Localized Deformation Bands Research Articles

Transformation pattern of shape-memory NiTi alloy during stress-biased thermal cycling

Despite the superelastic deformation of NiTi has been documented and analyzed elaborately, its shape-memory behavior during stress-biased thermal cycling has not been thoroughly unveiled. This paper examines the evolution of transformation pattern in NiTi upon thermal cycling over a wide range of biasing stress. For the present shape-memory NiTi, both forward and reverse transformations proceed via the growth of localized deformation band (LDB) under low biasing stress (σbias ≤ 150 MPa), while LDB only appears during forward transformation and reverse transformation is uniform under high biasing stress (σbias ≥ 200 MPa). This is the first time that delocalization (conversion of deformation mode from localization to homogeneity) is observed during stress-biased thermal cycling. We clarify that the intrinsic undercooling/overheating of martensitic transformation results in unstable thermomechanical response, and it is the origin of localization in shape-memory NiTi. It is found that transformation-induced plasticity (TRIP), which is dominated by dislocation slip and deformation twining, not only causes irreversibility in the present shape-memory NiTi but also leads to delocalization through stabilization of intrinsic thermomechanical response of reverse transformation.

Thermomechanics of phase transformation induced localization in NiTi tubes. Part II constitutive modeling and simulations

In an isobaric test, typically used for establishing the transformation temperatures of SMAs, a specimen is taken through a cool/heat cycle at a prescribed stress level. The companion paper, Part I, demonstrated that such tests on NiTi tubes result in rather complex interactions between the helical bands of localized deformation induced by phase transformations, the associated latent heat, and the thermal exchange with the environment. These interactions were quantified using accurately controlled testing conditions and full-field diagnostics, and provide a challenging platform for evaluating analyses. In Part II this challenge is met by first extending the thermomechanical constitutive framework developed by our group to include variations of transformation stress, strain, and latent heat with temperature. Unique features of the model include: modeling the reversible phase transformations of NiTi through a single surface in the deviatoric stress–temperature space with the transformation strain and entropy as the internal variables; and use of softening to model the inhomogeneous deformation exhibited in tension. The model is calibrated to the isothermal results of Part I. The constitutive model is then incorporated into a fully coupled static displacement-thermally transient finite element analysis that is used to simulate the isobaric experiments on NiTi tubes of Part I over a range of stresses. The clamped ends of the experiment are idealized as radial constraints. Heat exchange between the structure and the environment is strictly by convection. Isobaric testing is simulated by taking the model tube through the cool/heat cycle of the experiment at the prescribed stress level. A small thickness depression at one of the ends is used to initiate localized transformation. The temperature-strain response is reproduced with the two transformations initiating at essentially the same temperatures as in the experiments, producing the correct transformation strains for all stress levels. Transformation of M propagates via a helical band at similar speeds as in the experiments, while transformation of A is via multipronged fronts for all cases. Successful reproduction of the velocities of the banded transformations is governed by the value of the convection coefficient. Overall, the reproduction of the isobaric experiments over a range of stress levels, validates the constitutive and structural models. It also points to the limitations of modeling the heat exchange between the specimen and the environment only by convection.

Deformation behavior and crack mechanism of a first generation single-crystal Ni-based superalloys under thermomechanical fatigue loading

A research investigation was conducted to analyze thermomechanical fatigue (TMF) in a first-generation nickel-based single-crystal superalloy. The analysis was conducted within a temperature range of 450–950 °C while employing a strain ratio of −1 during two phases, in-phase (IP) and out-of-phase (OP). TMF life data were collected at varying mechanical strain amplitudes. The IP and OP TMF lifetimes were compared under identical strain conditions. A detailed analysis of the fractures and microstructures of the tested specimens was performed to investigate the crack mechanism. In the IP specimens, the formation of cracks closely followed the direction perpendicular to the principal stress within the material, which was primarily associated with the presence of micropores. Internal cracks tended to develop at locations with strain concentrations and subsequently propagate outward from the surface. The main mechanism behind these cracks was the interaction between creep and fatigue damage. Conversely, in the OP TMF specimens subjected to an oxide scale formed on the fracture surface, leading to fatigue cracking at multiple locations. The cracks gradually extended within the material before transitioning into the (111) crystallographic planes, where localized deformation bands occurred, affecting an area depleted of the γ' phase. This crack mechanism predominantly involved interactions between oxidation and fatigue. Among the various fatigue life prediction models, the Ostergren model exhibited strong agreement with the predicted data points for the fatigue life.

The mechanism for Li-addition induced homogeneous deformation in Mg-4.5wt.% Li alloy

The mechanism by which Li additions induce a more homogeneous deformation was systematically investigated in this study by comparing the deformation behavior of pure Mg and Mg-4.5 wt.% Li using experiments, crystal plasticity simulations, and theoretical calculations. The results of the digital image correlation measurements showed that under tension along the transverse direction (TD), strong localized deformation bands with an angle of approximately 20° with respect to the TD were observed in pure Mg, but were absent in Mg-4.5 wt.% Li. Slip trace analyses indicated that basal slip was the predominant deformation mode for pure Mg, whereas that for Mg-4.5 wt.% Li was prismatic slip. A high relative activity of pyramidal 〈c+a〉 slip, approximately 17 %, was observed in Mg-4.5 wt.% Li at a high strain of 10 %. Contraction or double twins were hardly detected during the entire tension process. Crystal plasticity simulations revealed that the 4.5 wt.% Li addition dramatically reduced the critical resolved shear stress (CRSS) ratio of pyramidal 〈c+a〉 slip: prismatic slip: basal slip, which was 31.5:21:1 for pure Mg and 4.3:1.3:1 for Mg-4.5 wt.% Li. Theoretical calculations using the CRSS ratios determined by simulations revealed that easy and uniform deformation propagation induced by a dramatically reduced CRSS ratio of prismatic slip to basal slip played a more important role in improving the deformation homogeneity, in addition to the enhanced activity of pyramidal 〈c+a〉 slip. The mechanisms for how Li additions improve the ductility of Mg are also revisited in this paper.

Study of the effect of shock wave loading on the structure and properties of bronze alloys BrAZh9-4 and BrAMts9-2

Bronze alloys, due to their resistance to mechanical abrasion and high corrosion resistance, are used for the manufacture of machine parts and mechanisms that are subject to friction during operation. We present the results of studying the effect of shock-wave loading on the structure and properties of bronze alloys of grades BrAZh9-4 and BrAMts9-2. Shock-wave loading experiments were carried out by throwing the flyer plate onto cylindrical samples and compressing by a sliding detonation wave. The method of throwing a flyer plate accelerated by the energy of an explosion is often used to determine the spall strength of materials and the method of compression by a sliding detonation wave is used to create a large dynamic pressure inside the material. It is shown that at a throwing speed of a flyer plate of 2.4 km/sec, the impact pressure of the plate with the sample is 15 – 16 GPa, which exceeds the bronze shear strength. Under indicated loading conditions, the hardness of bronze increases by 53 and 25% for BrAZh9-4 and BrAMts9-2, respectively. Studies of the microstructure using scanning electron and optical microscopy revealed multiple cracks and micropores present on the surface of transverse sections forming a zone of spall fracture and areas turning into bands of localized deformation. Moreover, it is shown that when the samples are loaded with a flyer plate in a clip and without it, a greater number of cracks and shear areas are observed. Compression by a sliding detonation wave with a different amount of explosive charge revealed small defects present in the structure at the grain boundaries. The results obtained can be used to developed technologies for modifying and restoring the properties of bronze parts subject to shock-wave destruction.

Kinematic fields measurement during Ti-6Al-4V chip formation using new high-speed imaging system

Orthogonal cutting on Ti-6Al-4V produces segmented chips which results from localized deformation within the Primary Shear Zone (PSZ). For in situ visualization of the material flow during Ti-6Al-4V chip formation, a new high-speed optical system is proposed. Difficulties arise from the submillimetric size of the cutting zone. Therefore, a dedicated optical system with coaxial illumination is designed allowing for local scale analysis of chip formation. In this paper, the cutting forces were measured highlighting the unsteady nature of Ti-6Al-4V chip segmentation. For kinematic fields measurement, a novel method of Digital Image Correlation (DIC) technique is applied on the recorded images from the cutting zone. It allows for the identification of the localized deformation bands. Then, the level of the cumulative strain fields reached in the PSZ was presented and analyzed. The effect of the rake angle on the strain fields was studied. Using a 0∘ rake angle, the segment chip was found to be more subjected to deformation than in the case using 15∘. Accuracy of the measured strain fields was discussed function of the main source of errors. In addition, the chip morphology and microstructure were investigated with a scanning electron microscopy (SEM). It shows crack opening along the PSZ and high material failure near the tool-chip interface which explains the difficulties of DIC application in the context of kinematic fields measurement during orthogonal cutting. Finally, correlation between the measured strain fields and the mean value of the chip segment width was made.

STRUCTURE, MARTENSITIC TRANSFORMATIONS AND MECHANICAL PROPERTIES OF AGING NANOCRYSTALLINE TI-50.9 AT % NI ALLOY

The effect of aging temperature in the range of 300-500°C on the structure, R martensitic transformations and mechanical characteristics of nanocrystalline Ti-50.9 at % Ni alloy with a grain/subgrain structure was studied. It was found that variation in the spatial distribution of coherent Ti3Ni4 particles in the nanostructure from their location on dislocations during low-temperature aging to precipitation at dislocation boundaries under intense aging is accompanied by a change in the morphology of the R phase from a nanodomain to a self-accommodating lamellar structure. The nanodomain structure of the R phase contributes to uniform deformation of the alloy during loading/unloading and stabilization of superelasticity. When loading the alloy with a lamellar R-phase morphology, localized deformation bands are formed by the R-phase reorientation in a Luders deformation manner.

Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50% uniform elongation

A new medium-Mn steel was designed to achieve unprecedented tensile properties, with a yield strength beyond 1.1 GPa and a uniform elongation over 50%. The tensile behavior shows a heterogeneous deformation feature, which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands. Multiple strain hardening mechanisms for excellent tensile properties were revealed. Firstly, non-uniform martensite transformation occurs only within a localized deformation band, and initiation and propagation of every localized deformation band need only a small amount of martensite transformation, which can provide a persistent and complete transformation-induced-plasticity effect during a large strain range. Secondly, geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening. Lastly, martensite formed by displacive shear transformation can inherently generate a high density of mobile screw dislocations, and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multiplication due to the dynamic strain aging effect, and these two effects provide a high density of mobile dislocations for strong strain hardening.

DISLOCATION STRUCTURE IN A LOCALIZED DEFORMATION BAND FORMED DURING TENSION OF A NORMALIZED 09G2S STEEL SPECIMEN

A transmission electron microscopy study was carried out to examine the dislocation structure of normalized steel 09G2S that exhibits a strain aging effect. In a specimen stretched to the yield point, the maximum dislocation density is observed in the center (active zone) of the localized deformation band and decreases by about an order of magnitude at the band edges. The structure of these band regions, which are at different stages of strain hardening, is analyzed. The Burgers vector of dislocations of the primary slip system is determined; the dislocation ends form ordered flat dipole walls, leading to slip polygonization. The relationship between the specimen structure at the yield point and at the ultimate load is considered.

Mechanical behavior of CP-Ti at high strain rates and under stress triaxiality

In this study, we investigated the influence of stress triaxiality on the mechanical behavior of commercially pure (CP-Ti) VT1-0 titanium (Grade 2) in the range of strain rates from 0.1 to 103 s−1. Tensile tests were carried out on flat notched and smooth specimens using an Instron VHS 40 / 50–20 servo-hydraulic testing machine. Phantom V711 was used to capture the deformation process of the specimens. The digital image correlation (DIC) method was employed to investigate the evolution of local fields in the gauge section of the specimen. Fracture surface topology analysis was performed using the Keyence VHX‐600D digital microscope. The results revealed that an increase in the strain rate from 102 to 103 s−1 leads to a significant discrepancy between the relative residual elongation δ and the strains that occur in the localization bands prior to crack initiation. CP-Ti undergoes fracture due to nucleation, growth, and coalescence of damage in bands of localized plastic deformation oriented along the maximum shear stresses. Fracture surface analysis indicated an increase in the surface roughness parameter with increasing stress triaxiality. The results confirm that the fracture of commercially pure titanium exhibits ductile behavior at strain rates from 0.1 to 103 s−1, for triaxiality stress parameter 0.333 ≤ η < 0.5, and at a temperature close to 295 K. The least influence of stress triaxiality on the strain-to-fracture appeared at a strain rate of 103 s−1.

A simple method enabling efficient quantitative analysis of the Portevin–Le Chatelier band characteristics

Localized deformation bands are often observed in materials exhibiting the Portevin–Le Chatelier (PLC) effect. However, efficient quantitative analysis of PLC bands remains challenging. A novel method is thus proposed in this work, where a multi-strain-jump function is introduced to capture the experimentally obtained staircase-like strain profile from digital image correlation (DIC). This approach is simple to implement and allows for: (i) automatically extracting the band strain throughout the test, which can be used to further evaluate the band velocity, and (ii) linking band characteristics with the corresponding local material properties. The efficiency of the method is demonstrated by analysing the band characteristics for different strain rates and temperatures. The results reveal that the relative band velocity is proportional to the work hardening rate, i.e., vb/vg∝Θ, for the continuously propagating type A bands.

Quasi-static and dynamic response of a Cu/Nb composite following equal channel angular extrusion

The article presents a study of the plastic properties of a Cu–18%Nb composite following equal channel angular extrusion under both low- and high-strain rate loadings. The microstructures are characterized by optical and scanning electron microscopy as well as electron backscatter diffraction. Comparisons are also made with a Cu–50%Nb laminate made by accumulative roll bonding. Textures obtained within the respective phases in the extruded composite closely mimic those previously reported for the pure components alone (Cu, Nb). Grain sizes of the major phase (Cu) exhibit broad distributions, due largely to partial recrystallization during the extrusion process. Further re-crystallization is obtained within localized deformation bands produced at high strain rates (3 × 103 s−1). While the degree of anisotropy and the strain rate sensitivity of the composite response are similar to those of pure Cu after extrusion, the strength levels of the composite are somewhat greater, falling broadly between those obtained in the pure components after extrusion to comparable plastic strains. The implications for selection of composite composition and processing for achieving higher strengths are briefly discussed.

Sea Ice Rheology Experiment (SIREx): 2. Evaluating Linear Kinematic Features in High‐Resolution Sea Ice Simulations

AbstractSimulating sea ice drift and deformation in the Arctic Ocean is still a challenge because of the multiscale interaction of sea ice floes that compose the Arctic Sea ice cover. The Sea Ice Rheology Experiment (SIREx) is a model intercomparison project of the Forum of Arctic Modeling and Observational Synthesis (FAMOS). In SIREx, skill metrics are designed to evaluate different recently suggested approaches for modeling linear kinematic features (LKFs) to provide guidance for modeling small‐scale deformation. These LKFs are narrow bands of localized deformation that can be observed in satellite images and also form in high resolution sea ice simulations. In this contribution, spatial and temporal properties of LKFs are assessed in 36 simulations of state‐of‐the‐art sea ice models and compared to deformation features derived from the RADARSAT Geophysical Processor System. All simulations produce LKFs, but only very few models realistically simulate at least some statistics of LKF properties such as densities, lengths, or growth rates. All SIREx models overestimate the angle of fracture between conjugate pairs of LKFs and LKF lifetimes pointing to inaccurate model physics. The temporal and spatial resolution of a simulation and the spatial resolution of atmospheric boundary condition affect simulated LKFs as much as the model's sea ice rheology and numerics. Only in very high resolution simulations (≤2 km) the concentration and thickness anomalies along LKFs are large enough to affect air‐ice‐ocean interaction processes.

High strain-rate compression behavior of polymeric rod and plate Kelvin lattice structures

The compressive high strain-rate behavior of polymeric Kelvin lattice structures with rod-based or plate-based unit cells was investigated through experimental techniques and finite element simulations. Polymeric lattice structures with 5x5x5 unit cell geometries were manufactured on the millimeter scale using vat polymerization additive manufacturing and tested at low (0.001/s) and high (1000/s) strain-rates. High strain-rate experiments were performed and validated for a viscoelastic split-Hopkinson (Kolsky) pressure bar system (SHPB) coupled with high-speed imaging and digital image correlation (DIC). Experimental results at both low and high strain-rates show the formation of a localized deformation band which was more prevalent in low relative density specimens and low strain-rate experiments. Strain-rate effects of lattice specimens strongly correlate with effects of the base polymer material; both bulk polymer and lattice specimen demonstrated strain-rate hardening, strain-rate stiffening, and decreased fracture strain under dynamic loading. Results show mechanical failure properties and energy absorption depended strongly on the relative density of the lattice specimen and exhibited distinct scaling between relative density and geometry type (rod, plate) and loading rate. High relative density plate-lattices demonstrated inferior mechanical properties to rod-lattices; however, there exists a critical relative density for a given mechanical property (17%–28%) below which plate-lattices outperform rod-lattices of similar mass. High strain-rate explicit finite element simulations were performed and showed good agreement with the mechanical failure trends and deformation modes observed in the experiments.

Autowave Criteria of Fracture and Plastic Strain Localization of Zirconium Alloys

Plastic deformation and fracture of Zr–1% Nb alloys exposed to quasi-static tensile testing have been studied via a joint analysis of stress-strain curves, ultrasound velocity and double-exposure speckle photographs. The possibilities of ductility evaluation through the εxx strain distribution in thin-walled parts of zirconium alloys are shown in this paper. The stress-strain state of zirconium alloys in a cold rolling site is investigated considering the development of localized deformation bands and changes in ultrasound velocity. It is established that the transition from the upsetting to the reduction region is accompanied by the significant exhaustion of the plasticity margin of the material; therefore, the latter is more prone to fracture in this zone exactly. It is shown that traditional methods estimating the plasticity margin from the mechanical properties cannot reveal this region, requiring a comprehensive study of macroscopically localized plastic strain in combination with acoustic measurements. In particular, the multi-pass cold rolling of Zr alloys includes various localized deformation processes that can result in the formation of localized plasticity autowaves. Recommendations for strain distribution division over the deformation zone length in the alloy in the pilger roll grooves are provided as well.

Studying Deformation Behaviors in Austenitic Stainless Steels within a Temperature Range of 143 K &lt; T &lt; 420 K

This work deals with studying staging and macroscopic strain localization in austenitic stainless steel 12Kh18N9T within a temperature range of 143 K &lt; T &lt; 420 K. The visualization and evolution of macroscopic localized plastic deformation bands at different stages of work hardening were carried out by the method of the double-exposure speckle photography (DESP), which allows registering displacement fields with a high accuracy by tracing changes on the surface of the material under study and then comparing the specklograms recorded during uniaxial tension. The shape of the tensile curves σ(ε) undergoes a significant change with a decreasing temperature due to the γ-α'-phase transformation induced by plastic deformation. The processing of the deformation curves of the steel samples made it possible to distinguish the following stages of strain hardening, i.e. the stage of linear hardening and jerky flow stage. A comparative analysis of the design diagrams (with the introduction of additional parameters of the Ludwigson equation) and experimental diagrams of tension of steel 12Kh18N9T for different temperatures is carried out. The analysis of local strains distributions showed that at the stage of linear work hardening, a mobile system of plastic strain localization centers is observed. The temperature dependence of the parameters of plastic deformation localization at the stages of linear work hardening has been established. Unlike the linear hardening, the jerky flow possesses the propagation of single plastic strain fronts that occur one after another through the sample due to the γ-α' phase transition and the Portevin-Le Chatelier effect. It was found that at the jerky flow stage, which is the final stage before the destruction of the sample, the centers of deformation localization do not merge, leading to the neck formation.

Kinetics of Plastic Deformation Localization Bands in Polycrystalline Nickel

Jerky flow has recently aroused interest as an example of complex spatiotemporal dynamics resulting from the collective behavior of defects in Al- and Mg-based alloys under loading. This paper presents the results of the study of the macroscopic strain localization kinetics in Nickel 200 (99.5 wt % purity). Uniaxial tension of flat samples is monitored at room temperature in the load–unload mode at a constant strain rate and total deformation increment up to 5%. The stress–strain curves reveal jerky flow from the yield point to the formation of the neck. The digital speckle correlation method evidences the movement of localized plastic deformation bands under the conditions of the Portevin–Le Chatelier effect (PLC). It is shown that stress drops during jerky flow in Ni are accompanied by the formation of morphologically simple single PLC bands. It is established that, with an increase in total deformation, the number of PLC bands and their velocity of motion along the sample decrease, while their time period increases. Moreover, an increase in total deformation leads to an increase in the parameters of the force response (i.e., time period and stress drop magnitude). It is found that the criterion of damage for PLC bands as a function of the total strain has a sigmoidal shape.

3D-Printing Damage-Tolerant Architected Metallic Materials with Shape Recoverability via Special Deformation Design of Constituent Material.

Architected metallic materials generally suffer from a serious engineering problem of mechanical instability manifested as the emergence of localized deformation bands and collapse of strength. They usually cannot exhibit satisfactory shape recoverability due to the little recoverable strain of metallic constituent material. After yielding, the metallic constituent material usually exhibits a continuous low strain-hardening capacity, giving the local yielded regions of architecture low load resistance and easily developing into excessive deformation bands, accompanied by the collapse of strength. Here, a novel constituent material deformation design strategy has been skillfully proposed, where the low load resistance of yielded regions of the architecture can be effectively compensated by the significant self-strengthening behavior of constituent material, thus avoiding the formation of localized deformation bands and collapse of strength. To substantiate this strategy, shape-memory alloys (SMAs) are considered as suitable constituent materials for possessing both self-strengthening behavior and shape-recovery function. A 3D-printing technique was adopted to prepare various NiTi SMA architected materials with different geometric structures. It is demonstrated that all of these architected metallic materials can be stably and uniformly compressed by up to 80% without the formation of localized bands, collapse of strength, and structural failure, exhibiting ultrahigh damage tolerance. Furthermore, these SMA architected materials can display more than 98% shape recovery even after 80% deformation and excellent cycle stability during 15 cycles. This work exploits the amazing impact of constituent materials on constructing supernormal properties of architected materials and will open new avenues for developing high-performance architected metallic materials.

The Microstructure and Properties of Laser Shock Peened CMSX4 Superalloy

The influence of laser shock peening on the surface morphology and microstructure of single-crystal CMSX4 nickel-based superalloy was investigated by optical profilometry and atomic force microscopy, scanning and transmission electron microscopy as well as scanning-transmission electron microscopy in high-angle annular dark-field mode. Maps of chemical elements distribution in the laser-affected areas were determined using energy-dispersive X-ray spectroscopy. Furthermore, after the LSP, nanohardness tests were conducted on the cross section of the treated samples as well as the untreated material. Laser shock peening caused an ablation and melting of the surface layer and hence enlarged the surface roughness. Beneath the surface, in the laser shock-peened areas, severe distortion of the regular {gamma mathord{left/ {vphantom {gamma {gamma^{prime}}}} right. kern-0pt} {gamma^{prime}}} microstructure was observed. In the surface layer, down to about 15 μm, shear bands of localized deformation were formed. Moreover, the result showed that the average nano-hardness value was obviously increased in the laser-treated region.

Dislocation-induced plastic instability in a rare earth containing magnesium alloy

An uncommon yield point phenomenon is observed in a Mg-1.5 wt.%Nd extrusion alloy deformed in tension. The stress-strain curve exhibits a drop at the onset of yielding, which is followed by a prolonged plateau stage prior to normal strain hardening. Localized deformation bands are observed to nucleate at the upper yield stress and propagate during the yield plateau. The yield plateau exists independent of texture and grain size of the material and shows a small dependence on strain rate. It is hypnotized that the yield point phenomenon is caused by the interactions between mobile dislocations and solute clusters. Specifically, the localized deformation at the front of deformation bands is a consequence of glide carried out by dislocations being unlocked from solute clusters by stress-strain fields from propagating bands, while dislocations away from the bands remain locked and material swept by the bands hardens.