Morphology Of Shear Bands Research Articles

List of Abstracts

. Production efficiency as well as the reduction of energy consumption and waste are the main drivers for the improvement of large-scale production in chemical industry. In addition, the demand for new processes – these days especially related to the chemical energy conversion as basis for the Energiewende – are additional triggers for innovation. Catalysis, well recognized by the public e.g. in form of the 3-way automobile catalyst, plays an important role for about 90% of all processes in chemical industry. The development of new and improved catalysts is a challenge for chemists – as is the search for new structural materials for mechanical engineers. Due to the often-necessary noble and thus expensive metals (e.g. palladium) in many catalysts, structural and chemical function have been developed separately in the two communities. Recent progress in catalytically active materials allows replacing expensive noble metals in selected catalytic processes by significant cheaper elements [1]. This offers the new perspective to combine structural and chemical functionality by innovative materials. Within this area, intermetallic compounds with their peculiar combination of crystal and electronic structure [2], represent an interesting class of materials to address this vision as will be shown in the contribution.

Shear Band Morphology of Indented Region in Cu-Based Metallic Glass

Plastic deformation after indentation of the metallic glass Cu47Ti35Zr11Ni6Si1 at different loading conditions was examined. Discontinuities on the loading curves were observed, the magnitude of which depends on the loading rate. The presence of these discontinuities is influenced by the precise shape of the indentation tip. At lower loading rates and using a cube corner indenter tip the discontinuities on the loading curves are more pronounced. An increase of the loading rate tends to diminish instantaneous plastic deformation as appear by pop-ins. Using a Berkovich type indenter tip the plastic deformation is more steady. It is concluded that the final morphology of the pile-up area strongly depends on the geometry of the indenter tip, whereas no correlation between discontinuities in the loading part of the indentation curve and the formation of shear band patterns was observed.

The impact of ultrasonic vibrations on shear banding in nanocrystalline titanium

The mechanical properties of nanocrystalline titanium of commercial purity were found to be influenced substantially by superimposed ultrasonic vibrations, since by this treatment, dislocation slipping and grain-boundary sliding under plastic deformation are modified. An effective decrease of the grain-boundary strength within the shear band was observed. The impact on the shear banding, on the shear-band morphology, and on its structure was investigated by transmission and scanning electron microscopy.

The shear band controlled deformation in metallic glass: a perspective from fracture.

Different from the homogenous deformation in conventional crystalline alloys, metallic glasses and other work-softening materials deform discontinuously by localized plastic strain in shear bands. Here by three-point bending test on a typical ductile Pd-Cu-Si metallic glass, we found that the plastic deformed region during fracture didn’t follow the yielding stress distribution as the conventional material mechanics expected. We speculated that such special behavior was because the shear bands in metallic glasses could propagate easily along local shear stress direction once nucleated. Based on a 3D notch tip stress field simulation, we considered a new fracture process in a framework of multiple shear band deformation mechanism instead of conventional materials mechanics, and successfully reproduced the as-observed complicate shear band morphologies. This work clarifies many common misunderstandings on metallic glasses fracture, and might also provide a new insight to the shear band controlled deformation. It suggests that the deformation of metallic glasses is sensitive to local stress condition, and therefore their mechanical properties would depend on not only the material, but also other external factors on stress condition. We hope that start from this work, new methods, criteria, or definitions could be proposed to further study these work-softening materials, especially for metallic glasses.

Bending property enhancements of Zr55Cu30Al10Ni5 bulk metallic glass: Effects of various surface modifications

Effects of various surface modifications on four-point bending property improvements of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) are systematically studied. Surface modifications investigated include thin-film metallic glass coating, artificial scratches and femtosecond laser patterning. After modifications, the negligible plastic strain of 3-mm-thick BMG is found to markedly enhance to ~9.2%, without sacrificing its extraordinary strength. Instead, bending yield and fracture strengths in the surface-modified samples are increased to ~2.8GPa and ~4.0GPa, respectively, corresponding to a rise of ~66% from those of as-cast samples. In addition, the samples with surface modifications exhibit distinct shear band morphologies. Relationships between the shear band offset, shear band spacing and plastic strain of bent BMG samples are established. Our experimental results demonstrate that BMG is able to survive plastic flow without abrupt fracture through proper designs of surface modifications, in which large numbers of shear bands are formed to distribute the strain effectively upon loading.

The hardness and related deformation mechanisms in nanoscale crystalline–amorphous multilayers

Built upon recent findings that nanocrystalline–amorphous multilayers could possess both high strength and ductility due to nano-layer-limited structures, depth-sensing nanoindentation was employed to evaluate the hardness and shear band deformation behavior of nanoscale crystalline–Cu/amorphous–CuZr (C/A) multilayers among a wide range of amorphous layer thickness with constant crystalline Cu layer thickness. By varying individual layer thickness of amorphous layers, both weakening and strengthening effects on the hardness of C/A multilayers were observed, comparing with that derived from the rule of mixture, for which discontinuity of dislocation motion played a crucial role. A critical amorphous layer thickness of 20nm was identified, above which interface–dislocation–absorption dominated plastic deformation, causing the weakening effect, below which, continuous-dislocation-impediment took over and was responsible for the strengthening effect. Mechanisms underlying the observed shear band morphologies under indentation were also extendedly discussed.

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.

Primary Recrystallization Texture in Rolled (Fe<sub>81</sub>Ga<sub>19</sub>)<sub>99</sub>B<sub>1</sub> Sheet

The recrystallization texture in a rolled polycrystalline (Fe81Ga19)99B1 alloy sheet was investigated using X-ray and EBSD (Electron backscattering diffraction) methods. The η fiber (//RD) with dominant Goss ({110}) component was successfully obtained through sheet thickness after primary recrystallization. Goss remains the dominant texture component during further grain growth. The development of Goss can be attributed to the specific rolling induced favorable shear band morphology and deformation texture. The results provide a prospective route for the efficient recrystallization texture optimization and economical production of Fe-Ga sheets.

Effect of pre-annealing treatment on the compressive deformation and damage behavior of ultrafine-grained copper

Abstract The effect of short-term pre-annealing treatment on the compressive deformation and damage behavior of ultrafine-grained (UFG) copper produced by equal channel angular pressing (ECAP) has been studied by mechanical tests and microstructural observations. It is found that a controlled 10-min pre-annealing treatment on the UFG copper below the recrystallization temperature can result obviously in a higher strength (higher flow stress level) and a higher flow stability (lower stress softening rate) under uniaxial compression, as compared to the case of as-ECAPed sample. Observations of surface and interior deformation damage features indicate that, compared to the as-ECAPed copper sample, the pre-annealed ECAPed copper at less than recrystallization temperature may exhibit a better compressive plastic deformation capacity, namely, the destructive deformation morphology of large-scale shear bands normally observed in compressed as-produced UFG copper sample would not appear in compressed pre-annealed sample, except for a relatively smooth surface morphology featuring some ‘step’ reliefs with a few discontinuous shear bands and non-propagation voids. Finally, the deformation mechanism of different kinds of severe plastic deformation (SPD) processed UFG materials as well as their pre-annealing effects are further discussed, and a general rule is summarized as follows: an appropriate selection of pre-annealing temperature and annealing time is needed for improving both strength and ductility of SPDed materials with a microstructure comprising equiaxed grains and well-defined grain boundaries.

Microstructural evolution and mechanical properties of niobium processed by equal channel angular extrusion up to 24 passes

We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress–strain curves exhibit elastic–nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress–strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate.

Plastic Deformation in an Amorphous Ni-P Coating

An experimental and numerical investigation of the hardness and associated plastic deformation in as-deposited and as-annealed nickel-phosphorus (Ni-P) coatings was conducted. In addition to the indentation-deformation behavior, the deformation morphology underneath the indenter was examined. The yield strength extracted from the indentation data is as high as 5.6 GPa, indicating pressure-sensitive plasticity. Results show that the as-deposited Ni-P coating was deformed appreciably through the shear-band mechanism with semicircular and radial shear-band morphologies. From the incremental loading-unloading cyclic experiments, the phenomena on hardening and recovery, which have scarcely been recognized in amorphous materials at room temperature, were observed in the amorphous coating using instrumented nanoindentation. A numerical simulation of the interfacial indentation test between the Ni-P coating and the substrate reveals the pileup and shear bands of the Ni-P coating that were observed during the indentation tests.

Strong Goss Texture in Recrystallized Fe<sub>81</sub>Ga<sub>19</sub> Sheet

Fe81Ga19 sheets were produced by conventional rolling procedure. Recrystallization texture dominated with strong Goss ({110}) was successfully obtained through sheet thickness by primary recrystallization annealing. The development of Goss can be attributed to the favorable shear band morphology and deformation texture derived from the applied specific rolling process.

Effect of phase composition on self-organization of shear bands in Ti-1300 titanium alloy

Abstract The distribution characteristics of shear bands were investigated in the Ti-1300 titanium alloy by the thick-walled cylinder (TWC) technique. Self-organization of shear bands was observed and investigated in the single β phase, larger size and content of α phase as well as smaller size and content of α phase in α + β phase. The experimental results indicated obvious differences in the morphology and self-organization behavior of shear bands in these specimens. At the early stage, the spacings of shear bands in the specimens are close. Cracks occur early in the shear bands of the specimen with less α phase. At the later stage, the shear bands in the specimen composed of the single β phase propagate faster than that in the other specimens. The specimen with more α phase has the maximum average nucleation rate. The spacing of shear bands in it is smaller than that in the other specimens. As cracks appear early with heavy unloading, the specimen with less α phase has the largest shear band spacing in the three specimens.

Effect of orientation on self-organization of shear bands in 7075 aluminum alloy

The spatial distribution of shear bands was investigated in the rolled 7075 aluminum alloy through the thick-walled cylinder (TWC) technique with 0°, 90° and 45° angles between the aluminum alloy cylinder axial direction and the rolling direction. Self-organization of multiple adiabatic shear bands was observed in different orientation specimens and investigated by using Schmid factor theories. The experimental results indicated obvious differences in the morphology and self-organization of shear bands for the specimens. At the initial stage, the spacing of the shear bands in the 0° specimen is smaller than in the other specimens. The nucleation of the shear bands in the 90° specimen is early. Due to the shielding effect, fast-developed shear bands block the development of the neighboring smaller shear bands in the 90° specimen. The spacing of the shear bands in the 45° specimen is much larger than in the other specimens under the similar effective strain. At the late stage, a large number of shear bands nucleate in the 0° specimen, and the spacing of the shear bands is small. The shear bands in the 90° specimen are well-developed with obvious shielding effect and the largest spacing. The 45° specimen has the maximum average nucleation rate of the shear bands. Owing to the close Schmid factors of the slip systems of the 45° specimen, the spacing of the shear bands in the 45° specimen is still larger than in the 0° specimen.

Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass

Instrumented indentation experiments on a Zr-based bulk metallic glass (BMG) in as-cast, shot-peened and structurally relaxed conditions were conducted to examine the dependence of plastic deformation on its structural state. Results show significant differences in hardness, H, with structural relaxation increasing it and shot peening markedly reducing it, and slightly changed morphology of shear bands around the indents. This effect is in contrast to uniaxial compressive yield strength, σy, which remains invariant with the change in the structural state of the alloys investigated. The plastic constraint factor, C = H/σy, of the relaxed BMG increases compared with that of the as-cast glass, indicating enhanced pressure sensitivity upon annealing. In contrast, C of the shot-peened layer was found to be similar to that observed in crystalline metals, indicating that severe plastic deformation could eliminate pressure sensitivity. Microscopic origins for this result, in terms of shear transformation zones and free volume, are discussed.

Microstructure-Level Machining Simulation of Carbon Nanotube Reinforced Polymer Composites—Part I: Model Development and Validation

A microstructure-level finite element machining model has been developed to simulate the machining of carbon nanotube (CNT) reinforced polymer composites. The model integrates a failure model with a previously developed microstructure-based material model. The competition between ductile and brittle modes of failure in the polymer phase (polycarbonate) is captured by implementing the Gearing and Anand failure model calibrated at different temperatures. The CNT phase is given a simple strain-to-failure criterion. The proposed machining model has been validated at different orthogonal machining conditions for the plain polycarbonate and for composites with two different percentage loadings of CNTs. On an average, the model is seen to successfully predict the cutting forces with an accuracy of 8% and the thrust forces with an accuracy of 13.4% for all the materials. The machining model also predicts the continuous chip morphology and formation of adiabatic shear bands in plain polycarbonate and for composites with lower loadings of CNTs. On an average, the chip thicknesses are predicted within an accuracy of 14% for plain polycarbonate and 10% for the CNT composites.

Stress-induced structural transformation and shear banding during simulated nanoindentation of a metallic glass

Simulated nanoindentation tests on a three-dimensional model of a binary metallic glass-forming alloy reveal how the stress field and material structure interact to control deformation beneath the indenter. Initially, the stress field follows the Hertzian solution with fluctuations due to heterogeneities. Homogeneous or localized plastic deformation arises, depending on the processing history of the material. In the case of localized deformation, the first shear band initiates sub-surface, relaxing the local shear stress. The subsequent shear band morphology is observed to be rate-dependent. Deformation induced changes in material structure are characterized in terms of short-range ordering and free volume, with the former providing significantly greater signal-to-noise.

Morphologies of three-dimensional shear bands in granular media

We present numerical results on spontaneous symmetry breaking strain localization in axisymmetric triaxial shear tests of granular materials. We simulated shear band formation using the three-dimensional distinct element method with spherical particles. We demonstrate that the local shear intensity, the angular velocity of the grains, the coordination number, and the local void ratio are correlated and any of them can be used to identify shear bands; however, the latter two are less sensitive. The calculated shear band morphologies are in good agreement with those found experimentally. We show that boundary conditions play an important role. We discuss the formation mechanism of shear bands in the light of our observations and compare the results with experiments. At large strains, with enforced symmetry, we found strain hardening.

Hardness and plastic deformation in a bulk metallic glass

An experimental investigation into the Vickers hardness and associated plastic deformation in as-cast and annealed Pd42Ni40P18 bulk metallic glass was conducted. In addition to the bulk indentation behavior, the deformation morphology underneath the indenter and its variation with annealing time was examined by employing the bonded interface technique. For both the bulk and the interface indentations, the trends in the shear band induced plastic deformation zone sizes with the indentation load agree well with those predicted from the expanding cavity model. However, the yield strength extracted from the indentation data is higher than that measured in uniaxial compression, indicating pressure sensitive plasticity. Results show that the as-cast as well as the partially crystallized alloys deform appreciably through the shear band mechanism, with semi-circular and radial shear band morphologies. The latter gets increasingly prominent with increasing annealing time. Atomic force microscopy of the deformation region reveals increasing shear band heights with load, consistent with nanoindentation results. Whereas the spacing between semicircular shear bands was found to be independent of their distance from the tip of the indenter for the as-cast alloy, it was found to increase linearly, after a small zone of constant spacing, in the annealed alloys. Implications of this study in understanding the mechanical behavior of metallic glasses and their derivatives are discussed.

Deformation morphology underneath the Vickers indent in a Zr-based bulk metallic glass

Hardness and plastic deformation during Vickers indentation in as-cast, annealed, and fully crystallized Zr 57Cu 27Al 11Ni 5 bulk metallic glass was examined. Subsurface deformation morphology under the indenter tip was studied at various loads and for different annealing time. Appreciable plastic deformation, through shear banding, occurs in the as-cast and annealed alloys. Two different types of shear bands are observed. Their occurrence depends upon the amount of annealing and hence on the extent of crystallization. The fully crystallized alloy exhibits extensive cracking. Trends in the deformation zone size with load are consistent with the expanding cavity model, while the shear band morphology, particularly for the as-cast sample, attests the qualitative applicability of the slip line field theory.