Formation Of Adiabatic Shear Bands Research Articles

Analysis of the Transformation-induced Plasticity Effect during the Dynamic Deformation of High-manganese Steel

The transformation-induced plasticity (TRIP) effect and resistance characteristics to adiabatic shear failure at high strain rates of high-manganese steel were investigated by using scanning electron microscopy and electron backscattering diffraction. Results showed that the high-manganese steel exhibited excellent strain hardening effect and resistance to adiabatic shear failure because of the TRIP effect. The TRIP effect occurred during dynamic deformation and showed two distinct stages, namely, the smooth TRIP process before the formation of adiabatic shear band (ASB) and the inhibited TRIP process during further deformation. In the first stage, the martensitic transformation showed slight orientation dependence and weak variant selection, which promoted the TRIP effect. In the second stage, reverse martensitic transformation occurred. Adiabatic shear bands (ASBs) developed typical shear microtextures {111} 1 ¯ 10 >. In microtextures, two groups of fine grains are in a twin relationship and uniform distribution, which restrained the formation of holes and cracks within the ASBs and enhanced damage resistance after ASB formation.

A systematic study of grain refinement during impact of 4340 steel

Inhomogeneous plastic deformation at high strain rates and large strains results in the concentration of deformation and strain localization in adiabatic shear bands (ASBs). In the current study, AISI 4340 steel specimens were impacted at increasing strain rates/strains (impact momentums) to capture the formation of adiabatic shear bands. Advanced specimen preparation techniques using Focused Ion Beam (FIB) and extensive electron microscopy were used to identify the sequence of evolution of the shear bands. It was observed that the structure of the shear bands that evolve after strain localization starts out with elongation of the grains due to grain reorientation in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. For higher strain rates/strains during impact, the elongated grains break along the initiated dislocation boundaries resulting in the creation of smaller elongated broken grains and nanograins. Boundary refinement of the broken grains occurring through grain rotation and adiabatic heating results in the evolution of refined grains, subgrains and nanograins. The presence of elongated grains, broken grains, refined grains, subgrains and nanograins within the shear band structures demonstrates that the local deformation is inhomogeneous and that these mechanisms occur concurrently. It is concluded that the evolution of the shear band structure can be considered as a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands.

Behavior of Transformation-Induced Plasticity during Adiabatic Shear Bands Formation in High Manganese Steels

The transformation-induced plasticity (TRIP) effect is examined at high strain rate of over 104 s−1 compression during adiabatic shear band (ASB) formation to determine whether the TRIP effect proceeds smoothly and whether it can restrain ASB formation. Results show two distinct stages of TRIP during dynamic straining: the smooth occurrence of TRIP before ASB formation and its suppression during ASB formation, which is attributed to grain refinement, accumulated crystal defects, and adiabatic temperature rising. Compared with copper and martensitic armor steel, TRIP steel demonstrates a higher work hardening effect attributed to the strong interaction of austenite (γ), ϵ-martensite, and α′-martensite. Such interaction effectively postpones ASB formation.

Characterization of the Hot Deformation Behavior of a Al-Si Alloy Using Processing Maps

The compression tests of solution treatment ZL109 alloy have been performed in the compression temperature range from 250°C to 450°C and the strain rate range from 0.0005s-1 to 0.5s-1. A processing map has been developed on the basis of flow stress data obtained as a function of temperature and strain rate, which revealed two domains of hot working for the alloy: one is situated at temperature between 270°C and 340°C with strain rate between 0.05s-1 and 0.5s-1, the other is situated at the temperature between 380°C and 450°C with strain rate between 0.0005s-1 and 0.004s-1. Combining with the processing map, the optimum parameters of hot working for ZL109 alloy are that 300°C/0.5s-1 and 450°C/0.0005s-1, respectively. Microstructure observations indicated that DRX occurred in both these domains. The instable zones, i. e., adiabatic shear bands formation, wedge cracking, were also identified in the processing map and microstructural examination was performed for validation.

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M3C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350 °C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.

Compressive strength and hot deformation mechanisms in as-cast Mg-4Al-2Ba-2Ca (ABaX422) alloy

The behaviour of an as-cast ABaX422 Mg alloy has been evaluated with regard to its compressive strength in the temperature range 25–250 °C and hot working characteristics in the range 260–500 °C. The microstructure of the as-cast alloy has intermetallic phases Mg17Ba2 and (Al, Mg)2Ca at the grain boundaries and is fine grained. The alloy has compressive strength better than AZ31 with Ca and Zn, which was attributed to the finer grain size. A processing map developed to characterize its hot working behaviour revealed two dynamic recrystallization domains in the temperature and strain rate ranges of (1) 300–390 °C/0.0003–0.001 s−1 and (2) 400–500 °C/0.0003–0.5 s−1. In the first domain, basal + prismatic slip occurs along with recovery by climb while in the second domain, second-order pyramidal slip dominates and recovery occurs by cross-slip. The apparent activation energy estimated in Domains 1 and 2 are 169 and 263 kJ/mol respectively, both being higher than that for self-diffusion suggesting that the intermetallic particles in the matrix cause considerable back stress. Bulk metal working of this alloy may be done in Domain 2 which ensures high workability while finish working may be done in Domain 1 in order to achieve a fine grained component. The alloy exhibits flow instability regimes at higher strain rates, in both the lower and higher temperature regions of the processing map, the manifestation being adiabatic shear band formation and flow localization respectively.

Microstructure Evolution of Ti-6Al-4V Alloy during High Speed Cutting: An Investigation Using Finite Element Simulation

In this paper, a novel model combining the microstructure prediction model and a modified constitutive model of the Johnson-Cook (JC) model was developed and embedded into FEM software via the user subroutine. The chip formation and microstructure evolution in high speed cutting of Ti-6Al-4V alloy were simulated based on the presented model. The results indicated that dynamic recrystallization mainly happened in ASBs, where the grain size had a big decline. According to the variation of cutting temperature and grain size of microstructure, the mechanism of the adiabatic shear bands (ASBs) formation was investigated deeply and concluded that dynamic recrystallization was the root cause of the serrated chip formation.

Effect of preloading on the formation of adiabatic localized shear in copper

This paper presents the results of a study of the formation of localized shear in M1 copper of two types: as-received and after preloading by a quasi-entropic compression wave. The experiments were performed with hat-shaped samples using the split Hopkinson bar method. For both types of copper, dynamic compression diagrams were obtained at strain rates of 2100–2500 s−1. The copper structure was subjected to metallographic analysis, and the effect of preliminary shock deformation on the dynamic mechanical properties of the material was estimated. It is shown that preloaded higher-strength metals with a smaller degree of strain hardening are more prone to the formation of adiabatic shear bands.

Shear band formation in AISI 4340 steel under dynamic impact loads: Modeling and experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

Forming limit diagram and void coalescence analysis of AA5052 coated with molybdenum-based ceramic nanocomposites

Aluminium 5052 alloy sheets of size 75×25mm and 3mm thickness with double edge semicircular notches of diameter from 2–8mm have been coated with MoSi2–SiC nanocomposite coatings by sputtering process. The structural morphology of the ceramic coatings was explored by Fourier Transform Infrared (FT-IR) spectroscopy, X-ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM). The coated and uncoated sheet samples were subjected to tension–compression state of strain up to fracture by varying the notch sizes using INSTRON Universal Testing Machine (UTM). Fracture behaviour studies of the sheets were performed and forming limit diagram (FLD) was drawn. The void coalescence analysis was also carried out by using SEM images and the effect of coating behaviour of combined forming and fracture were analyzed. In the coated sheet, the L/W ratio was very close to 1.0. Thus, no oblate/prolate voids were observed and the heat generated during deformation was retained for longer time and thereby adiabatic shear band formation has occurred with increased formability.

An experimental investigation into the micro-mechanics of spall initiation and propagation in Ti–6Al–4V during shock loading

Titanium alloys have long been considered candidate materials for use in monolithic armor systems due to their excellent specific mechanical properties and relative ease of manufacture. However, high production costs combined with a propensity to fail due to adiabatic shear band formation and spallation have limited the adoption of titanium alloys in armor systems. In this paper, the spall behavior of the workhorse titanium alloy, Ti–6Al–4V, is investigated via a series of plate impact experiments. Back-scatter electron (BSE) microscopy and electron back-scatter diffraction (EBSD) are carried out on recovered specimens to investigate the nature of spall initiation and propagation. It is found that spall initiation occurs by the nucleation of voids at the grain boundaries between plastically hard/soft grains of the dominant hcp α phase. Further, it is shown that spall then propagates via the growth/coalescence of the nucleated voids into facets, followed by the coalescence of the facets through highly localized ductile bridging. A number of suggestions are made to improve the spall resistance of titanium alloys based on these observations.

Experimental and theoretical research on critical characteristics for adiabatic shear localization fracture in high-speed machining

Adiabatic shear localization fracture (ASLF), which is a catastrophic fracture of the adiabatic shear band (ASB), will occur inevitably with the further increase of cutting speed. High-speed machining experiment of hardened AISI 1045 steel (HRC 45) is carried out by using a PCBN cutter with rake angle −10° at various cutting speeds up to 1,400 m/min and various feeds from 0.2 to 0.4 mm/r. Chip morphology (including discontinuously serrated chip and isolated segments), crack propagation, and blue brittleness effect in isolated segments formation are observed microscopically. Energy convergence in ASB with cutting conditions is calculated. Considering the effect of shear wave on the formation of ASB in high-speed machining, the ASLF theory under triaxial stress state is proposed and compared with the experimental results. The influences of shear band properties and loading conditions on ASLF are also discussed. The energy saturation limit of the shear band can be reasonably estimated by using the ASLF theory, which lays a theoretical and experimental foundation for further prediction study of isolated segments formation in high-speed machining.

Formation and evolution of adiabatic shear bands in zirconium alloy impacted by split Hopkinson pressure bar

The formation and evolution of the adiabatic shear bands in the zirconium alloy impacted by split Hopkinson pressure bar were investigated by means of the experimental design. The different strain levels in the zirconium alloy subjected to high strain rate loading were first designed by two times impact at almost same strain rate. Only deformed bands were formed in the zirconium alloy impacted once, and two types of adiabatic shear bands, namely deformed bands and transformed bands, were distinguished in the zirconium alloy impacted twice. Microstructural observation shows that the deformed bands showed the severe strain localization, and the transformed bands composed the ultrafine and equiaxed grains. The formation of the ultrafine and equiaxed grains in the transformed bands could be attributed to the rotational dynamic recrystallization mechanism. Based on the characterization of the ASBs formed at different strain stages, the evolution process of the ASBs is proposed. The deformed bands are suggested as the initial stage of the transformed bands formation, and the transformed bands are considered as the further development of the deformed bands. The microhardness measurements show that the microhardness values in the transformed bands were higher than that of the deformed bands and original sample because of strain hardening and grain refining.

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200s−1 and quasi-static compressive loading at a strain rate of 0.0032s−1. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.

Ballistic impact behaviour of β-CEZ Ti alloy against 7.62 mm armour piercing projectiles

The present study describes the ballistic impact behaviour of a high strength metastable near beta Ti alloy, β-CEZ in different microstructural conditions against 7.62 mm armour piercing projectiles. Ballistic trials were carried out on 10 mm thick β-CEZ and Ti–6Al–4V plates clamped with 50 mm thick 7017 Al alloy backing. The ballistic performance of different conditions was evaluated by measuring residual depth of penetration (DOP) in 50 mm thick Al-7017 alloy backing. In the present study, even though the β-CEZ titanium alloy in as-rolled and α + β solution treated + aged conditions had significantly higher strength and hardness compared to conventionally used Ti–6Al–4V, significant improvement in ballistic impact resistance was not observed. Post ballistic microstructural observation of crater cross section showed formation of adiabatic shear bands (ASBs) and ASB induced cracks in all conditions. As-rolled, α + β solution treated and aged conditions of β-CEZ alloy showed more ASBs, ASB induced cracks in comparison to Ti–6Al–4V. The β-solution treated and quenched condition of β-CEZ had less number of ASB and highest residual depth of penetration. In this study the ballistic results are discussed based on the post ballistic microstructural observations.

Microstructure of adiabatic shear bands in Ti6Al4V

Abstract Microstructural deformation mechanisms in adiabatic shear bands in Ti6Al4V are studied using traditional TEM and selected area diffraction, and more advanced microstructural characterisation techniques such as energy dispersive X-ray spectroscopy, high angle annular dark field STEM and conical dark field TEM. The shear bands under investigation are induced in Ti6Al4V samples by high strain rate compression of cylindrical and hat-shaped specimens in a split Hopkinson pressure bar setup. Samples from experiments interrupted at different levels of deformation are used to study the evolution of the microstructure in and nearby the shear bands. From the early stages of adiabatic shear band formation, TEM revealed strongly elongated equiaxed grains in the shear band. These band-like grains become narrower towards the centre of the band and start to fraction even further along their elongated direction to finally result in a nano-crystalline region in the core. In fully developed shear bands, twins and a needle-like martensite morphology are observed near the shear band.

Multiple Adiabatic Shear Bands in Ti–6Al–4V Targets under Ballistic Impact

In order to research the formation and spread of adiabatic shear bands in Ti-6Al-4V targets, LS-DYNA code is used to simulate the ballistic impact process. The projectile used in the impact test is a flat-headed steel cylinder with diameter of 7.62mm and length of 39mm. The results of simulated and impact test are in good agreement. Multiple adiabatic shear bands form in Ti-6Al-4V targets under high-speed ballistic impact. Adiabatic shear bands were found to extend parallel with a certain distance. The formation and distribution of adiabatic shear bands was related to the breaking-off of projectiles, which was caused by the distribution of maximum shear stress in Ti-6Al-4V targets and projectiles.

Analysis of adiabatic shearing failure mechanism for aluminum matrix composites based on experimental and numerical simulation

Adiabatic shear behavior and the corresponding mechanism of TiB2/Al composites were researched by split Hopkinson pressure bar (SHPB). Results show that the flow stresses of the TiB2/Al composites exhibit softening tendency with the increasing of strain rates. All the composites fail in splitting and cutting with a 45 degree, and the phase transformed bands of molten aluminum are found on the adiabatic shear layers. The deformation behavior and shear localization of the TiB2/Al composites specimens were simulated by finite element code MSC. Marc. The Johnson-Cook model was used to describe the thermo-viscoplastic response of the specimen material. There was unanimous between the numerical result and the experimental result on the location of the adiabatic shear band. From the numerical simulation and experiment, it was concluded that the instantaneous failure of the composite was ascribed due to the local low strength area where the formation of adiabatic shear band was, and the stress condition had significant effect on the initiation and propagation of adiabatic shear band (ASB).

Formation of Adiabatic Shear Bands in a Tempered Steel under Dynamic Loading and their Damage Effects

Abstract In this study a low alloyed steel was selected and heat treatments including austenization, quenching and tempering were applied to it to obtain a hardened matrix. After heat treatments two steel plates ha-ving hardnesses of 315 and 320 HV were subjected to ballistic shots as target materials. The shots were carried out at 0° from a distance of 30 m to the target materials using API 55-7.62 mm armor piercing projectiles with a velocity of 835–840 m/s. Metallographical and fractographical examinations were carried out after the shots to investigate the behaviour of matrix under dynamic loading and to observe the formation as well as type of adiabatic shear bands and their effects on the nucleation and propagation of cracks within the matrix, in addition to the inclusions. In the etched samples a degenerated matrix compared to the original tempered martensitic-bainitic matrix was observed in the regions close to the penetration. On the other hand deformed and transformed types of adiabatic shear bands were observed as white bands. It is concluded that both types of adiabatic shear bands directly play an important role in the nucleation and propagation of cracks in addition to the inclusions within the matrix.

Deformation and fracture behavior of laser processed dense and porous Ti6Al4V alloy under static and dynamic loading

Abstract Deformation and fracture behavior of laser processed dense and porous Ti6Al4V alloy under uniaxial static and dynamic compression loadings were experimentally investigated in this study. Samples with 0%, 10% and 20% porosities were fabricated using a continuous wave Nd–YAG laser in a controlled environment. The dynamic tests were performed at the strain rates of 1 × 103, 4 × 103 and 8 × 103/s using the split Hopkinson pressure bar (SHPB) test system and the static tests were done at the strain rate of 1 × 10−3/s. In addition, SEM and EBSD analyses were also carried out to study the failure mechanisms and texture development due to solidification and deformation. Both deformation and fracture exhibited appreciable rate sensitivity. For the former, the material strength increased, and for the latter, the failure strains decreased with the increased strain rates. The 20% porous Ti6Al4V showed lower strength, but higher ductility compared to the 10% material, but the 10% porous material had both less strength and ductility than the dense (0% porosity) material and the ductility of 20% porous material was only comparable to that of the dense material, i.e., did not exhibit appreciable increase of ductility. The texture analysis indicated that all the samples had reasonably consistent textures due to solidification. Hence the texture effect is essentially negligible and the observed material behavior is mainly a manifestation of the induced porosity. Both the macroscopic experiments and the microstructure analysis suggest that the formation of adiabatic shear band (ASB) could likely be the major failure mechanism for Ti6Al4V and the pores likely be the sites for nucleating ASB and subsequent catastrophic failure. For initially dense Ti6Al4V, the failure and the corresponding failure strain seem to depend on the duration of the incubation period for developing a nucleation site.