Elastoplastic Transition Research Articles

Unusual deformation mechanisms evoked by hetero-zone interaction in a heterostructured FCC high-entropy alloy

Understanding the synergistic mechanical effects of heterostructured materials remains challenging due to the complexities in the underlying deformation mechanisms, which are usually diverse, activated at different length scales and possibly interacting. Here, we unravel a deformation fundamental for heterostructures: in addition to the direct contribution on strength, hetero-zone interaction and the development of long-range internal stress could assist in evoking extra plastic mechanisms that are difficult to activate in their homogeneous counterparts. Specifically, the deformation of a heterostructure in Al0.1CoCrFeNi alloy, featuring nanostructured hard lamellae embedded in fine-grained soft matrix, is taken as an example for study. Drawing from experimental insights, the long-range internal stress buildup at elastoplastic transition stage due to intense dislocation pile-ups against hetero-zone boundary, which increases yield strength significantly, is theoretically analyzed. At the plastic stage, the high internal stress helps to activate phase transformation in the fine-grained zones, and the inter-zone constraint leads to form dispersed stable strain bands in the nanostructured zones. These extra mechanisms, together with enhanced deformation twinning, facilitate work hardening and coordinate the strain partitioning between zones, imparting improved ductility at high flow stress. These findings indicate a principle for heterostructural design: introducing strong hetero-zone interaction to enhance internal stress and thereby invoke new deformation mechanisms.

Tailoring size and fraction of coherent L12 nanoprecipitates to achieve strong hardening in medium entropy alloys with heterogeneous grain structures

Heterogeneous grain structures with coherent L12 nanoprecipitates were constructed in a medium entropy alloy with chemical composition of Co34.5Cr32Ni27.5Al3Ti3 (at. %). The volume fraction and size of L12 nanoprecipitates are observed to become higher and larger, and the interspacing is found to become smaller with increasing aging time, resulting in a severer heterogeneity. The domain of tensile properties was expanded by varying the size and volume fraction of coherent L12 nanoprecipitates after aging treatment. The samples after longer-time aging treatment show stronger strain hardening as compared to those after shorter-time aging treatment. The hetero-deformation-induced (HDI) hardening plays a more vital role in the longer-time aged samples than the shorter-time aged samples, especially at the elasto-plastic transition stage. The dominant precipitation hardening mechanism is observed to transit from dislocation-cutting to Orowan dislocation-looping with increasing precipitation size, as predicted by a theoretical model and validated by experimental results. Multiple deformation defects, such as deformation twins, stacking faults and Lomer-Cottrell locks, are the dominant deformation mechanisms for all samples, while interactions between these defects and L12 nanoprecipitates were observed to be more intensive in the longer-time aged samples than in the shorter-time aged samples, resulting in stronger precipitation and HDI hardening.

Crystal-plasticity modelling of the yield surfaces and anelasticity in the elastoplastic transition of metals

The paper presents a full-field crystal-plasticity computational investigation of 10με small-strain-offset yield surfaces with pointed vertexes that are seen in the elastoplastic transition of pre-strained polycrystal metals. It is concluded that the shape of these yield surfaces obtained with a full-field spectral solver compares reasonably well with calculated ones by a simple aggregate Taylor model. The influence of material strength, work hardening, and texture are discussed. An assessment is made of the origin of anelasticity and Bauschinger effects at small strains, considering two mechanisms. Firstly, there is a built-in composite effect in crystal elastoplastic simulations due to the mixture of elastically and plastically loaded grains. Secondly, kinematic hardening of reverse slip systems will contribute to the Bauschinger effect. Based on analyses of the computed selected cases and comparison to previously published measurements, it is concluded that both mechanisms are important.

Analysis of compression response and coordinated deformation mechanism of bi-crystal Al-Cu-Mg-Ag alloy using CPFEM

In this paper, the compression behavior of Al-Cu-Mg-Ag bi-crystal samples at room temperature under different orientation combinations was studied. The stress-strain distribution of bi-crystal samples during the compression process was studied based on the crystal plastic finite element method (CPFEM). The deformation mismatch and slip transfer caused by the orientation combination of bi-crystal are discussed. The results show that the Al-Cu-Mg-Ag alloy bi-crystal samples with different orientations exhibit an anisotropic hardening response under compression. The combination of soft and hard orientation can coordinate the dislocation drive of the yield point, delay the elastoplastic transition of the yield point, and greatly improve the strength of the alloy. The mechanism of high strength of heterogeneous structure is proposed in this paper, which provides an important reference for the design of high strength heat resistant aluminum alloy for automobiles.

The influence of alien dislocations on the plasticity of [formula omitted]-axis Mg single crystals under tension and compression

Uniaxial tension and compression tests were conducted on as-grown [0001] Mg single crystals and on [0001] single crystals containing an alien distribution of basal dislocations to elucidate their effect on the deformation mechanisms and strain hardening. During tensile deformation, the basal dislocations induce elasto-plastic transition and micro-yielding that precede the gross deformation by twinning. They decrease the stress for twin nucleation, the tensile strength, and the failure strain compared to the virgin single crystals. The weakening of the microstructure and early failure are attributed to the higher density of micro cracks formed in the crystals with alien basal dislocations. The extraneous basal dislocations alter entirely the deformation mechanisms and strain hardening characteristic of [0001] single crystals under compression. They suppress the pyramidal slip as a dominant deformation mode and promote the gross deformation by cooperative basal slip and twinning during extended deformation plateau. The deformation twinning is of anomalous type, nucleating and propagating with the assistance of extraneous basal dislocations at the negative Schmid Factor and against the imposed compressive strain. τtw≈2.1±0.2MPa is found as the basal slip-assisted twinning stress in Mg.

Probing atomistic deformation behavior of graphene-coated Al0.3CoCrFeNi high-entropy alloy under nanoindentation

This study employs a series of molecular dynamics (MD) simulations to investigate the atomistic deformation mechanism of graphene (Gr)-coated Al[Formula: see text]CoCrFeNi high-entropy alloy (HEA) during nanoindentation. In the elastic region, both substrates exhibit similar indentation performance. However, at greater indentation depths, the Gr-coated HEA offered higher resistance against indentation, leading to a remarkable 78% increase in hardness compared to pristine HEA. The graphene-coated HEA also shows extended unloading time (52.3[Formula: see text]ps) compared to the pristine HEA (44.7[Formula: see text]ps), indicating improved structural recovery. Importantly, graphene coating raises the critical indentation depth for the emergence of the initial embryonic plastic deformation loop (PDL) from 0.628[Formula: see text]nm in pristine HEA to 0.86[Formula: see text]nm in the Gr-coated HEA, signifying its hindrance to the elastoplastic transition. Gr coating suppresses dislocation nucleation at first, but eventually stimulates dislocation formation at greater depths, resulting in a higher dislocation density in Gr-coated HEA, which contributes to increased material strength. Additionally, the post-indentation surface morphology of the Gr-coated HEA substrate displays reduced pile-up formation due to the restraining action of the Gr coating. The findings of this study provide a comprehensive understanding of the deformation mechanism in the Gr-coated Al[Formula: see text]CoCrFeNi HEA and its potential implications for surface engineering.

Effect of strain amplitude and confining pressure on the velocity and attenuation of <i>P</i> and <i>S</i> waves in dry and water-saturated sandstone: an experimental study

In rock physics, much attention has been paid to the study of the processes of strain of natural materials at small strains. Experiments using high-precision measurements have allowed new knowledge at micro/nano level to be acquired. The microplasticity of solids is studied in materials science, but there is also data regarding some rocks. The property of microplasticity of natural materials is still little studied. The study was carried out on rock samples. The effect of strain amplitude and confining pressure on the velocity and attenuation of P and S waves in dry and water-saturated sandstone has been studied. The method of reflected waves was used in the frequency range of 0.5–1.4 MHz at four strain amplitudes (0.5–1.67)·10−6 Amplitude cycling causes an open and closed hysteresis effect for wave velocity and attenuation. This has been observed for both dry and water-saturated sandstone. The hysteresis loop overlaps in both states. The amplitude changes in the velocity of P-wave in dry sandstone is 1.12 %, and the attenuation of P-wave in dry sandstone is 5.43 %. As for S-wave, its maximum attenuation in dry sandstone reaches 8.81 %. The behavior of a wave velocity and attenuation can be explained by the combined effect of viscoelasticity and microplasticity. Elastoplastic transition strongly depends on the details of the microstructure, its defectiveness, and other parameters. The characteristics of the complications of wave parameters can be the signs of the internal structure of the subject.

Effect of Temperature on the Kinetics of Localized Plasticity Autowaves in Lüders Deformation

The paper analyzes the elastoplastic transition in Fe–0.025 wt. % C at a temperature of 296–503 K and strain rate of 6.67·10−6–3.33·10−3 s−1. The analysis shows that the lower yield stress increases by a power law with increasing the strain rate, and that its rate sensitivity decreases linearly with increasing the test temperature. At temperatures lower than 393 K, the rate sensitivity of the lower yield stress is normal, and at 393–503 K, it is zero. In the range 393–503 K, the kinetics of the Lüders bands is changed from steady to discrete, and the higher the strain rate, the higher the temperature of this transition. Using the available data on the dynamics of dislocations and diffusion of interstitial impurities in the test alloy, it is demonstrated that the kinetics of Lüders bands are controlled by the effect of dynamic strain aging. If the arrest time of mobile dislocations tw at barriers which are overcome via thermal activation is comparable with the precipitation time of interstitial atoms ta at these dislocations, the motion of a Lüders band is discrete, and the band represents an excitation wave of localized plasticity; its refractory period is determined by the time of dynamic strain aging. If ta >> tw, the band moves monotonically and represents a switching autowave. The results of the analysis suggest that the effect of serrated yielding at the lower temperature boundary of blue brittleness can be suppressed by increasing the strain rate. When the arrest time of dislocations tw decreases, the comparability of tw and ta is broken, and no excitation autowave is formed. The data reported in the paper can be used to develop warm rolling technologies for materials with a sharp elastoplastic transition.

Modeling for predicting triaxial mechanical properties of recycled aggregate concrete considering the recycled aggregate replacement

To theoretically investigate the triaxial mechanical behavior of recycled aggregate concrete (RAC), four series of samples, a C30 conventional concrete (CC) and three RACs with different recycled aggregate (RA) replacement ratios (20 %, 50 % and 80 %), were prepared. The triaxial compression tests with 0, 2, 4 and 8 MPa confining pressures were performed. Experimental results showed that the difference in failure stress decreased from 30.0 % to 2.7 % with increasing the confining pressure for the four series of concretes. This means that for failure stress, the influence of RA replacement ratios on the low confining pressure condition should be taken into account while its effect trends to decrease or even can be neglected with increasing the confining pressure. Furthermore, the elastic properties and the evolution of plastic characteristics were also related to the RA replacement ratio. Based on the test results, this study develops a modified elastoplastic model to accurately predict the triaxial mechanical behavior of RAC. The proposed non-integer high order failure criterion can better reflect the confining pressure sensitivity, while the associated flow rule can well describe the elastoplastic transition with increasing the confining pressure. Moreover, the influence of RA replacement was analyzed, and the parameters related to RA replacement ratios were considered in this model. Comparing the numerical simulations with the experimental results, it was concluded that the present modeling can well predict the main mechanical behavior of both CC and RACs by introducing RA dependent parameters.

Atomistic tensile deformation mechanisms in a CrCoNi medium-entropy alloy with gradient nano-grained structure at cryogenic temperature

Large-scale molecular dynamics (MD) simulations have been utilized to reveal the atomistic deformation mechanisms of a CrCoNi medium-entropy alloy (MEA) with gradient nano-grained (GNG) structure in the present study. Strong strain hardening was observed in the gradient nano-grained structure at the elasto-plastic transition stage, which could be attributed to the Masing hardening. After yielding, obvious partitioning of tensile strain was detected in the gradient nano-grained structure, which indicates the existence of hetero-deformation induced (HDI) hardening effect and could account for the higher flow stress of the gradient nano-grained structure than that calculated by the rule of mixture from its homogenous nano-grained (NG) structured counterparts. Moreover, partitioning of stacking fault factor (corresponding to the partitioning of tensile strain), which demonstrates the intensity of dislocation behaviors, was also observed in the gradient nano-grained structure. The differences of factors for each grain size area were found to be smaller in the gradient nano-grained structure than those of its homogeneous nano-grained structured counterparts, which indicates the influence of forward stress and back stress on dislocation motion near the zone boundary between the hard zone with smaller grains and the soft zone with larger grains, further verifying the presence of hetero-deformation induced hardening in the gradient nano-grained structure.

Study of Elasto-Plastic Transition, Phase Transition and Equation of State of Ceramics

Study of Elasto-Plastic Transition, Phase Transition and Equation of State of Ceramics

Kinetics of Lüders deformation as an autowave process

The authors investigated the nature and kinetics of the moving fronts of localized deformation, which are formed at the elastoplastic transition in materials with dislocation and martensitic micromechanisms of plastic deformation under active tension at different velocity. Digital image correlation was used for registration and quantitative specification of front movement. Attained results were discussed under synergistic approach. A deformed subject is considered as open and far-from-equilibrium system (active medium) containing distributed potential energy source stress, which are microconcentrators. In process of external influence these concentrators relax through microslip and cause a form change of the object itself. Each microconcentrator can be considered as an active element, it has two states: metastable elastically stressed and stable relaxed. In external influence, transition is possible only from the first state to the second. Such elements are characterized as trigger elements and active medium is characterized as a bistable medium. In bistable media, switching autowaves propagate. They represent moving boundaries, which separate metastable and stable states. Within this concept considered fronts of localized deformation can be interpreted as switching autowaves. The study showed that shape and kinetic parameters of fronts of localized deformation do not depend on chemical composition, structure and micromechanisms of deformation, it confirms their autowave nature. On the other hand, the kinetics of switching autowaves should be determined by characteristics of the external influence. Genuinely, velocity of localized deformation fronts increases with deformation velocity. It is found that dependence of these fronts on deformation velocity is non-linear parabolic with index less than one and equally for all examining materials.

Characterization of elasto-plastic transition of sheet metal by using large-scale four-point bending test

The elastic properties of a material do play an important role in the spring back behaviour of sheet metal and have a dominant influence on the depth of where the bending tool has to go. In industry, the elastic behaviour is often approximated to a pure linear behaviour defined by two coefficients: Young’s modulus (E) and yield stress (Re). This modelling hypotheses result in geometrical errors in the angle and flange lengths of bended part. In order to better determine material behaviour at very small strains, a large-scale four-point bending test machine is presented in this study. Generating pure bending and measuring the residual radius allow observation of the transition between elastic and plastic deformation behaviour. By deforming the test beam in the supposed pure elastic area, it appears that the pure elasticity behaviour of the material is indeed non-existent. Plastic strains can be detected as soon as the very first bending of the sample, at very low equivalent stress well under usual accepted elastic yield stresses. Perspectives are addressed in order to solve this difficulty, which hinders the possibility of correctly simulating the bending process, particularly with regard to tight geometry requirements.

Enhanced tensile properties by heterogeneous grain structures and coherent precipitates in a CoCrNi-based medium entropy alloy

A dual heterogeneous structure with both heterogeneous grain structure and coherent L12 nanoprecipitates was obtained in a CoCrNi-based medium entropy alloy (MEA) with chemical composition of Co34.5Cr32Ni27.5Al3Ti3 (in at%). The volume fraction of L12 phase is observed to become higher and the corresponding interspacing becomes smaller after aging, resulting in a more severe heterogeneity. The unaged samples are found to have better tensile properties as compared to those for the CoCrNi MEA with homogeneous and heterogeneous grain structures. The aged samples display an even better synergy of strength and ductility than the corresponding unaged samples. The hetero-deformation-induced hardening plays a more important role in the aged samples than in the unaged samples, especially at the elasto-plastic transition stage, producing higher density of geometrically necessary dislocations for better tensile properties. Multiple deformation twins, stacking faults and Lomer-Cottrell locks are the dominant deformation mechanisms for the unaged samples, while interactions between these defects and L12 nanoprecipitates play important roles in the aged sample, the shearing hardening mechanism is observed for L12 nano-particles.

Large rotations of the grain-scale stress tensor during yielding set the stage for failure

The stress-state within individual grains in a polycrystal determine the fate of the aggregate including mechanical failure. By tracking the evolution of the stress tensor throughout the elastoplastic transition, large rotations of the stressstate, which have long been theorized to occur, are observed experimentally for the first time. These stress rotations (∼15°) are more than an order of magnitude larger than the concomitant crystallographic lattice reorientations (∼0.9°) well-known to occur during metal plasticity. Furthermore, these rotations are accompanied by a decrease in stress triaxiality within certain grains, promote strain softening, and set the stage for failure at an early stage of deformation. These results provide a completely new perspective through which to contemplate the question of “hot-spots” responsible for failure of high-performance structural materials.

Predicting the mechanical behaviour of a sandy clay stabilised with an alkali-activated binder

There is a growing interest in the geomechanical behaviour of low performing soils strengthened with alkali-activated materials, which have been promoted as low-carbon-footprint binders. This paper focuses on the performance of a sandy clay stabilised with NaOH-activated blast furnace slag after short (28 days) and long (90 days) curing periods. Triaxial compression experiments were conducted at a range of mean effective stresses (41 to 600 kPa) and overconsolidation ratios (1 to 12.2). The experimental data was used to calibrate a kinematic hardening constitutive model and the ability of the model to capture the behaviour of artificially stabilised sandy clay was investigated. Triaxial results carried out on the stabilised soil at both curing periods showed a behaviour resembling that observed for cement-mixed clays. The model, which had never been tested in artificially stabilised soils, successfully predicted the smooth elastoplastic transition observed on the non-stabilised soil specimens and the peak/residual shear strains and strain-softening behaviour after peak strengths in its stabilised state after 28 and 90 curing days.

Mechanical properties and deformation mechanisms of a Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy at elevated temperatures

A Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy (MEA) with a single-phase face-centered cubic (fcc) crystal structure are casted. The as-cast MEA possesses remarkable work hardening ability, yield strength (σy), ultimate tensile strength (σUTS) and uniform elongation (σu) at high temperatures up to 800 °C. Up-turn of strain hardening rate after elasto-plastic transition (secondary strain hardening) is observed for the MEA during tensile deformation in the temperature range of 25-800 °C. A high density of dislocation forests with solute atmospheres in concentrated solid solution is considered sufficient for back stress strengthening, thus causing the up-turn of strain hardening rate. Post-mortem microstructural analysis reveals that the temperature effects on mechanical properties of the MEA are closely related to dislocation structures, dislocation densities and dynamic-strain aging (DSA). With increasing deformation temperature from 25 °C to 800 °C, the dislocation density undulates and the fraction of screw dislocations increases. Strong solute pinning effect in conjunction with dominant forest strengthening mechanism (indicated by negative strain rate sensitivity and large activation volume) boost the strain hardening rate at the temperatures from 400 ℃ to 700 ℃. At 800 °C, the dominant deformation mechanism changes from the forest dislocation cutting mechanism to dislocation cross-slip with increasing strain, resulting in the sharp up-turn of strain hardening rate. On the other hand, DSA poses plastic instability to deteriorate the strain hardening of the MEA. These phenomena are considered to have material impacts on mechanical properties and fracture mechanisms of MEAs and high-entropy alloys at high temperatures.

Effect of Deformation Rate on the Elastic-Plastic Deformation Behavior of GH3625 Alloy

GH3625 alloy is a typical polycrystalline material. The mechanical properties of a crystal within the alloy depend on the single crystal properties, lattice orientation, and orientations of neighboring crystals. However, accurate determination of single crystal properties is critical in developing a quantitative understanding of the micromechanical behavior of GH3625. In this study, the effect of deformation rate on the elastoplastic deformation behavior of GH3625 was investigated using in situ neutron diffraction room-temperature compression experiments, EBSD, and TEM. The results showed that the microscopic stress–strain curve included elastic deformation (applied stress, σ ≤ 300 MPa), elastoplastic transition (300 MPa 350 MPa) stages, which agreed with the mesoscopic lattice strain behavior. Meanwhile, the deformation rate was closely related to the crystal elastic and plastic anisotropy. The results of the lattice strain, peak width, and intensity reflected by the specific hkl showed that the deformation rate had little effect on the elastic anisotropy of the crystal, but had a significant effect on the plastic anisotropy of the crystal. With the increase in the deformation rate, the high angle grain boundaries gradually changed to the low angle grain boundaries, and the proportion of twin boundaries gradually reduced. Also, the grains transformed from uniform deformation to nonuniform deformation. Moreover, with the increase in deformation rate, the total dislocation density (ρ) of the alloy first decreased and then increased, whereas the geometrically necessary dislocation density (ρGND) monotonically increased, and the statistically stored dislocation (SSD) density (ρSSD) monotonically decreased. Meanwhile, the abnormal work hardening behavior of the sample at a deformation rate of 0.2 mm/min was mainly related to the SSD generated by uniform deformation. Additionally, the contribution of dislocation strengthening and TEM observation confirmed that the dominant deformation of GH3625 was dislocation slip, and its work hardening mechanism was dislocation strengthening.

Molecular dynamics simulation of microstructure evolution during the fracture process of nano-twinned Ag

In this paper, the evolution of microstructure and the role of twin boundary during the fracture process of nano-twinned Ag are studied through molecular dynamics simulation. The fracture process of nano-twinned Ag with multiple twin boundaries is compared with both the fracture process of single crystal Ag and nano-twinned Ag with single twin boundary. Results demonstrate that multiple twin boundaries can significantly improve the strength of material by influence the propagation of crack and dislocation as observed by experiments. By comparing the stress–strain relationship and evolution of microstructure, we found that the elastoplastic transition in single crystal Ag is characterized by dislocation emission from the crack tip, while the elastoplastic transition in nano-twinned Ag is characterized by dislocation crossing the twin boundary. According to crack-resistance curves and observation results of microstructure evolution, multiple twin boundaries can significantly improve the fracture toughness of nano-twinned Ag compared to single twin boundary. The role of twin boundary in the fracture process is mainly to influence the propagation of crack and dislocation. The crack tip in single crystal Ag gradually gets blunt with the nucleation and emission of dislocations, while the crack in nano-twinned Ag propagates by cleavage. To explain this phenomenon, the generalized stacking fault energies and surface energies of both nano-twinned Ag and single crystal Ag are calculated and compared. It is shown that twin boundary can influence the energy barrier for atomic slip while have no influence on the generation of fracture surface. Twin boundary can also change the propagation direction of dislocations and stacking faults. Stacking faults propagate along (111) planes in matrix grain and propagate along (111¯) planes in twin grain, showing a tortuous shape. In this process, dislocations accumulate on the twin boundary, leading to high local dislocation density and high strength of nano-twinned material, which is agreement with experiments and simulations. Our study is helpful for understanding the relationship between microscopic deformation and macroscopic properties of nano-twinned crystal material.

Elastoplastic transition in a metastable β-Titanium alloy, Timetal-18 – An in-situ synchrotron X-ray diffraction study

The elastoplastic transition of a metastable β-Ti alloy, Timetal-18, is studied using in-situ high energy synchrotron X-ray diffraction microscopy (HEDM). The measured evolutions of the complete elastic strain (and stress) tensor(s), resolved shear stress, lattice rotation and rotation of the stress state of the grains are compared with the predictions of the elasto-viscoplastic Micromechanical Analysis of Stress-Strain Inhomogeneities with fast Fourier transform (MASSIF) code instantiated with an experimentally measured microstructure which matched that of the sample. The preferred glide plane of dislocations with ½<111> Burgers vectors of the BCC alloy was explored. It was found that the polycrystalline stress-strain response could be equally well described by any of the candidate glide planes or combinations thereof (i.e., pencil glide). However, simulations involving slip on {112} planes yielded a marginally better description of the individual grain-level responses, as compared to the simulation involving only the {110} planes. The small (typically <1°) crystallographic reorientations that the grains undergo during the elastoplastic transition, are insufficient to permit discrimination between candidate slip modes. The resolved shear stress (RSS) distributions showed a sharp increase in skewness around macroscopic yield and it was found that the hardening during the elastoplastic transition is primarily due to intergranular interactions. Analysis of “hard” and “soft” grains suggests non-Schmid effects may be present, even in these low strain rate, room temperature experiments. Finally, examination of the individual responses revealed “strain softening” in some of the grains. Intragranular heterogeneity in the orientation and stress state are highlighted as important areas for future investigations, which may reveal answers to unresolved questions in this research.