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.     

Correlation between the elastic modulus and the intrinsic plastic behavior of metallic glasses: The roles of atomic configuration and alloy composition

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.     

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.     

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.     

Non-crystallographic shear banding in crystal plasticity FEM simulations: Example of texture evolution in α-brass

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.     

Local order influences initiation of plastic flow in metallic glass: Effects of alloy composition and sample cooling history

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.     

Plastic deformability of metallic glass by artificial macroscopic notches

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.     

Water soluble conducting polymer composite of polyvinyl alcohol and leucine: An effective acid corrosion inhibitor for mild steel

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 (E_a, ΔH_o, ΔS_o for corrosion and ΔG_ads, ΔH_ads, ΔS_ads for adsorption) reveal a strong interaction between inhibitor and mild steel surface. The negative values of ΔG_ads 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.     

Role of nanocrystals on the plasticity of amorphous alloy

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.     

Cryogenic temperature plasticity of a bulk amorphous alloy

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.     

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.     

Enhancement of plasticity in Zr-based bulk metallic glasses electroplated with copper coatings

Electrodeposition was used to coat copper films on the surface of the BMG pillars (bulk metallic glasses) of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (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.     

A nanoindentation study of serrated flow in bulk metallic glasses

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.     

Structural origins for the high plasticity of a Zr–Cu–Ni–Al bulk metallic glass

The structural origins for the high plasticity of a Zr₅₃Cu_18.7Ni₁₂Al_16.3 (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.     

Shear bands of as-cast and semi-solid Ti48Zr27Cu6Nb5Be14 bulk metallic glass matrix composites

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.     

The application of radiation diffuse scattering to the calculation of phase diagrams of F.C.C. substitutional alloys

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.     

The dependence of shear modulus on dynamic relaxation and evolution of local structural heterogeneity in a metallic glass

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 G_I: one (μ) is related to the atomic bonding strength of solid-like regions and the other (G_II) 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 = G_I + G_II) 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.     

Low-temperature specific heat of the superconductor Mo3Sb7

The low-temperature specific heat of a superconductor Mo₃Sb₇ with T_c = 2.2 ± 0.05 K has been measured in magnetic fields up to 5 T. In the normal state, the electronic specific heat coefficient γ_n, and the Debye temperature Θ_D are found to be 34.5(2) mJ mol⁻¹ K⁻² and 283(5) K, respectively. The enhanced γ_n 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/k_BT_c = 4.0 and 2Δ₂/k_BT_c = 2.5, and relative weights of the molar electronic heat coefficients γ₁/γ_n = 0.7 and γ₂/γ_n = 0.3. Some characteristic thermodynamic parameters for the studied superconductor, like the specific heat jump at T_c, ΔC(T_c)/γ_nT_c, the electron–phonon coupling constant, λ_e−ph, the upper H_c2 and thermodynamic critical H_c0 fields, the penetration depth λ, coherence length ξ and the Ginzburg–Landau parameter κ are evaluated. The estimated values of parameters such as 2Δ₀/k_BT_c, ΔC(T_c)/γ_nT_c, N(E_F) and λ_e−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.     

Oxidation features of plastic deformed Zr-based bulk metallic glass

The surface morphology of in situ deformed, pre-deformed, undeformed and crystallized Zr_47.9Ti_0.3Ni_3.1Cu_39.3Al_9.4 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.     

Dynamic response of a Pd40Ni40P20 bulk metallic glass in tension

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.