Ti 5 bulk metallic glass (BMG) alloy samples in both rod and plate geometry were prepared. Different free volume states were obtained through thermal treatment. The plastic deformation ability of the BMGs was investigated through both a three-point bending test and compression test. The three-point bending results reveal the important role of free volume content on the formation of multiple shear bands, as the shear band propagation can be efficiently stopped due to the existence of the stress gradient from the surface to the neutral plane. In compression, the sample size rather than free volume controls the shear banding behavior.
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...
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, monolithic BMGs usually present no yielding and strain hardening during room-temperature deformation due to highly localized shear bands, which significantly limits the range of possible applications. Recent works show that impressive improvements in plasticity can be achieved for some Zr-, [3] Pd-, [4] Pt-, [5] and Cu-Zr [6][7][8][9][10][11][12][13][14] -based BMGs. In particular, Cu-Zr (or Zr-Cu)-based BMGs such as exhibited large compressive plastic strains. The presence of small nanocrystals [6][7][8][9]13,15] and the deformation-induced nanocrystallization [10,14] have been given for factors contributing to the intrinsic ductility of these Cu-Zr-based BMGs, but the mechanism is still not fully understood. At the same time, the excellent plasticity of Ti-based BMG sample with 1 mm diameter [16] and Cu-Zr-based BMG [17] with 2 mm diameter [14] under compression are also attribute to larger amount of free volume introduced by high cooling rate and minor alloying, respectively. Since these as cast Cu-Zr-based BMGs [6][7][8][9][10][11][12][13][14][15] usually have small size in diameter of 1-3 mm, they should contain larger amount of free volume after casting. Then, the enhanced plasticity of Cu-Zr-based alloys might also be concerned with the free volume during the casting. Furthermore, in our recent paper, we have clearly demonstrated the great effect of free volume on the ductility of Cu 46 Zr 47 Al 7 alloy with fully glassy structure. [18] Therefore, the goal of the present work is to clarify the effects of both small nanocrystals and free volume on the plasticity of Cu-Zr-based BMG alloys. In this paper, Cu 46 Zr 47 Al 7 BMG and its composite containing small in situ precipitated nanocrystals were prepared through copper mold casting. Different free volume states were obtained through thermal treatment such as annealing and quenching. Three-point bending tests for these alloys investigated were undertaken to evaluate their plastic deformation ability. It is found that both the presence of small nanocrystals and free volume play important roles in the ductility of Cu-Zr-based BMG and its composites. Furthermore, the coupling effects of small nanocrystals and free volume should be responsible for the enhanced ductility of Cu-Zr-based BMGs.In this study, a Cu 46 Zr 47 Al 7 (in at.%) in plate with dimensions of 50 Â 20 Â h mm À3 (h ¼ 1.5 and 2) were prepared and different thickness h leads to different cooling rates upon solidification for the casting plates. The as cast 1.5 mm plate has the fully glassy structure and the 2.0 mm plate is composite consisting of main glassy structure and some small in situ precipitated nanocrystals with size of 5-20 nm. [15] Different free volume states were achieved by different thermal treatment techniques. [18] The cast samples were sealed in an evacuated quartz capsule and annealed for 9...
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