Materials Science and Engineering A 449–451 (2007) 111–113
Ductile Zr-base bulk metallic glass D.H. Bae a,∗ , S.W. Lee a , J.W. Kwon a , X.D. Wang b , S. Yi b a
Department of Materials Science and Engineering, Yonsei University, Seoul, Republic of Korea Department of Materials Science and Metallurgy, Kyungpook National University, Daegu, Republic of Korea Received 23 August 2005; received in revised form 27 December 2005; accepted 15 February 2006
Abstract Bulk metallic glasses have a frozen-liquid structure comprising multi-component elements. Plastic deformation of bulk metallic glasses is highly localized into shear bands. As the shear band propagates, excess free volume is generated and spontaneously coalesced, leading to the formation of nanometer-scale voids. The voids evolve to be cracks that immediately cross the whole sample area causing a catastrophic fracture even under compression. Therefore, to prohibit the catastrophic failure, atomic clustering kinetics should be retarded. Here, we report a new monolithic zirconium-base metallic glass that is highly deformable without global failure under compression. Due to high thermal activation energy for the atomic movement, the crack formation attributed to void coalescence within shear bands can be prevented exhibiting extraordinarily high ductility under compression. © 2006 Elsevier B.V. All rights reserved. Keywords: Metallic glass; Ductility; Crystallization; Mechanical properties
1. Introduction Bulk metallic glasses having liquid-like atomic structure [1,2] show, approximately, elastic–perfectly plastic behavior with an extended region of elastic strain (around 2%) [3,4]. Plastic deformation process of bulk metallic glasses under quasi-static compressive loading includes the formation of shear bands along the shear plane, which rapidly propagate across the sample [5,6]. This behavior results in a very limited global plastic strain (typically 0–2%). Within shear bands containing a relatively high free volume concentration , nanometer-scale voids are developed through the local atomic movement enhanced by adiabatic heating and the voids progressively coalesce to be cracks [8,9]. The immediate propagation of the cracks along the shear band leads to the catastrophic fracture frustrating the bulk metallic glasses as viable engineering materials. In this paper, through systematic alloy designing techniques for Zr-base metallic glasses, we present a new zirconium-base metallic glass, Zr61.7 Al8 Ni13 Cu17 Sn0.3 , that exhibits no macroscopic failure under compression at ambient temperature, while most bulk metallic glasses deform in a brittle manner as observed in Z41 Ti14 Cu13 Ni10 Be22 (Vit1) (shown later). The underlying ∗
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mechanism of exceptional plasticity is investigated in terms of structural characteristics and atomic movement kinetics. 2. Experimental The master alloy was prepared by an arc-melting technique under a Ti-gettered Ar atmosphere using high-purity elements (99.8% Zr, 99.99% Al, 99.99% Ni, 99.99% Cu and 99.99% Sn). The master alloy was remelted several times to ensure even distribution of the alloying elements, and then cast into amorphous rods of up to 4 mm in diameter by the suction casting method. A differential scanning calorimeter (Perkin-Elmer, DSC7) was used for both continuous heating (heating rate = 20 K/min) and isothermal annealing experiments to investigate thermal properties as well as crystallization kinetics of the amorphous rods of 1 mm in diameter. For isothermal experiments, specimens were heated to desired temperatures with a fast heating rate (80 K/min) to minimize structural relaxation during heating and then isothermal annealing was performed between the glass transition and the onset temperature of crystallization. In addition, phase identification of the annealed specimens was carried out using X-ray diffractometry (Rigaku, CN2301) with monochromatic Cu K␣ radiation source. The morphology and surface of the deformed samples were observed by secondary electron microscopy (SEM, Hitachi S2700, 10 kV). We also
D.H. Bae et al. / Materials Science and Engineering A 449–451 (2007) 111–113
used high-resolution transmission electron microscopy (HREM, JEOL, 4000EX) to examine the section of the metallic glass rods. Thin foil specimens for electron microscopy were carefully prepared by ion beam milling method. For mechanical property measurements, themomechanical analyses were conducted with the heating rate of 20 K/min up to the temperature of 823 K under the constant stress of 320 kPa for the amorphous rods of 2 mm in diameter. Uniaxial compression tests were conducted for the samples of 1 mm in diameter with the aspect ratio of 2:1 at room temperature at an initial strain rate of 1 × 10−4 s−1 with an interrupted manner. 3. Results and discussion The alloy shows yield stress 1.9 GPa, followed by permanent plastic deformation with an almost constant stress level (calculated engineering stress is also plotted by a dash line with an assumption of perfectly plastic behavior) (Fig. 1). A large number of shear bands are developed at an angle of approximately 45◦ to the loading axis and between them branching of shear bands occurs redistributing the localized plastic strain (Fig. 2(a)). The serration in the plastic deformation curves is attributed to the propagation of shear bands that causes load drops  (Fig. 1). However, geometrical instability becomes prevail in the strain range of 17–42% (inserted image in Fig. 2(a)). One of the large shear bands slides macroscopically lowering the flow stress level to about 1 GPa. After the shear band touches the specimen holder, the load increases again due to a continuous increase of load-carrying area. Since the aspect ratio of the sample is less than 1.4:1 above ∼25% strain (inserted image in Fig. 2(b)), the newly formed shear bands cannot propagate across the entire diameter of the sample. The serration observed in the high strain region (Fig. 1) is a strong evidence to support that the specimen still deforms with the continuous formation of shear bands (Fig. 2(b)). Surprisingly, the alloy never shows any fatal failure unlike in the case of most bulk metallic glasses.
Fig. 2. SEM images on the surface of the 20% strained sample (a) and 65% strained sample (b) for the Zr61.7 Al8 Ni13 Cu17 Sn0.3 metallic glass in which the appearance of the deformed samples are inserted, respectively.
No failure indicates that any critical cracks, which would be formed by the coalescence of excess free volume, are not developed within shear bands during the whole deformation process of the alloy. Indeed, the heavily deformed sample releases the same amount of heat as that of the undeformed one during crystallization (H values in Table 1), demonstrating that atomic clustering induced by atomic configurational change during deformation is trivial. Also, in the study of transmission electron microscopy, we also do not observe any long-range ordering within shear bands. Based upon the kinetic analysis results, the thermal activation energy for crystallization is 2.8 times higher than that of Vit1 (Fig. 3). Furthermore, the Table 1 Thermal properties of Zr61.7 Al8 Ni13 Cu17 Sn0.3 for as-cast and 65% deformed samples
Fig. 1. Engineering stress–strain curves of Z41 Ti14 Cu13 Ni10 Be22 (Vit1) and Zr61.7 Al8 Ni13 Cu17 Sn0.3 metallic glasses under uniaxial compression at room temperature (calculated engineering stress is plotted by a dash line).
Zr61.7 Al8 Ni13 Cu17 Sn0.3 As-cast 65% strained
Tg , glass transition temperature; Tx , crystallization temperature; H, the amount of heat flow.
D.H. Bae et al. / Materials Science and Engineering A 449–451 (2007) 111–113
be the reason of large plasticity . However, all materials mentioned above fail catastrophically and it raises a serious reliability issue for bulk metallic glasses as structural materials. However, as demonstrated with the new bulk metallic glass Zr61.7 Al8 Ni13 Cu17 Sn0.3 , the failure of it can be effectively prevented by significantly retarding the atomic movement kinetics. 4. Conclusions
Fig. 3. Temperature dependence of the rate of 50% crystallization (1/t0.5 ) of Vit1 and Zr61.7 Al8 Ni13 Cu17 Sn0.3 . The calculated thermal activation energy (Q) for crystallization of Zr61.7 Al8 Ni13 Cu17 Sn0.3 is 2.8 times higher than that of Vit1.
compressive strain rate of the alloy measured under constant stress in the supercooled liquid region is much lower than that of Vit1 (Fig. 4). Therefore, kinetic and thermal analysis results consistently confirm the fact that atomic rearrangement in the alloy is very sluggish compared with that in Vit1. For the purpose of developing reliable structural bulk metallic glasses, several attempts have been made to enhance ductility. A variety of composite materials have been synthesized by introducing a second phase as crystalline phase to generate multiple shear bands and to prevent the propagation of shear bands through the sample [10,11]. In addition, by small changes in composition of fully disordered alloys, greater elongation to failure is achieved probably due to the effects of strong branching of the shear bands . Furthermore, a large Poisson’s ratio together with a low glass transition temperature may
A new highly deformable Zr–Al–Cu–Ni–Sn BMG reliable for structural applications is developed through alloy designing techniques for sluggish atomic movement kinetics. The metallic glass deforms with the formation of abundant shear bands and exhibits no catastrophic failure. The thermal activation energy of the Zr61.7 Al8 Ni13 Cu17 Sn0.3 alloy for 50% crystallization is around 3.96 eV, which seems to be very high. At the same time, the dynamic mass flow behavior investigated under constant stress shows the very sluggish atomic movement kinetics. Therefore, it appears that the exceptional ductility of the Zr61.7 Al8 Ni13 Cu17 Sn0.3 BMG is closely related to the sluggish atomic movement kinetics, which prevents voids formation under applied loading. Also, the deformed sample releases the same amount of heat as that of the undeformed one, demonstrating that atomic configurational changes after deformation are trivial for the BMG system. Therefore, the catastrophic failure of bulk metallic glasses can be effectively prevented by significantly retarding the atomic movement kinetics together with the formation of abundant shear bands. Acknowledgement This work was financially supported by MOCIE (Ministry of Commerce, Industry and Energy) under the project named development of structural metallic materials and parts with super strength and high performance. References           
Fig. 4. Themomechanical thermogram determined by heating with 20 K/min under compressive stress of 320 kPa in the supercooled liquid region for Vit1 and Zr61.7 Al8 Ni13 Cu17 Sn0.3 . The compressive strain rate of Zr61.7 Al8 Ni13 Cu17 Sn0.3 is much lower than that of Vit1.
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