the high resolution images in Figure 2(b). The inset (SAED) pattern clearly demonstrates the presence of the tiny nanocrystals in the amorphous matrix.…”

Effect of Alloying Elements with Positive Heat of Mixing on the Free Volume and Compressive Plasticity in ZrCoCuAl Bulk Metallic Glasses

“…32) For metallic glasses with different compositions, even through their free volumes are identical, they may have a distinct degree of metallic bonding and dissimilar plasticity. In metallic glass with the same chemical composition but different processing conditions such as quenching rate, 33,34) annealing 35,36) and mechanical pre-deformation, 37,38) a good correlation between the free volume content and the plasticity can usually be found. Contrasting with the findings in this work, it can be concluded that although free volume in metallic glass changes with chemical composition, free volume alone cannot fully describe the influence of the composition on the structure and the mechanical properties of metallic glasses with different compositions.…”