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M A Meyers - One of the best experts on this subject based on the ideXlab platform.
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Shear Localization in metallic materials at high strain rates
Progress in Materials Science, 2021Co-Authors: Na Yan, M A MeyersAbstract:Abstract Three factors govern adiabatic Shear Localization: strain hardening (or softening), strain-rate hardening, and thermal softening. It is typically associated with large Shear strains (>1), high strain rates (103–107 s−1), and high temperatures (40–100% of melting point), all of which happen within narrow regions with widths of about 1–200 μm. It is often an undesirable phenomenon, leading to failure, but there are situations where it is desirable, e. g., the generation of machining chips. Here, we review the development of both theoretical and experimental achievements, from the initiation of Shear bands to their propagation with emphasis on three aspects: novel experimental techniques, novel materials, and nano/microstructural effects. The principal characteristics of adiabatic Shear bands in metallic materials at the nano- and micro-scale are described. Bands that were formerly identified as transformed actually consist of nanocrystalline/ultrafine grains. These grains result from the breakup of the microstructure by a rotational recrystallization process. The evolution of the microstructure inside Shear bands and their interactions for hcp, bcc, and fcc alloys, high-entropy alloys, nanocrystalline alloys, and metallic glasses are analyzed mechanistically. The gaps in the field and opportunities for future research are identified. Modern experimental characterization and computational techniques enable a more profound and predictive understanding of adiabatic Shear Localization and its avoidance in advanced materials.
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Shear Localization in metallic materials at high strain rates
Progress in Materials Science, 2020Co-Authors: Na Yan, M A MeyersAbstract:Abstract Three factors govern adiabatic Shear Localization: strain hardening (or softening), strain-rate hardening, and thermal softening. It is typically associated with large Shear strains (>1), high strain rates (103-107 s-1), and high temperatures (40-100% of melting point), all of which happen within narrow regions with widths of about 1-200 μm. It is often an undesirable phenomenon, leading to failure, but there are situations where it is desirable, e. g., the generation of machining chips. Here, we review the development of both theoretical and experimental achievements, from the initiation of Shear bands to their propagation with emphasis on three aspects: novel experimental techniques, novel materials, and nano/microstructural effects. The principal characteristics of adiabatic Shear bands in metallic materials at the nano- and micro-scale are described. Bands that were formerly identified as transformed actually consist of nanocrystalline/ultrafine grains. These grains result from the breakup of the microstructure by a rotational recrystallization process. The evolution of the microstructure inside Shear bands and their interactions for hcp, bcc, and fcc alloys, high-entropy alloys, nanocrystalline alloys, and metallic glasses are analyzed mechanistically. The gaps in the field and opportunities for future research are identified. Modern experimental characterization and computational techniques enable a more profound and predictive understanding of adiabatic Localization and its avoidance in advanced materials.
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the effects of ultra fine grained structure and cryogenic temperature on adiabatic Shear Localization in titanium
Acta Materialia, 2019Co-Authors: Shiteng Zhao, Bingfeng Wang, Shuang Cui, Renkun Chen, R Z Valiev, M A MeyersAbstract:Abstract The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic Shear Localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that Shear Localization in dynamic deformation is also altered. The width of the Shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of Shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano- recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40–250 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.
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adiabatic Shear Localization in the crmnfeconi high entropy alloy
Acta Materialia, 2018Co-Authors: Shiteng Zhao, Bingfeng Wang, Senhat M Alotaibi, Yong Liu, M A MeyersAbstract:Abstract The mechanical behavior of the single phase (fcc) CrMnFeCoNi high-entropy alloy (HEA) is examined in the dynamic regime. A series of experiments by dynamic-loading hat-shaped specimens using stopper rings to control the displacement are performed, and the alloy resists adiabatic Shear-band formation up to a very large imposed Shear strain of ∼7. It is proposed that the combination of the excellent strain-hardening ability and moderate thermal-softening effect retard Shear Localization. Recrystallized ultrafine-grained grains (diameters of 100–300 nm) with twins are revealed inside the Shear band. Their formation is explained by the rotational dynamic recrystallization mechanism. The stability of the structure at high strain rates strongly suggests a high ballistic resistance for this class of alloys.
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high velocity deformation of al 0 3 cocrfeni high entropy alloy remarkable resistance to Shear failure
Scientific Reports, 2017Co-Authors: Shiteng Zhao, H Diao, P K Liaw, M A MeyersAbstract:The mechanical behavior of a single phase (fcc) Al0.3CoCrFeNi high-entropy alloy (HEA) was studied in the low and high strain-rate regimes. The combination of multiple strengthening mechanisms such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning leads to a high work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to Shear Localization was studied by dynamically-loading hat-shaped specimens to induce forced Shear Localization. However, no adiabatic Shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to Shear Localization, which makes this material an excellent candidate for penetration protection applications such as armors.
Bingfeng Wang - One of the best experts on this subject based on the ideXlab platform.
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the effects of ultra fine grained structure and cryogenic temperature on adiabatic Shear Localization in titanium
Acta Materialia, 2019Co-Authors: Shiteng Zhao, Bingfeng Wang, Shuang Cui, Renkun Chen, R Z Valiev, M A MeyersAbstract:Abstract The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic Shear Localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that Shear Localization in dynamic deformation is also altered. The width of the Shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of Shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano- recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40–250 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.
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Shear Localization of fcc high-entropy alloys
EPJ Web of Conferences, 2018Co-Authors: Zezhou Li, Shiteng Zhao, Bingfeng WangAbstract:Dynamic behavior of the single phase (fcc) Al0.3 CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs) was examined. The combination of multiple strengthening mechanisms such as solid solution hardening, cutting forest dislocation, as well as mechanical nano-twinning leads to a high work-hardening rate, compared with conventional alloys. The resistance to Shear Localization was studied by dynamicallyloading hat-shaped specimens to induce forced Shear Localization. However, no adiabatic Shear band could be observed for Al0.3CoCrFeNi HEA at a large Shear strain ~1.1. Additionally, Shear Localization of the CoCrFeMnNi HEA was only found at an even larger Shear strain ~7 under dynamic compression. It is therefore proposed that the combination of the excellent strain-hardening ability and modest thermal softening of these two kinds of high-entropy alloys gives rise to remarkable resistance to Shear Localization, which makes HEAs excellent candidates for impact resistance applications.
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adiabatic Shear Localization in the crmnfeconi high entropy alloy
Acta Materialia, 2018Co-Authors: Shiteng Zhao, Bingfeng Wang, Senhat M Alotaibi, Yong Liu, M A MeyersAbstract:Abstract The mechanical behavior of the single phase (fcc) CrMnFeCoNi high-entropy alloy (HEA) is examined in the dynamic regime. A series of experiments by dynamic-loading hat-shaped specimens using stopper rings to control the displacement are performed, and the alloy resists adiabatic Shear-band formation up to a very large imposed Shear strain of ∼7. It is proposed that the combination of the excellent strain-hardening ability and moderate thermal-softening effect retard Shear Localization. Recrystallized ultrafine-grained grains (diameters of 100–300 nm) with twins are revealed inside the Shear band. Their formation is explained by the rotational dynamic recrystallization mechanism. The stability of the structure at high strain rates strongly suggests a high ballistic resistance for this class of alloys.
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adiabatic Shear Localization in ultrafine grained 6061 aluminum alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2016Co-Authors: Bingfeng Wang, Shiteng Zhao, Jindian Zhou, Xiaoxia HuangAbstract:Abstract Localized Shear is an important mode of deformation; it leads to catastrophic failure with low ductility, and occurs frequently during high strain-rate deformation. The hat-shaped specimen has been successfully used to generate Shear bands under controlled shock-loading tests. The microstructure in the forced Shear band was characterized by optical microscopy, microhardness, and transmission electron microscopy. The true flow stress in the Shear region can reach 800 MPa where the strain is about 2.2. The whole Shear Localization process lasts for about 100 μs. The Shear band is a long and straight band distinguished from the matrix by boundaries. It can be seen that the grains in the boundary of the Shear band are highly elongated along the Shear direction and form the elongated cell structures (0.2 µm in width), and the core of the Shear band consists of a number of recrystallized equiaxed grains with 0.2−0.3 µm in diameters, and the second phase particles distribute in the boundary of the ultrafine equiaxed new grains. The calculated temperature in the Shear band can reach about 667 K. Finally, the formation of the Shear band in the ultrafine grained 6061 aluminum alloy and its microstructural evolution are proposed.
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Shear Localization and microstructure in coarse grained beta titanium alloy
Materials Science and Engineering: A, 2016Co-Authors: Bingfeng Wang, Shiteng Zhao, Xiaoyan Wang, Fangyu Xie, Xiaoyong ZhangAbstract:Abstract Adiabatic Shear Localization plays an important role in the deformation and failure of the coarse grained beta titanium alloy Ti-5 Al-5 Mo-5 V-1 Cr-1 Fe with grain size about 1 mm at high strain rate deformation. Hat shaped specimens with different nominal Shear strains are used to induce the formation of Shear bands under the controlled shock-loading experiments. The true stress in the specimens can reach about 1040 MPa where the strain is about 1.83. The whole Shear Localization process lasts about 35 μs. The microstructures within the Shear band are investigated by optical microscopy, scanning electron microscopy / electron backscatter diffraction, and transmission electron microscopy. The results show that the width of the Shear bands decreases with increasing nominal Shear strain, and the grains in the transition region near the Shear band are elongated along the Shear band, and the core of the Shear band consists of the ultrafine deformed grains with width of 0.1 μm and heavy dislocations. With the aims of accommodating the imposed Shear strain and maintaining neighboring grain compatibility, the grain subdivision continues to take place within the band. A fiber texture is formed in the core of the Shear band. The calculated temperature rise in the Shear band can reach about 722 K. Dynamic recovery is responsible for the formation of the microstructure in coarse grained beta titanium alloy.
V F Nesterenko - One of the best experts on this subject based on the ideXlab platform.
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Shear Localization in dynamic deformation of materials microstructural evolution and self organization
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2001Co-Authors: M A Meyers, J C Lasalvia, V F Nesterenko, Q. XueAbstract:The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to Shear Localization. Shear Localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics, polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of Shear bands has been studied in metals (Ti, Ta, Ti–6Al–4V, and stainless steel), granular and prefractured ceramics (Al2O3 and SiC), a polymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the Shear bands. A fine recrystallized structure is observed in Ti, Cu, Al–Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are greater than a critical size ac. When they are smaller than ac, particle deformation takes place. For the granular SiC, a novel mechanism of Shear-induced bonding was experimentally identified inside the Shear bands. For all materials, Shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models. © 2001 Elsevier Science B.V. All rights reserved.
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spontaneous and forced Shear Localization in high strain rate deformation of tantalum
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1999Co-Authors: Y J Chen, M A Meyers, V F NesterenkoAbstract:High-strain-rate Shear Localization was induced in tantalum by (a) lowering the deformation temperature or (b) subjecting it to high strains by dynamic deformation (up to ot 0.8) or (c) pre-shocking (at oeff 0.22) and then deforming it. Although at ambient temperature the deformation of tantalum is macroscopically uniform to high strains (ot$ 0.8), at 77 K Shear Localization under the same loading condition was developed at a critical strain of 0.2 to 0.3. This higher propensity to Shear Localization at low temperatures is a direct consequence of the combination of lower heat capacity and higher rate of thermal softening. At the three temperatures investigated (77, 190 and 298 K), Localization occurs at strains significantly higher than the instability strains (the maxima of the adiabatic stress‐strain curves for these three temperatures). The thicknesses of the forced Localization regions and Shear bands were found to be a function of temperature, and decreased with decreasing temperature (at the same strain) in accord with the equation proposed by Y. Bai et al. (Y. Bai, C. Cheng, S. Yu, Acta Mechanica Sinica 2 (1986) 1). Shock deformation of tantalum enhances its predisposition to subsequent Shear Localization, and this was demonstrated by subjecting shocked and unshocked specimens to high strain, high strain rate deformation through the collapse of a thick-walled cylinder assembly. © 1999 Elsevier Science S.A. All rights reserved.
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Shear Localization and chemical reaction in high strain high strain rate deformation of ti si powder mixtures
Acta Materialia, 1998Co-Authors: H C Chen, J C Lasalvia, V F Nesterenko, M A MeyersAbstract:Ti-Si mixtures were subjected to high-strain-rate deformation at a pressure below the threshold for shock-wave initiation. Whereas the collapse of interparticle pores did not initiate reaction, regions of localized macro-deformation initiated reaction inside Shear bands at suAciently high strains (g010), and propagation of the reaction through the entire specimen at higher strains (g020-40). This study demon- strates that temperature increases in Shear Localization regions can initiate chemical reaction inside a reac- tive powder mixture. The Shear band spacing was 00.6-1 mm. Thermodynamic and kinetic calculations yield the reaction rate outside the Shear bands, in the homogeneously deformed material, which has a
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Shear Localization and recrystallization in high strain high strain rate deformation of tantalum
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1997Co-Authors: V F Nesterenko, M A Meyers, J C Lasalvia, Y J Chen, M P Bondar, Ya L LukyanovAbstract:Tantalum was subjected to high plastic strains (global effective strains between 0 and 3) at high strain rates (>104 s−1) in an axisymmetric plane strain configuration. Tubular specimens, embedded in thick-walled cylinders made of copper, were collapsed quasi-uniformly by explosively-generated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains achieved generated temperatures which produced significant microstructural change in the material; these strains and temperatures were computed as a function of radial distance from the cylinder axis. The microstructural features observed were: (i) dislocations and elongated dislocation cell (eeff 2.5, T > 1000 K). Whereas the post-deformation (static) recrystallization takes place by a migrational mechanism, dynamic recrystallization is the result of the gradual rotation of subgrains coupled with dislocation annihilation. A simple analysis shows that the statically recrystallized grain sizes observed are consistent with predicted values using conventional grain-growth kinetics. The same analysis shows that the deformation time is not sufficient to generate grains of a size compatible with observation (0.1–0.3 μm). A mechanism describing the evolution of the microstructure leading from elongated dislocation cells, to subgrains, and to micrograins is proposed. Grain-scale Localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at the higher plastic strains (eeff > 1). Residual tensile ‘hoop’ stresses are generated near the central hole region upon unloading; this resulted in ductile fracturing along Shear Localization bands.
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Shear Localization in high strain rate deformation of granular alumina
Acta Materialia, 1996Co-Authors: V F Nesterenko, M A Meyers, H C ChenAbstract:Abstract Dynamic deformation of densified granular alumina of two different particle sizes was investigated by the radial symmetric collapse of a thick-walled cylinder. The densified granular alumina was used to model the flow in ballistic impact and penetration of fragmented ceramic armor. Shear Localization was a well developed deformation mode at an overall radial strain of ∼0.2–0.4 and strain rate of 10 4 s −1 . The following qualitative features of Shear bands were established: • Shear bands have clear boundaries and their thickness does not depend on the initial particle size and has a typical value ∼10 μm. • The structure of the Shear bands was dependent on initial particle size, suggesting differences in the mechanisms of flow. For the ∼4 μm alumina, comminution (break-up) and softening of particles were observed. For the ∼0.4 μm particles, a peculiar structure consisting of a central crack with two lateral cracks was formed. • Distributions of Shear bands and displacement magnitudes were dependent on initial particle size. The observed differences in powder behavior are associated with different mechanisms of powder repacking. For large particles (∼4 μm), additional hardening resulting from microfracture and subsequent repacking of different size particles in the powder takes place. The small-sized (∼0.4 μm) ceramic does not go through the particle fracturing stage and the hardening is due to “classical” repacking.
Shiteng Zhao - One of the best experts on this subject based on the ideXlab platform.
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the effects of ultra fine grained structure and cryogenic temperature on adiabatic Shear Localization in titanium
Acta Materialia, 2019Co-Authors: Shiteng Zhao, Bingfeng Wang, Shuang Cui, Renkun Chen, R Z Valiev, M A MeyersAbstract:Abstract The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic Shear Localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that Shear Localization in dynamic deformation is also altered. The width of the Shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of Shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano- recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40–250 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.
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Shear Localization of fcc high-entropy alloys
EPJ Web of Conferences, 2018Co-Authors: Zezhou Li, Shiteng Zhao, Bingfeng WangAbstract:Dynamic behavior of the single phase (fcc) Al0.3 CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs) was examined. The combination of multiple strengthening mechanisms such as solid solution hardening, cutting forest dislocation, as well as mechanical nano-twinning leads to a high work-hardening rate, compared with conventional alloys. The resistance to Shear Localization was studied by dynamicallyloading hat-shaped specimens to induce forced Shear Localization. However, no adiabatic Shear band could be observed for Al0.3CoCrFeNi HEA at a large Shear strain ~1.1. Additionally, Shear Localization of the CoCrFeMnNi HEA was only found at an even larger Shear strain ~7 under dynamic compression. It is therefore proposed that the combination of the excellent strain-hardening ability and modest thermal softening of these two kinds of high-entropy alloys gives rise to remarkable resistance to Shear Localization, which makes HEAs excellent candidates for impact resistance applications.
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adiabatic Shear Localization in the crmnfeconi high entropy alloy
Acta Materialia, 2018Co-Authors: Shiteng Zhao, Bingfeng Wang, Senhat M Alotaibi, Yong Liu, M A MeyersAbstract:Abstract The mechanical behavior of the single phase (fcc) CrMnFeCoNi high-entropy alloy (HEA) is examined in the dynamic regime. A series of experiments by dynamic-loading hat-shaped specimens using stopper rings to control the displacement are performed, and the alloy resists adiabatic Shear-band formation up to a very large imposed Shear strain of ∼7. It is proposed that the combination of the excellent strain-hardening ability and moderate thermal-softening effect retard Shear Localization. Recrystallized ultrafine-grained grains (diameters of 100–300 nm) with twins are revealed inside the Shear band. Their formation is explained by the rotational dynamic recrystallization mechanism. The stability of the structure at high strain rates strongly suggests a high ballistic resistance for this class of alloys.
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high velocity deformation of al 0 3 cocrfeni high entropy alloy remarkable resistance to Shear failure
Scientific Reports, 2017Co-Authors: Shiteng Zhao, H Diao, P K Liaw, M A MeyersAbstract:The mechanical behavior of a single phase (fcc) Al0.3CoCrFeNi high-entropy alloy (HEA) was studied in the low and high strain-rate regimes. The combination of multiple strengthening mechanisms such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning leads to a high work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to Shear Localization was studied by dynamically-loading hat-shaped specimens to induce forced Shear Localization. However, no adiabatic Shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to Shear Localization, which makes this material an excellent candidate for penetration protection applications such as armors.
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adiabatic Shear Localization in ultrafine grained 6061 aluminum alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2016Co-Authors: Bingfeng Wang, Shiteng Zhao, Jindian Zhou, Xiaoxia HuangAbstract:Abstract Localized Shear is an important mode of deformation; it leads to catastrophic failure with low ductility, and occurs frequently during high strain-rate deformation. The hat-shaped specimen has been successfully used to generate Shear bands under controlled shock-loading tests. The microstructure in the forced Shear band was characterized by optical microscopy, microhardness, and transmission electron microscopy. The true flow stress in the Shear region can reach 800 MPa where the strain is about 2.2. The whole Shear Localization process lasts for about 100 μs. The Shear band is a long and straight band distinguished from the matrix by boundaries. It can be seen that the grains in the boundary of the Shear band are highly elongated along the Shear direction and form the elongated cell structures (0.2 µm in width), and the core of the Shear band consists of a number of recrystallized equiaxed grains with 0.2−0.3 µm in diameters, and the second phase particles distribute in the boundary of the ultrafine equiaxed new grains. The calculated temperature in the Shear band can reach about 667 K. Finally, the formation of the Shear band in the ultrafine grained 6061 aluminum alloy and its microstructural evolution are proposed.
Zhanqiang Liu - One of the best experts on this subject based on the ideXlab platform.
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Shear Localization sensitivity analysis for johnson cook constitutive parameters on serrated chips in high speed machining of ti6al4v
Simulation Modelling Practice and Theory, 2015Co-Authors: Bing Wang, Zhanqiang LiuAbstract:Abstract This research aims to investigate the influence of material constitutive parameters on the serrated chip formation during high speed machining (HSM) of Ti6Al4V alloys with finite element simulations and cutting experiments. The Johnson–Cook (JC) constitutive model and JC fracture model with an energy-based ductile failure criterion are adopted to simulate the HSM process. Five JC constitutive model parameters such as initial yield stress, hardening modulus, strain hardening coefficient, strain rate dependency coefficient, and thermal softening coefficient are included in this research. Shear Localization sensitivity is novelly proposed to describe variations of serrated chips under different JC constitutive model parameters. Shear Localization sensitivity is subdivided into chip serration sensitivity and chip bending sensitivity. The research finds that the influences of initial yield stress and thermal softening coefficient parameters on the chip serration and bending are much more prominent than those of the rest three JC constitutive model parameters. With initial yield stress or hardening modulus in JC constitutive model increasing, the chip serration sensitivity increases and the chip bending sensitivity decreases. However, the influences of the rest three parameters on chip serration sensitivity are opposite. High speed orthogonal cutting experiments of Ti6Al4V are carried out to validate the simulation results under different cutting speeds ranging from 50 m/min to 3000 m/min and fixed uncut chip thickness with 0.1 mm. The results show that the serrated degree of chips increases with the cutting speed increasing until the chips become completely fragmented. The cutting speed break point of chip morphology from serrated to fragmented ones for Ti6Al4V is about 2500 m/min. The average cutting force decreases with the cutting speed increasing, which is a prominent advantage for HSM. This paper can help to get deeper insights into the serrated chip formation mechanism in HSM.
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investigations on the chip formation mechanism and Shear Localization sensitivity of high speed machining ti6al4v
The International Journal of Advanced Manufacturing Technology, 2014Co-Authors: Bing Wang, Zhanqiang LiuAbstract:This study develops a combined numerical and experimental approach to get deeper insights into chip formation mechanism for high-speed machining of titanium alloy Ti6Al4V. The numerical investigation of high-speed machining is implemented with the aid of finite element analysis software Abaqus/Explicit, in which the Johnson-Cook (JC) fracture model with an energy-based ductile failure criterion is adopted. Meanwhile, the experiments of high-speed orthogonal cutting are carried out to validate the numerical results. The cutting speeds are selected ranging from 50 to 3,000 m/min, and the uncut chip thickness is fixed at 0.1 mm. The variables investigated include the serrated degree and serrated frequency of chips in addition to the cutting force. The results show that both the serrated degree and serrated frequency have positive correlations with the cutting speed. An important regularity for the transformation of chip morphology from serrated to unit at a critical cutting speed has been achieved, and the critical value for Ti6Al4V is about 2,500 m/min. The research also finds that the cutting force decreases with the increasing cutting speed, while its fluctuant frequency and amplitude increase sharply. Furthermore, the influences of JC fracture constants (the five constants in JC fracture model) on chip formation are investigated based on the finite element method, which is the main original and innovative highlight of this study. The Shear Localization sensitivity is firstly proposed to describe the influences of JC fracture constants on the chip formation process. When the JC fracture constants decrease, the Shear Localization sensitivity is positive which means that the serrated degree increases and vice versa. The sensitivity analyses indicate that the influences of initial failure strain D 1 and exponential factor of stress triaxiality D 2 on chip formation process are more conspicuous than the rest three ones. This paper is enticing from both the engineering and the analytical perspectives aimed at predicting the evolution of serrated chip formation and chip morphology transformation in metal cutting process.
