Formation Of Adiabatic Shear Bands Research Articles

Solid Particle Erosion of Nanocrystalline Nickel Coatings: Influence of Grain Size and Adiabatic Shear Bands

The primary objective of the present study is to investigate the influence of nanocrystalline grain size on the solid particle erosion behavior of nickel. For the above purpose, 450-μm-thick nanocrystalline Ni coatings having the average grain sizes of 21, 42, 70, and 195 nm were obtained using pulsed electrodeposition (PED). All these samples along with bulk annealed Ni samples (43 μm grain size) were subjected to solid particle erosion using SiO2 particles as an erodent at a constant impact velocity of 45 m/s and two impact angles (30 and 90 deg). Erosion results indicate that bulk Ni and PED Ni coatings of grain sizes 195 and 70 nm exhibit the same erosion rate, while PED Ni coatings of 42 and 21 nm grain size exhibit marginally higher erosion rates with a clear trend of increasing erosion rate with decreasing grain size. It was also observed that the higher erosion rates exhibited by 21- and 42-nm-grain size PED Ni samples were associated with the formation of adiabatic shear bands (ASBs) originating from the eroded surface and propagating into the eroded sample. The experimental observations have been understood on the basis of a transition from a localization model for erosion for coarse-grained Ni (> 70 nm) to an ASB-induced erosion model for grain sizes less than 70 nm.

Microstructure evolution and mechanical properties of an AA6061/AZ31B alloy plate fabricated by explosive welding

In this paper, both experimental and theoretical studies were conducted to investigate the bonding interface formation, microstructure evolution, and the interface strength of an AA6061/AZ31B alloy cladding plate fabricated by explosive welding. The evolution of microstructures including adiabatic shear bands (ASBs) structure, recrystallized grains, and elongated grains were analyzed using optical microscope (OM), electron back scatter diffraction (EBSD), and transmission electron microscope (TEM). A model was also proposed to study the ASBs formation and recrystallized grains in the ASBs. In the study, during explosive welding, a periodic wavy bonding interface was observed and the formation of such an interface was found due to the periodic jetting caused by the high impact stresses at the impact points during welding. The ASBs were found formed along the directions where the stress waves were concentrated due to high energy accumulation. Near the bonding interface, many crystallized grains were found in the AZ31B alloy plate, while elongated grains were dominant in the AA6061 alloy plate. Difference in the crystal structure of the Al and Mg alloy was believed to be the reason causing such microstructure evolution difference. Nanoindentation tests on the ASBs showed that, due to the existence of fine grains resulted from the recrystallization in the ASBs structure, the hardness of the ASBs structure (1.22 GPa) was higher than that of its surrounding structure. The shear strength of the bonding interface of the explosively-welded AA6061/AZ31B cladding plate can reach up to 201.2 MPa.

A Numerical Simulation of Adiabatic Shear Bands Formation in Hollow Cylinder

The process of torsion deformation of a hollow cylinder is considered. The mathematical model describing this process is formulated. Numerical algorithm which allows one to simulate the fully localized plastic flow is proposed. The influence of the geometrical parameters of the problem of plastic flow localization is studied. The limiting case when the problem of torsion deformation of a hollow cylinder transforms to the plane problem of deformation of infinite slab is considered. An influence of cylinder size to the formation of adiabatic shear bands is considered. It is shown that declining of inner size of cylinder results to a significant change of solution and displacement of localization area to the inner surface of a hollow cylinder.

Numerical Simulation of Adiabatic Shear-Band Formation in Composites

The process of plastic flow localization in a composite material consisting of welded steel and copper plates under shear strain is considered. The mathematical model of this physical process is formulated. A new numerical algorithm based on the Courant–Isaacson–Rees scheme is proposed. The algorithm is verified using three test problems. The algorithm efficiency and performance are proved by the test simulations. The proposed algorithm is used for numerical simulation of plastic strain localization on composite materials. The influence of boundary conditions, the initial plastic strain rate, and the width of the materials forming the composite bar on the localization process is studied. It is demonstrated that at the initial stage, the shear velocity for the material layers varies. Theoretical estimates of the oscillation frequency and period are proposed; the calculations using these estimates agree completely with the numerical experiments. It is established that the deformation is localized in the copper part of the composite. One or two localization regions situated at a typical distance from the boundaries are formed, depending on the width of the steel and copper parts, as well as the initial plastic strain rate and the chosen boundary conditions. The dependence of this distance on the initial plastic strain rate is demonstrated and corresponding estimates for boundary conditions of two types are obtained. It is established that in the case of two localization regions, the temperature and deformation in one of them increase much faster than in the other, while at the initial stage these quantities are nearly equal in both regions.

Multiple strengthening sources and adiabatic shear banding during high strain-rate deformation of AISI 321 austenitic stainless steel: Effects of grain size and strain rate

The dynamic impact response of AISI 321 steel at strain rate of 4000, 5500, 6500 and 7500s−1 was investigated using the split Hopkinson pressure bar system. The alloy samples processed to have grain size of 0.24, 3, 13 and 34µm were studied. While the yield strength and hardness increases with decrease in grain size, strain hardening rate is comparable for all grain sizes. Microstructural evaluation of the impacted specimens using high-resolution electron backscattered diffraction (HR-EBSD) technique showed grain boundary strengthening, deformation twinning, deformation-induced martensitic transformation, dislocation multiplication during slip and precipitation of carbides that act as barriers to dislocation motion as additional source of strengthening. Slip and twinning were the dominant deformation mechanisms observed in the steel. Twinning, dislocation multiplication during slip and carbide precipitation contributed more strongly to strain-hardening in coarse-grained (CG) specimen while stain-induced martensite and grain boundary strengthening are the most beneficial to strengthening in the ultra-fine-grained (UFG) specimens. The temperature rise in the specimens during impact increases with strain rate. This slowed the kinetics of twinning, phase transformation and dislocation interaction especially in CG structure. Both XRD and HR-EBSD texture results confirmed the development of {110}||CD (CD: compression direction) texture in the austenite phase at the expense of {100}||CD and {111}||CD fibres. Thermomechanical instability leading to the formation of adiabatic shear band (ASB) occurred in the test specimens as they deformed at high strain rates. While the amount of deformation twinning and αʹ-martensite decreases towards the ASB, only the carbides and small fraction of αʹ-martensite are observed inside the ASB. Grain refinement via rotational dynamic recrystallization occurred within the ASB. The extent of grain refinement increased with increase in initial grain size of the test specimen.

The influence of temper condition on adiabatic shear failure of AA 2024 aluminum alloy

The influence of temper condition on the adiabatic shear failure of AA 2024 aluminum alloy under compressive loading was investigated. The alloy in T351 and T651 temper conditions were subjected to quasi-static compressive loading at a strain rate of 3.2 × 10−3s−1 and dynamic impact loading at strain rates between 2 × 103 and 6 × 103s−1. Adiabatic shear bands (ASBs) are formed under dynamic impact loading in the two temper conditions. However, the T351 temper specimen is more susceptible to the formation of ASB. All dynamic stress-strain plots exhibit two peak flow stresses, PFS I and PFS II. The PFS I is higher for T651 specimens than for T351 specimens while PFS II is higher for T351 specimens than for T651 specimens. PFS II is higher than PFS I for T351 specimens, for all strain rates, whereas PFS II is mostly lower than PFS I for T651 specimens. The results of EBSD measurements on the alloy in both temper conditions after high strain-rate deformation indicate a significant texturing of the grains around < 110 > ||CD, where CD stand for compression direction. Yield strength in quasi-static loading is higher for T651 than T351 specimens while plastic strain hardening rate is higher for T351 than T651 specimens.

High Temperature Strength and Hot Working Technology for As-Cast Mg–1Zn–1Ca (ZX11) Alloy

Cast Mg–1Zn–1Ca alloy (ZX11) has been tested to evaluate its compressive strength between 25 °C and 250 °C, and workability in the range of 260–500 °C. The ultimate compressive strength of this alloy is about 30% higher than that of creep-resistant alloy Mg–3Sn–2Ca (TX32) between 25 °C and 200 °C, and exhibits a plateau between 100 °C and 175 °C, similar to TX32. This is attributed to Mg2Ca particles present at grain boundaries that reduce their sliding. The processing map, developed between 260 and 420 °C in the strain rate limits of 0.0003 s−1 to 1 s−1, exhibited two domains in the ranges: (1) 280–330 °C and 0.0003–0.01 s−1 and (2) 330–400 °C and 0.0003–0.1 s−1. In these domains, dynamic recrystallization occurs, with basal slip dominating in the first domain and prismatic slip in the second, while the recovery mechanism being climb of edge dislocations in both. The activation energy estimated using standard kinetic rate equation is 191 kJ/mol, which is higher than the value for lattice self-diffusion in magnesium indicating that a large back stress is created by the presence of Ca2Mg6Zn3 intermetallic particles in the matrix. It is recommended that the alloy be best processed at 380 °C and 0.1 s−1 at which prismatic slip is favored due to Zn addition. At higher strain rates, the alloy exhibits flow instability and adiabatic shear band formation at &lt;340 °C while flow localization and cracking at grain boundaries occurs at temperatures &gt;400 °C.

On adiabatic shear localization in nanostructured face-centered cubic alloys with different stacking fault energies

We have investigated the dynamic (or high strain rate) response of nanostructured face-centered-cubic alloys with different stacking fault energies after severe plastic deformation under uniaxial compression. It is found that adiabatic shear bands were easier to appear in those alloys with lower stacking fault energy, even though under quasi-static loading more obvious hardening behavior was observed with decreasing stacking fault energy. To understand the formation mechanism of adiabatic shear bands, firstly thermal softening during dynamic loading was considered. Dynamic tests within a wide range of temperature and interrupted loading revealed that adiabatic temperature rise prior to plastic instability has only minor effect on the onset of shear localization. Unlike medium/high stacking fault energy materials, strong textures have always been reported after severe plastic deformation in the metals with low stacking fault energy. Hence, influence of texture on the propensity to adiabatic shear bands was taken into account. The dynamic anisotropic behaviors show that the pre-existing texture formed during severe plastic deformation is responsible for the initiation of localized deformation, and the adiabatic temperature rise in the localized region promotes the formation of adiabatic shear bands. To verify the effect of texture on the formation of adiabatic shear bands we have incorporated preferred orientation of grains into finite element modeling based on crystal plasticity. An elastic-viscoplastic continuum slip constitutive relation was adopted to describe the mechanical response of materials, in which the dependence of slip systems' resistance on the temperature evolution was also considered. The simulation results are in good accordance with the experimental results. As such a more comprehensive picture of the formation of adiabatic shear bands in such materials has been uncovered along with the underlying mechanism.

Mechanical responses and dynamic failure of nanostructure Cu–Al alloys under uniaxial compression

Three kinds of nanostructure (NS) Cu–Al alloys (Cu-2.57%Al, Cu-6.87%Al and Cu-11.14%Al) were produced via equal channel angular pressing at room temperature and the smallest grain size achieved was 93 nm. Mechanical properties of NS Cu–Al alloys were studied by compression tests over a wide range of strain rates (5 × 10−4–4 ×103 s−1). The microstructure of NS Cu–Al alloys before-and-after mechanical testing were examined by transmission electron microscope (TEM). Deformation mechanisms of NS Cu–Al alloys with different Stacking Fault Energy (SFE) were also discussed. It is worth mentioning that the post-loading observations of NS Cu-11.14%Al (with the lowest SFE loaded at a high strain rate of 4 × 103 s−1) by Optical and Scanning Electron Microscope verified the appearance of Adiabatic Shear Band (ASB) which is found rarely in nanostructured FCC metals. TEM observation showed that the nanoscale grains in ASBs were much smaller and cleaner in contrast to the grains with high density of defects outside the bands. It should be a consequence of the recrystallization because of adiabatic temperature rise inside the band. The reasons for the formation of ASBs in NS FCC alloys were also explored and explained.

Microstructural observations on the terminal penetration of long rod projectile

Abstract Present study focuses on the terminal penetration of tungsten heavy alloy (WHA) long rod penetrator impacted against armour steel at an impact velocity of 1600 m/s. The residual penetrator and armour steel target recovered after the ballistic test have been characterized using optical microscope, scanning electron microscope (SEM) and electron probe micro analyzer (EPMA). Metallurgical changes in target steel and WHA remnant have been analysed. Large shear stresses and shear localization have resulted in local failure and formation of erosion products. Severe plastic deformation acts as precursor for formation of adiabatic shear band (ASB) induced cracks in target steel. Recovered WHA penetrator remnant also exhibits severe plastic deformation forming localized shear bands, ASB induced cracks and shock induced cracks.

Effect of strain rate on the mechanical properties of a gum metal with various microstructures

In this work, a bulk gum metal (GM) was fabricated via arc melting from high purity powders. The ingots were first extruded using a conventional route followed by equal channel angular pressing (ECAP). The mechanical behavior of the extruded GM and ECAP-processed GM was studied under both quasi-static and high strain rate compression conditions to evaluate the influence of strain rate. In addition, the associated mechanical anisotropy, or the lack thereof, was investigated through loading in different orientations with respect to the extrusion or ECAP direction. Precipitous stress drops were observed under dynamic compression of both extruded and ECAP-processed GM specimens when loading perpendicular to the extrusion direction. Adiabatic shear banding (ASB) was found to be associated with the precipitous stress drops on the dynamic stress-strain curves. The details of the ASBs were characterized by optical and scanning electron microscopy, with emphasis on electron backscattered diffraction (EBSD). The mechanisms responsible for the formation of ASB were examined both from thermal softening and geometrical softening perspectives. Significant microstructure refinement within ASBs was established, and a possible grain refinement mechanism was proposed.

High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process

In the automotive industry, advanced high strength steels (AHSS) are widely used as sheet part components to reduce weight, even though this leads to several challenges. The demand for high-quality shear cutting surfaces that do not require reworking can be fulfilled by adiabatic shear cutting: High strain rates and local temperatures lead to the formation of adiabatic shear bands (ASB). While this process is well suited to produce AHSS parts with excellent cutting surface quality, a fundamental understanding of the process is still missing today. In this study, compression tests in a Split-Hopkinson Pressure Bar with an initial strain rate of 1000 s-1 were performed in a temperature range between 200 °C and 1000 °C. The experimental results show that high strength steels with nearly the same mechanical properties at RT may possess a considerably different behavior at higher temperatures. The resulting microstructures after testing at different temperatures were analyzed by optical microscopy. The thermo-mechanical material behavior was then considered in an analytical model. To predict the local temperature increase that occurs during the adiabatic blanking process, experimentally determined flow curves were used. Furthermore, the influence of temperature evolution with respect to phase transformation is discussed. This study contributes to a more complete understanding of the relevant microstructural and thermo-mechanical mechanisms leading to the evolution of ASB during cutting of AHSS.

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.

Microstructural mechanism in adiabatic shear bands of Al-Cu alloy bars using electromagnetic impact upsetting

The Al-Cu alloy bars were subjected to electromagnetic upsetting. The microstructural evolution of the adiabatic shear band (ASB) was characterized and revealed. The results showed that multiple slip systems were activated at high strain rates and contribute to the formation of ASB. Dynamic nucleation particles in the boundary of slip bands were formed under the continuous dynamic recrystallization mechanism. In addition, equiaxed dynamic recrystallization grains were produced by a rotating dynamic recrystallization mechanism.

Modeling spontaneous adiabatic shear band formation in electro-magnetically collapsing thick-walled cylinders

The ability to simulate shear bands evolution in thick-walled-cylinder (TWC) experiments is required to understand their spontaneous formation and propagation. Recently we presented experiments on electro-magnetically collapsing metallic cylinders (Lovinger et al., 2015). Here we present numerical simulations that reproduce the experimental results for multiple shear bands in those TWC's. We present a detailed study of the initiation and propagation of the shear bands and their mutual interactions, which replicates many of the experimental observations. We investigate the influence of initial perturbations and pressure history on the initiation and final stages of the process using an energy-based failure model which incorporates a positive feedback mechanism. The numerical model is calibrated for four different materials to reconstruct the number of shear bands and their experimentally determined distribution. The results indicate that the number of shear bands is related to deformation micromechanisms operating in the material, such as twinning and martensitic transformations, which may hold back and eventually stall the shear bands evolution. The numerical simulations provide a reliable quantitative description of the shear bands distribution and spacing, thus paving the way for future predictive work of this failure mode.

Ballistic Limit Thickness and Failure of Steel Targets with Different Strengths under EFP Impact

The protective performance of different armour steels and structural steels against the impact of an explosively formed projectile was investigated. For the tests the impact velocity of the EFP projectiles was kept constant at 1400 m/s. Machinely formed EFP simulants have been accelerated with a powder gun instead of using explosively formed and accelerated EFPs. First tests have been done with EFP simulants with a mass of about 140g, for later tests downscaled projectiles with a scaling factor of ½ have been used. To evaluate the protective performance of the different steels target plates of different thicknesses have been prepared. In the case of perforation the residual velocity of the projectile after perforation has been determined as a measure of the protective performance of the steel.First, a classical RHA armour steel with hardnesses of about 350 HBW has been tested. In the case of the EFP projectile impacting a RHA steel plate with a thickness in the ballistic limit range, the penetration and perforation process of this kind of projectile seems to consist of two parts: a ductile hole enlargement process of a deformable projectile in the first part and shear failure leading to plugging in the second, dominant part. The shear failure leading to the formation of a massive plug is governed by adiabatic shear banding, what was observed from metallographic analysis.To investigate the influence of the hardness and strength on the protective performance against the EFP, additionally some steels with higher and with lower hardness than the RHA steel were tested. For the steels of higher hardness, classical armour steels with hardnesses of 500 HBW and 600 HBW have been chosen, also as new armour steel grades with hardnesses of 440 HBW and 600 HBW with increased toughness for blast applications. For the steels with lower hardness, the structural steels S355J2+N, S690QL and S960QL have been chosen.It was found, that the mechanical properties of the steels do not influence their protective performance, at least if the residual velocity after perforation is taken as a measure for the protective performance. In the case where the plate thickness was sufficient to stop the projectile it seems, that the structural steels perform at least as good as RHA steel. The S690QL steel was able to stop the projectile at thicknesses lower than the RHA steel. This behavior was assigned to its lower sensitivity to the formation of adiabatic shear bands.

Adiabatic shear bands localization in materials undergoing deformations

We consider the adiabatic shear banding phenomenon in composite materials undergoing the high speed shear deformations. The mathematical model of adiabatic shear banding in thermo-visco-plastic material is given. New two step numerical algorithm which is based on the Courant-Isaacson-Rees scheme that allows one to simulate fully localized plastic flow from initial stage of localization is proposed. To test this numerical algorithm we use three benchmark problems. The testing results show the accuracy and efficiency of proposed algorithm. The features of adiabatic shear bands formation in composites are studied. The existence of characteristic depth of localization in composites is shown. Influence of initial temperature distribution on the processes of adiabatic shear bands formation in composites is considered.

An Experimental Study on the Deformation Behavior of Aluminium Armour Plates Impacted by Two Different Non-deformable Projectiles

The present study describes the experimental results pertinent to the penetration of AA-2024, AA-6061 and AA-7017 plates with two different non-deformable steel projectiles. The diameters of the projectiles used for ballistic impact testing are 7.62 and 12.7mm. The projectiles are impacted on the aluminium targets of 70mm thickness with velocities in the range of 840±10 m/s. The results presented include variation in damage pattern in experimental alloys with respect to different projectiles. The microstructures and micro-hardness values along the projectile penetration path have been investigated to understand the material deformation behaviour. Some observations relating to the adiabatic shear band formation have also been presented. From the ballistic testing experiments, it is observed that AA-7017 plates display higher ballistic resistance among the tested aluminium alloys. The ballistic performance of the aluminium alloy plates have been correlated with their respective mechanical properties.

An examination of adiabatic shearing behavior in ZK60 alloy with different states of heat treatment

Adiabatic shearing behavior and fracture were investigated in ZK60 magnesium alloy by means of radial collapse of a thick-walled cylinder. The alloy was tested in the solid solution (T4), as-extruded (T5) and solid solution plus aged (T6) conditions to examine adiabatic shearing behavior. Results showed that the alloy was less susceptible to the formation of Adiabatic Shear Bands (ASBs) under the T4 condition due to the number of observed adiabatic shear bands and their corresponding average lengths. On the other hand, the alloy under the T5 condition was more susceptible to the formation of adiabatic shear bands with 39 and 41 shear bands observed under effective strains of 0.72 and 0.95 respectively. Nevertheless, the shear bands observed in the T6 condition were longer compared to all the other heat treated conditions. The widths of the ASBs are 3.8µm, 8.56µm and 8.62µm at the T4, T5 and T6 states respectively. It was shown that the alloy under the different conditions provide a wide range of microstructural parameters and mechanical properties. It was shown that the ZK60 magnesium alloy under the different conditions provide a wide range of microstructural parameters (grain size, precipitates) and mechanical properties that affect its adiabatic shearing behavior. It is concluded that ASB is one of the damage fracture models under this kind of high strain rate loading where cracks nucleated on the edge of the ASBs and propagated along the shear direction.

Formation mechanism of the high-speed deformation characteristic microstructure based on dislocation slipping and twinning in α-titanium

Abstract