Nucleation Of Shear Bands Research Articles

The role of the interface in a Ti‐based metallic glass matrix composite with in situ dendrite reinforcement

Many in situ metallic glass matrix (MGM) composites have been developed as promising engineering materials having distinguished properties because of sharing virtues of high strength of metallic glasses and large plasticity of crystal phase, for example, Ti47Zr19Be15V12Cu7 MGM composites (the yield strength: 1600 MPa, the fracture strength: 3024 MPa and the total strain 32.6%). Although the toughening mechanisms in these materials have been investigated, the role of the interface bridging the ductile dendrite and the glass phase is still unclear. To this aim, specimens of the as‐received and after compression of this Ti‐based MGM composite were investigated to by using the transmission electron microscopy and the high‐resolution transmission electron microscopy. The results of the microstructure investigation indicated that the interface in the composite consists of a nanosized transform layer with an approximate width of 4 nm. During plastic deformation, the interface either suppresses plastic deformation caused by dislocations on the dendrite side or initiates nucleation of multiple shear bands throughout nanocrystallization on the glass phase side nearby the interface, which is favorable for the plastic deformation of the material. Copyright © 2014 John Wiley & Sons, Ltd.

Metallic glass–steel composite with improved compressive plasticity

A Zr52.5Cu18Ni14.5Al10Ti5 bulk metallic glass toughened with a commercially available spring-shaped steel wire has been produced by centrifugal casting. The addition of the steel spring significantly affects shear band nucleation and propagation through the blockage, deflection and multiplication of shear bands at the glass–spring interface. As a result of the more homogeneous distribution of the plastic strain, the room temperature plasticity increases from 0.9% for the monolitic glass to about 4% for the glass–spring composite. Given the low volume fraction of the spring used in the composite (4.2vol.%), these results demonstrate the extreme effectiveness of the steel spring for improving the plasticity of the metallic glass.

Nucleation of shear bands in amorphous alloys

The initiation and propagation of shear bands is an important mode of localized inhomogeneous deformation that occurs in a wide range of materials. In metallic glasses, shear band development is considered to center on a structural heterogeneity, a shear transformation zone that evolves into a rapidly propagating shear band under a shear stress above a threshold. Deformation by shear bands is a nucleation-controlled process, but the initiation process is unclear. Here we use nanoindentation to probe shear band nucleation during loading by measuring the first pop-in event in the load-depth curve which is demonstrated to be associated with shear band formation. We analyze a large number of independent measurements on four different bulk metallic glasses (BMGs) alloys and reveal the operation of a bimodal distribution of the first pop-in loads that are associated with different shear band nucleation sites that operate at different stress levels below the glass transition temperature, Tg. The nucleation kinetics, the nucleation barriers, and the density for each site type have been determined. The discovery of multiple shear band nucleation sites challenges the current view of nucleation at a single type of site and offers opportunities for controlling the ductility of BMG alloys.

Influence of electropulsing treatment on the recrystallization and texture of Ni9W alloy strip

Abstract

Varying tensile fracture mechanisms of Cu and Cu–Zn alloys with reduced grain size: From necking to shearing instability

In ultrafine-grained (UFG) or nanocrystalline (NC) materials, achieving high strength often induces loss of ductility due to the formation of shear fracture without obvious necking feature. To investigate mechanical properties and fracture mechanism with reduced grain size, the macroscopic tensile fracture behaviors of UFG or NC Cu and Cu–Zn alloys were systematically investigated. It is found that the tensile strength and uniform elongation of UFG or NC Cu and Cu–Zn alloys display simultaneously increasing trend. The limitation of ductility can be attributed to the occurrence of shear bands in these materials; their main characteristic, the shear fracture angle, decreases with decreasing grain size as well as the degree of necking. The macroscopic fracture mechanisms are explained using an Ellipse criterion in terms of modification of the stress state in the fracture zone. It is suggested that the reduction of necking degree results from the geometrical hardening effect due to the change of the stress state. As the degree of necking decreases with decreasing grain size, the tensile shear fracture angle decreases too in the UFG/NC materials. Thus improving the geometrical hardening ability may effectively inhibit the formation of shear fracture, as the nucleation of shear bands becomes difficult.

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.

On the nucleation and growth of kink and shear bands

Similarities and differences between the phenomena of kink banding in compressed layered structures and shear banding in compressed granular media are explored. Simple models are introduced for both, and the focus is directed onto how they can nucleate from the perfectly flat state. A convincing scenario is found for each in which a mode develops from an initial bifurcation into a periodic state, followed by rapid localization under falling load, while retaining decaying but wavy tails. At a certain lower critical load, the tails lose their waviness, and the expected form of the kink or shear band appears. In each case, good numerical evidence is provided for the existence of this form of behaviour. A second potential instability for the layered case is also explored, linked to the appearance of a critical force dipole that overcomes bending stiffness locally at some point along the length. This mode, which should appear with non-wavy decaying tails at the lower of the two critical loads mentioned earlier, proves somewhat elusive. Evidence is found for its existence in the linearized approximation to the layered model, but the search for numerical solutions to the underlying nonlinear equation is hindered by a shortage of suitable boundary conditions.

Enhancing the plasticity of metallic glasses: Shear band formation, nanocomposites and nanoglasses investigated by molecular dynamics simulations

We investigate the influence of various microstructural features on the deformation behavior of binary Cu64Zr36 glasses by molecular dynamics computer simulations and discuss how and why the very same modifications established for enhancing the strengths of crystalline materials, namely the insertion of solutes, precipitates and grain boundaries, can be used for tuning the mechanical properties of metallic glasses. First, by testing bulk samples with and without open surfaces under tensile load, we show that the condensation of shear transformation zones into shear bands can occur as heterogeneous but also as a homogeneous nucleation process. Then, the influence of crystalline nanoprecipitates on shear band nucleation and propagation is investigated. Finally, we study the effect of grain size and composition on the deformation behavior of nanoglasses and nanoglass composites. The results reveal that glass–glass interfaces act as structural heterogeneities ,which promote shear band formation and prevent strain localization.

Collective evolution dynamics of multiple shear bands in bulk metallic glasses

The collective behavior of multiple shear bands was investigated under in situ four-point bending tests of a Zr-based bulk metallic glass (BMG) over a wide range of sample scales. The self-organization of shear band pattern, characterized by shear band spacing and shear offset, is observed with the variation of sample size and bend curvature, which presents significant size effect and tension–compression asymmetry. To unveil these fundamental behaviors, an analytical model for the evolution dynamics of multiple shear banding is developed for BMGs. In this model, both micro-structural evolution and pressure sensitivity are taken into account by introducing a new law for the stress softening of BMGs within the framework of continuum mechanics; the collective evolution of shear bands is regarded as the coupling result of the structural softening, the momentum diffusion, and the energy conservation. Applying the proposed theoretical model to the bending deformation of BMGs, the analytical solutions of shear band spacing, shear offset and failure strain are obtained. The fundamental behaviors of multiple shear bands are uncovered, in line with the experimental observations: notable scaling laws are found in the evolution of shear band spacing and shear offset, and the inhomogeneous size effect of plasticity is revealed by a transition from weak to strong size-dependence of failure strain with decreasing sample thickness. To be further, a competing map of shear band nucleation and propagation is established based on energy dissipation. The underlying mechanism of these size dependent behaviors of multiple shear bands in BMGs is found to be ascribed to the energy dissipation competition between the nucleation and propagation of shear bands.

Recent developments in ductile bulk metallic glass composites

Abstract

Bending behavior of electrodeposited glassy Pd–P and Pd–Ni–P thin films

The bending behavior of electrodeposited Pd–P and Pd–Ni–P glassy films was investigated. The critical thickness for the transition from localized to non-localized deformation depends on temperature and strain rate, whether the yield strength is size dependent or not. The drop in Young’s modulus induced by the decreased strain rate and increased temperature results in a smaller decrease in the stress for shear band propagation in comparison with the stress for shear band nucleation, leading to an increased critical thickness for non-localized deformation.

Ti-based amorphous/nanocrystal composite with high ductility and strain-hardening

The mechanical behavior of a Ti-based amorphous/nanocrystal composite was studied by micro-compression testing. The results show that this composite has high ductility and strain-hardening. The deformability improvement is attributed to the nucleation, multiplication and interaction of localized shear flows and shear bands during deformation. Microscopic observations indicate that compressive loading facilitates nanocrystal precipitation in the amorphous matrix, which interact with crack propagation through the amorphous matrix, resulting in remarkable plasticity.

On the microstructure–tensile property correlations in bulk metallic glass matrix composites with crystalline dendrites

Bulk metallic glass (BMG) matrix composites with crystalline dendrites as reinforcements exhibit a wide variance in their microstructures (and thus mechanical properties), which in turn can be attributed to the processing route employed, which affects the size and distribution of the dendrites. A critical investigation on the microstructure and tensile properties of Zr/Ti-based BMG composites of the same composition, but produced by different routes, was conducted so as to identify “structure–property” connections in these materials. This was accomplished by employing four different processing methods—arc melting, suction casting, semi-solid forging and induction melting on a water-cooled copper boat—on composites with two different dendrite volume fractions, Vd. The change in processing parameters only affects microstructural length scales such as the interdendritic spacing, λ, and dendrite size, δ, whereas compositions of the matrix and dendrite are unaffected. Broadly, the composite’s properties are insensitive to the microstructural length scales when Vd is high (∼75%), whereas they become process dependent for relatively lower Vd (∼55%). Larger δ in arc-melted and forged specimens result in higher ductility (7–9%) and lower hardening rates, whereas smaller dendrites increase the hardening rate. A bimodal distribution of dendrites offers excellent ductility at a marginal cost of yield strength. Finer λ result in marked improvements in both ductility and yield strength, due to the confinement of shear band nucleation sites in smaller volumes of the glassy phase. Forging in the semi-solid state imparts such a microstructure.

Intrinsic and extrinsic size effects on deformation in nanolayered Cu/Zr micropillars: From bulk-like to small-volume materials behavior

By using microcompression methodology, deformation of nanolayered Cu/Zr micropillars was systematically investigated within wide ranges of intrinsic layer thickness (5–100nm) and extrinsic sample size (300–1200nm). The intrinsic size effect, extrinsic size effect and their interplay were respectively revealed. Competition between the intrinsic and extrinsic size effects leads to experimental observation of a critical layer thickness of ∼20nm, above which the deformation is predominantly intrinsic-size-related and insensitive to sample size, while below which the two size effects are comparable. The underlying deformation mechanisms were proposed to transform from bulk-like to small-volume materials behavior. Deformation mode is correspondingly transited from homogeneous extrusion/barreling to inhomogeneous shear banding, but the two competing modes coexist in the layer thickness range from ∼50 to 20nm. In the regime of shear deformation, the extrinsic size dependence is displayed in that the deformation was controlled by shear bands nucleation in larger pillars while controlled by shear bands propagation in smaller pillars. A deformation mode map is developed to clearly elucidate the coupling intrinsic and extrinsic size effects on the deformation mode of nanolayered pillars.

Ambient temperature embrittlement of a Zr-based bulk metallic glass

Three-point bending experiments were performed on Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105) bulk metallic glass (BMGs) over a wide range of ambient temperatures varying from the liquid nitrogen temperature (77 K) to about boiling water temperature (370 K). The results demonstrated that the flexural strength of the Zr-based BMG monotonously increased with decreasing of test temperature, whereas plastic deformation ability displayed a maximum at a moderate temperature (about room temperature). That is, the plastic deformation ability was deteriorated by either higher or lower temperature than room temperature through affecting shear band nucleation and propagation within materials respectively.

Evolution of shear banding in fully dense nanocrystalline Ni sheet

Compared with the coarse-grained counterpart, nanocrystalline (NC) metals have higher strength simultaneously with a decrease in ductility, strain localization is a main factor contributed to the early failure of NC metals during plastic deformation. This work deals with the study of shear banding in fully dense electrodeposited NC Ni sheet with sample dimensions at tens of millimeters under quasi-static uniaxial tensile load through the use of a strain gage calculated by digital image correlation technique. Shear band nucleation, broadening process and failure point were recognized. It is identified that maximum shear strain happens in the middle of the shear band where crack initiates first in this experiment. This indicates that the shear banding induces the failure of the NC Ni sample. Meanwhile, physical characteristics of the shear band, such as inclination and width of single full-developed shear band, were determined quantitatively. The results show that the inclination of shear band is about 63°, as well as the width of shear band is in sub-micrometer range. To investigate the micro-mechanisms during the shear banding process in the NC Ni sample, in situ tensile testing in a transmission electron microscope was conducted, the results suggest that grain boundary migration and grain coalescence are the main carriers during the propagation of shear band.

Interface Constraints on Shear Band Patterns in Bonded Metallic Glass Films Under Microindentation

When using the bonded interface technique for indentation tests, the semicircular and radial shear bands can be observed on the top surfaces and bonded interfaces in bulk metallic glasses (BMGs). In addition to the stress relaxation effects at the bonded interface, indentation tests on bonded BMG films on the steel platen further demonstrate the effects of the film/substrate interface on shear band patterns. The understanding of these shear band patterns will help design internal constraints to confine shear bands and thus to prevent brittle failure of BMGs. In contrast to previous studies, which connect shear band directions to principal shear stress or effective stress, as in the Mohr–Coulomb model, this article adopts the Rudnick–Rice instability theory—shear bands are a result of loss of material stability but are not a yield phenomenon. Shear band directions depend on material constitutive parameters (including Poisson’s ratio, coefficient of internal friction, and dilatancy factor) and principal stresses. Consequently, internal constraints such as the bonded interface and film/substrate interface may redistribute the stress fields and thus affect the shear band propagation directions. Finite element simulations were performed to determine the contact stress fields using continuum plasticity model. It is found that semicircular shear bands on the bonded interface follow the direction of the second principal stress, while radial shear band patterns depend on the two in-plane principal stresses. With the presence of film/substrate interfaces, the radial shear bands will be “reflected” at the interface, and the semicircular shear bands change directions and end at the interface. It should be noted that the actual stress field differs from the continuum plasticity simulations because of the strain localizations associated with shear bands. To this end, an explicit history of shear band nucleation and propagation is simulated by the free volume model, which reproduces the change from radial to semicircular shear bands when interface relaxation is introduced. These predictions agree well with our experimental observations of microindentation tests on two Zr-based BMG films laterally bonded and placed on a steel platen.

1024 MD法・連続場変換法の併用による非晶質金属の変形機構解析(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

1024 MD法・連続場変換法の併用による非晶質金属の変形機構解析(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

Nucleation reactions during deformation and crystallization of metallic glass

Nucleation reactions play a central role in the synthesis of both bulk metallic glasses and nanostructured materials. For nanostructured materials it is necessary to promote a high nucleation density without significant growth or coarsening. Beyond crystallization reactions nucleation of shear bands is critical for promoting a homogeneous flow and useful ductility for structural applications of bulk metallic glass. The study and analysis of nucleation reactions for these different situations requires a consideration of the stochastic nature of nucleation, the influence of heterogeneous sites, and the controlling transport properties. For shear band nucleation, the stochastic nature can be effectively probed by instrumented nanoindentation tests. The analysis of a statistically significant number of measurements of the first pop-in shear band nucleation events reveals at least two main nucleation sites. In nanostructured composites, the initial nucleation stage is influenced by transient effects as reflected in the delay time prior to steady state nucleation and by heterogeneous nucleation sites that are related to medium range order regions in Al-base amorphous alloys. Moreover, the early growth characteristics are linked to the maximum achievable particle density. The new developments and insight on the fundamental understanding of nanostructure reaction mechanisms offer valuable guidance for control of nanoscale microstructures and for promoting ductile deformation behavior.

Single shear-band plasticity in a bulk metallic glass at cryogenic temperatures

At cryogenic temperatures bulk metallic glasses can sustain higher plastic strains than at room temperature. This is generally believed to result from an intrinsic shear-band nucleation rate that increases with decreasing temperature. Here we report on inhomogeneous flow operating via a single shear band even at cryogenic temperatures, challenging the presupposition of increased shear-band activity. The results provide a new interpretation of non-serrated flow and explain, via a simple viscosity law, the correspondingly observed strength increase with decreasing temperature.