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1.
《Intermetallics》2017
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. 相似文献
2.
Recent reports suggest that Poisson’s ratio (ν), or the related ratio of shear modulus G to bulk modulus B, indicates the potential of metallic glasses (MGs) to sustain plastic strain. Using molecular dynamics simulations of the Cu64Zr36 MG as a representative, we demonstrate why and how these elastic and plastic properties are correlated, in terms of the common structural origin underlying these mechanical behaviors in MGs. The full icosahedral ordering has been identified as the key structural feature in the Cu–Zr MGs that controls not only the G and the G/B (or ν), but also the initiation of shear localization and the intrinsic plasticity. Additional analysis of the Cu–Zr MGs of different compositions and MGs in different alloy systems reveals a general correlation of the plasticity with the G/B ratio, as the latter is able to represent and couple the effects of both the atomic configuration and the alloy composition. 相似文献
3.
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. 相似文献
4.
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. 相似文献
5.
We present crystal plasticity finite element simulations of the texture evolution in α-brass polycrystals under plane strain compression. The novelty is a non-crystallographic shear band mechanism [Anand L, Su C. J Mech Phys Solids 2005;53:1362] that is incorporated into the constitutive model in addition to dislocation and twinning. Non-crystallographic deformation associated with shear banding leads to weaker copper and S texture components and to a stronger brass texture compared to simulations enabling slip and twinning only. The lattice rotation rates are reduced when shear banding occurs. This effect leads to a weaker copper component. Also, the initiation of shear banding promotes brass-type components. In summary the occurrence of non-crystallographic deformation through shear bands shifts face-centered-cubic deformation textures from the copper type to the brass type. 相似文献
6.
《Acta Materialia》2008,56(18):5263-5275
Bulk metallic glasses (MGs) with tunable plasticity and strength have been reported recently. Using Cu–Zr and Cu–Zr–Al MG models, here we illustrate how and why alloy composition and cooling history influence the initial flow behavior in the early stage of plasticity. Starting from Cu46Zr54, either increasing the Cu concentration, or substituting Al for a few percent of Zr, increases the resistance to the initiation of plastic flow, the softening after the local yielding, and the propensity for strain localization. These effects are shown to be intrinsic to the uniform, fully amorphous MGs and rooted in their internal structure. Our quantitative monitoring of the local environment, especially the role of full-icosahedral clusters in shear transformations, identifies the fertile and resistant structural entities controlling deformation. The structural mechanisms have implications for macroscopic plasticity, and the alloy dependence of the MG structure reveals a microscopic origin underlying the varying mechanical properties. 相似文献
7.
This paper reports that the plasticity of Zr-based metallic glass can be improved by creating two symmetrical semi-circular notches. Unlike the experimental findings of the samples without notches, a steady shear deformation can be created by the large-scale stress gradient around the two symmetrical notches and the plasticity of metallic glass can be enhanced to a high value of ~10% under compression tests. The improved plasticity may be due to the easy initiation of shear bands around the notches, and the consequent blocking effect of notches on the propagation of shear bands, similar to the dislocation mechanism in crystalline materials. To reveal the particular plastic deformation behavior of metallic glass, Ti3SiC2 ceramic and high-strength steel specimens with two symmetrical semi-circular notches were also conducted under compressive loadings; however, no enhancement in plasticity was found. It is suggested that creating a stress gradient is a particular strategy for designing metallic glasses in order to improve their plasticity. 相似文献
8.
A new corrosion inhibitor namely poly(vinyl alcohol‐leucine) composite (PVAL) has been synthesized and its influence on corrosion inhibition of mild steel in 1 M hydrochloric acid solution has been studied by weight loss and potentiodynamic polarization techniques. The composite (PVAL) showed more than 95% inhibition efficiency (IE) at an optimum concentration of 0.6% by weight. The inhibition efficiency of inhibitor has been found to vary with inhibitor concentration, solution temperature, and immersion time. Various kinetic and thermodynamic parameters (Ea, ΔHo, ΔSo for corrosion and ΔGads, ΔHads, ΔSads for adsorption) reveal a strong interaction between inhibitor and mild steel surface. The negative values of ΔGads indicate the spontaneous adsorption of the inhibitor on mild steel surface. Potentiodynamic polarization studies showed PVAL as mixed type inhibitor. It inhibits mild steel corrosion by blocking the active sites of the metal. Electrochemical impedance spectroscopic (EIS) techniques were also used to investigate the mechanism of corrosion inhibition. 相似文献
9.
Chang-Myeon Lee Soo-Won Chae Hwi-Jun Kim Jae-Chul Lee 《Metals and Materials International》2007,13(3):191-196
The role of nancocrystals on the plasticity of the Cu64Zr36 amorphous alloy was studied in terms of the initiation of shear bands and the propagation of cracks. The plastic deformation
behaviors of the fully amorphous and the partially crystallized samples with various degrees of crystallinity were compared.
The partially crystallized alloys with an adequate amount of nanocrystals showed a considerable enhancement in their plasticity.
Finite element calculations were conducted to qualitatively examine the role of the nanocrystals on the formation of the shear
bands, while high-resolution electron microscopy was used to directly observe the crack propagation behavior through the matrix
with (or without) nanocrystals. 相似文献
10.
Unlike crystalline alloys, bulk amorphous alloys exhibit a significantly enhanced plastic strain when tested at cryogenic temperatures. It was observed that both the retarded band propagation and the multiple shear banding are the characteristic features of the shear banding behaviors that are responsible for the enhanced plasticity at cryogenic temperatures. This study explores the retarded band propagation behavior at cryogenic temperatures in terms of the increased viscosity of a material inside a propagating band. The formation of multiple shear bands at cryogenic temperatures is also clarified by evaluating the role played by the cryogenic environment in facilitating the creation of disorder, where local deformation nucleates. 相似文献
11.
Intermittently plastic compression tests of the Zr52.5Cu17.9Ni14.6Al10Ti5 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. 相似文献
12.
《Intermetallics》2015
Electrodeposition was used to coat copper films on the surface of the BMG pillars (bulk metallic glasses) of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit. 105) with the film thicknesses of 71.5 and 161.1 μm. The experimental results of the compression tests of the bare Vit. 105 pillars and the coated Vit. 105 pillars revealed that the copper costing increased the density of shear bands in the Vit. 105 pillars formed during the tests, resulting in the improvement of plasticity. The plastic strain was 6.1% for the coated pillars with a coating thickness of 161.1 μm, which is 3.59 times of 1.7% of the bare Vit. 105 pillars. The deformation of the copper films dissipated the strain energy and limited the propagation of shear bands, which led to the initiation and formation of multiple shear bands. The technique developed in this work provides an effective way to enhance the plasticity of BMGs at room temperature. 相似文献
13.
《Acta Materialia》2003,51(1):87-99
Plastic deformation of two Pd- and two Zr-based bulk metallic glasses (BMGs) is investigated through the use of nanoindentation, which probes mechanical properties at the length scale of shear bands, the carriers of plasticity in such alloys. These materials exhibit serrated flow during nanoindentation, manifested as a stepped load-displacement curve punctuated by discrete bursts of plasticity. These discrete “pop-in” events correspond to the activation of individual shear bands, and the character of serrations is strongly dependent on the indentation loading rate; slower indentation rates promote more conspicuous serrations, and rapid indentations suppress serrated flow. Analysis of the experimental data reveals a critical applied strain rate, above which serrated flow is completely suppressed. Furthermore, careful separation of the plastic and elastic contributions to deformation reveals that, at sufficiently low indentation rates, plastic deformation occurs entirely in discrete events of isolated shear banding, while at the highest rates, deformation is continuous, without any evidence of discrete events at any size scale. All of the present results are consistent with a kinetic limitation for shear bands, where at high rates, a single shear band cannot accommodate the imposed strain rapidly enough, and consequently multiple shear bands must operate simultaneously. 相似文献
14.
D.D. Qu K.-D. Liss Y.J. Sun M. Reid J.D. Almer K. Yan Y.B. Wang X.Z. Liao J. Shen 《Acta Materialia》2013,61(1):321-330
The structural origins for the high plasticity of a Zr53Cu18.7Ni12Al16.3 (at.%) bulk metallic glass are explored. Under plastic flow conditions, in situ synchrotron high-energy X-ray diffraction reveals that the atomic strain saturates to the closest packing in the longitudinal direction of the applied load while atoms yield in the transverse plane. Scanning electron microscopy investigation reveals that global plasticity benefits from abundant shear band multiplication and interactions. Atomic level flows are seen to accompany profuse shear bands. The plasticity enhancement of this metallic glass benefits from such atomic level flows. Atomic level flow facilitates the activation of shear transformation zones that further self-assemble to promote shear band multiplication. On the other hand, it also mitigates the shear band propagation that prevents catastrophic shear band extension. 相似文献
15.
The as-cast Ti48Zr27Cu6Nb5Be14bulk metallic glass matrix composites(BMGMCs)were fabricated using a copper mold suction casting method.Then,the semi-solid BMGMC samples were obtained following an isothermal treatment(heating at 900°C for 10 min,then cooling with water).The microstructure and compression property were investigated by scanning electronic microscopy(SEM)and universal mechanical tester.As a result of the isothermal treatment,the crystal shapes change from fine,granular,and dendritic to spherical or vermicular,and the average crystal size of the as-cast and semi-solid samples is 2.2μm and 18.1μm,respectively.The plasticity increases from 5.31%in the as-cast to 10.23%in the semi-solid samples,with an increase of 92.66%.The shear bands from different areas of the side surfaces of as-cast and semisolid compression fracture samples were observed.The characteristic changes of multiplicity,bend,branch and intersection of shear bands in different areas indicate that the deformation of as-cast and semi-solid samples is non-uniform during compression.It is found that poor plasticity of the as-cast samples or good plasticity of the semi-solid samples are reflected by characteristics of the shear bands.The semi-solid isothermal treatment improves the plasticity by forming large crystals which can block the expansion of shear bands and promote the multiplicity of shear bands. 相似文献
16.
By using quantitative information about the radiation diffuse-scattering intensity of the disordered f.c.c. substitutional alloy Me′1-cMe″c (c—concentration) the Fourier component,
, of mixing energies of Me′ and Me″ atoms may be estimated. We have to use the measurement data of the diffuse-scattering intensities at the corresponding reciprocal-space points k of the disordered phase and then determine the parameter
(k). The statistical thermodynamics of the non-ideal solid solution is determined by these energy parameters {
(k)}. Therefore, one can obtain the configuration free energy of an alloy, F=U-TS (U—internal energy, S—entropy), and then determine its fundamental thermodynamic characteristics, including not only its phase diagram, but also the concentration-dependent order–disorder transformation temperature, temperature and concentration long-range order parameter dependences, chemical activity, heat capacity etc. Some thermodynamic properties are calculated within the framework of the statistical-thermodynamic approach for f.c.c.-Ni–Fe alloy. The diffuse-scattering intensity values are taken from data in the literature. 相似文献
17.
Starting from the nanoscale structural heterogeneities intrinsic to metallic glasses (MGs), here we show that there are two concurrent contributions to their microscale quasi-static shear modulus GI: one (μ) is related to the atomic bonding strength of solid-like regions and the other (GII) to the change in the possible configurations of liquid-like regions (dynamic relaxation). Through carefully designed high-rate nanoscale indentation tests, a simple constitutive relation (μ = GI + GII) is experimentally verified. On a fundamental level, our current work provides a structure–property correlation that may be applicable to a wide range of glassy materials. 相似文献
18.
《Acta Materialia》2008,56(19):5694-5700
The low-temperature specific heat of a superconductor Mo3Sb7 with Tc = 2.2 ± 0.05 K has been measured in magnetic fields up to 5 T. In the normal state, the electronic specific heat coefficient γn, and the Debye temperature ΘD are found to be 34.5(2) mJ mol−1 K−2 and 283(5) K, respectively. The enhanced γn value is interpreted as due to a narrow Mo-4d band pinned at the Fermi level. The electronic specific heat in the superconducting state can be analyzed in terms a phenomenological two BCS-like gap model with the gap widths 2Δ1/kBTc = 4.0 and 2Δ2/kBTc = 2.5, and relative weights of the molar electronic heat coefficients γ1/γn = 0.7 and γ2/γn = 0.3. Some characteristic thermodynamic parameters for the studied superconductor, like the specific heat jump at Tc, ΔC(Tc)/γnTc, the electron–phonon coupling constant, λe−ph, the upper Hc2 and thermodynamic critical Hc0 fields, the penetration depth λ, coherence length ξ and the Ginzburg–Landau parameter κ are evaluated. The estimated values of parameters such as 2Δ0/kBTc, ΔC(Tc)/γnTc, N(EF) and λe−ph suggest that Mo3Sb7 belongs to an intermediate-coupling regime. The electronic band structure calculations indicate that the density of states near the Fermi level is formed mainly by the Mo-4d orbitals and that there is no overlap between the Mo-4d and Sb-sp orbitals. 相似文献
19.
The surface morphology of in situ deformed, pre-deformed, undeformed and crystallized Zr47.9Ti0.3Ni3.1Cu39.3Al9.4 bulk metallic glass in air at elevated temperatures are investigated to improve the understanding of the effect of plastic deformation on the oxidation. The plastic deformation is performed through instrumental macroindentation, while could provide a well developed shear offset pattern around the indents at lower temperatures. The oxidation experimental results show that the in situ formed and pre-existed shear bands are more susceptible to oxidation. The plastic deformation region during homogenous deformation has also a significant oxidation sensitivity compared with undeformed material. To clarify the oxidation mechanism of shear band, specimens with surface scratches were also studied. The mechanism for the preferential oxidation of shear bands and of the plastic deformation region is discussed. 相似文献
20.
《Scripta materialia》2002,46(1):43-47
Tensile behavior of a bulk metallic glass Pd40Ni40P20 was characterized under both quasi-static and dynamic strain rate conditions. No major difference was observed. Multiple shear bands formed in samples tested at the dynamic strain rate. However, shear band interaction appears to have an insignificant effect on the plasticity of the alloy. 相似文献