Transformed Shear Band Research Articles

Mechanical behavior and microstructure evolution during high strain rate deformation of AA7075-T651

The current study presents the effects of strain and temperature on the mechanical response and microstructure evolution in AA7075-T651 at high strain rates. Compression tests have been performed at room temperature (RT), 200, 300 and 400 °C using a Split-Hopkinson pressure bar (SHPB) setup with strain rates ranging between 1400 and 5300 s−1. For deformation at RT, the flow stress increases with increase in strain rate. Whereas deformation at elevated temperatures show a non-monotonous behavior of the flow stress with respect to the strain rate. This trait is attributed to the pronounced effects from the adiabatic shear bands (ASBs); namely, distorted shear bands (DSBs) and transformed shear bands (TSBs); and cracks resulting from the plastic deformation instability during hot deformation. The sequence of microstructure evolution is: inhomogeneity in the initial microstructure – DSB – TSB – crack –fracture. The feasibility of formation and growth of ASBs and cracks increases with increase in strain and temperature, neglecting any significant effect from the strain rate. During the compression tests, temperature of the material rises due to adiabatic heating. Considering a certain strain developed in the material, this adiabatic temperature rise decreases as the deformation temperature is increased. Furthermore, during individual deformation processes, the temperature rise increases with increasing strain. The adiabatic temperature leading to the formation of TSB is approximated to be 0.7 times of the melting temperature of the alloy. These results from the current study are to be used in developing a physics-based material model for the alloy.Article HighlightsAt elevated temperatures, compression with Split-Hopkinson bars produce two types of shear bands and cracks.Evolution of shear bands and cracks is promoted by increase in strain and temperature irrespective of strain rate.Adiabatic temperature approximating to 70% of the melting point forms refined grain structure of transformed band.

Atomic insights into the size effect of glassy domain on the propagation of plastic deformation carriers in crystal-glass nanocomposite

Crystal-glass nanocomposites with the synergy of high strength and exceptional ductility are promising for future applications in micro-electromechanical systems. Deformation behaviors of crystal-glass nanocomposites are governed by the formation and propagation of their plastic deformation carriers, namely, dislocations in the crystalline phase and strain-activated atomic clusters (e.g., shear transformation zones and shear bands) in the glassy phase. Yet, it is challenging to unveil the size effect of a glassy domain on the propagation of plastic deformation carriers in crystal-glass nanocomposites. To clarify the above issue, in this work, we perform molecular dynamics simulation on simple configurations fabricated by embedding a series of cylinder glass domains with different radii into the single-crystal matrix. Their stress–strain response and microstructures, especially the deformation carriers in the two phases evolving with the applied compressive strain, are quantitively analyzed. The average shear strain of glassy atoms is found to significantly decrease with the increased glassy domain volume, accordingly alleviating the strain localization in the glassy phase. The formation and propagation of strain-activated atomic clusters are also suppressed by enlarging the glassy domain volume due to the lowered shear strains sustained by glassy atoms. Moreover, dislocation densities in the crystalline matrix also decrease in the configuration with a larger-volume glassy domain, which can be ascribed to the enhanced dislocation absorption effect from the amorphous-crystal interfaces. This work indicates that the mechanical properties of multi-phase nanocomposites can be improved by rationally optimizing the phase contents and provides new knowledge on designing high-performance nanocomposites.

Research on dynamic mechanical properties and plastic constitutive relation of Ti3Al intermetallic compounds under mechanical-thermal coupling

Ti3Al intermetallic compounds is a lightweight, high-temperature resistant rare aerospace materials, whose dynamic mechanical properties concern the accuracy of simulation analysis and further development in material manufacturing. To obtain the plastic constitutive relation of Ti3Al under the action of mechanical-thermal coupling, material rheological behavior was investigated by using the separation Hopkinson pressure bar (SHPB) system with a high-temperature synchronous assembly. Meanwhile, the Johnson-Cook plastic constitutive model was calculated and modified by introducing strain compensation and adiabatic temperature rise at the range of strain rate of 0.001–6500 s−1 and deformation temperature of 293–1173 K. Furthermore, the microstructure evolution of Ti3Al fracture surface morphology was discussed. Experimental results showed that the flow stress had a weak positive strain rate enhancement effect, significant negative temperature thermal softening effect and strain rate plasticizing effect. Adiabatic temperature rise was accompanied by the high strain rate, and its value increased exponentially with the strain rate. The adiabatic deformed shear band (ADSB) appeared at an edge of 45° along the loading direction at the strain rate 4500 s−1 and the deformation temperature 273 K, respectively. ADSB tended to expand internally along with the strain rate and evolved into an adiabatic transformed shear band (ATSB) when the temperature rose to 973 K. The maximum relative errors in the modified J-C model at the high strain rate and the room temperature, the high strain rate and the high temperature were 3.11% and 14.91%, respectively. Consequently, the modified J-C model described in dynamic mechanical properties of Ti3Al was more accurate than the conventional J-C model.

Adiabatic shear bands produced in a metallic armor, after dynamic compression with a Hopkinson pressure bar

A Brazilian high hardness armor (HHA) steel was, for the first time dynamically tested and microstructurally analyzed for the occurrence of shear bands. The HHA was initially quenched at 910 °C for 10 min and tempered at 310, 425 and 610 °C for 1 h to produce different microstructures as well as phases and precipitates. After characterizing the microstructures, a dynamic compression test on a Hopkinson bar was performed on special samples to simulate high deformation rate in a ballistic impact. Then, a search for adiabatic shear bands, using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) was performed on the impacted samples. The transformed shear bands generated were characterized by X-ray diffraction (XRD) and electron backscattering diffraction (EBSD). The results showed that the microstructure generated in the tempering heat treatment at 310 °C is the most susceptible to the appearance of shear bands. In addition, it can be indicated that the formation of these band is related to a possible dynamic recrystallization mechanism.

Effect of aging status on susceptibility of adiabatic shear localization in Al–Zn–Mg–Cu alloy

The effect of aging status on adiabatic shear band (ASB) in Al–Zn–Mg–Cu alloy is systematically studied under dynamic shear loading. The microstructure within shear bands is characterized using EBSD method. It is found that peak-aged (PA) status shows the highest susceptibility to produce ASBs, followed by natural-aged (NA) status, while over-aged (OA) status exhibits the strongest resistance to shear localization. The localized bands in PA and NA states are characterized as transformed shear bands, composed of dynamically recrystallized (DRXed) ultrafine grains in the center, whereas the deformed shear band in OA status contains elongated subgrain laths and few ultrafine grains, which indeed confirms the previous finding, that DRX precedes the onset of ASB and may serve as the trigger for ASB. Based on the microstructural characteristics from the edge to the center of ASBs, it is demonstrated that progressive subgrain misorientation model can explain the DRX mechanism inside the ASBs, which involves a progressive evolution from lamellar structures to elongated laths, and eventually being transformed into ultrafine equiaxed grains. Precipitates play significant roles in DRX process by altering dislocation multiplication and subsequent evolution of substructures, thereby causing the different susceptibility of adiabatic shear localization.

Structural and mechanical properties of densified (Li2O)0.2(SiO2)0.8 glasses: A molecular dynamics simulations study

Molecular dynamics (MD) simulations have been carried out to study the structural and mechanical properties of densified (Li2O)0.2(SiO2)0.8 glasses under uniaxial tension. Fractions of SiOx(x = 4−6) and LiOy(y = 3−8) units change with the density. There is a relationship between the changes in Qn species and in SiOx unit with increasing density. The sample of 2.28 g cm−3 exhibits brittle behavior, at which fraction of SiO4 units is almost maintained and the breakages of LiO bonds cause the transformation of LiOy units from higher- to lower- fold Li coordination. Big-voids are formed during the tension process. LiO voids play important roles in the evolution and coalescence of voids causing the crack propagation. The densified samples of 2.96 and 3.81 g cm−3 exhibit ductile behavior, at which the transformations of SiOx and LiOy occur. Shear transformation zones (STZs) and shear bands (SBs) are formed during plastic deformation.

High deformation capacity and dynamic shear band propagation of imprinted amorphous Cu50Zr50/crystalline Cu multilayered nanofilms

Molecular dynamics simulations are used to investigate the plastic deformation behavior of Cu50Zr50/Cu amorphous/crystalline (A/C) multilayered nanofilms with different interface directions, number of layers and temperatures under the imprinting process. The results show that the loading force of multilayered nanofilms grows with increasing thickness of layers and decreasing temperature. Specifically, the maximum stress is larger with increasing layer thickness, this shows an inverse Hall-Petch relationship between the stress and the layer thickness. The local stress is focused around the stamp and rises up as increasing imprinting depth. The plastic deformation behaviors are realized by dislocations or stacking faults in crystalline layers and shear transformation zones (STZs) or shear bands in amorphous layers. Additionally, the propagation of shear bands or spatial correlation of STZs in an amorphous layer can be disrupted at the amorphous/crystalline interfaces (ACIs). The centrosymmetry parameter (CSP) and dislocation analysis (DXA) reveal the nucleation of stacking faults at the ACIs and the Shockley partial dislocation account for 80% in the total dislocations in all cases. The affected region extends and displacement of atoms becomes more irregular as increasing temperature. Besides, the radial distribution function (RDF) reveals the structure of the material is more stable at low temperature. The transverse interface specimens and thinner layer thickness, and high temperature are facilitated to material formability.

Mechanism of high-pressure torsion-induced shear banding and lamellar thickness saturation in Co–Cr–Fe–Ni–Nb high-entropy composites

Abstract

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.

Grain structure and precipitate variations in 7003-T6 aluminum alloys associated with high strain rate deformation

The variation of the grain and precipitate structure plays a rather important role on the response of the 7003-T6 Al-Zn-Mg-Cu alloys to high speed deformation but related reports are limited. This paper represents a detailed investigation about the flow behavior and microstructure evolution of these alloys under high strain rate impacting. Methods such as split Hopkinson pressure bar testing, optical and TEM microscopy, Vickers hardness testing, etc. were employed. Features such as plastic compression, tilting, distortion, elongation and refinement manners of the grains as deformation proceeds were studied. The changes of the precipitates in phase type, size and density, and their contribution to the flow stress during this thermo-mechanical process was investigated. The results show that the motion of dislocations and temperature rising induced by the impact work were responsible for these microstructure changes. Deformed shear bands with elongated grains formed in the diagonal direction of the sample when total strain reached 0.17. Prototype of the refined grain structure in these bands was constructed when the dislocation walls accumulate in the triangle grain boundary and form closed cells. With the assist of localized temperature rising and further increase of the strain to above 0.19, the deformed shear band was finally changed into transformed shear band with fine recrystallized grain structure in the center. Dynamic re-precipitation including phase transformation from GPII zones to ƞ′ and then to ƞ, coarsening in size and then dissolving was promoted. Elevated temperature and multiplied dislocation facilitated these processes. Precipitation coarsening in the matrix and dissolving in the ASB played a significant role of softening in the flow behavior as a counterbalance of the deformation and geometric hardening. A schematic model considering both of the grain structure and precipitate evolutions was established and discussed.

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.

Tensile and nanoindentation deformation of amorphous/crystalline nanolaminates: Effects of layer thickness and interface type

We perform molecular dynamics simulations to investigate plastic deformation of alternative Cu64Zr36/Cu amorphous-crystalline nanolaminates (ACNLs) with different layer thicknesses and interface types under uniaxial tension and nanoindentation. Plastic deformation is characterized by shear transformation zones (STZs) or shear bands in amorphous layers and dislocations in crystalline layers, respectively. Nucleation of STZs or shear bands in an amorphous layer can be triggered at glass-glass interfaces (GGIs) in the same layer or at the intersections of amorphous-crystalline interfaces (ACIs) and dislocations from the neighboring Cu layers. Decreasing layer thickness and introducing grain boundaries (GBs) or GGIs into ACNLs are effective methods to improve ductility and facilitate the transition from inhomogeneous deformation to homogeneous deformation or co-deformation. With the decrease of layer thickness, more ACIs are introduced and behave as nucleation sites of STZs and dislocation. These STZs at the adjacent interfaces can interact with each other directly, promoting the deformation transition. Additionally, the introduction of GBs and GGIs facilitates crystal plasticity and glass plasticity in corresponding layers, which again boosts the plasticity of nearby layers and contributes to homogeneous strain distribution and co-deformation. However, the improved ductility of ACNLs is at the price of strength and hardness. Thus, keeping a balance among them can be useful for the synthesis of novel nanolaminate with superior ductility, high strength and high hardness.

A comparative study of the compressive behaviour of AISI 321 austenitic stainless steel under quasi-static and dynamic shock loading

The mechanical response of AISI 321 austenitic stainless steel under both dynamic and quasi-static compression was studied and compared. Compression in both loading conditions led to strain-induced phase transformation which follows the FCC ɣ-austenite→BCC ά-martensite sequence. During deformation in both loading conditions, precipitation of nano-sized carbide and evolution of strain-induced martensite contributed to strengthening while plastic deformation mechanisms occurred in the specimens by slip and mechanical twinning mechanisms. Specimens subjected to quasi-static compression exhibited lower yield strength and higher strain hardening capacity than the dynamically impacted specimen. While deformed shear band (DSB) characterized by elongated grains developed in the quasi-statically compressed specimen, transformed shear band (TSB) characterized by nano-grains (average size of 0.28μm) were formed in the dynamically impacted specimen. The texture of the FCC austenite in all regions of the specimen under both loading conditions is predominantly 〈110〉||CD (compression direction) with minor 〈111〉||CD component. Large strain localization in the TSB of an impacted specimen led to off-〈110〉||CD texture in the FCC austenite. In BCC martensite, 〈111〉||CD and 〈100〉||CD fibres are the predominant texture components in specimens subjected to dynamic impact and quasi-static loading, respectively.

Improved ductility of Cu64Zr36 metallic glass/Cu nanocomposites via phase and grain boundaries

We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix.

Effects of temper condition on high strain-rate deformation of AA 2017 aluminum alloy in compression

Cylindrical specimens of AA 2017 alloy in T451, T651 and O temper conditions were subjected to quasi-static compressive loading at a strain rate of 3.2×10−3 s−1 and dynamic impact loading at strain rates between 3000s−1 and 8000s−1. The effects of strain rates and temper condition on damage evolution were investigated. Deformation under quasi-static loading was dominated by strain hardening while high strain rates deformation was characterized by two-peak stress–strain curves, indicating simultaneous occurrence of hardening and softening controlling deformation. The degree of softening after the first peak is highest in the annealed sample while the age-hardened specimens showed higher degree of softening beyond the second peak. Both deformed and transformed shear bands were observed in the impacted age-hardened specimens, whereas only deformed bands developed in the annealed specimens. The critical strains for the onset of the thermomechanical instability leading to the formation of adiabatic shear bands are about the same for both T451 and T651 specimens, but much higher for the annealed (O) specimens. The age-hardened specimens fractured in a manner consistent with ductile fracture mode that is characterized by the sequential nucleation, growth and coalescence of micro-voids inside transformed shear bands.

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.

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.

X-ray photoemission electron microscopic study of transformed shear bands in AISI 4340 steel

Failures of structural materials under dynamic mechanical loading at high strain rates are commonly initiated by shear strain localisation along adiabatic shear bands, which act as preferential sites for crack initiation and propagation. We have used synchrotron based X-ray photoemission electron microscopy (XPEEM) to identify chemical elements and measure compositional contrast in the shear band in AISI 4340 steel. The high spatial resolution of XPEEM combined with near edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to study the microstructural evolution in transformed shear bands that formed in quench hardened and tempered AISI 4340 steel under dynamic impact loading. We compared our XPEEM findings with other complementary techniques such as energy dispersive X-ray spectroscopy and scanning electron microscopy to get a complete picture. As in the case of optical and scanning electron microscopy, XPEEM images show a featureless transformed band devoid of the martensite laths found in the parent metal. Interestingly, XPEEM images and corresponding Cr 2p→3d and Ni 2p→3d NEXAFS spectra confirms a compositional contrast in the transformed shear band that resulted in more nickel inside the shear band than in the adjacent region.Les défaillances des matériaux structuraux sous charge mécanique dynamique à des vitesses élevées de déformation sont communément initiées par la localisation de la déformation de cisaillement le long des bandes de cisaillement adiabatique, lesquelles agissent comme sites préférentiels pour l’amorçage et la propagation de fissure. Nous avons utilisé le synchrotron basé sur la spectromicroscopie d’électrons photo-excités par rayons X (XPEEM) pour identifier les éléments chimiques et pour mesurer le contraste de composition dans la bande de cisaillement de l’acier AISI 4340. La haute résolution spatiale par XPEEM, combinée avec la spectroscopie des structures fines d’absorption X proches du seuil (NEXAFS), est utilisée pour étudier l’évolution de microstructure dans les bandes de cisaillement transformées qui se forment dans l’acier AISI 4340 durci par trempe et revenu sous charge dynamique par impact. Nous avons comparé nos trouvailles par XPEEM à d’autres techniques complémentaires comme la spectroscopie aux rayons X à dispersion d’énergie et la microscopie électronique à balayage afin d’obtenir une image complète. Comme dans le cas de la microscopie optique et électronique à balayage, les images XPEEM montrent une bande transformée sans particularité et sans martensite massive trouvée dans le métal parent. Ce qui est intéressant, c’est que les images XPEEM et les spectres correspondants de NEXAFS Cr 2p →3d et Ni 2p →3d confirment un contraste de composition dans la bande de cisaillement transformée qui a pour résultat la présence de plus de nickel à l’intérieur de la bande de cisaillement que dans la région adjacente.

The effects of microstructure, strain rates and geometry on dynamic impact response of a carbon–manganese steel

Abstract The effects of strain rates, microstructure and geometry on the occurrence of adiabatic shear bands in AISI 1340 steel during impact loading were investigated. Cylindrical and cuboidal specimens of the steel were heat treated to have microstructure consisting of tempered martensite, ferrite + martensite (dual-phase) or ferrite + pearlite. Depending on the applied impact momentum, both deformed and transformed shear bands were observed in the specimens with martensitic and dual-phase structures. Deformed bands first formed and then metamorphosed into transformed bands as the intensity of localized shear strain increased. The tendency for formation of a transformed band is higher in the martensitic steel specimens. Within the range of the applied impact momentum of between 30 and 42 kg m/s, which generated strain-rates of between 630 and 3400/s, only deformed bands were observed in the normalized steel specimens containing ferrite and pearlite. The width of these deformed bands increased as the applied impact momentum was raised. Whereas circular shear bands were observed on the transverse section of the impacted cylindrical specimens, the shape of shear bands in the same section of the cuboidal specimens was rectangular. The hardness of the shear bands, which was measured to be higher than that of the bulk material, increased as the impact momentum was raised. Fractographic study of fractured specimens showed ductile shear failure and knobby fracture modes along the transformed bands.

Effect of prior heat treatment on the dynamic impact behavior of 4340 steel and formation of adiabatic shear bands

Abstract Adiabatic shear bands are observed in many metallic materials when subjected to both high strain rates (>103 s−1) and large strains. These bands are a manifestation of damage and may lead to catastrophic failure because they are susceptible to crack nucleation and propagation due to their high internal stresses. Minimizing their occurrence is important in many applications and processes. In this study, the effect of prior heat treatment on evolution of adiabatic shear bands in a typical heat treatable steel was investigated. Cylindrical AISI 4340 steel specimens were heat treated at 315 °C, 425 °C and 620 °C, for 1 h or 2 h and impacted using the direct impact Hopkinson pressure bar (DIHPB). Tempering the specimens at 315 °C and 425 °C for 1 h led to the formation of distinct and well-developed adiabatic shear bands after impact. Diffused shear bands were formed in the specimens tempered at 315 °C and 425 °C for 2 h. Specimens tempered at 315 °C and 425 °C for 1 h exhibited higher impact resistance than those tempered at 315 °C and 425 °C for 2 h. The 1 h tempered specimens absorbed much of the impact energy without failing at the impact momentums at which those tempered at 315 °C and 425 °C for 2 h were failing. These observations demonstrate that tempering the steel specimens at 315 °C and 425 °C for 1 h may lead to catastrophic failure through the formation of adiabatic shear bands, while they exhibit higher impact resistance than when they are tempered for 2 h. Tempering the steel specimens at 620 °C for 2 h before impact resulted in the formation of deformed shear bands, which are less susceptible to crack nucleation and propagation. However, at very high impact momentum, a distinct transformed shear band was observed, indicating that catastrophic failure through the formation of adiabatic shear bands can be reduced by increasing the tempering temperature of the steel specimens even though at very high impact momentums, failure through the formation of adiabatic shear bands is likely to occur. Thus the initial conditions (tempering temperature and time) of a heat treatable steel before impact are shown to have a significant influence on the formation of adiabatic shear bands.