Propagation Of Deformation Bands Research Articles

Nonlinear response to contact impact on the surface of an aluminum alloy AlMg6 exhibiting the Portevin-Le Chatelier effect

Intermittent plastic flow known as the Portevin-Le Chatelier effect is a pronounced nonlinear phenomenon in a material science. One of the traditional approaches to the study of nonlinear system is to identify and analyze its responses to an external impulsive action. In the present work, the influence of impact indentation on the evolution of the spatio-temporal patterns of localized strain in an AlMg alloy deformed under the Portevin-Le Chatelier effect is studied with using an acoustic emission technique and high-speed video recording of propagating deformation bands. Dynamic and nonlinear responses to impact indentation of the surface of an alloy deformed by uniaxial tension above the conditional yield strength are revealed; the first include high values of dynamic hardness and local strain and loading rates, while the second involve the threshold and multiple nature of the force and acoustic responses. It has been established that there is a threshold impact energy of the indenter at which an induced strain jump of minimum amplitude occurs, accompanied by the formation of bands of macrolocalized plastic deformation. The conditions are established under which the work of plastic deformation during the development of the induced strain jump significantly, by more than two orders of magnitude, exceeds the energy of the initiating impact, which acts only as a trigger for the “premature” release of the elastic energy accumulated in the material before the moment of impact. It is shown that the impact-induced bands of macrolocalized deformation represent a hidden three-dimensional type of erosion damage that reduces the mechanical stability and durability of the alloy. A phenomenological model is proposed for the formation of bands, strain jumps, and bursts of acoustic emission induced by an indenter impact, which qualitatively explains the experimental results obtained in the work.

In situ EBSD study of formation and propagation of deformation bands in a single-crystal superalloy during tensile deformation

In situ EBSD study of formation and propagation of deformation bands in a single-crystal superalloy during tensile deformation

Influence of deformation banding instabilities on small scale yielding of a Mg–Nd alloy revealed by in-situ digital image correlation

Propagating deformation bands are observed to accommodate the initial plasticity in an as-extruded Mg–1.5Nd alloy under tension using digital-image-correlation. The propagating bands cause an uncommon plateau in the stress–strain response of the alloy prior to restoring a common decreasing work hardening with further straining. Effects of the deformation banding and underlying plateau in the flow stress on small scale yielding are investigated during low cycle fatigue (LCF) and tension of notched specimens. Alternating formation/disappearance of deformation bands in the gauge section of as-extruded LCF specimens during testing is observed to reduce life compared to annealed specimens exhibiting no instabilities. In contrast, the bands deflect the plastic zone ahead of the notch from the principal plane orthogonal to the applied loading inducing positive effect on toughness of the alloy.

In situ investigation of the mechanism of propagation of Portevin-Le Chatelier deformation bands

Based on the analysis of high-speed video data of propagating deformation bands in an aluminum-magnesium alloy, it was found that the movable deformation band contains an excess of dislocations of one mechanical sign, and the main mechanism for the propagation of the localized plastic deformation front along the sample axis is a relay transmission of sliding in the antiparallel direction to relax the bending moment created by the primary deformation band. It is shown that the "instantaneous" rate of plastic deformation during ~1 ms in the growing band reaches a value of ~103 s-1, comparable to the rate of deformation during shock tests. Keywords: intermittent deformation, vocalization, dislocations, mechanical charge, aluminum-magnesium alloy.

Origin, parameters, and underlying deformation mechanisms of propagating deformation bands in irradiated 316L stainless steel

Lüders-type propagating deformation bands were observed in specimens of irradiated 316L stainless steel samples removed from Spallation Neutron Source target vessels after service. Mechanical testing with digital image correlation (DIC) and in-situ tensile testing with scanning electron microscopy electron backscatter diffraction showed that the observed Lüders-type behavior was not related to the known transformation-induced plasticity or twinning-induced plasticity behavior. Instead, the phenomenon occurs at small local strain values before a significant amount of martensite or deformation twins appear in the microstructure. Microstructural analysis and in-situ mechanical test results suggest Lüders-type band formation and propagation were related to the appearance and evolution of defect-free channels—analogous to slip bands. A modified Swift equation with a Ludwigson-like component was offered to rationalize the phenomenon and model the strain-softening processes at small strain values. The results indicate complex microstructural processes and deformation mechanisms were active at small strain values and underline the benefits of advanced mechanical test approaches such as DIC.

Microstructure and mechanical properties of WNbMoTaZrx (x = 0.1, 0.3, 0.5, 1.0) refractory high entropy alloys

WNbMoTa refractory high entropy alloy (RHEA) with single BCC phase has high strength, high hardness and excellent high-temperature mechanical properties. However, the limited plasticity greatly affects its engineering applications. In this work, WNbMoTaZrx (x = 0.1, 0.3, 0.5, 1.0) RHEAs were prepared by vacuum arc melting. The microstructure, hardness, compressive properties and fracture behavior were characterized. The results have shown that WNbMoTaZrx RHEAs have a single BCC1 solid solution phase at low Zr content, and a second BCC2 phase appeared in the RHEAs with a higher Zr content. With the increase of Zr content, the microstructure of WNbMoTaZrx RHEAs changed to a typical dendrite structure, and simultaneously increase of strength, hardness and compressive plasticity was observed. The improvement of the mechanical properties was mainly attributed to the solid solution strengthening of Zr elements, the formation of more interdendritic regions, and the refinement of dendrite structures. The deformation mechanisms of the RHEAs with increased compressive plasticity were examined using step-by-step observations and discussed related to the evolution of microstructures. The results have shown that under applied loadings, deformation bands were initiated within the dendrite regions, while the propagation of deformation bands was impeded by interdendritic regions, resulting in significantly increased compressive plasticity. The fracture mode also changed from the intergranular fracture in WNbMoTa RHEA to the transgranular fracture in WNbMoTaZr1.0 RHEA. The present work not only gives more insight into the strengthening and deformation mechanisms of the WNbMoTaZrx (x = 0.1, 0.3, 0.5, 1.0) RHEAs, but also explores the application potential of WNbMoTa-based RHEAs.

Strain localization criteria for viscoplastic geomaterials

AbstractThis work presents a viscoplastic localization criterion to detect quasi‐instantaneous (i.e., load‐induced) and delayed (creep‐induced) strain localization in rate‐dependent solids. The study is based on the theory of controllability and a viscoplastic description of the mechanical response. Analytical precursors of unstable states are defined through systems of ordinary differential equations (OEDs). The use of the proposed criteria is illustrated at the material point level through a set of strain localization analyses simulating active strain localization of a porous rock. In addition, full‐field finite element simulations of compression tests conducted under various pressures are reported to demonstrate the role of local unstable viscoplasticity in the spontaneous propagation of deformation bands under stationary boundary conditions. The study shows that the viscoplastic localization criterion maintains a negative sign as long as the behavior is unstable, that is, the rate of deformation is accelerating. The sign switch coincides with the transition to decelerating deformation. The analyses revealed that pulses of overstress always emerge in correspondence with the growth of unstable behavior, and the peak matches the transition to stable behavior. The local responses recovered from full‐field analyses were consistent with those observed in analyses at material point level and the predictions of the presented theory.

Mechanical stratigraphy influences deformation band pattern in arkosic sandstones, Rio do Peixe Basin, Brazil

This study shows how mechanical stratigraphy influences deformation bands' structural attributes in faulted, heterolithic arkosic sandstones in the Rio do Peixe Basin, northeastern Brazil. We combined the analyses of sedimentary units with rock properties (porosity, permeability, Young's modulus, Poisson's ratio, and uniaxial compressive strength) and structural data to characterize the mechanical units and related deformation band pattern. Our results show that deformation bands have differences in frequency, geometry, thickness, and dip in different mechanical units. In addition, sedimentary parameters as grain size, layer thickness, stratification, cementation, and mineral composition impact the vertical propagation of deformation bands. In each layer, deformation bands frequency increases approaching a normal fault, and the dip of the deformation bands varies when passing through layers. In the studied mechanical units, deformation band density increases with increasing host rock porosity and decreasing Young's modulus. Finally, we document that the development of deformation bands increases the overall cohesion of the fault zone, particularly in units with coarser grain sizes. Our results have implications for mechanical stratigraphy in faulted siliciclastic reservoirs.

Experimental study of the Portevin-Le Chatelier effect under complex loading of Al-Mg alloy: procedure issues

The aim of this work is to solve the methodological issues of the experimental study of the nucleation and propagation of deformation bands due to the Portevin-Le Chatelier effect under conditions of complex loading. It is of interest to determine the boundaries of unstable plastic deformation of the AMg6 alloy under complex loading conditions. A technique for controlling the loading process with a given rate of deformation intensity of materials has been worked out. The results showed that short-term stops and unloading during loading influence on the value of critical deformation, at which the manifestation of the jerky flow begins. The evolution of inhomogeneous strain fields and local strain rates under conditions of manifestation of jerky flow during tension with torsion tests of Al-Mg alloy samples.

Strain Hardening of Low-Carbon Steel in the Area of Jerky Flow

Purpose. The aim of this work is to assess the effect of ferrite grain size of low-carbon steel on the development of strain hardening processes in the area of nucleation and propagation of deformation bands. Methodology. Low-carbon steels with a carbon content of 0.06–0.1% C in various structural states were used as the material for study. The sample for the study was a wire with a diameter of 1mm. The structural studies of the metal were carried out using an Epiquant light microscope. Ferrite grain size was determined using quantitative metallographic techniques. Different ferrite grain size was obtained as a result of combination of thermal and termo mechanical treatment. Vary by heating temperature and the cooling rate, using cold plastic deformation and subsequent annealing, made it possible to change the ferrite grain size at the level of two orders of magnitude. Deformation curves were obtained during stretching the samples on the Instron testing machine. Findings. Based on the analysis of stretching curves of low-carbon steels with different ferrite grain sizes, it has been established that the initiation and propagation of plastic deformation in the jerky flow area is accompanied by the development of strain hardening processes. The study of the nature of increase at dislocation density depending on ferrite grain size of low-carbon steel, starting from the moment of initiation of plastic deformation, confirmed the existence of relationship between the development of strain hardening at the area of jerky flow and the area of parabolic hardening curve. Originality. One of the reasons for decrease in Luders deformation with an increase of ferrite grain size of low-carbon steel is an increase in strain hardening indicator, which accelerates decomposition of uniform dislocations distribution in the front of deformation band. The flow stress during initiation of plastic deformation is determined by the additive contribution from the frictional stress of the crystal lattices, the state of ferrite grain boundaries, and the density of mobile dislocations. It was found that the size of dislocation cell increases in proportion to the diameter of ferrite grain, which facilitates the development of dislocation annihilation during plastic deformation. Practical value. Explanation of qualitative dependence of the influence of ferrite grain size of a low-carbon steel on the strain hardening degree and the magnitude of Luders deformation will make it possible to determine the optimal structural state of steels subjected to cold plastic deformation.

Influence of neutron irradiation on Al-6061 alloy produced via ultrasonic additive manufacturing

Samples of aluminum alloy 6061 produced via ultrasonic additive manufacturing (UAM) were irradiated in the High Flux Isotope Reactor (HFIR) up to 17.3 dpa at ~70°C while in contact with water using perforated rabbit capsules. The irradiation campaign included as-received (AR) material, specimens subjected to various post-weld heat treatments (PWHTs, including hot isostatic pressing [HIP]), and reference (wrought) alloy samples. Mechanical tensile tests, accompanied by digital image correlation (DIC) analysis, fractography, and metallography, were performed as a part of the post-irradiation evaluation. The X- and Y-specimens (i.e., oriented in the sonotrode moving and vibration directions, respectively) showed pronounced radiation hardening and ductility decrease. Specific serration flow behavior and propagation of deformation bands were observed under various material conditions up to 3.5 dpa but disappeared at 17.3 dpa. In all cases, the fracture mechanism of X- and Y-specimens was ductile; ductile dimples dominated the fracture surface. Irradiated X- and Y-specimens showed good performance, regardless of material conditions (AR or PWHT). The performance of Z-specimens oriented in the build direction was strongly dependent on the PWHT. The AR and aged specimens showed fracture stress decrease with dose, and they experienced fracture under irradiation after 3.5 dpa; specimen cross section analysis revealed specific interface degradation that was likely related to corrosion. Recrystallization significantly improved in-reactor performance. HIP suppressed interface degradation due to recrystallization and pore removal, which led to good in-reactor performance for Z-specimens.

Deformation-induced ultrafine grains near fatigue crack tip and correlative fatigue damage in Al matrix composite

New insight into the microstructural evolution and the correlative damage mechanisms near fatigue crack tip in a TiB2/Al composite was conducted by transmission kikuchi diffraction and transmission electron microscopy. It was found that the micro deformation band formed from the crack tip and ran along slip planes. Ultrafine grains were induced in deformation band, where the fatigue crack preferring to grow along these deformation-induced boundaries was first demonstrated as the essential process of crack across parent grains and grain boundaries. Ahead of crack tip, the TiB2 particles can affect crack growth by impeding the successive propagation of deformation band.

Experimental study of macroscopic localization of plastic flow in al-mg alloy under complicated stress-strain state

The main objective of this work is to investigate the spatial-time inhomogeneity due to the jerky flow during plastic deformation of Al-Mg alloy under complex stress conditions. The use of specimens in the form of plates with a rigid circular rim and a reverse curvature rim was considered. Experimental data on the evolution of inhomogeneous strain and temperature fields have been obtained, illustrating the processes of initiation and propagation of deformation bands of the localized plastic flow by using the digital image correlation and infrared thermography.

Characterization of localized plastic deformation behaviors associated with dynamic strain aging in pipeline steels using digital image correlation

Localized deformation behavior in X52 and X70 pipeline steels during dynamic strain aging (or DSA, caused by interstitial atom-dislocation interactions during plastic deformation) were systematically assessed using digital image correlation with incremental strain analysis during elevated temperature tensile testing in the range of 25–350 °C. Similar to materials that use substitutional atoms for DSA, deformation band propagation and nucleation behaviors in the steels were directly related to the morphologies of flow curves from the tensile data. Comparisons between the steels and substitutional DSA systems were made. The type of localized deformation behavior was dependent on the temperature and strain rate conditions of the tensile test and the total amount of strain in the sample. Plastic flow was primarily dominated by deformation band propagation and nucleation at relatively low and high temperatures respectively within the DSA regime. It was theorized that the changing serration morphologies and deformation behaviors at different temperature-strain rate conditions were due to relative differences between the mobility of the solute atoms and the velocity of the mobile dislocations throughout the DSA regime. The dislocation density (which increases due to strain hardening during a tensile test) also influenced deformation behavior at relatively low and high strains.

Combined deformation twinning and short-range ordering causes serrated flow in high-manganese steels

The isolated impact of deformation twinning and short-range ordering on the serrated flow phenomena in two high-manganese twinning-induced plasticity steels, an X60Mn18 and an X30MnAl17-1 alloy, was studied via quasistatic tensile tests at room temperature and at 150 °C. In order to identify the effect of prior deformation twins, both alloys were investigated in recrystallized and non-recrystallized states. The experimental data suggest that the inhibition of the movement of trailing partial dislocations by short-range ordered clusters results in a promotion of deformation twinning. A multi-scale model is introduced to connect short-range ordering theory, deformation twinning, the formation and propagation of deformation bands and the occurrence of serrated flow.

Peculiar Spatiotemporal Behavior of Unstable Plastic Flow in an AlMgMnScZr Alloy with Coarse and Ultrafine Grains

The work addresses the effects of nanosize particles and grain refinement on the patterns of stress serrations and kinematics of deformation bands associated with the Portevin–Le Chatelier instability of plastic flow. Ultra-fine-grained microstructure was obtained using equal-channel angular pressing of the initial coarse-grained alloy. Tensile tests were carried out on flat specimens at strain rates in the range from 3 × 10−5 to 1.4 × 10−2 s−1. Using local extensometry techniques, it was found that the presence of nanoscale precipitates promotes quasi-continuous propagation of deformation bands in the entire strain-rate range. The grain refinement leads to a transition to relay-race propagation at high strain rates and static strain localization at low rates. The results are discussed from the viewpoint of competition between various dynamical modes of plastic deformation associated with collective dynamics of dislocations.

Propagating compaction bands in confined compression of snow

When deforming snow slowly, it resists. But when applying a deformation rapidly, it gives in more easily. Experiments now reveal propagating deformation bands and the localization of strain in compressed snow — both natural and artificial.

Investigation of the effect of electric current on serrated deformation and acoustic emission in the aluminum-magnesium alloy 5056

The effect of direct electric current on the serrated deformation of the aluminum-magnesium alloy 5056 has been studied using the acoustic emission method and high-speed video filming of propagating deformation bands. The phenomenon of the electric current-induced suppression of low-frequency acoustic emission signals has been revealed in the range of 1 Hz–2 kHz, which is connected with the development of Portevin-Le Chatelier deformation bands. The characteristic times of damping and growth of plastic instabilities and acoustic signals caused by them after current turn-on and turn-off, respectively, have been estimated.

Effect of orientations on in situ tensile deformation and fracture behaviours of nickel-base single-crystal superalloys

After systematically investigating in situ tensile deformation and fracture behaviours of nickel-base single-crystal superalloys with four different orientations, their slip and fracture modes in relation to the orientations are obtained and shown as follows. In [0 0 1] oriented specimen, four slip systems were activated and the crack initiated along the interactive location between the third slip bands (SBs) and deformation bands in priority. The crack propagation involves all three crack opening types of I, II and III. [0 1 1] oriented specimens exhibited obvious low strain hardening rate and high ductility, which can be attributed to the balance between the softening caused by the propagation of SBs and the latent hardening caused by the propagation of deformation bands. Except for case, the critical resolved shear stress (CRSS) of the other oriented superalloys at room temperature is approximate, which is due to non-Schmid behaviour in superalloys. Lall–Chin–Pope model is used to explain the orientation dependence of CRSS. Furthermore, the fracture modes of different oriented specimens depend on the intensity of intrusion and extrusion and the degree of match between normal stress and shear stress. The shear stress is beneficial to the crack initiation and the normal stress contributes to the crack propagation.

Serrated creep and spatio-temporal structures of macrolocalized plastic deformation

The dynamics and morphology of macrolocalized deformation bands have been investigated using a complex of high-speed in situ methods under the conditions of serrated creep of flat samples of the aluminum-magnesium alloy 5456 with different aspect ratios. It has been found that, at the front of a macroscopic plastic deformation jump, a complex structure of propagating deformation bands, which are considered as macrolocalized deformation “quanta,” is spontaneously formed in the material. It has been shown that, with an increase in the sample length, the deformation behavior of the alloy tends to the state of self-organized criticality.