Formation of adiabatic shearing band for high-strength Ti-5553 alloy:A dramatic thermoplastic microstructural evolution

By using split Hopkinson pressure bar, optical microscopy and electronic microscopy, we investigate the influence of initial microstructures on the adiabatic shear behavior of high-strength Ti-5Al-5V-5Mo-3Cr(Ti-5553) alloy with lamellar microstructure and bimodal microstructure. Lamellar alloy tends to form adiabatic shearing band(ASB) at low compression strain, while bimodal alloy is considerably ASBresistant. Comparing with the initial microstructure of Ti-5553 alloy, we find that the microstructure of the ASB changes dramatically. Adiabatic shear of lamellar Ti-5553 alloy not only results in the formation of recrystallized β nano-grains within the ASB, but also leads to the chemical redistribution of the alloying elements such as Al, V, Cr and Mo. As a result, the alloying elements distribute evenly in the ASB.In contrast, the dramatic adiabatic shear of bimodal alloy might give rise to the complete lamination of the globular primary a grain and the equiaxial prior β grain, which is accompanied by the dynamic recrystallization of a lamellae and β lamellae. As a result, ASB of bimodal alloy is composed of a/β nanomultilayers. Chemical redistribution does not occur in ASB of bimodal alloy. Bimodal Ti-5553 alloy should be a promising candidate for high performance armors with high mass efficiency due to the processes high dynamic flow stress and excellent ASB-resistance.

Adiabatic Shear Bands in 30CrNi_3MoV Structural Steel Induced during High Speed Cutting

The width and spacing of adiabatic shear bands (ASBs) in the serrated chips generated during high speed orthogonal cutting of 30CrNi3MoV structurai steel were measured by opticai microscopy (OM), the temperature rise in the shear band was estimated. The microstructures of the ASBs were also characterized by SEM and TEM. The results show that the width and spacing of ASBs decrease with the increase of the cutting speed. The further observations show that the microstructure between the matrix and the center of the ASB gradually changes, and that the martensitic phase transformation, carbide precipitation and recrystallization may occur in the ASB.

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）.

Adiabatic shear sensitivity of ductile metal based on gradient-dependent JOHNSON-COOK model

Based on the expression proposed by WANG for the local plastic shear deformation distribution in the adiabatic shear band（ASB） using gradient-dependent plasticity,the effects of 10 parameters on the adiabatic shear sensitivity were studied.The experimental data for a flow line in the ASB obtained by LIAO and DUFFY were fitted by use of the curve-fitting least squares method and the proposed expression.The critical plastic shear strains corresponding to the onset of the ASB for Ti-6Al-4V were assessed at different assigned ASB widths.It is found that the proposed expression describes well the non-linear deformation characteristics of the flow line in the ASB.Some parameters in the JOHNSON-COOK model are back-calculated using different critical plastic shear strains.The adiabatic shear sensitivity decreases as initial static yield stress,work to heat conversion factor and strain-rate parameter decrease,which is opposite to the effects of density,heat capacity,ambient temperature and strain-hardening exponent.The present model can predict the ASB width evolution process.The predicted ASB width decreases with straining until a stable value is reached.The famous model proposed by DODD and BAI only can predict a final stable value.

Adiabatic shear localization evolution for steel based on the Johnson-Cook model and gradient-dependent plasticity

Gradient-dependent plasticity is introduced into the phenomenological Johnson-Cook model to study the effects of strainhardening, strain rate sensitivity, thermal-softening, and microstructure. The microstructural effect （interactions and interplay among microstructures） due to heterogeneity of texture plays an important role in the process of development or evolution of an adiabatic shear band with a certain thickness depending on the grain diameter. The distributed plastic shear strain and deformation in the shear band are derived and depend on the critical plastic shear strain corresponding to the peak flow shear stress, the coordinate or position, the internal length parameter, and the average plastic shear strain or the flow shear stress. The critical plastic shear strain, the distributed plastic shear strain, and deformation in the shear band are numerically predicted for a kind of steel deformed at a constant shear strain rate. Beyond the peak shear stress, the local plastic shear strain in the shear band is highly nonuniform and the local plastic shear deformation in the band is highly nonlinear. Shear localization is more apparent with the increase of the average plastic shear strain. The calculated distributions of the local plastic shear strain and deformation agree with the previous numerical and experimental results.

Adiabatic Shear Localization for Steels Based on Johnson-Cook Model and Second-and Fourth-Order Gradient Plasticity Models

To consider the effects of the interactions and interplay among microstructures, gradient-dependent models of second- and fourth-order are included in the widely used phenomenological Johnson-Cook model where the effects of strain-hardening, strain rate sensitivity, and thermal-softening are successfully described. The various parameters for 1006 steel, 4340 steel and S-7 tool steel are assigned. The distributions and evolutions of the local plastic shear strain and deformation in adiabatic shear band （ASB） are predicted. The calculated results of the second- and fourth- order gradient plasticity models are compared. S-7 tool steel possesses the steepest profile of local plastic shear strain in ASB, whereas 1006 steel has the least profile. The peak local plastic shear strain in ASB for S-7 tool steel is slightly higher than that for 4340 steel and is higher than that for 1006 steel. The extent of the nonlinear distribution of the local plastic shear deformation in ASB is more apparent for the S-7 tool steel, whereas it is the least apparent for 1006 steel. In fourth-order gradient plasticity model, the profile of the local plastic shear strain in the middle of ASB has a pronounced plateau whose width decreases with increasing average plastic shear strain, leading to a shrink of the portion of linear distribution of the profile of the local plastic shear deformation. When compared with the sec- ond-order gradient plasticity model, the fourth-order gradient plasticity model shows a lower peak local plastic shear strain in ASB and a higher magnitude of plastic shear deformation at the top or base of ASB, which is due to wider ASB. The present numerical results of the second- and fourth-order gradient plasticity models are consistent with the previous numerical and experimental results at least qualitatively.

Microstructural Characterization of the Shear Bands in Fe-Cr-Ni Single Crystal by EBSD

An investigation has been made into the microstructural characterization of the shear bands generated under high-strain rate （≈10^4 s^-1） deformation in Fe-15%Cr-15%Ni single crystal by EBSD-SEM （electron backscatter diffraction-scanning electron microscopy）, TEM （transmission electron in microscopy） and HREM （high- resolution electron microscopy）. The results reveal that the propagation of the shear band exhibits an asymmetrical behavior arising from inhomogenous distribution in plasticity in the bands because of different resistance to the collapse in different crystallographic directions; The γ-ε-α′phase transformations may take place inside and outside the bands, and these martensitic phases currently nucleate at intersections either between the twins and deformation bands or between the twins and ε-sheet. Investigation by EBSD shows that recrystallization can occur in the bands with a grain size of an average of 0.2μm in diameter. These nano-grains are proposed to attribute to the results of either dynamic or static recrystallization, which can be described by the rotational recrystallization mechanism. Calculation and analysis indicate that the strain rate inside the shear band can reach 2.50×10^6 s^-1, which is higher, by two or three orders of magnitude, than that exerted dynamically on the specimen tested.

Adiabatic shear banding of hot-extruded tungsten heavy alloy under cryogenic temperature

The effect of cryogenic temperature on adiabatic shear banding (ASBing) of tungsten heavy alloy (WHA) processed by hot-hydrostatic ex-trusion was investigated.Results show that,when the initial temperature is decreased,the dynamic flow stress,the critical failure time,and the dynamic failure energy of specimens show an increasing tendency,while the susceptibility to ASB of WHA shows a decreasing tendency,which is characterized by decreased shear strain and increased width of shear bands.Microstructure analysis shows that the number of mi-crocracks within ASB exhibits an increasing tendency with decreased initial temperature,and the dynamic recrystallization (DRX) process within ASB is evidently suppressed at the lower temperature.As a result of the lower temperature,the motion and rearrangement of disloca-tion are effectively suppressed,which is mainly responsible for the incomplete DRX process within ASB and decreases susceptibility to ASB.

Failure mode change and material damage with varied machining speeds:a review

High-speed machining(HSM) has been studied for several decades and has potential application in various industries, including the automobile and aerospace industries. However,the underlying mechanisms of HSM have not been formally reviewed thus far. This article focuses on the solid mechanics framework of adiabatic shear band(ASB) onset and material metallurgical microstructural evolutions in HSM. The ASB onset is described using partial differential systems. Several factors in HSM were considered in the systems, and the ASB onset conditions were obtained by solving these systems or applying the perturbation method to the systems. With increasing machining speed, an ASB can be depressed and further eliminated by shock pressure. The damage observed in HSM exhibits common features. Equiaxed fine grains produced by dynamic recrystallization widely cause damage to ductile materials, and amorphization is the common microstructural evolution in brittle materials. Based on previous studies, potential mechanisms for the phenomena in HSM are proposed. These include the thickness variation of the white layer of ductile materials. These proposed mechanisms would be beneficial to deeply understanding the various phenomena in HSM.

Effect of deep cryogenic treatment on the microstructural,mechanical and ballistic properties of AA7075-T6 aluminum alloy

The study focused on investigating the effect of Deep Cryogenic Treatment(DCT)on the mechanical and ballistic properties of AA7075-T6 aluminum alloy.The microstructure,microhardness,tensile strength,and impact strength of the Base Material(BM)and DCT-treated 7075 samples were analyzed through metallographic analysis and mechanical tests.The microstructure of the DCT-treated 7075 samples revealed fine grains and a distribution of secondary phase particles.The tensile strength,impact strength,and microhardness of DCT-treated samples increased by 7.41%,4%,and 9.68%,respectively,compared to the BM samples.The fractography analysis of the tensile samples showed cleavage facets,microvoids,and dimples in both the samples.The ballistic behavior of the BM and DCT target plates were studied by impacting hard steel core projectiles at a velocity of 750±10 m/s.The target plates failed due to petaling and ductile hole enlargement,and the depth of penetration(DOP)of the DCT target was less than that of the BM target,indicating a higher ballistic resistance.The post-ballistic microstructure examination of the target plates showed the formation of an Adiabatic Shear Band(ASB)without any cracks.It was concluded that the DCT treatment improved the mechanical and ballistic properties of the aluminum alloy due to grain refinement and high dislocation density.

Ballistic tests on hot-rolled Ti-6Al-4V plates:Experiments and numerical approaches

Superior ballistic performance and the lightweight character of Ti alloys are considered as main reasons for their use in armour applications against a broad spectrum of ballistic threats,e.g.bullet,fragment or blast impact.Because dynamic loading caused by typical penetrators is characterized by high strain rates,only specific test methods allow a closer investigation of the respective material behaviour.In the present study,quasi-static and dynamic compression tests as well as ballistic tests were conducted on a twophase a+βalloy Ti-6Al-4V(in m%)manufactured by hot-rolling.Post-deformation heat treatments,influencing microstructure and mechanical properties were applied in order to compare three different microstructural configurations:as-rolled,mill-annealed and bimodal.While,on the one hand,ballistic tests were employed for the determination of the ballistic limit velocity v_(50),compression tests,on the other hand,delivered essential input parameters for the application of the Johnson-Cook constitutive model in a finite element simulation of the impact event.The comparison of experimental results to simulation results was supplemented by means of microstructural characterization of tested samples with the focus set on the prevalently observed deformation and damage mechanisms,as for example adiabatic shearing.

Dynamic fracture of Ti-6Al-4V alloy in Taylor impact test

The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose ＂tips＂ are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional ＂stagger ridge＂ structure.

Microstructural evolution of 2519-T87 aluminum alloy obliquely impacted by projectile with velocity of 816 m/s

Target made of 2519-T87 aluminum alloy was obliquely impacted by a projectile. Microstructural evolution around the crater was investigated by optical microscopy （OM）, transmission electron microscopy （TEM）, and electron backscattered diffraction （EBSD）. The micro-hardness distribution near the crater after impact was studied. The results indicate that at the entering stage, the amount of adiabatic shear band （ASB） is the most, and the precipitates are as fine as those of the target material; the micro-hardness is higher than that at the other stages. At the stable-running stage, the amount of ASB reduces as the micro-bands increase; the precipitates tend to coarsen, which leads to the decrease of the micro-hardness. At the leaving stage, there is a large amount of micro-bands; the precipitates are refined, and the micro-hardness is higher than that at the stable-running stage. The difference in the micro-hardness of the impact stages is due to work hardening and precipitate coarsening, which is caused by adiabatic temperature rise in the alloy.

Evaluation of dynamic performance and ballistic behavior of Ti-5Al-5Mo-5V-3Cr-1Zr alloy

Terminal ballistic tests using 7.62 mm armor-piercing incendiary （API） projectiles were performed to evaluate the resistance to penetration of Ti-5Al-5Mo-5V-3Cr-IZr （Ti-55531） alloy. The dynamic properties were determined by a split Hopkinson pressure bar （SHPB） test system. Ti-55531 plates were subjected to two kinds of heat treatments, leading to the formation of high-strength and high-toughness plates. The results of SHPB test exhibit that the maximum impact absorbed energy of the high-strength plate at a strain rate of 2200 s^-1 is 270 MJ/m^3; however, the maximum value for the high-toughness plate at a strain rate of 4900 s^-1 is 710 MJ/m^3. The ballistic limit velocities for the high-strength and high-toughness plates with dimensions of 300 mm×300 mm×8 mm are 330 and 390 m/s, respectively. Excellent dynamic properties of Ti-55531 alloy correspond to good resistance to penetration. The microstructure evolution related to various impact velocities are observed to investigate the failure mechanism.

Effect of heat treatment conditions on ballistic behaviour of various zones of friction stir welded magnesium alloy joints

Ballistic behaviour of different zones of post-weld heat-treated(PWHT)magnesium alloy(AZ31B)target against 7.62 mm×39 mm armour-piercing(AP)projectile with a striking velocity of(430±20)m/s was determined.Magnesium alloy(AZ31B)welded joints were prepared by using friction stir welding(FSW)process and subjected to different heat treatment conditions.The microhardness values of non-heat-treated and heat-treated FSW joints were investigated.The results indicated that PWHT process(250°C,1 h)has improved the microhardness of heat-treated FSW joints.Scanning electron microscope(SEM)microstructure showed that heat treatment has caused the formation of fineα-Mg grains and tiny precipitates and made the dissolution ofβ-Mg17Al12 phase into the Mg matrix.The ballistic behaviour of PWHT zones was estimated by measuring the depth of penetration(DOP)of the projectile.Lower DOP value was observed for the base metal zone(BMZ)of a heat-treated welded joint.Post ballistic SEM examinations on the cross-section of all three zones of crater region showed the formation of adiabatic shear band(ASB).

The modified temperature term on Johnson-Cook constitutive model of AZ31 magnesium alloy with {0002} texture

The dynamic compression experiments with Split-Hopkinson Pressure Bar(SHPB)were performed on AZ31 magnesium alloy rolled sheet specimens in the normal direction(AZ31-ND)with{0002}texture at the temperature of 293-523 K and the strain rate of 0.001-2200 s^−1.The temperature term in Johnson-Cook(JC)constitutive model had been reasonably modified.This advantage made constitutive model promising for decribing the dynamic deformation behavior of AZ31-ND with{0002}texture more accurately.The obtained true stress-true plastic strain curves agreed well with the measured results in a wide range of strain rates and temperatures.The thermal softeninging,strain and strain rate hardening effect on the AZ31-ND with{0002}texture were discussed.The adiabatic shear band(ASB)of AZ31-ND with{0002}texture hat shaped specimen was successfully predicted by combining modified JC constitutive model and numerical simulation,which was also validated by Electron Back-Scattered Diffraction(EBSD)map under the same boundary condition.

A Review of Microstructural Evolution in the Adiabatic Shear Bands Induced by High Speed Machining

Investigations made by the authors and collaborators into the microstructural and fracture aspects of adiabatic shear bands （ASBs） of the hardened steels and Ti alloys induced by high speed machining （HSM） are briefly reviewed. The principal findings are the following： （a） the microstructure inside the ASBs varies from the band center to the normal chip material, the gradient microstructures are found; （b） the HSM can produce two types of ASBs with increasing in cutting speed, the deformed shear bands formed at lower cutting speed and the transformed shear bands formed at higher cutting speed; （c） the very small equiaxed recrystallized grains are observed in the center of the ASBs, the dynamic recrystallization and phase transformation may occur simultaneously during the formation of the transformed ASBs; （d） The dynamic rotational recrystallization is the origin of the equiaxed grains in the center of the ASBs. A microstructural evolution model in ASBs produced during HSM for the harden steel is proposed; （e） the microstructural pattern of fracture surface is characterised by the elongated dimples. A microcosmic adiabatic shear fracture model during HSM of the hardened steel is built up.

Shear localization behavior in hat-shaped specimen of near-αTi−6Al−2Zr−1Mo−1V titanium alloy loaded at high strain rate

The microstructure characteristics in early stage shear localization of near-αTi−6Al−2Zr−1Mo−1V titanium alloy were investigated by split Hopkinson pressure bar(SHPB)tests using hat-shaped specimens.The microstructural evolution and deformation mechanisms of hat-shaped specimens were revealed by electron backscattered diffraction(EBSD)method.It is found that the nucleation and expansion of adiabatic shear band(ASB)are affected by both geometric and structural factors.The increase of dislocation density,structure fragment and temperature rise in the deformation-affected regions provide basic microstructural conditions.In addition to the dislocation slips,the extension twins detected in shear region also play a critical role in microstructural fragmentation due to twin-boundaries effect.Interestingly,the sandwich structure imposes a crucial influence on ASB,which finally becomes a mature wide ASB in the dynamic deformation.However,due to much larger width,the sandwich structure in the middle of shear region is also possible to serve as favorable nucleation sites for crack initiation.

Adiabatic shear banding of hot-rolling Ti–6Al–4V alloy subjected to dynamic shearing and uniaxial dynamic compression

Effect of stress state including dynamic shearing and uniaxial dynamic compression on adiabatic shear banding(ASBing) of hot-rolling Ti–6Al–4V(TC4) alloy was investigated. The absorbed energy of specimen before failure was calculated to evaluate the susceptibility to adiabatic shear band(ASB) of TC4 alloy quantitatively.Results show that the susceptibility to ASB of hot-rolling TC4 alloy exhibits obvious anisotropy under both dynamic shearing and uniaxial dynamic compression conditions, but the anisotropy of susceptibility to ASB under dynamic shearing condition exhibits an opposite tendency with that under uniaxial dynamic compression condition. Under the condition of uniaxial dynamic compression, material shows the highest susceptibility to ASB when loaded along transverse direction(TD) of the hot-rolling TC4, while the lowest susceptibility when loaded along rolling direction(RD). However, under the condition of dynamic shearing,the material behaves in the opposite way, demonstrating the lowest susceptibility when loaded along TD of the hotrolling TC4, while the highest susceptibility when loaded along RD.

Multi-scale crystal plasticity finite element simulations of the microstructural evolution and formation mechanism of adiabatic shear bands in dual-phase Ti20C alloy under complex dynamic loading

A dynamic compression test was performed on α+β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar.The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements.The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model.Subsequently,the evolution histories of the grain shape,size,and orientation inside the adiabatic shear band were quantitatively simulated.The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction.Furthermore,by calculating the grain rotation and temperature rise inside the adiabatic shear band,the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled.The results revealed that the microstructural softening stress was triggered and then stabilized(in general)at a relatively high value.This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation.Subsequently,thermal softening increased linearly and became the main softening mechanism.Noticeably,in the final stage,the thermal softening stress accounted for 78.4% of the total softening stress due to the sharp temperature increase,which inevitably leads to the stress collapse and potential failure of the alloy.