Radial Shear Bands Research Articles

Shear band evolution related with thermal annealing revealing ductile-brittle transition of Zr35Ti30Be27.5Cu7.5 metallic glass under complex stress state

The ductile-brittle transition (DBT) caused by thermal annealing was systematically investigated by means of small punch test (SPT), accompanied by finite element simulation, uniaxial compression and nanoindentation in Zr-based metallic glasses (MGs) annealed for 1 h in a wide temperature range, i.e., sub-Tg, Tg ∼ Tx and above-Tx. In multiaxial loading mode, three deformation mechanisms, i.e., (ⅰ) circumferential shear bands (CSBs) + radial shear bands (RSBs), (ⅱ) RSBs and (ⅲ) radial cracks (RCs), can be found separately on MGs annealed sub-Tg, Tg ∼ Tx and above-Tx, leading to a transition in deformation mode from ductility to brittleness. First, for MGs with sub-Tg, Tg ∼ Tx annealing, the reduction in the number of CSBs and RSBs, as well as the disappearance of CSBs, are responsible for the ductile-brittle transition, which is attributed to the annihilation of free volume caused by structural relaxation and the reduction of released elastic energy. Based on simulation results, the decrease in plasticity of MGs is accompanied by the reduction in stress distribution area, which is in agreement with the change in the number and distribution area of shear bands. The initiation of multiple shear bands and their strong interaction dramatically improve the plasticity of MGs under the synergistic effect of large stress distribution area and high content of free volume. Second, the embrittlement of MGs with above-Tx annealing is mainly attributed to the rapid formation and extension of RCs induced by the occurrence of crystallization. Meeanwhile, simulation results indicate that stress distribution area decreases gradually and facilitate the formation of stress concentration, leading to the rapid extension of RCs and the occurrence of catastrophic fracture of MGs. In addition, MGs with sub-Tg, Tg ∼ Tx and above-Tx annealing not only exhibit such DBT in uniaxial compression, but also show a soft-hard transition in nanoindentation test.

Size dependence of ductile to brittle transition of Zr-based metallic glasses under multiaxial loading

The size dependence of ductile to brittle transition of Zr-based metallic glasses was systematically investigated by small punch test. It was found that the metallic glasses with thickness ranging from 50 to 700 μm displayed an obvious transition from ductility to brittleness. Compared with thick samples, thin samples exhibit higher strength and better plasticity, which is attributed to the size-dependent transformation from soft state to hard state dominated by the deformation factor ( α = τ / σ ). Specifically, two significant transitions in deformation behavior at sample thickness of 200 and 500 μm are attributed to two critical deformation factors of α c 1 and α c 2 , which are responsible for the initiation of circumferential and radial shear bands, respectively. Specifically, only several micro-cracks can be observed on sample surface with α ≤ α c 2 . While α c 1 ≥ α ≥ α c 2 , multiple radial shear bands are activated and uniformly distributed on the sample surface. Further, when α ≥ α c 1 , multiple circumferential shear bands primarily initiate under high shear stress and large number of radial and circumferential shear bands can be observed on sample surface. The initiation and interaction of multiple radial and circumferential shear bands significantly enhance the ductility of the samples with α ≥ α c 1 .

Indentation and deformation behavior of FeCo-based bulk metallic glass alloys

FeCo-based bulk metallic glass with the compositions (Fe0.5Co0.5)69−xNb6B25+x (x=0, 2, 4) were cast using electric arc melting and suction casting technique. The samples were tested and analyzed in as-cast, sub-Tg annealed and partially crystalline conditions. Vicker’s microhardness was found to increase after annealing and partial crystallization. For high boron content hardness values approaching 1400 HV were obtained in partially crystalline samples. A compressive strength value of 4776MPa was calculated using hardness data. Sub-Tg annealing resulted in a decrease in the size of deformation zone formed underneath the hardness indenter after indentation. The bulk metallic glass alloys were embrittled by the partial crystallization treatment. It resulted in the formation of corner cracks during indentation. The size of the deformation zone increased with an increase in load. Semicircular and radial type of shear bands were observed as in the deformation zone with a higher number of radial shear bands observed in the annealed samples compared to as-cast samples. A comparison is made between the size of the deformation zone obtained experimentally and that predicted by the available models.

Mechanical response of Ti-based bulk metallic glass under static and dynamic indentations

Vickers hardness and plastic deformation of Ti-based bulk metallic glass (Ti-BMG) were investigated under both static and dynamic indentations. Dynamic hardness is obviously greater than that under static indentation, which is attributed to the combination effects of energy barriers and separation of nanocrystallizations from the metallic glass during dynamic indentation. Although dynamic indentation induced more severe deformation than that under static indentation, the deformation characteristics in the two loading cases are nearly the same: both exhibiting semi-circular shear bands on the top surface and a mixture of semi-circular and radial shear bands underneath the indentation tips. The most obvious difference between the two kinds of indentations is that substantial successive shear bands accompanied by numerous shear-offsets formed in dynamic indentation while obviously less shear bands and shear-offsets formed in static indentation.

On the subsurface deformation of two different Fe-based bulk metallic glasses indented by Vickers micro hardness

Bonded interface technique was employed to examine the nature of subsurface deformation during Vickers micro indentation in two iron-base bulk metallic glasses, (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 (BMG-1) and Fe40Co8Cr15Mo13Y2C16B6 (BMG-2). Quantitative information such as the subsurface deformation zone size was recorded for indentation loads ranging from 200 to 5000 gr. The results showed that the BMG-2 specimens had an average hardness value higher than those observed in the BMG-1 specimens. The trends of the hardness vs. indentation load in the BMG-1 specimens were found to be related to the pressure sensitive index, while in the BMG-2 specimens, the cracking events and deformation-induced creation of free volume were responsible for the hardness tendency change. Observations of the deformation zones indicated that they deformed noticeably through two types of semi-circular and radial shear bands and the density of the radial shear bands was much more in the annealed specimens compared to the as-cast specimens. The relaxed and partially crystallized BMG-2 specimens exhibited cracking, ripple-like pattern as well as cracking and fragmentation, respectively. A simplified R = CP0.5 model was used to analyze the shear band zone size in the subsurface and specimens brittleness.

The effect of liquid phase separation on the Vickers microindentation shear bands evolution in a Fe-based bulk metallic glass

The Vickers microindentation experiments and associated plastic deformation in as-cast and annealed (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass was conducted. In addition to the bulk indentation behavior, the shear band morphology underneath the Vickers microindenter was examined by employing the bonded interface technique. Microstructural characterization revealed that a liquid phase separation occurred during melting process. Atomic force microscopy of the glassy matrix of the as-cast specimen reveals the composition inhomogeneity induced by the liquid phase separation. This effect generates shear band branching or deflection during the shear band propagation. For the bulk indentation, the trends in the hardness vs. indentation load were found related to the pressure sensitive index and the phase separation process simultaneously. The results show that the as-cast as well as the annealed specimens are deformed through semi-circular and radial shear bands. In addition, in the partially crystalized specimen, the change in the properties and microstructure of the BMG induced by the partial crystallization treatment and phase separation process resulted in tertiary shear bands formation.

Plasticity enhancement of Zr-based bulk metallic glasses by direct current electropulsing

Direct current electropulsing was used to improve the plasticity of (Zr53Cu30Ni9Al8)99.5Si0.5 bulk metallic glasses. After the electropulsing treatment, the specimen showed reductions in both the glass transition and the crystallization temperatures while retaining its amorphous structure, and both Young's modulus and the hardness decreased while the nanoindentation loading curve became more serrated. Using the bond-interface method and Vickers indentation, the treated specimen showed more branching of semi-circular shear bands and less radial shear bands compared to its as-cast counterpart. The possible plasticity enhancement mechanism of the electropulsing treatment was also discussed.

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.

Internal microstructures in shearing giant electrorheological fluids

The macroscopic physical properties of giant electrorheological (GER) fluids are crucially dependent on their internal particle microstructure. However, GER fluids are often sheared in work, which makes it difficult to clearly observe their internal microstructure via existing experimental tools. Here, an experimental method has been developed to observe the lamellar microstructures of GER fluids, which fill the space between two relatively rotating parallel disk electrodes (disk–disk geometry). Nevertheless, only microstructures in surface regions can be observed in the shearing process via some experimental approaches. We incorporated two fluid phase models of ER fluids and the mechanism of GER fluids in order to study the GER fluids in the shearing process. Internal microstructures of GER fluids shearing in the whole region simultaneously were discovered for the first time. The internal system produces a variety of complex patterns and dynamics. The patterns are caused by the fluctuation frequency of radial shear bands; and there exist vertical shear bands which give rise to delamination perpendicular to the flow direction. The distribution of radial and vertical shear bands in the inner geometry is governed by the Onsager principle of minimum energy dissipation rate.

Mechanical response of amorphous ZrCuTi/PdCuSi nanolaminates under nanoindentation

In this paper, the relationship between mechanical response and shear band evolution for ZrCuTi (ZCT)/PdCuSi (PCS) nanolaminates under nanoindentation is discussed. Comparing to the monolithic amorphous ZCT and PCS films, the ZCT/PCS nanolaminates exhibit enhanced hardness and a more homogenous deformation mode. Cross-sectional transmission electron microscopy observations show that the shear-band patterns beneath the indent in ZCT/PCS nanolaminates are irregular and convoluted, and appear to be a mixture of the semi-circular shear bands and radial shear bands.

On the deformation morphology of bulk metallic glasses underneath a Vickers indentation

The techniques commonly used for observing the deformation mechanisms underneath a Vickers indentation in metallic glasses (chemical etching, bonded interface) induce artefacts such as cracks or semi-circular shear-bands. We propose an alternative technique based on the propagation of indentation corner cracks through a pre-existing imprint, which is possible in metallic glasses such as iron-based compositions. With this procedure, only radial shear-bands are observed. Comparisons between the chemical etching or the bonded interface techniques and the new technique are made.

Vickers indentation tests in a Zr 62.55Cu 17.55Ni 9.9Al 10 bulk amorphous alloy

Vickers indentation tests were conducted on a Zr 62.55Cu 17.55Ni 9.9Al 10 bulk amorphous alloy to investigate the evolution of shear bands and its plastic deformation dimension via a bonded interface technique. Under all indentation loads, the plastic deformation is accommodated through semi-circular and radial shear bands. The plastic deformation dimension increases with increasing the indentation loads. A simplified λ = C( P) 0.5 model was put forward to predict and estimate the plastic deformation dimension characterized by shear bands in the subsurface. For the Zr 62.55Cu 17.55Ni 9.9Al 10 amorphous alloy, C is about 15.314 µm/N 0.5. The normalized shear band zone is independent to the indentation load.

Direct observations on the evolution of shear bands into cracks in metallic glass

The evolution of shear bands (SBs) into cracks was observed by using a high-resolution scanning electron microscope in Zr59Cu20Al10Ni8Ti3 metallic glassy samples after a small punch test with different strain rates. As shear strain increased along a radial SB, three distinctive regions of morphologies were found (I) bonded SB, (II) microcrack plus bonded SB, and (III) full crack. In region II with moderate shear strain, some glassy “extrusions” were also observed. Once shear offset increases to a critical value, the SB becomes a full crack. For two different SBs in one specimen, the critical shear offsets maintain approximately the same value, which sheds light on the critical shear failure condition of metallic glass. The critical shear offset was also found to be sensitive to the strain rate and a higher strain rate led to less critical shear offset. It is suggested that the structure evolution and heat evolution within a shearing SB should be responsible for the previous results.

Assessment of Localized Deformations in Sand Using X-Ray Computed Tomography

Abstract The internal fabric and localized deformation patterns of triaxial sand specimens were investigated using computed tomography (CT). Three displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses (0.05, 0.52, and 1.30 kPa) in a microgravity environment aboard the Space Shuttle during the NASA STS-79 mission. CT scanning was performed on these flight specimens, as well as on an uncompressed specimen and a specimen tested in a terrestrial laboratory at 1.30 kPa effective confining stress. CT demonstrated high accuracy in detecting specimen inhomogeneity and localization patterns. Formation of deformation patters is dependent on the effective confining stress and gravity. Multiple symmetrical radial shear bands were observed in the specimens tested in a microgravity environment. In the axial direction, two major conical surfaces were developed. Nonsymmetrical spatial deformation was observed in the 1-G specimen. Analysis tools were developed to quantify the spatial density change. Void ratio variation within and outside the shear bands is calculated and discussed.