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

Abstract

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...

Shear transformation zone volume determining ductile–brittle transition of bulk metallic glasses

“…3a displays the flexural stress-deflection curves of as-cast and 15 min annealed samples (Group 1) under three-point bending tests at room temperature. The plastic deflection values were used to reflect their plastic deformation ability [19,23]. The plastic deflection decreased (Fig.…”

“…The smooth region corresponds to the critical shear displacement for crack nucleation occurring in shear bands [25][26][27]. This shear displacement is, to some extent, proportional to the plastic deflection [19,23].…”

Shear band multiplication aided by free volume underthree-point bending

“…12,39,40 Similarly, the ductile second phases such as Ta particle 41 and CuZr martensite 42 can act as obstacles to stop the shear band propagation and introduce stress concentration to initiate new shear bands in other sites. However, for the three-point bending, the shear band propagation can be efficiently stopped due to the existence of the stress gradient from the surface to the neutral plane, the factors such as free volume 21 and small nanocrystals, 43 which are beneficial to the shear band multiplication, will play important roles on the bending ductility. However, for most monolithic BMGs, the above mechanisms to halt the shear band propagation and initiate new shear bands on other sites under compression are not available.…”

Computer simulation of the matrix–inclusion interphase in bulk metallic glass based nanocomposites

“…The introduction of nanoparticles into bulk metallic glasses (BMGs) is a promising way to improve their mechanical properties [1][2][3][4][5][6][7][8][9][10][11]. This concerns especially the enhancement of the ductility.…”