Gradual shear band cracking and apparent softening of metallic glass under low temperature compression

Metallic glasses (MGs) usually exhibit synchronously enhanced plasticity and strength with decreasing the testing temperature. Although great efforts have been made, why MGs show better plasticity at low temperature remains unclear. In this work, the shear band cracking and fracture mechanism of a relatively brittle TiZr-based MG was investigated through methods of low temperature compression and 3D X-ray tomography. Pronounced apparent softening and profuse internal shear-band cracks were observed along with the improved plasticity, enhanced yield strength, decreased average softening rate, and reduced area of vein pattern as decreasing the testing temperature. Moreover, the fracture features can be correlated well with the compressive properties; and the true rupture stress that is carried by the still-bonded part of major shear band was found to be very close to the yield strength, demonstrating that the apparent softening should mainly originate from the cracking rather than dilation of shear band. The decreased softening rate can be fitted by a diffusion model, implying reduced atomic mobility and increased cracking resistance. Consequently, the improved plasticity of TiZr-MG at low temperature was attributed to the suppression of instant fracture and the enhanced resistance to shear band cracking, rather than the change of shear band density.     

Analyses of shear band emission in a Mg-based bulk metallic glass deformed at different nanoindentation rates

Nanoindentation behavior of Mg₅₇Cu₃₁Y_6.6Nd_5.4 bulk metallic glass was characterized at different loading rates. The load–displacement curves exhibit significant displacement serrations, apparently associated with discrete shear band emission, at low loading rates but disappear at high rates. Analyses based on displacement serration, strain rate serration and hardness serration were carried out to determine the critical strain rate beyond which the transition from inhomogeneous to homogeneous deformation actually took place. It was concluded that the hardness serration analysis probably provides the most reasonable result as the other two were limited by the instrument noises. Based on a shear band nucleation model, the critical nucleus size was estimated to be a sphere of about 25 nm in diameter.     

Analysis of plastic strain and deformation mode of a Zr-based two-phase bulk metallic glass in compression

A ductile phase-separated Zr-based bulk metallic glass (BMG) deformed to different strains at room temperature and low strain rate was characterized. The BMG samples were compressed to nominal strains of 3%, 7%, and 10%, after which the samples were unloaded for morphological observation using scanning electron microscopy. The morphological observation was subsequently used for the interpretation of the measured load–displacement curves. It was found that the BMG exhibited apparent uniform deformation initially (at plastic strain <1%) and, then, visible local shear bands began to developed. Afterwards, a principal shear band was soon developed and dominated the deformation process until fracture. In this study, we also found that the local shear strain varies along the principal shear plane and decreases monotonically from the shear band initiation site.     

基于独立分量分析理论的大块非晶合金切削信号检测

研究了在大块非晶合金切削力信号检测时利用独立分量分析法对检测信号进行去噪处理技术。在试验中,采用独立分量分析法对切削测量系统测量的大块非晶合金切削力信号进行迭代分离,从而提取出主切削力信号。并针对大块非晶合金在不同切削深度下的变形特征,运用扫瞄式电子显微镜观察了大块非晶合金的切削带特征。主切削力信号频谱的快速傅里叶变换分析表明,随着切削深度的增加,切削力信号高频部分的振幅越来越大,而大块非晶合金切削力信号高频部分是由切削带形成过程的特征引起的,并随着切削深度的增加而增加,且主切削力Fz 的频率为115 Hz。研究结果表明：采用独立分量分析法进行噪声分离后更能精确识别切削力信号中的主要信息,减少噪声造成的误判。     

Effect of the nanoindentation rate on the shear band formation in an Au-based bulk metallic glass

This study investigated the nanoindentation behavior of Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass samples at loading rates ranging from 0.03 to 300 mN s⁻¹. Notable shear band pop-in events were observed. The pop-in size was observed to increase linearly with the load and decreased exponentially with the strain rate. A free-volume mechanism was proposed for interpreting these observations quantitatively. The results and analyses also shed light on the shear band nucleation and evolution processes in bulk metallic glasses.     

Dynamic mechanical behavior of a Zr-based bulk metallic glass during glass transition and crystallization

The dynamic mechanical behaviors of the Zr41Ti14Cu12.5Ni8Be22.5Fe2 bulk metallic glass (BMG) during continuous heating at a constant rate were investigated. The glass transition and crystallization of the Zr-based BMG were thus characterized by the measurements of storage modulus E and internal friction Q-1. It was found that the variations of these dynamic mechanical quantifies with temperature were interre-lated and were well in agreement with the DSC trace obtained at the same heating rate. The origin of the first peak in the internal friction curve was closely related to the dynamic glass transition and subsequent primary crystallization. Moreover, it can be well described by a physical model, which can characterize atomic mobility and mechanical response of disordered condense materials. In comparison with the DSC trace, the relative position of the first internal friction peak of the BMG was found to be dependent on its thermal stability against crys-tallization.     

Wetting angle and infiltration velocity of Zr base bulk metallic glass composite

The contact angles of Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ alloy on tungsten substrate were measured by sessile drop technique at the temperature range from 1023 to 1323 K and different processed time. Based on the morphology of the melt drop, the surface tension as function of processing temperature and holding time was obtained. Diffusion band was observed at the fringe of metallic drop, which confirms that the wetting between the melt and tungsten belongs to reactive wetting. The relationship between the flow velocity of melt and processing parameters, such as infiltration pressure, volume fraction of fibers and the filtration length, was calculated comprehensively.     

Dynamic fragmentation induced by network-like shear bands in a Zr-based bulk metallic glass

Dynamic fragmentation induced by network-like shear bands is observed in a Zr-based bulk metallic glass subjected to impact loading, which is different from shear failure via one dominant shear band under quasi-static compression. Further, the influence of elastic strain energy on the evolution of shear bands is investigated. It is shown that the shear-band pattern occurring in one dominated mode or in multiple modes strongly depends on the loading rates. Dynamic fragmentation is due to the competition between the elastic strain energy driving a shear band and the dissipated energy along the shear band.     

Viscous flow in sliding shear band formed during tensile deformation of hypoeutectic Zr-based metallic glass

The Zr₇₀Ni₁₆Cu₆Al₈ bulk metallic glass (BMG) was tested in tension mode using high-speed imaging in order to monitor the viscous flow inside a sliding shear band. Assuming the shear band thickness to be 15 nm, we estimated the shear viscosity to be ～3.8 × 10³ Pa s. The apparent temperature in the shear band was estimated to reach approximately 852 K on the basis of the relationship between the viscosity and the strain rate in the supercooled liquid region and was found to be considerably lower than the melting temperature (1293 K).     

Atomistic simulation of shear localization in Cu–Zr bulk metallic glass

Shear deformations of Cu₅₇Zr₄₃ bulk metallic glass (BMG) model systems are performed using molecular dynamics simulation. The results suggest that both the hydrostatic stress and the stress normal to the shear plane should affect the shear response (modified Mohr–Coulomb yield criterion). We see shear localization and shear band nucleation in both a small system of 2000 atoms, and large systems of 524,288 atoms, and analyze local atomic structure evolution.     

Simulation of shear banding and crack propagation in bulk metallic glass matrix composites

Different modes of shear banding in bulk metallic glass (BMG) matrix composites are identified from the systematic simulation studies based on a mesoscopic phase-field model for deformation in glassy alloys. We characterize the interaction between shear band and crystalline reinforcements by considering the residual stress and atomic bonding condition at the interface between BMG-matrix and the reinforcement. The simulation demonstrates that compressive residual stress assists to impede the shear bands propagating toward the reinforcements, while tensile residual stress accelerates such process. In addition, the effect of atomic bonding at the interface on shear banding is investigated by the simulation. The relations between the fracture toughness and the residual stress and atomic bonding condition at the interface are quantitatively determined.     

Effect of pre-existing shear bands on the tensile mechanical properties of a bulk metallic glass

Bulk Zr_64.13Cu_15.75Ni_10.12Al₁₀ metallic glass has been rolled at room temperature in two different directions, and the dependences of microstructure and tensile mechanical property on the degree of deformation and rolling directions have been investigated. No deformation-induced crystallization occurs except for shear bands. Shear band formation in conjugated directions is achieved in the specimen rolled in two directions, while rolling in one direction induces shear band formation only in a single direction. Pre-existing properly spaced soft inhomogeneities can stabilize shear bands and lead to tensile plastic strain, and the efficient intersection of shear bands in conjugated directions results in work-hardening behavior, which is further confirmed by in situ tensile scanning electron microscopic observation. Based on the experimental results obtained in two different specimen geometries and finite element analysis, it is deduced that a normal-stress-modified maximum shear stress criterion rather than a shear plane criterion can describe the conditions for the formation of shear bands in uniaxial tension.     

Fracture surface morphology of compressed bulk metallic glass-matrix-composites and bulk metallic glass

The fracture morphology of Zr-based bulk metallic glass-matrix-composites (BMGCs) and Cu-based bulk metallic glass (BMG) after compression testing has been studied. The quasi-static compression fracture surface displays a mixture of three different distinct patterns: vein-like, smooth featureless and river-like features. The last one corresponds to the morphology known from tensile tests of BMGs. Moreover, randomly distributed transversal steps on the fracture plane are also present. This is in contrast to previous studies where a characteristic vein-like pattern is considered a unique feature of the fracture of BMGs under quasi-static uniaxial compression. The presence of different fracture features indicates that the development of the fracture plane occurs in a stepwise mode.     

Hardness and shear band evolution in bulk metallic glasses after plastic deformation and annealing

Strain-induced hardening and annealing-induced softening are typical in crystalline metals. Bulk metallic glasses (BMG) exhibit the opposite behavior, namely, strain-induced softening and annealing-induced hardening. In addition, reversible softening–hardening–softening occurs in a BMG subjected to a three-step deformation–annealing–deformation process. The hardness changes after deformation and annealing can be correlated with the shear band patterns around/underneath Vickers indents. Shear bands produced during indentation of as-cast BMG are semicircular and radial, consistent with the stress distribution beneath the indenter. In contrast, the shear bands in the pre-strained BMG are irregular and convoluted, and appear to be a mixture of the shear bands produced during the prior compression and those in the as-cast BMG. After annealing, the shear bands tend to recover their semicircular and radial shapes consistent with the annealing-induced hardening.     

Experimental characterization of plastic flow initiation in a bulk metallic glass via intermittent compression test

Intermittently plastic compression tests of the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk metallic glass were performed under a geometrically constrained condition at different cross-head displacement rates. A flow stress-overshoot phenomenon was observed during the macroscopically plastic deformation. It was found that there is a close correlation between the stress-overshoot intensity and the spatial density of shear bands. The stress-overshoot intensity can be used as a parameter to characterize the ability of plastic flow initiation caused by the spatial nucleation of shear bands in the bulk metallic glass. The micromechanisms of shear band multiplication were discussed.     

Electrochemical micromachining of a Zr-based bulk metallic glass using a micro-tool electrode technique

For the first time the possibility of electrochemical micromachining (ECMM) of a Zr-based bulk metallic glass (BMG) using a micro-tool electrode technique is reported. It is demonstrated that the choice of the electrolyte chemistry is substantial for a successful ECMM processing. For the bulk glassy Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy a concentrated aqueous NaNO₃ standard machining solution is not suitable due to formation of thick and dense corrosion product layers which hinder the machining of structures with high aspect ratio. On the other hand, a commercial methanolic HClO₄ solution is shown to be very promising. In the first experiments with this electrolyte micro-hole structures were machined with aspect ratios of about 1 at depths of ～40 μm. The effect of process parameters such as pulse voltage and pulse length was investigated and their influence on the machined structure morphology is established. In a first approach the potential and challenges of this electrochemical micromachining technique for microforming of multi-component bulk metallic glass surfaces will be discussed.     

Boss formability of a Zr-based bulk metallic glass

Macroscopic solid-to-solid hot formability of a Zr-based bulk metallic glass has been investigated in this study by applying a boss forming process within supercooled liquid region (SLR). The morphology and microstructures after boss forming were first examined by using an optical microscopy (OM), a field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The variation of thermal properties before and after boss forming was also analyzed by using a differential scanning calorimeter (DSC). The glass transition temperature, T_g and the crystallization onset temperature, T_{x, onset} were found to decrease with increasing test time and temperature. Macroscopic extrusion formability was found to match well with the results predicted through a processing map based on a dynamic materials model (DMM). The specimens were found to form well under the conditions of high temperature and slow punch jig speed. A FEM simulation study has also been carried out to understand the causes of piping problem. A sound boss without a pipe could be formed by reducing the flow rate difference in the contact and the core sites.