An electron microscopy appraisal of tensile fracture in metallic glasses

Three glass-forming alloy compositions were chosen for ribbon production and subsequent electron microscopy studies. In situ tensile testing with transmission electron microscopy (TEM), followed by ex situ TEM and ex situ scanning electron microscopy (SEM), allowed the deformation processes in tensile fracture of metallic glasses to be analysed. In situ shear band propagation was found to be jump-like, with the jump sites correlating with the formation of secondary shear bands. The effect of structural relaxation by in situ heating is also discussed. Nanocrystallization near the fracture surface was observed; however, no crystallization was also reported in the same sample and the reasons for this are discussed. Both the TEM and the SEM observations confirmed the presence of a liquid-like layer on or near the fracture surface of the ribbons. The formation of a liquid-like layer was characterized by the vein geometries and vein densities on the fracture surfaces and its dependence on shear displacement, δ, is discussed. A simple model is adapted to relate the temperature rise during shear banding to the glass transition and melting temperatures and this is used to explain the variety of fracture surfaces which are developed for macroscopically identical tensile testing of metallic glasses together with features which exhibit local melting.     

An instability index of shear band for plasticity in metallic glasses

Catastrophic failure along a dominant shear band sets the limit on how much plasticity can be achieved in metallic glasses (MGs) under uniaxial compression. Here we show that this instability process is governed by a single system parameter, called the shear-band instability index (SBI), which is proportional to sample size and inversely proportional to machine stiffness. We provide extensive experimental proof of this concept by conducting a series of tests with a range of controlled values of sample size and machine stiffness. The theory of SBI has led us to a more comprehensive understanding of the mechanisms of plastic deformation in MGs via simultaneous operation of multiple shear bands versus a single dominant one. This concept provides a theoretical basis to design systems which promote plasticity/ductility in MGs by suppressing or delaying shear-band instability.     

Improved plasticity and fracture toughness in metallic glasses via surface crystallization

Mechanical crystallization was induced in the monolithic bulk metallic glasses by surface mechanical attrition treatment (SMAT) to create the isolated crystallite islands in the top surface layer. Inside the isolated crystallite islands, microstructure consists of the crystallites with a gradient grain size evolution and the residual amorphous phase. Moreover, isolated crystallite islands, acting as the obstacles to restrict the highly localized deformation of shear bands/cracks, effectively limit the shear bands extension, suppress the shear bands opening, and avoid the cracks developing, which significantly enhance the overall plasticity and fracture toughness. They were suggested by the secondary shear bands in the glassy matrix and fine-shearing together with micro-cracking inside the isolated crystallite islands. Finally, the improved plasticity and fracture toughness were systematically discussed. Based on the current results, surface crystallization is proposed to optimize the mechanical properties of metallic glasses.     

具有网络结构的Zr-Al-Ni-Cu块体金属玻璃的塑性和断裂模式转变

研究网络结构对Zr?Al?Ni?Cu块体金属玻璃的塑性和断裂模式的影响。采用化学腐蚀法显示并用SEM观察横截面和纵截面的微观结构,采用室温单向压缩试验测定力学性能。结果表明,网络结构显著地影响Zr?Al?Ni?Cu块体金属的塑性和断裂模式,当网络结构的尺寸达到某一临界值时,塑性和断裂模式发生转变。当胞状结构的尺寸约为3μm时,Zr基块体金属玻璃表现出塑性,并且塑性随胞状尺寸的增加而降低。断裂模式随胞状尺寸的增加逐渐由单一45°剪切面断裂向双45°剪切面断裂,最后转变为劈裂断裂。另外,探讨这些Zr基块体金属玻璃的塑性和断裂模式发生改变的机理。     

Yielding and shear banding of metallic glasses

In this study the yielding and subsequent shear banding evolution process of Zr-, Cu-, Fe- and Mg-based metallic glasses (MGs) were investigated using carefully designed cyclic microcompression tests. It was found that yielding starts from a stable plastic flow with a viscosity of the order of 10¹² Pa s, resembling that of the glass transition. This provides critical evidence that yielding is caused by a stress-induced glass transition with internal randomly distributed liquid-like cores get connected. Up to a critical point the liquidized layer penetrates the entire sample with an initiation viscosity of 10⁸ Pa s, comparable with that of the liquid-like cores. Along the liquid-like layer shear band propagation involves shear band sliding and is succeeded by shear band arrest. Dynamic softening leads to an increase in the velocity of shear band sliding, with resultant shear offset, which can be successfully captured by a linear softening model. Once the elastic energy in the shear band is dissipated its internal structure begins to recover, with the solid-like matrix being reconstructed, resulting in shear band arrest. A simple diagram elucidating the yielding and shear banding dynamics is constructed, which sheds light on the fundamental nature of the deformation mechanisms of MGs.     

Effects of drawing on the tensile fracture strength and its reliability of small-sized metallic glasses

The effects of drawing on the structure and mechanical properties of a Co-based metallic glass under tension were thoroughly investigated. Surface changes induced by drawing, including removal of surface flaws, surface chemical homogenization and generation of compressive residual stress tend to increase the fracture strength, whilst open volumes created during drawing, particularly nano-voids, are likely to soften the wires. Initially, the surface changes are decisive factors, but as drawing proceeds, the open volumes gradually become dominant, yielding a maximum fracture strength in the wires with an area reduction ratio of 22%. Moreover, it was found that the fracture strength reliability was enhanced by the drawing, which is due not only to the surface perfection but also to the increase of plastic deformation capability, manifested by the decrease in the activation energy of individual shear transformation zones. Our results imply that the drawing technique could be a promising approach to continuously producing small-sized glassy wires with improved overall properties.     

The fracture toughness of bulk metallic glasses

Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.     

Mutual interaction of shear bands in metallic glasses

Shear banding is the main plastic deformation mode in metallic glasses. Even though there are many researches focused on the initiation and propagation of shear bands, the interaction among them has not been systematically studied. Using an atomic force microscope, we investigated the mutual interaction of shear bands at the surface of Cu₅₀Zr₅₀ metallic glass ribbons at the nanoscale. At the sites of the interaction, the propagation direction of one shear band can be changed by the pre-existing one, and the offset is the vector sum of the two bands. Under external stress, one shear band can be decomposed into several tiny bands and more materials could be taken into the deformation zones. Therefore, more energy can be dissipated and the deformation could be more homogeneous for the mutual interaction process. These results are useful for a mechanistic understanding of the evolution and suppression of shear band propagations, as well as the design of metallic glasses with improved plasticity.     

Molecular dynamics of shear transformation zones in metallic glasses

Molecular dynamics simulations have been employed to investigate the dynamics of atomic rearrangements in Ni₅₀Zr₅₀ amorphous alloys sheared at different pressures. Local atomic dynamics has been characterized in terms of number of displacing atoms as well as of apparent activation volume and energy. The dimensionality of rearranging atom clusters has been also evaluated. Numerical findings, although not conclusive, indicate that atomic rearrangements are probably triggered by a free volume redistribution. Highly localized and cooperative, such plastic rearrangements occur in regions tending to a two-dimensional occupancy of space.     

延性金属圆柱棒的拉伸断裂数值模拟

216 9钢在工业产品中经常用到 ,是一种奥氏体不锈钢 ,属于FCC金属。 2 16 9钢为延性金属 ,在拉伸状态下表现出延性断裂 ,从试验分析看 ,断口主要为韧窝形式。本文应用Gurson细观损伤模型 ,采用显式动力法分析了 2 16 9钢圆柱棒在拉伸状态下的损伤、紧缩和断裂。分析 2 16 9钢的静态拉伸表明 ,采用Gurson模型可以较好地模拟这种材料的延性断裂形式。分析结果与试验现象相符。     

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔG_c, the shear band stability index k₀, and the critical shear band length l_c, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔG_c, large k₀, and small l_c. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.     

Dual character of stable shear banding in bulk metallic glasses

In this work, a systematical study of the stable shear banding behavior is performed across a wide range of alloy compositions of bulk metallic glasses (BMGs). Through microcompression, it can be demonstrated that the stable shear banding behavior could exhibit a dual character of stochastic and deterministic propagations. Different from the stochastic character, which is found insensitive to sample sizes, the deterministic character displays a clear trend of a sample size effect, which can be captured by the energy balance principle. Based on the theoretical framework laid out in this work, a correlation between the plastic energy dissipation and the sample size effect is established, which can be then utilized to explore the influence of the alloy’s chemical compositions on the behavior of shear-induced material softening. Finally, the energy-based formalisms aimed to quantify the shear banding size effect in MGs are discussed.     

Controlling shear band behavior in metallic glasses through microstructural design

Plastic deformation in metallic glasses is governed by the initiation and propagation of shear bands. The successful use of bulk metallic glasses in structural applications will depend on controlling these processes to improve ductility and toughness. In Zr–Cu–Ni–Al metallic glasses, the addition of Ta can influence the structure of the material and hence the shear band behavior in two ways. At low Ta contents (<4 at.%), the material is amorphous but has enhanced order over length scales of 5–15 Å Higher levels of Ta result in the precipitation of bcc Ta-rich solid solution particles in a metallic glass matrix. Under uniaxial compression, both of these materials show greater apparent plastic strain to failure than the glass without Ta. This appears to be the result of the influence of the structure on the initiation and propagation of shear bands in the amorphous matrix.     

Theoretical prediction of temperature dependent shear modulus of bulk metallic glasses

A model of temperature dependent shear modulus and Young's modulus in bulk metallic glasses is established. The inherent relationship between the glass transition temperatures, the Debye temperature and shear modulus of bulk metallic glasses is revealed. The temperature dependent shear modulus can be predicted by our model without any fitting parameter. The model is presented based on a critical energy density criterion for plastic yielding which is derived from fundamental thermodynamics. This critical energy density consists of two parts: the heat added to the system and the input of mechanical energy, which are not completely equivalent. The agreement between theoretical results and experimental results is striking. And it is found that the temperature dependent Young's modulus could also be predicted pretty well by our model.     

Stable flowing of localized shear bands in soft bulk metallic glasses

This paper reports a novel class of Zr–Cu–Fe–Al bulk metallic glasses (BMGs) with high Zr content and unusual deformation behavior. Higher content of Zr results in lower hardness, but favors the generation of highly localized shear bands, even if nanocrystalline phases precipitate in the glassy matrix. Moreover, flow of the dominant shear band proceeds slowly under compressive stress without rapid shear rupture and facture. The unusual deformation behavior suggests that designing BMG with a high content of solvent element and low hardness might be a promising new way of developing ductile BMG.     

Hidden order in the fracture surface morphology of metallic glasses

We report the fractal nature of dimple structures on the fracture surface of various metallic glasses (MGs) with significantly different mechanical properties. The analyzed fractal dimension increments of MGs lie in a narrow range of 0.6–0.8. The results indicate that the MGs with marked differences in plasticity and toughness may have a unified local softening mechanism and similar nonlinear plastic behavior in front of the crack tip during fracture. We present a physical picture for the dimple structure formation from the plastic zone in front of the crack tip. The evolution of the plastic zone from the interaction of the shear transformation zones is theoretically modeled as a stochastic process far from thermodynamic equilibrium, and the model can capture the formation and fractal nature of the dimple structures on the fracture surface of metallic glasses.