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Jörg F. Löffler - One of the best experts on this subject based on the ideXlab platform.
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Shear-Band dynamics in metallic glasses
Advanced Functional Materials, 2015Co-Authors: Robert Maaß, Jörg F. LöfflerAbstract:The future of metallic glasses as an advanced structural and functional material will to a great extent depend on the understanding and control of their mesoscopic flow defects called Shear Bands. These defects are sweet-and-sour; sweet because they mediate macroscopic plasticity at room temperature, and sour because they quickly promote failure. In the past decade, fundamental research generated great progress in characterizing the role that Shear Bands play during plastic deformation of disordered systems, including metallic glasses. Similar to those in many other materials, Shear Bands in metallic glasses are only active for a very short time, which directed research focus towards topological, structural, chemical, and thermal properties of formed, but inactive Shear Bands. In this paper, recent progress in directly characterizing the Shear-Band dynamics in situ during straining experiments is presented. Various Shear-Banding stages are outlined, including formation, propagation, and arrest, as well as Shear-Band creep and aging. The results are discussed in a more general context of disordered materials, concluding with a summarizing overview of time-scales involved in Shear Banding, and describing future research directions that may lead to controlled Shear-Band plasticity in metallic glasses. Dynamic properties of Shear Bands are a key element for the design of plastically stable bulk metallic glasses. In this Feature Article, recent progress on in situ characterization of Shear-Band dynamics is summarized. The aim is to provide a comprehensive understanding of Shear-Band initiation, propagation, arrest, creep, and aging, and how they determine the plastic flow behavior of bulk metallic glasses.
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compositional dependence of Shear Band dynamics in the zr cu al bulk metallic glass system
Applied Physics Letters, 2014Co-Authors: Peter Thurnheer, R Maas, Stefan Pogatscher, Jörg F. LöfflerAbstract:Shear-Band velocities of individual Shear Bands in ZrxCu90−xAl10 (x = 45–65) metallic glasses were investigated as a function of temperature and Zr-content. The apparent activation energy of the Shear-Band propagation dynamics increases with Zr-content, giving evidence for the sensitivity of inhomogeneous plastic flow and related time scales to chemical composition. The Shear-Band viscosities derived display a Zr-content trend which contrasts with equilibrium viscosities in the supercooled liquid regime. These findings are discussed in terms of effective temperature and short-range order.
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single Shear Band plasticity in a bulk metallic glass at cryogenic temperatures
Scripta Materialia, 2012Co-Authors: R Maas, David Klaumunzer, E I Preis, P M Derlet, Jörg F. LöfflerAbstract: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.
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temperature dependent Shear Band dynamics in a zr based bulk metallic glass
Applied Physics Letters, 2010Co-Authors: David Klaumunzer, R Maas, Florian Dalla H Torre, Jörg F. LöfflerAbstract:Flow serrations recorded during inhomogeneous deformation of Zr52.5Ti5Cu17.9Ni14.6Al10 (Vit105) were studied during compression testing at temperatures between −40 and 60 °C. The Shear Band velocities determined exhibit a pronounced temperature dependence covering nearly two orders of magnitude. The velocities follow an Arrhenius-type behavior with an associated activation energy of 0.3±0.05 eV. The results demonstrate a thermally activated mechanism of Shear Band propagation, which is similar to the behavior of other, nonmetallic amorphous materials.
Z F Zhang - One of the best experts on this subject based on the ideXlab platform.
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Shear Band fracture in metallic glass sample size effect
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2019Co-Authors: Shaogang Wang, Z F Zhang, X D WangAbstract:Abstract Previous studies have revealed that the Shear Band propagation of metallic glasses can be strongly influenced by decreasing the sample size, leading to the dramatically enhanced plasticity. Here we show that the sample size also affects the Shear Band fracture behavior. When reducing the sample size, the compressive Shear Band fracture mode is changed from the typical instant fracture with vein patterns into a gradual fracture mode without the formation of vein pattern. This can be explained by the marked reduction of energy dissipation density during Shear Band sliding in small samples and thus the enhanced Shear Band stability against the instable fracture.
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gradual Shear Band cracking and apparent softening of metallic glass under low temperature compression
Intermetallics, 2017Co-Authors: X D Wang, H F Zhang, Z W Zhu, Zisen Liu, Z F ZhangAbstract:Abstract Metallic glasses (MGs) usually exhibit synchronously enhanced plasticity and strength with decreasing the testing temperature. Although great efforts have been made, why MGs show better plasticity at low temperature remains unclear. In this work, the Shear Band cracking and fracture mechanism of a relatively brittle TiZr-based MG was investigated through methods of low temperature compression and 3D X-ray tomography. Pronounced apparent softening and profuse internal Shear-Band cracks were observed along with the improved plasticity, enhanced yield strength, decreased average softening rate, and reduced area of vein pattern as decreasing the testing temperature. Moreover, the fracture features can be correlated well with the compressive properties; and the true rupture stress that is carried by the still-bonded part of major Shear Band was found to be very close to the yield strength, demonstrating that the apparent softening should mainly originate from the cracking rather than dilation of Shear Band. The decreased softening rate can be fitted by a diffusion model, implying reduced atomic mobility and increased cracking resistance. Consequently, the improved plasticity of TiZr-MG at low temperature was attributed to the suppression of instant fracture and the enhanced resistance to Shear Band cracking, rather than the change of Shear Band density.
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evolution of Shear Band cracking in metallic glass under cyclic compression
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: X D Wang, Zeng Qian Liu, Z F ZhangAbstract:Abstract The evolution of Shear-Band cracking in a typical Zr-based metallic glass under high stress level was explored by interrupted cyclic compression-compression experiments. It was found that the fatigue crack was obviously easier to form along the Shear Band under cyclic compression than under monotonic loading. Furthermore, the critical Shear offset for fatigue crack propagation was obtained, which increases firstly and then decreases, depending on the competition between the accumulation of dilatation damage in the Shear Band and the release of stress concentration caused by plastic deformation. Moreover, to elucidate the cracking mechanisms, a concept of “fatigue Shear-Band propagation rate”, an analogy with the fatigue crack propagation rate, is proposed and characterized. These results may enhance the understanding on fatigue damage mechanism of MG and offer quantitative guidance on future design for MG and MG matrix composites with excellent fatigue property.
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progressive Shear Band propagation in metallic glasses under compression
Acta Materialia, 2015Co-Authors: Zhuoyi Liu, Gang Wang, Z F ZhangAbstract:Abstract Shear Band plays a key role in dominating the strength and plasticity of metallic glasses, and exhibits two kinds of propagation modes under compression, i.e., progressive and simultaneous propagation. These two different propagation modes of Shear Bands lie in different stages of plastic deformation. Prior to macroscopic yielding, Shear Bands have already been initiated yet not penetrated through the sample. These inserting Shear Bands exhibit linearly decreasing plastic strain from the end to tip, demonstrating a progressive propagation mode. Once the macroscopic yielding occurs, the major Shear Band fully transects the sample and propagates in a simultaneous sliding manner. The progressive propagation of Shear Bands causes an apparent work-hardening behavior, which can be well explained by assuming a higher critical stress for Shear Band initiation than propagation. The results demonstrate that metallic glasses with a smaller difference between critical conditions for initiation and propagation of Shear Band should have better plastic deformability, which can be reflected by the plastic strain to macroscopic yielding read from stress–strain curves.
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Shear Band evolution during large plastic deformation of brittle and ductile metallic glasses
Philosophical Magazine Letters, 2010Co-Authors: Jingping Cui, Z F Zhang, B L Shen, M Stoica, J EckertAbstract:Four monolithic metallic glasses (MMGs) with different plasticities varying from brittle to ductile behavior under unconstrained loading were subjected to small punch testing. All specimens undergo large plastic deformation with multiple cobweb-like Shear Bands under these conditions. The process of Shear Band evolution was carefully controlled and investigated. Plasticity of MMGs is characterized by equivalent plastic strain e* (product of Shear Band density and critical Shear offset). Thus, this article provides an experimental basis for a better understanding of the Shear Band evolution during plastic deformation of MMGs.
T G Nieh - One of the best experts on this subject based on the ideXlab platform.
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direct measurements of Shear Band propagation in metallic glasses an overview
Intermetallics, 2011Co-Authors: S X Song, T G NiehAbstract:Abstract A series of experimental efforts have recently made in an attempt to gain understanding of Shear Band propagation in metallic glasses. It was found that plastic flow serration observed in compression was actually caused by successive (intermittent) Shear along a single Shear plane, not random Shear Band emission. Several experimental techniques, including conventional Instron, attaching linear voltage differential transducer, strain gage, and high-speed camera, were employed to investigate Shear Band propagation during flow serration. The test results showed that the Shear Band propagation consisted of the acceleration, deceleration, and the final arrest. The maximum velocity of a propagating Shear Band was about 4 mm s −1 , which corresponds to a high strain rate of about 10 5 s −1 . The viscosity of a propagating Shear Band was evaluated to be only about 1 × 10 4 –5 × 10 5 Pa s, indicating the Shear Band was very fluidic. Video images capture from a high-speed camera also revealed that the Shear was simultaneous, rather than in a progressive fashion.
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on the Shear Band direction in metallic glasses
Acta Materialia, 2011Co-Authors: Yanfei Gao, Lu Wang, Hongbin Bei, T G NiehAbstract:Abstract This paper shows that it is inappropriate to relate the angle between the loading axis and the Shear-Band (or fracture) plane in metallic glasses under uniaxial loading conditions to the coefficient of internal friction in the Mohr–Coulomb model. Shear Bands in metallic glasses are a result of material instability (which can be predicted from constitutive parameters and loading conditions), which does not correspond to the material yield condition. Specifically, the Shear-Band directions depend on the Poisson’s ratio, the ratios of three deviatoric principal stresses to the von Mises stress, the coefficient of internal friction, and the dilatancy factor. The last parameter describes whether the plastic flow is associative or non-associative. Theoretical predictions based on the classic Rudnicki–Rice model agree well with a compilation of observations in uniaxial mechanical tests. Furthermore, using the elastic contact solutions and the Rudnicki–Rice model, we identify three (two) regimes under the two-dimensional cylindrical (three-dimensional spherical) contact where different Shear-Band directions may occur. When using a bonded-interface technique to visualize Shear Bands under three-dimensional contacts, it should be noted that the stress component normal to the bonded interface is released, resulting in the commonly observed semicircular Shear Bands whose directions are predicted to follow the larger in-plane principal stress.
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flow serration and Shear Band viscosity during inhomogeneous deformation of a zr based bulk metallic glass
Intermetallics, 2009Co-Authors: S X Song, T G NiehAbstract:Abstract Uniaxial compressive behavior of Zr 64.13 Cu 15.75 Ni 10.12 Al 10 bulk metallic glass at room temperature was characterized with high-sensitivity strain gauges directly attached to test samples. Displacement–time curves exhibited micron-size serrations (or bursts) after the onset of yielding, apparently associated with discrete Shear Band formation. Each displacement burst disclosed three-step (acceleration, steady-state, and deceleration) process in Shear Band propagation. The viscosity of a propagating Shear Band was found to be relatively low and, actually, in a similar range usually measured in the supercooled liquid region. A detailed analysis of the experimental results using a self-consistent Vogel–Fulcher–Tamann (VFT) equation based on free volume model suggested that Shear Band propagation was mainly resulted from free volume accumulation.
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Flow serration and Shear-Band propagation in bulk metallic glasses
Applied Physics Letters, 2009Co-Authors: Hongxia Chen, T G Nieh, S X Song, J.c. Huang, Jason S.c. JangAbstract:Flow serration in bulk metallic glasses (BMGs) was analyzed using high-sensitivity strain gauges. Based on the displacement-time profile for one serration, Shear-Band propagating speed was determined and found to be insensitive to the applied strain rates. The disappearance of serration at high strain rates is a result that the signal of displacement burst was overwhelmed by the applied strain rate. In comparison with the ductile Pd-based and brittle Mg-based BMGs, the ductility of BMGs appears to be closely related to the dynamics during Shear-Band propagation.
R Maas - One of the best experts on this subject based on the ideXlab platform.
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long range stress fields and cavitation along a Shear Band in a metallic glass the local origin of fracture
Acta Materialia, 2015Co-Authors: R Maas, K Samwer, Pascal Birckigt, C Borchers, Cynthia A VolkertAbstract:Nanomechanical properties along a single Shear Band in a Zr-based metallic glass were studied. Spatial mapping of both indentation hardness and modulus reveal complex long-range softening patterns that are indicative of internal stress fields along the Shear Band. These internal stresses reach values of the order of the yield strength of the tested metallic glass. Time dependent stress relaxation along the Shear Band is observed, and Shear-Band cavitation at the micron scale is found. Both the cavitation and the internal stresses are attributed to the non-planar Shear plane that during Shear-Band propagation leads to the development of off-axis stress components relative to the Shear direction. The cavities are a signature of a Shear-Band-to-crack transition, which is supported by stress fields known to develop ahead of mixed mode I and II crack tips.
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a single Shear Band in a metallic glass local core and wide soft zone
Applied Physics Letters, 2014Co-Authors: R Maas, K Samwer, W Arnold, Cynthia A VolkertAbstract:Two dimensional mapping of structural properties near a single Shear Band in a Zr-based bulk metallic glass reveals the presence of hardness and modulus reductions at a micrometer length scale. The isolated Shear Band had carried all the macroscopic plastic strain and the material near the Shear-Band exhibits structural variations both along and normal to the Shear plane. Analyzing the nanoindentation data indicates that long range internal stresses are the primary cause of the spatially varying structure. The results demonstrate that a nano-scale defect in a metallic glass may have a micrometer range signature.
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compositional dependence of Shear Band dynamics in the zr cu al bulk metallic glass system
Applied Physics Letters, 2014Co-Authors: Peter Thurnheer, R Maas, Stefan Pogatscher, Jörg F. LöfflerAbstract:Shear-Band velocities of individual Shear Bands in ZrxCu90−xAl10 (x = 45–65) metallic glasses were investigated as a function of temperature and Zr-content. The apparent activation energy of the Shear-Band propagation dynamics increases with Zr-content, giving evidence for the sensitivity of inhomogeneous plastic flow and related time scales to chemical composition. The Shear-Band viscosities derived display a Zr-content trend which contrasts with equilibrium viscosities in the supercooled liquid regime. These findings are discussed in terms of effective temperature and short-range order.
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single Shear Band plasticity in a bulk metallic glass at cryogenic temperatures
Scripta Materialia, 2012Co-Authors: R Maas, David Klaumunzer, E I Preis, P M Derlet, Jörg F. LöfflerAbstract: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.
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temperature dependent Shear Band dynamics in a zr based bulk metallic glass
Applied Physics Letters, 2010Co-Authors: David Klaumunzer, R Maas, Florian Dalla H Torre, Jörg F. LöfflerAbstract:Flow serrations recorded during inhomogeneous deformation of Zr52.5Ti5Cu17.9Ni14.6Al10 (Vit105) were studied during compression testing at temperatures between −40 and 60 °C. The Shear Band velocities determined exhibit a pronounced temperature dependence covering nearly two orders of magnitude. The velocities follow an Arrhenius-type behavior with an associated activation energy of 0.3±0.05 eV. The results demonstrate a thermally activated mechanism of Shear Band propagation, which is similar to the behavior of other, nonmetallic amorphous materials.
S X Song - One of the best experts on this subject based on the ideXlab platform.
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how hot is a Shear Band in a metallic glass
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2016Co-Authors: S X Song, Gang Wang, J G Wang, Ye Pan, B A Sun, Qijie Zhai, Kang Cheung Chan, W H WangAbstract:Abstract Due to the localization in space and the transience in time, investigations on the Shear Bands in metallic glasses are extremely difficult. The liquid-like layer frozen on fracture surfaces suggests a decreased viscosity in the Shear Band. Whether it is resulted from locally heating remains controversial. In this paper, the temperature rise in Shear Bands is profiled as a function of the duration of Shear Banding event, the distance from the Shear Band center and the thickness of Shear Band. The elastic energies released from the specimen and the testing machine are estimated regarding the serrations with different load drops in the compressive load–displacement curve of a Zr-based metallic glass. The duration of Shear event and the released energy by serration are the two main factors determining the temperature rise in Shear Bands. It is found that both “cold” and “hot” Shear Bands are attainable. Then the sliding speed, the viscosity and the crystallization probability of Shear Band are studied. These results can help to better understand and describe the operation of Shear Band in a quantified and analytical way.
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direct measurements of Shear Band propagation in metallic glasses an overview
Intermetallics, 2011Co-Authors: S X Song, T G NiehAbstract:Abstract A series of experimental efforts have recently made in an attempt to gain understanding of Shear Band propagation in metallic glasses. It was found that plastic flow serration observed in compression was actually caused by successive (intermittent) Shear along a single Shear plane, not random Shear Band emission. Several experimental techniques, including conventional Instron, attaching linear voltage differential transducer, strain gage, and high-speed camera, were employed to investigate Shear Band propagation during flow serration. The test results showed that the Shear Band propagation consisted of the acceleration, deceleration, and the final arrest. The maximum velocity of a propagating Shear Band was about 4 mm s −1 , which corresponds to a high strain rate of about 10 5 s −1 . The viscosity of a propagating Shear Band was evaluated to be only about 1 × 10 4 –5 × 10 5 Pa s, indicating the Shear Band was very fluidic. Video images capture from a high-speed camera also revealed that the Shear was simultaneous, rather than in a progressive fashion.
-
flow serration and Shear Band viscosity during inhomogeneous deformation of a zr based bulk metallic glass
Intermetallics, 2009Co-Authors: S X Song, T G NiehAbstract:Abstract Uniaxial compressive behavior of Zr 64.13 Cu 15.75 Ni 10.12 Al 10 bulk metallic glass at room temperature was characterized with high-sensitivity strain gauges directly attached to test samples. Displacement–time curves exhibited micron-size serrations (or bursts) after the onset of yielding, apparently associated with discrete Shear Band formation. Each displacement burst disclosed three-step (acceleration, steady-state, and deceleration) process in Shear Band propagation. The viscosity of a propagating Shear Band was found to be relatively low and, actually, in a similar range usually measured in the supercooled liquid region. A detailed analysis of the experimental results using a self-consistent Vogel–Fulcher–Tamann (VFT) equation based on free volume model suggested that Shear Band propagation was mainly resulted from free volume accumulation.
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Flow serration and Shear-Band propagation in bulk metallic glasses
Applied Physics Letters, 2009Co-Authors: Hongxia Chen, T G Nieh, S X Song, J.c. Huang, Jason S.c. JangAbstract:Flow serration in bulk metallic glasses (BMGs) was analyzed using high-sensitivity strain gauges. Based on the displacement-time profile for one serration, Shear-Band propagating speed was determined and found to be insensitive to the applied strain rates. The disappearance of serration at high strain rates is a result that the signal of displacement burst was overwhelmed by the applied strain rate. In comparison with the ductile Pd-based and brittle Mg-based BMGs, the ductility of BMGs appears to be closely related to the dynamics during Shear-Band propagation.
