Ideal shear banding in metallic glass

As the most fundamental deformation mechanism in metallic glasses (MGs), the shear banding has attracted a lot of attention and interest over the years. However, the intrinsic properties of the shear band are affected and even substantially changed by the influence of non-rigid testing machine that cannot be completely removed in real compression tests. In particular, the duration of the shear banding event is prolonged due to the recovery of the stressed compliant frame of testing machine and therefore the temperature rise at the operating shear band is, more or less, underestimated in previous literatures. In this study, we propose a model for the ‘ideal’ shear banding in metallic glass. The compliance of the testing machine is eliminated, and the intrinsic shear banding process is extracted and investigated. Two important physical parameters, the sliding speed and the temperature of shear band, are calculated and analysed on the basis of the thermo-mechanical coupling. Strain-rate hardening is proposed to compensate thermal softening and stabilise the shear band. The maximum value of the sliding speed is found to be on the order of 10 m/s at least, and the critical temperature at which strain-rate hardening begins to take effect should reach as high as 0.9T_g (T_g is the glass transition temperature) for a stable shear banding event in metallic glass according to the early experimental data. This model can help to understand and control the shear banding and therefore the deformation in MGs.     

On the nature of the Kaiser effect in metallic glasses

The Kaiser effect was observed in the measurement of acoustic emission (AE) during the course of uniaxial compression of the bulk samples of metallic glasses Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 and Zr_52.5Ti₅Cu_17.9Ni_14.6Be_22.5. The field-ion microscopy study of bulk Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 shows that this glass has a polycluster structure. This fact allows one to reveal the nature of AE appearing during the course of deformation of metallic glasses and to interpret the observed Kaiser effect. The dislocations generated at the intercluster boundaries and moving through the glass bulk are the sources of AE.     

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr_65–xCu₂₅Al₁₀Fe_x (x = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.     

Isothermal kinetics and relaxation recovery of high-frequency shear modulus in the course of structural relaxation of Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> bulk glass

Isothermal kinetics of relaxation of the high-frequency (1.4 MHz) shear modulus during structural relaxation of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass below the glass transition temperature is studied by an in situ method of contactless electromagnetic acoustic transformation. The kinetic law of relaxation is established. It is shown that quenching of aged samples from the supercooled liquid state leads to a decrease in the absolute value of shear modulus to below the initial value; the degree of subsequent isothermal relaxation of the modulus may be several times higher than the initial value. Possible reasons for relaxation and recovery of the shear modulus are considered.     

Time and amplitude dependences of the damping decrement and shear modulus upon irreversible structural relaxation in a Zr-Cu-Ni-Al-Ti bulk metallic glass

The time dependences of the irreversible relaxation of the damping decrement and the shear modulus of a Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass are investigated using an inverse torsion pendulum in the range from room temperature to ～650 K. The spectrum of activation energies of irreversible structural relaxation is evaluated from the results obtained. Analysis of the amplitude dependences of the damping decrement and the shear modulus allows the conclusion that the relaxation centers responsible for the amplitude dependence differ from those associated with the irreversible structural relaxation at temperatures below and in the vicinity of the glass transition point.     

Thermal effects induced during heating of the bulk metallic glass Zr46Cu46Al8

It has been shown that thermal effects measured calorimetrically during heating of the bulk metallic glass Zr₄₆Cu₄₆Al₈ can be interpreted as a result of the annihilation or generation of structural “defects,” which are similar in their properties to interstitial dumbbells in simple crystalline metals. The shear susceptibility, which characterizes the sensitivity of the shear modulus of the glass to the concentration of “defects” of the type of interstitial dumbbells, has been calculated.     

Comparison of shear banding in BMGs due to thermal-softening and free volume creation

This paper reports a comparative study of shear banding in BMGs resulting from thermal softening and free volume creation. Firstly, the effects of thermal softening and free volume creation on shear instability are discussed. It is known that thermal softening governs thermal shear banding, hence it is essentially energy related. However, compound free volume creation is the key factor to the other instability, though void-induced softening seems to be the counterpart of thermal softening. So, the driving force for shear instability owing to free volume creation is very different from the thermally assisted one. In particular, long wave perturbations are always unstable owing to compound free volume creation. Therefore, the shear instability resulting from coupled compound free volume creation and thermal softening may start more like that due to free volume creation. Also, the compound free volume creation implies a specific and intrinsic characteristic growth time of shear instability. Finally, the mature shear band width is governed by the corresponding diffusions (thermal or void diffusion) within the band. As a rough guide, the dimensionless numbers: Thermal softening related number B, Deborah number (denoting the relation of instability growth rate owing to compound free volume and loading time) and Lewis number (denoting the competition of different diffusions) show us their relative importance of thermal softening and free volume creation in shear banding. All these results are of particular significance in understanding the mechanism of shear banding in bulk metallic glasses (BMGs). Supported by the Chinese Academy of Sciences under the project “Multi-Scale Complex System” (Grant No. KJCX-SW-L08), the National Natural Science Foundation of China (Grant Nos. 10725211 and 10721202), and the Doctorial Start-up Fund of Hunan University of Science and Technology (Grant No. E50840)     

Effect of deformation and heat treatment on the damping constant and shear modulus of a bulk Zr-Cu-Ni-Al-Ti metallic glass

The effect of deformation by rolling or quenching from temperatures close to the glass transition temperature on the damping constant and the shear modulus of preliminarily annealed bulk samples of a Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass was studied. These treatments are found to result in recovery of the “irreversible” contributions to the damping constant and the shear modulus, and the deformation treatment is shown to lead to an increase in the amplitude-dependent internal friction.     

"Work-Hardenable" ductile bulk metallic glass

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (< 1%) at room temperature. We present a new class of bulk metallic glass, which exhibits high strength of up to 2265 MPa together with extensive "work hardening" and large ductility of 18%. Significant increase in the flow stress was observed during deformation. The "work-hardening" capability and ductility of this class of metallic glass is attributed to a unique structure correlated with atomic-scale inhomogeneity, leading to an inherent capability of extensive shear band formation, interactions, and multiplication of shear bands.     

Irreversible structural relaxation in a bulk Pd-Cu-Ni-P metallic glass

The evolution of the shear modulus and the damping decrement during irreversible structural relaxation in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in a temperature range below the glass transition temperature has been studied with an inverse torsion pendulum at a frequency of ～25 Hz. It is shown that the irreversible relaxation can be recovered via quenching from temperatures above the glass transition temperature. The spectrum shape, the characteristic activation energies, and the attempt frequencies of the irreversible structural relaxation are estimated.     

Microstructure controlled shear band pattern formation and enhanced plasticity of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions

Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb beta phase, with bcc structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading organized shear band patterns develop throughout the sample. This results in a dramatic increase in the plastic strain to failure, impact resistance, and toughness of the metallic glass.     

Kinetics of structural relaxation of bulk and ribbon Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> glasses determined from electrical resistance measurements

The electrical resistances of ribbon and bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glasses, whose quenching rates differ by four orders of magnitude, were precisely measured during cyclic heating. Three stages of electrical resistance relaxation are detected as the maximum heating temperature increases. The first and third stages decrease the electrical resistance, and the second stage increases it. The first stage is shown to be caused by the relaxation of deformation-induced internal stresses and not to be related to the excess free volume concentration, which differs by a factor of about 2 in the ribbon and bulk samples. The second stage reflects structural relaxation in the glass and is only partly related to its free volume. The third relaxation stage is assumed to be caused by fine precrystallization phenomena like phase separation. The effect of deformation by rolling or quenching from the temperature range of a supercooled melt on the resistance relaxation kinetics was also studied.     

Stress relaxation and viscosity of a bulk Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> metallic glass under isochronous heating conditions

Isochronous relaxation of tensile stresses is measured in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in the initial state and after certain thermal treatments. The results of measurements are used to find the energy spectrum of irreversible structural relaxation, from which the temperature dependence of shear viscosity is then calculated. This dependence is also found independently from measurements of creep in the same glass. The calculated viscosity is shown to agree well with the experimental data.     

永磁性大块金属玻璃Nd60Al10Fe20Co10低 温磁性的研究

研究了Nd₆₀Al₁₀Fe₂₀Co₁₀ 大块金属玻璃 磁性随温度的变化关系，结果表明Nd₆₀Al₁₀Fe₂₀Co₁₀ 在室温下 表现为永磁性，随着温 度的降低，矫顽力和磁滞回线形状都有很大的变化.交流磁化率在18 K左右出现尖峰而且峰 值温度随频率变化，表明该大块非晶体系中存在自旋玻璃态. 关键词： 大块金属玻璃 低温磁化率 自旋玻璃     

冲击加载下Zr51Ti5Ni10Cu25Al9金属玻璃的塑性行为

进行了10—27 GPa应力范围内Zr₅₁Ti₅Ni₁₀Cu₂₅Al₉金属玻璃的平面冲击实验以研究其高压-高应变率加载下的塑性行为.由样品自由面粒子速度剖面的分析获得了冲击加载过程的轴向应力,并通过轴向应力与静水压线的比较获得剪应力.实验结果表明,尽管存在明显的松弛效应,但Zr基金属玻璃的Hugoniot弹性极限随着冲击应力的增加而增加.然而,塑性波阵面上的剪应力则显示先硬化而后软化现象,而且软化的幅度随冲击应力的增加而增加.冲击加载下Zr基金属玻璃的上述剪应力变化特征与分子动力学模拟结果比较一致,但与压剪实验结果和一维应力冲击实验结果明显不同.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

Effect of heat treatment on the elastic characteristics of a bulk amorphous Zr-Cu-Ni-Al-Ti alloy

The changes in the elastic properties of a bulk Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ metallic glass exposed to heat treatments at temperatures of up to 700°C were studied by acoustic methods. An anomalous decrease in the alloy shear modulus (to values lower than that in the amorphous state) was found after the first stages of nanocrystallization.     

Initial stages in the decay of the amorphous phase in the bulk metallic glass Zr-Cu-Ti

Using methods of x-ray diffraction analysis, differential scanning calorimetry, dilatometry, and transmission electron microscopy, we have investigated the initial stages of decay of the amorphous phase in a bulk metallic glass based on zirconium. We found that crystallization of the bulk metallic glass proceeds in several stages, where in the first stage the bulk conversion effect is equal to more than 1.6%, or about 80% of the total bulk crystallization effect. We showed that the first stage of decay of the amorphous phase in the bulk metallic glass Zr₂₉Ti₁₁Cu₆₀ leads to the formation of a nanocrystalline structure with a grain size of 1–5 nm. We have analyzed the change in the shape of the diffraction maximum during the formation of the very fine nanocrystalline structure. Fiz. Tverd. Tela (St. Petersburg) 41, 1129–1133 (July 1999)     

Structural,elastic and electronic properties of XNCa3 (X = Ge,Sn and Pb) compounds

Using ab initio calculations, we have studied the structural, elastic and electronic properties of XNCa₃, with X=Ge, Sn and Pb. Geometrical optimization of the unit cell are in agreement with the available experimental data. The band structures show that all studied materials are electrical conductors. The analysis of the site and momentum projected densities, charge transfer and total valence charge density shows that the chemical bonding in XNCa₃ compounds is of covalent–ionic nature with the presence of metallic character. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal polycrystalline XNCa₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that XNCa₃ compounds are brittle in nature. We estimated the Debye temperature of XNCa₃ from the average sound velocity.     

Isothermal creep of bulk glassy Zr52.5Ti5Cu17.9Ni14.6Al10 below T g

The results of an isothermal creep investigation of Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass at temperatures below the glass transition temperature are presented. The long-time (t > (2–4) × 10³?s) creep regularities were found to be the same as those known for ribbon metallic glasses, in spite of the pronounced difference in the production quenching rates. It is argued that creep behaviour of bulk metallic glass is determined by the rate of irreversible structural relaxation. The apparent activation energy spectrum reconstructed from isothermal creep measurements agrees well with that determined from linear heating creep data.