Formation Of Adiabatic Shear Bands Research Articles

Comparison of various high-stress wear conditions and wear performance of martensitic steels

The demanding environments typically encountered by the wear resistant steels create challenges for the materials selection, because the hardness grades of the steels alone do not reveal the true nature of their wear behavior. In this study, five commercial wear resistant steels were tested using three application oriented test methods with five different test variables for abrasion, impact-abrasion, and slurry erosion. All the used test methods produced high-stress conditions that crushed the used mineral abrasive, plastically deformed the sample surfaces, and led to the formation of adiabatic shear bands. When the results produced by the chosen methods were compared, the normalization of the wear losses by the wear area and test time revealed well the differences between the methods. The test methods ranked the steels similarly, but there were clear differences in the wear rates and wear mechanisms between the tests. In addition, the abrasive methods produced surface adiabatic shear bands, while subsurface shear bands were initiated by the more impacting methods. In the studied conditions, the work hardening ability of the steel had a clear influence on its wear resistance, which largely explains the marked differences in the wear rates of the studied commercial 500HB grade steels.

High strain-rate deformation of T8-tempered, cryo-rolled and ultrafine grained AA 2099 aluminum alloy

The mechanical response of as-received coarse-grained (CG) AA 2099-T8 aluminum alloy under high strain-rate loading was compared with those of cryo-rolled and ultrafine grain (UFG) specimens. The UFG alloy was developed using thermomechanical processing that involved solution heat treatment of the as-received AA 2099-T8 alloy, followed by cryo-rolling and subsequent annealing. High strain-rate testing of cylindrical specimens of the UFG, cryo-rolled and CG alloy was carried out using the split Hopkinson pressure bar system, while scanning and transmission electron microscopic techniques were used for microstructural characterization of the specimens before and after the test. The true strain and true strain rate of the specimens increased as the impact momentum (IM) was increased. Grain refinement via thermomechanical processing did not improve the dynamic yield strength of AA 2099 aluminum alloy, which is attributed to the development of more non-equilibrium UFG boundaries during the dynamic deformation of UFG AA 2099 specimens. However, the formation of adiabatic shear bands (ASBs) which trigger dynamic fracture in alloys during high strain-rate deformation was highly suppressed by grain refinement. Whereas only deformed shear band (DSB) developed in cryo-rolled and UFG specimens, the as-received CG AA 2099-T8 alloy developed transformed shear band (TSB), along which crack nucleated and propagated. The dissolution of precipitates due to adiabatic heating increased towards the TSB. Of the possible strengthening sources in AA 2099 alloy, precipitation strengthening contributed mostly to the yield strength under dynamic impact loading at IM≤22 kg. ms−1, while strengthening by dislocation-dislocation interactions dominated at IM≥22 kg. ms−1.

Investigations on microstructure evolution of TA1 titanium alloy subjected to electromagnetic impact loading

In this work, the adiabatic shear band of TA1 titanium alloy subjected to electromagnetic impact loading was investigated. The formation of adiabatic shear band and microstructure evolution within it were revealed by microstructure characterizations. Deformation results showed an adiabatic shear band with the width of 10μm located in shear deformation zone, and most deformations mainly concentrated in the narrow band. The compressive instability and the hardness difference contributed to the formation of adiabatic shear band. Severe shear deformations led to high location density within the adiabatic shear band. A large amount of dislocations distributed in the form of dislocation cells and random dislocations. The rotational dynamic recrystallization mechanism caused that many dynamic recrystallization grains with the size of 100–200nm were found inside the adiabatic shear band. Adiabatic temperature rise and distortion energies stored by high dislocation densities provided sub-grain rotations with the driving forces.

Finite element simulations on the relation of microstructural characteristics and the formation of different types of adiabatic shear bands in a β-titanium alloy

Deformation at high strain rates in combination with local thermal softening can lead to the formation of so-called adiabatic shear bands. These shear bands occur when the macroscopic deformation is concentrated in microscopic areas. The fundamental thermomechanical processes and micromechanical mechanisms acting during adiabatic shear band formation are not fully understood yet. In particular, the influence of different initial microstructures on shear band formation has not sufficiently clarified. In an earlier investigation, adiabatic shear bands formed in a β-titanium alloy with different initial microstructures were characterized in detail. It was shown that different volume fractions of the α-phase result in the formation of different types of shear bands. In the present study, a simple modeling approach based on the finite element (FE) method is used to investigate in a systematic parameter study (with a special focus on mechanical and thermal variables) how these different types of adiabatic shear bands can occur in the same material. The FE simulations clearly show that microstructural characteristics – such as size and volume fraction of the α-phase in the β-titanium alloy – have a strong influence on shear band formation. This study contributes to a more detailed understanding of the microstructural and thermomechanical mechanisms leading to the formation of different types of adiabatic shear bands in titanium alloys.

Effect of impurities on adiabatic shear bands (ASB) formation and evolution in U-2.5 wt%Nb alloy under impact

Local stress concentration sites and/or microstructural inhomogeneities result in the nucleation and initiation of adiabatic shear bands (ASB). This paper reports the influence of impurities for the initiation and evolution of ASBs in U-2.5 wt%Nb alloy. Both cylindrical specimen and hat-shaped specimen were used to investigate the formation and evolution mechanism of ASBs in U-2.5 wt%Nb alloy using a split Hopkinson pressure bar apparatus. It was observed that brittle inclusions such as carbides in all of the specimens break up to form microcracks at very small strain during impact, then the microcracks coalesce and grow with increasing strain, which revealed that the brittle impurities in specimen not only precedes adiabatic shear failure, but also likely to be a dominant micromechanical factor in the very generation of the band. Adiabatic shear fracture occurs in the top end of hat-shaped specimen, it also appears that the microstructural inhomogeneities play more important role in occurrence of ASBs than the geometrical inhomogeneities. Additionally, it was demonstrated that the ductile inclusion of niobium was susceptible to resist the propagation of shear bands.

Temperature Rise Associated with Adiabatic Shear Band: Causality Clarified.

One of the most important issues related to adiabatic shear failure is the correlation among temperature elevation, adiabatic shear band (ASB) formation and the loss of load capacity of the material. Our experimental results show direct evidence that ASB forms several microseconds after stress collapse and temperature rise reaches its maximum about 30 μs after ASB formation. This observation indicates that temperature rise cannot be the cause of ASB. Rather, it might be the result of adiabatic shear localization. As such, the traditional well-accepted thermal-softening mechanism of ASB needs to be reconsidered.

WITHDRAWN: Dynamic failure of titanium: Temperature rise and adiabatic shear band formation

WITHDRAWN: Dynamic failure of titanium: Temperature rise and adiabatic shear band formation

Failure modes in a carbon / titanium fibre metal laminate under hyper-velocity impact

Titanium and carbon-fibre reinforced epoxy fibre metal laminates (FMLs) are investigated to determine their potential as a mass efficient passive armour for countering the threat of explosively formed projectiles (EFPs) travelling in the region of 2000 m/s. A proposed methodology for predicting the effect of the impact is presented, together with the results from test shots. There is good correlation between the predictions and the test results. The tests show that the failure modes of the composite material constituent are influenced by the lamination sequence. The tests also show that one of the contributory mechanisms to the failure in the titanium is the formation of adiabatic shear bands.

A study of the dynamic impact behaviour of IN 718 and ATI 718Plus® superalloys

ABSTRACTThe dynamic impact response of IN 718 and ATI 718Plus®, in both the solution heat treated and age-hardened conditions, were investigated at different deformation temperatures and strain rates using a direct impact Hopkinson pressure bar. Analyses of the results provide a vital but previously not reported information that the ATI 718Plus® offers a higher resistance to damage during high strain rate ballistic impact deformation compared to the most widely used Iron-nickel based superalloy, Inconel 718. ATI 718Plus® showed higher strain hardening and strain rate sensitivity, in both heat treatment conditions, than IN 718. The difference in the deformation behaviour of both alloys, in the annealed condition, is attributable to the compositional modification in ATI 718Plus® which has been reported to lower its stacking fault energy and increases the tendency for deformation twinning. However, in the age-hardened condition, the difference is believed to be related to the disparity in the operative strengthening mechanism, of the precipitates present in both alloys. Furthermore, a higher susceptibility to strain location and the formation of adiabatic shear band, in aged IN 718, is attributable to the stronger temperature-softening characteristics observed in the alloy and to the limited strain hardening tendency under dynamic impact loading.

Dynamic mechanical properties and adiabatic shearing behavior of the shock prestrained Ti–6Al–4V alloy having bimodal microstructure

Plate impact experiments and dynamic reloading compression tests at 3 × 103 and 4 × 103 s−1 were conducted on the Ti–6Al–4V alloy with bimodal microstructure to investigate the effect of shock prestrain on the dynamic mechanical properties and adiabatic shearing behavior of this alloy. The shock prestrained alloy has higher flow stress but lower uniform plastic strain and impact absorption energy at both reloading strain rates due to the multiplication of dislocations during shock pulse. The Bauschinger effect and the lack of more effective strengthening factors limit the shock-induced strengthening effect in Ti–6Al–4V at an equivalent strain level during reloading at a strain rate of 3 × 103 s−1. However, the shock-induced strengthening effect is somewhat improved at a higher reloading strain rate. Shock prestrain facilitates the formation, bifurcation and interaction of adiabatic shear bands (ASBs) in Ti–6Al–4V through the promotion of the dynamic recovery process at the primary stage of the formation of ASBs. Plugs and clusters of ASBs can form in the shock prestrained Ti–6Al–4V with bimodal microstructure at a relatively low strain rate due to the high density and frequent interactions of ASBs.

Characterization of coarse and ultrafine-grained austenitic stainless steel subjected to dynamic impact load: XRD, SEM, TEM and EBSD analyses

A comparative study of the dynamic impact responses of coarse (37 µm) and ultrafine-grained (0.24 µm) AISI 321 austenitic stainless steel was conducted using a split Hopkinson pressure bar system. An increase in firing pressure of the system's projectile results in the corresponding increase in impact momentum, strain rate, total strain and maximum flow stress of the specimens. Although ultrafine-grained (UFG) specimens show higher compressive strength, coarse-grained (CG) specimens possess higher strain hardenability than the UFG specimens under the same impact loading conditions. UFG specimens exhibit lower critical strain and strain rate at which adiabatic shear bands (ASBs) are observed; hence, UFG specimens are more susceptible to ASB formation than CG specimens. Using the XRD, SEM, TEM and EBSD characterization techniques, the underlying deformation and strengthening mechanisms were studied. Slip and twinning are the active deformation mechanisms in CG specimens and they are highly suppressed in UFG specimens due to spatial restriction effect. At higher strain rates, CG specimens are only susceptible to the formation of multiple transformed shear band (MTSB), while UFG specimens are susceptible to both MTSB and TSB bifurcation. Although the mechanism behind the formation of MTSB is not completely clear, bifurcation of TSB was geometrically necessary and the presence of precipitate at the bifurcation point could play major role. EBSD analysis of inside, interface and outside the TSB revealed the development of equiaxed ultra-fine grain structure (∼0.17–0.21 µm in CG and ∼0.14–0.15 µm in UFG specimens) inside TSB by rotational dynamic recrystallization mechanism.

Numerical simulations of adiabatic shear localization in textured FCC metal based on crystal plasticity finite element method

Experimental investigations of adiabatic shear localization in the nanostructured face-centered-cubic (FCC) alloys have revealed that micro-texture has a key role in advancing the formation of adiabatic shear bands (ASBs) (see reference [35]). In this work, we present the crystal plasticity finite element simulations (CPFEM) of dynamic uniaxial compression and simple shear in polycrystalline models with different initial textures. The aim is to study the effects of typical textures, which are often observed in FCC metals after severe plastic deformation or recrystallization, on the formation of ASBs under high strain rate loading. The materials response is described by an elastic-viscoplastic continuum slip constitutive relation, in which the dependence of slip systems’ resistance on the temperature evolution is also considered. Simulation results show that, under high-rate compression, except for some textures which are not favorable for the formation of ASBs, different textures lead to various orientation of shear bands and different critical strains at which the shear localizations occur. High-rate simple shear loading is found to facilitate ASB formation. Even in the texture-free, namely Random texture, model obvious shear localization was observed. However, the dependence of slip system activation on texture leads to considerable variation in the critical shear strain level for ASB initiation. Additionally, the effects of some factors, such as strain rate, temperature rise and strength of materials, on the formation of ASBs in FCC textured metals are discussed to interpret the formation process in more depth. We also combined the experimental observations and numerical simulations to elaborate the reason why ASBs have rarely been reported in FCC materials.

Shear localization and recrystallization in high strain rate deformation in Ti-5Al-5Mo-5V-1Cr-1Fe alloy

To study the dynamic behavior and microstructural evolution in high strain rate deformation of Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy, a series of forced shear tests of hat shaped specimens have been conducted using a split Hopkinson pressure bar combined with “strain frozen” technique. Localized shear band is induced in these tests. This paper indicates that the flow stress of Ti-55511 alloy is independent on the punching depth, and thermal softening has a minor effect on the onset of adiabatic shear band and dynamic recrystallization formation. The concept of “adhesive fracture” can be identified as the dynamic failure mechanism for Ti-55511 alloy based on the crack propagation path.

Dynamic mechanical behavior of 93W–4.9Ni–2.1Fe/95W–2.8Ni–1.2Fe–1Al2O3 heavy alloy composite

93W–4.9Ni–2.1Fe/95W–2.8Ni–1.2Fe–1Al2O3 (93WHA/95WHA) composites were fabricated by powder metallurgical routes to meet the demands pertaining to both the penetration and fragmentation performances. Mechanical behavior, adiabatic shear band (ASB) formation and fracture characteristics of the 93WHA/95WHA composites at high strain rate were evaluated using split Hopkinson bars. The relation between composition, microstructure and dynamic mechanical properties was investigated. The dynamic compression and tensile behavior of the 93WHA/95WHA composite are dependent on the 93WHA part and 95WHA part, respectively. The 93WHA part in the 93WHA/95WHA composite is more easily to form narrow and deep ASBs than the 95WHA part, showing excellent penetration performance. Brittle fracture occurs in the 95WHA part of the 93WHA/95WHA composite by W–W grain boundary separation and matrix rupture under dynamic tensile loading because of high W–W contiguity, residual porosity and addition of Al2O3 particles in the 95WHA part, providing conceivable fragmentation performance and secondary destruction.

Microstructural Evolution in High-Strain-Rate Deformation of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy.

To study the microstructural evolution in high-strain-rate shear deformation of Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy, a series of forced shear tests of hat-shaped specimens have been conducted using a split Hopkinson pressure bar combined with the “strain-frozen” technique. A localized shear band is induced in Ti-55511 alloy in these tests. The experimental results demonstrate that the flow stress in hat-shaped specimens remains constant (about 600 MPa) and is independent of punching depth. The width of the adiabatic shear band increases with increasing punching depth and tends to saturate at 30 μm, and the estimation of the adiabatic shear band (ASB) width in hat-shaped (HS) specimens has been modified. Relying on the experimental results, thermal softening has a minor effect on the onset of the adiabatic shear band and dynamic recrystallization formation, and the nucleation mechanism for dynamic recrystallization is strain-induced boundary migration and subgrain rotation and coalescence. In addition, we suggest the concept of adhesive fracture as the dynamic failure mechanism for Ti-55511 alloy.

Dynamic Mechanical Responses of Ultrafine-Grained IF Steel Over a Wide Range of Temperatures

The ultrafine-grained interstitial free (UFG-IF) steel was fabricated using the equal channel angular pressing (ECAP) method. Transmission electronic microscope (TEM) observations showed that the grains were refined to sub-micrometer scale and the average grain size was several hundred nanometers (nm). Uniaxial compression tests were conducted over a wide range of strain rates (up to 6000/s) and temperatures (from −196[Formula: see text]C to 300[Formula: see text]C) to understand the dependence of mechanical behaviors on the strain rate and temperature. It is found that the UFG-IF steel showed a weak strain rate sensitivity and displayed a strong temperature sensitivity only within a very narrow temperature range. Microstructural observations on the specimens deformed dynamically at temperatures below −20[Formula: see text]C via scanning electronic microscopy (SEM) and TEM indicated the occurrence of adiabatic shear localization. In order to explain the reason why adiabatic shear localization formed at the low temperatures and high strain rates, the susceptibility factor to adiabatic shear band (ASB) was estimated. It is proposed that more heat generated during the adiabatic plastic deformation process due to the enhanced flow stress of the UFG-IF steel at lower temperatures may play a dominant role in the formation of ASB. Meanwhile, the experimental results also provide strong evidence that for the UFG-IF steel the dynamic recrystallization which results from the severe plastic deformation and high temperature rise only occurs at the later stage of ASB evolution. With regard to the formation of ASB in the UFG IF steel, the localized plastic flow at a lower stress level indicates that localized deformation happens firstly and the temperature rise in the localized shearing zone leads to the final plastic instability.

Adiabatic shear localization in the CrMnFeCoNi high-entropy alloy

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.

Hot deformation behavior of a Sr-modified Al-Si-Mg alloy: Constitutive model and processing maps

Abstract Isothermal compression experiments were conducted to study the hot deformation behaviors of a Sr-modified Al-Si-Mg alloy in the temperature range of 300–420 °C and strain rate range of 0.01–10 s−1. A physically-based model was developed to accurately predict the flow stress. Meanwhile, processing maps were established to optimize hot working parameters. It is found that decreasing the strain rate or increasing the deformation temperature reduces the flow stress. The high activation energy is closely related to the pinning of dislocations from Si-containing dispersoids. Moreover, the deformed grains and the Si-containing dispersoids in the matrix are elongated perpendicular to the compression direction, and incomplete dynamic recrystallization (DRX) is discovered on the elongated boundaries in domain with peak efficiency. The flow instability is mainly attributed to the flow localization, brittle fracture of eutectic Si phase, and formation of adiabatic shear band. The optimum hot working window is 380–420 °C and 0.03–0.28 s−1.

An Investigation on the Adiabatic Shear Bands in Depleted U-0.75 wt % Ti Alloy under Dynamic Loading

Adiabatic shear bands in uranium alloy projectiles/penetrators, during penetration, allow them to “self-sharpen,” a process that is absent in most tungsten alloy projectiles/penetrators. U-0.75 wt % Ti alloy samples have been accelerated to impact steel targets, and the distribution of adiabatic shear bands in residual samples has been studied in detail to understand the effect of self-sharpening on penetration. In our study, self-sharpening was evidenced by the distribution of the shear bands in the recovered sample. The shear bands formed during impact were observed to change direction when they crossed grain boundaries, which indicated that the grain boundaries had an influence on the adiabatic shear bands of U-0.75 wt % Ti. Micro-hardness test results showed that the Vickers micro-hardness in the adiabatic shear zone was 18% lower than that in the matrix. In the split-Hopkinson pressure bar (SHPB) experiment, a strain rate of around 2891 s−1 was the threshold strain rate that triggered the formation of adiabatic shear bands in the U-0.75 wt % Ti alloy.

Deformation Mechanisms and Formability Window for As-Cast Mg-6Al-2Ca-1Sn-0.3Sr Alloy (MRI 230D)

The hot deformation characteristics of MRI 230D alloy have been evaluated in the temperature range 260-500 °C and strain rate range 0.0003-10 s−1, on the basis of processing map. The processing map exhibited two domains in the ranges: (1) 300-370 °C and 0.0003-0.001 s−1 and (2) 370-480 °C and 0.0003-0.1 s−1. Dynamic recrystallization occurs in the both domains with basal slip dominating in the first domain along with climb as recovery process and second-order pyramidal slip dominating in the second with the recovery by cross-slip. In Domains (1) and (2), the apparent activation energy values estimated using the kinetic rate equation are 143 and 206 kJ/mole, respectively, the first one being close to that for lattice self-diffusion confirming climb. It is recommended that the alloy is best processed at 450 °C and strain rates less than 0.1 s−1, where non-basal slip and cross-slip occur extensively to impart excellent workability. The alloy exhibits flow instability in the form of adiabatic shear band formation and flow localization at lower temperatures and higher strain rates. Forging of a cup-shaped component was performed under various conditions, and the results validated the predictions of the processing map on the workability domains as well as the instability regimes.