Dominant Plastic Deformation Mechanism Research Articles

Molecular dynamics simulation of deformation behavior of nanocrystal CuNiAlCo medium-entropy alloys

Applying the molecular simulation method, the melting, elastic, and plastic properties of nanocrystal CuNiAlCo medium entropy alloy are investigated in this work. The results show that the melting point in nanocrystal CuNiAlCo is decreased with decreasing crystallite size from 5.0 to 3.1 nm. Compared to the bulk modulus, the crystallite size has a discernible influence on both the shear and Young's modulus. The uniaxial tension calculations suggest that the tensile strength decreases with decreasing crystallite size, and the inverse Hall–Petch relationship is found in nanocrystal CuNiAlCo. The polyhedral template matching analysis illustrates that the FCC constitution of CuNiAlCo gradually decreases with the decrease in crystallite size. When the strain is 30%, the amorphization would occur in uniaxial tension. The atomic structural evolution of CuNiAlCo with the crystallite size of 5.0 nm under the uniaxial strain verifies that the stacking fault reconstruction and amorphization are the dominant plastic deformation mechanism for nanocrystal CuNiAlCo medium entropy alloy. The shockley dislocation plays an essential role in uniaxial tension.

Tensile deformation behavior of high-strength TA18 titanium alloy tube under warm forming conditions and constitutive model based on dislocation density

Understanding the deformation behavior of high-strength TA18 titanium alloy tube (HS-TA18 tube) under warm forming conditions and the accurate description of flow stress is the basis of tube bending forming research. To this end, in this study, the HS-TA18 tube of 18 mm × 1.5 mm (out diameter × wall thickness) as the objective, uniaxial tensile tests were conducted under a wide temperature range (25 ℃−500 ℃). Then the macroscopic mechanical behavior and microstructure evolution were analyzed. The results show that the flow stress decreases with the increase of temperature. In the temperature range of 200–450 ℃, the degree of decline slows down. Moreover, the effect of strain rate on flow stress is not obvious, which indicates that dynamic strain aging (DSA) occurs in this region. Dislocation slip is the dominant mechanism of plastic deformation in the studied temperature range. Although dynamic recrystallization at 500 ℃ leads to softening trend of flow stress, dynamic recovery (DRV) is still the main softening mechanism during deformation. Then, a constitutive model based on dislocation density is established, which includes common thermal and athermal stresses. In addition, back stresses associated with geometrically necessary dislocations (GNDs) and additional stress caused by DSA are also taken into account. The model is used to predict the flow stress of the HS-TA18 tube under warm forming conditions. The prediction results are in good agreement with the experimental results, indicating that the established model has excellent prediction ability.

Atomic insight into mechanical behavior of AuPt alloys

AuPt alloys are widely applied in electronics, aerospace, and precision equipment fields. A thorough understanding of the mechanical behavior of AuPt alloys is critical for their potential applications. Here, we leverage molecular dynamics (MD) method to for the first reveal the effects of microstructure and temperature on mechanical behavior of AuPt alloys from atomic level. The nanoindentation of monocrystalline (MC), nanotwinned (NT), and nanocrystalline (NC) AuPt alloys are performed. The results demonstrate that the mechanical properties of the MC AuPt alloy are excellent at 77 K because of dislocation strengthening while poor at high temperature due to amorphous behavior. The introduction of twinning boundary (TB) greatly improves the mechanical properties of AuPt alloy by preventing the stress spread and promoting dislocation activity. The mechanical behavior of NC AuPt alloy is strongly dependent on grain size. With increasing average grain size, the dominant plastic deformation mechanism of NC AuPt alloy changes from GB migration to dislocation motion and then to GB destruction. An appropriate grain size should be selected to inhibit GB migration or GB destruction, thereby achieving the enhancement of NC AuPt alloy through dislocation strengthening. This work provides an important theoretical basis for the subsequent design of AuPt alloys with desirable mechanical performances to extend the application prospects of noble metals.

Atomistic study on the nano-scratch mechanism of CoCrFeMnNi high-entropy alloy at different morphology densities

The physical nature of the scratch behavior of CoCrFeMnNi HEA and its deformation mechanism at different morphology densities are investigated by molecular dynamics simulations. The results show that the groove morphology contributes to the reduction of surface plastic deformation and exhibits a friction-reducing effect. As the morphology density decreases, the surface deformation and atom pile-up decrease, and the plastic deformation in the scratch region decreases, resulting in a further enhancement of the friction reduction effect. The increase of scratch depth intensifies the plastic deformation of the specimens, and the average scratch coefficient of friction increases with the increase in scratch depth. The dominant plastic deformation mechanism in the scratch deformation of CoCrFeMnNi HEA with different morphology densities is the slip deformation of Shockley partial dislocations. The MD simulations are verified further by qualitatively comparing them with corresponding experimental observations of CoCrFeMnNi HEA.

On cyclic plasticity of nanostructured dual-phase CoCrFeNiAl high-entropy alloy: An atomistic study

In this study, we have employed molecular dynamics simulations to examine plastic deformation mechanisms of a “supra-nanometer-sized dual-phase glass-crystal” (SNDP-GC) high-entropy alloy (HEA) composite under cyclic loadings. This composite is produced by embedding CoCrFeNi HEA crystalline grains into a softer CoCrFeNiAl2 HEA glass. Cyclic loadings of different amplitudes are applied on one polycrystalline CoCrFeNi sample and two SNDP-GC HEA samples mentioned above. For the polycrystalline sample, dislocation motion and grain boundary (GB)-mediated deformation control the plastic deformation process, and the sample loses its original structure after a few cycles of the set amplitude due to GB migration. However, for the two SNDP-GC samples, as the grain boundary is replaced by the metallic glass (MG) phase, the dominant plastic deformation mechanism has changed to concentrating shear transformation in the MG phase. Our results also show that structural stability is highly dependent on the MG phase thickness. For the sample with a thinner MG layer, MG cannot accommodate sufficient deformation—so voids are generated in it. However, for the sample with a thicker MG phase, MG can store adequate deformations, thus dislocation initiations in crystalline grains are reduced and void generation is prevented in the MG phase.

Grain boundary relaxation induced ultrastrong-and-ductile bulk pure Ni

Both strength and ductility are essential for high-performance engineering structural materials, thus great endeavors have been invested to solve the strength-ductility trade-off of them during recent two decades. Here, we utilized grain boundary (GB) relaxation to circumvent the trade-off in bulk pure Ni through optimizing grain size when literatures tells that GB relaxation can improve strength but ductility. Both tensile strength and uniform elongation of Ni were elevated from 1450 MPa to 2013 MPa and from 2.33% to 5.17%, respectively. The combination of strength and ductility extends beyond the range established by the strongest bulk pure Ni known. Our investigation unraveled that the increased strength was produced by GB relaxation which remarkably mitigated GB softening produced by GB sliding, and that partial dislocation emission from GBs became the dominant plastic deformation mechanism. The dislocation activities inside the grains increased the strain hardening rate (θ). At the same time, enhanced probability of interaction between dislocations and GBs improved the strain rate sensitivity (m). It was the GB relaxation induced dislocation activities that improved θ and m, which stabilized the plastic deformation to enhance ductility. The present work offers a foundation for the ongoing of more advanced engineering structural materials with the advisable design of GB complexion.

Calcite Deformation Twins: From Crystal Plasticity to Applications in Geosciences

E-twinning is the dominant mechanism of plastic deformation of calcite at low temperature (<300 °C), and in most limestones, e-twins are, at the crystal scale, the dominant microstructures [...]

Study on deformation mechanisms and stretch formability of zirconium alloys with different textures under biaxial tensile stress

Compared to the uniaxial stress loading of zirconium alloys, biaxial tensile stress is more complicated, and its influence on the deformation mechanisms and stretch formability of such alloys is rarely reported. In this study, uniaxial and biaxial tensile tests were performed on commercially pure zirconium with strong basal texture (BT) and weakened texture (WT) to clarify the deformation mechanisms occurring under different stress states by analyzing the microstructures, textures, and Schmid factors. The prismatic slip was found to be the dominant plastic deformation mechanism under uniaxial tensile stress, while it was suppressed under biaxial tensile stress in BT, and pyramidal slip was activated. Because the orientations of WT grains were dispersed, some of the grains under biaxial stress underwent both prismatic slip and tensile twinning to coordinate deformation, which improved the room-temperature stretch formability of WT, despite its low uniaxial elongation.

Three-dimensional microstructure and texture evolution of Ti35 alloy applied in nuclear industry during plastic deformation at various temperatures

Microstructure and texture evolution of Ti35 alloy, which was mainly applied in spent nuclear fuel reprocessing industry, during plastic deformation at various temperatures, was systematically investigated by means of electron microscopy. During plastic deformation at room temperature, the alpha colonies were severely fragmented or elongated viewed along three directions, while containing strong texture of [0001]α//ND. Interestingly, in the subsequent annealing processes, the grains viewed along normal direction (ND) transformed into brick-like morphology with rectangular grain boundaries, while the intensity of texture decreased obviously. At the same time, the grains were equiaxed viewed both along rolling direction (RD) and transverse direction (TD). By contrast, Ti35 alloy went through hot rolling contained numerous 85°<1‾21‾0> tensile and 64°<11‾00> compressive twinning, especially viewed along ND, indicating that deformation twinning, together with dislocation slipping, was the dominant plastic deformation mechanism of Ti35 alloy during hot rolling. After annealing at 923K, these twinning were completely recrystallized and led to an equiaxed microstructure with {0001}α planes perpendicular with ND. Micro-hardness and room temperature tensile tests revealed that deformation twinning played beneficial roles in mechanical property of Ti35 alloy by improving its yield strength as well as elongation.

Breakthrough the strength-ductility trade-off in a high-entropy alloy at room temperature via cold rolling and annealing

Simultaneous strength–ductility enhancement of a high-entropy alloy via cold rolling and annealing is difficult. Cold rolling and annealing were performed on an as-cast Fe35Co21Ni6Cr18Mn20 high entropy alloy (HEA), the microstructure showed single phase face centered cubic (fcc) solid solution without phase transformation and the grains were fully recrystallized accompanied by additional grain growth. The grain size decreased significantly from 179.4 ± 15.3 μm to 15.2 ± 1.4 μm. A large number of annealing twins and twin boundaries (TBs) (fraction up to 44.2%) were observed. The dislocation slip and twinning were observed as the dominant plastic deformation mechanism. Meanwhile, dislocations plugging was also seen near the twin boundaries. Tensile tests revealed for the first time that the rolling-annealing HEA overcame the strength-ductility trade-off because the yield strength dramatically increased from 516.7 ± 3.4 MPa to 725.4 ± 4.7 MPa and ductility retained excellent (from 0.51 ± 0.01 to 0.53 ± 0.01) as compared with as-cast HEA, this behavior was attributed to the grain refinement and substantial annealing twins including transgranular annealing twins and suspened annealing twins.

Deformation mechanisms of zirconium alloys under biaxial tension at room temperature

Even though the plastic deformation behavior of zirconium alloy under uniaxial loading has been well understood, the plastic deformation mechanisms under biaxial stress state are rarely reported. In this study, the stress-strain responses of zirconium alloy plate after uniaxial tension and various biaxial tension paths are investigated. The texture and grain boundary misorientation are analyzed to clarify the plastic deformation mechanism at different biaxial stress states. It is found that while prismatic slip is the dominant plastic deformation mechanism under uniaxial loading condition, the plastic strain under biaxial loading is accommodated predominantly by tensile twinning and supplemented by slip.

Microscopic mechanism exploration and constitutive equation construction for compression characteristics of AZ31-TD magnesium alloy at high strain rate

Uniaxial compression tests were conducted with strain rates in the range of 0.001–2000 s−1 at room temperature for transverse direction (TD) of AZ31 magnesium hot-rolled sheet alloy. The true stress-true strain curves were characterized by inflection points between convexity and concavity. The deformation mechanism of AZ31-TD was systematically discussed by combining Schmid Factor (SF), critical resolved shear stress (CRSS), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The results showed that the dominant mechanism of plastic deformation before inflection point of true stress-true strain curve was {101‾2} tension twinning, while the dominant mechanism of plastic deformation after inflection point was non-base slip. Based on the deformation mechanism of AZ31-TD analysis, a modified JC model was proposed to predict the true stress-true plastic strain curves of AZ31-TD magnesium hot-rolled sheet alloy, which agreed well with the measured curves. This solved the difficult problem of predicting the change of curve shape caused by tension twinning in the initial stage of plastic deformation.

Gradient structure regulated plastic deformation mechanisms in polycrystalline nanotwinned copper

Molecular dynamics simulations are performed to investigate the effects of grain size gradient and twin thickness gradient on uniaxial tensile deformation behaviors of gradient-structured polycrystalline copper. Simulation results reveal that there exist an inverse Hall–Petch effect and gradient structure regulated plastic deformation mechanisms. That is, the average flow stress of the gradient nanograined (GNG) model or the gradient nanotwinned (GNT) model decreases with decreasing the grain size or twin boundary (TB) spacing, exhibiting the breakdown in the Hall-Petch relationship when the grain size or TB spacing is smaller than a critical size. The dominant plastic deformation mechanism is found to transform from the grain boundary (GB)-mediated one to the dislocation-based counterpart with the increase of the applied strain in the GNG model. The TB migration and annihilation dominate the plastic deformation in the grains with small TB spacing; while in the grains with large TB spacing, the dislocation multiplication and cutting across TB are mainly responsible for accommodating the deformation in the GNT model. Furthermore, the gradient distribution of strain and incompatibility of deformation induced by GB or TB gradient distribution are observed by monitoring the microstructural evolution.

Lattice strain and texture development in coarse-grained uranium – a neutron diffraction study

A neutron diffraction study of deformation in a cast uranium has been conducted for the first time. Lattice-scale plasticity in this coarse-grained material initiates at a lower stress than in a previous study of fine-grained material in the literature. This is attributed to a combination of larger thermal residual stresses in the coarse-grained material and the Hall-Petch effect making twinning easier in large grains. Asymmetry between the tensile and compressive response shows that twinning is the dominant plastic deformation mechanism at low strains. Axial texture changes for the cast uranium were calculated by post processing of the full diffraction spectra, which shows that lattice rotations associated with twinning occurred at yield. This lattice rotation was observed to disappear after unloading, which indicates that de-twinning can occur in uranium. ©British Crown Owned Copyright 2018/AWE

Grain-scale Stress and GND Density Distributions around Slip Traces and Phase Boundaries in a Titanium Alloy

For TA15 titanium alloy, slip is the dominant plastic deformation mechanism because of relatively high Al content. In order to reveal the grain-scale stress field and geometrically necessary dislocation (GND) density distribution around the slip traces and phase boundaries where the slip lines are blocked due to Burgers orientation relationship (OR) missing. We experimentally investigated tensile deformation on TA15 titanium alloy up to 2.0% strain at room temperature. The slip traces were observed and identified using high resolution scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) measurements. The grain-scale stress fields around the slip traces and phase boundaries were calculated by the cross-correlationbased method. Based on strain gradient theories, the density of GND was calculated and analyzed. The results indicate that the grain-scale stress is significantly concentrated at phase/grain boundaries and slip traces. Although there is an obvious GND accumulation in the vicinity of phase and subgrain boundaries, no GND density accumulation appears near the slip traces.

Response of oxide nanoparticles in an oxide dispersion strengthened steel to dynamic plastic deformation

The behavior of oxide nanoparticles in an oxide dispersion strengthened (ODS) steel subjected to dynamic plastic deformation (DPD) was investigated by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Contrary to the motivation for dispersing oxides in a ferritic matrix that the hard particles would be non-deformable and constitute obstacles of plastic deformation, it is discovered that oxide nanoparticles with sizes smaller than 20 nm were appreciably deformed to an average equivalent strain of 1.2 in the sample after DPD to a strain of 2.1. The plastic distortion of the oxide nanoparticles by compression increases with the externally applied strain. HRTEM analysis demonstrates that deformation twinning is the dominant mechanism of plastic deformation for the oxide nanoparticles. In addition, experimental results show that the deformation of oxide nanoparticles does not only occur at high strain rates, but also at lower strain rates, and does not rely on the interfacial coherency between the oxide nanoparticle and the ferritic steel matrix. Due to the incompatible deformation between the oxide nanoparticles and matrix, nanoscale voids form at their interface during deformation at low strains, and evolve with increasing deformation in distinctively different manner around larger and smaller particles. The reasons for the size effect on the deformation of oxide nanoparticles and on the co-deformation between oxide nanoparticles and ferritic matrix in the ODS steel are discussed.

Atomistic simulation of tension-compression asymmetry and its mechanism in titanium single-crystal nanopillars oriented along the [1 1 [formula omitted] 0] direction

Molecular dynamics simulations were performed with a Finnis-Sinclair many-body potential to investigate the mechanical properties and deformation mechanisms under applied uniaxial tensile and compressive loads in α-titanium (Ti) single-crystal nanopillars oriented along the 〈 1 1 2¯ 0 〉 direction. The results indicate that the mechanical properties and plastic deformation mechanism display tension–compression asymmetry. The non-linear elastic behavior is attributed to the difference in friction between the neighboring atomic planes at the elastic deformation stage under push and pull loading conditions. Increasing the friction leads to hardening with compression. Decreasing the friction leads to softening with tension. Increasing the friction may also lead to higher yield stress with compression compared with tension. Perfect nanopillars are yielded via the nucleation and propagation of {1¯ 0 1 0} 〈1 2¯ 1 0 〉 dislocations on the surface and corners of the nanopillars. Prismatic slip is the dominant mode of plastic deformation in the Ti nanopillars under compressive loading. However, {1 0 1¯ 1} 〈1 0 1¯2¯〉 twinning is the dominant plastic deformation mechanism together with prismatic slip under tensile loading. Two typical intrinsic stacking faults (SFs) with different propensities exist in the nanopillars. The microstructural evolution of the SFs was also simulated.

Mechanical properties of ultrafine-grained AX41 magnesium alloy at room and elevated temperatures

Mechanical properties of ultra-fine grained AX41 magnesium alloy were investigated at ambient and elevated temperatures. The study focuses on the effect of the grain size, dislocation density and crystallographic texture on the strength and formability. Commercial AX41 magnesium alloy was processed by hot extrusion followed by eight passes of equal-channel angular pressing at temperatures of 220 and 250 °C. Material was tested in tension in the direction parallel to the ECAP processing at temperatures up to 200 °C. In this temperature range, the ultra-fine grained structure is thermally stable. The strength of the material at room temperature is significantly affected by texture. In particular, basal texture component formed in ECAP samples is tilted by 40° from the processing direction and therefore facilitates plastic deformation by basal slip. In contrast, the texture of extruded material is not suitable for basal slip deformed in the same direction. The effect of texture on strength can outweigh the effect of reduced grain size and increased dislocation density in ultrafine-grained AX41 alloy. At elevated temperatures, starting from 100 °C, the deformation behaviour is controlled simultaneously by dislocation slip and grain boundary sliding (GBS). Mechanical properties of the as-cast coarse grained material are virtually independent of the temperature. On the other hand, the reduced grain size of UFG material caused the progressive decrease of the proof stress and, simultaneously, considerably improves the ductility. The study systematically shows that the effect of microstructural features on mechanical properties of coarse grained and UFG AX41 alloy strongly depends on the dominant mechanism of plastic deformation.

Hetero interface and twin boundary mediated strengthening in nano-twinned Cu//Ag multilayered materials

Based on molecular dynamics simulations, tensile mechanical properties and plastic deformation mechanisms of nano-twinned Cu//Ag multilayered materials are investigated in this work. Simulation results show that, due to the stronger strengthening effect of the twin boundary than both the cube-on-cube and hetero-twin interfaces between Cu and Ag layers, the strength increases with the increase of layer thickness for nano-twinned Cu//Ag multilayered materials with a constant twin spacing, while it decreases with the increase of layer thickness for twin-free ones. The strength of hetero-twin multilayered materials is higher than that of the cube-on-cube samples due to the different hetero interfacial configurations. The confined layer slip of dislocation is found to be the dominant plastic deformation mechanism for twin-free hetero-twin multilayered materials and the strength versus twin spacing in nano-twinned samples follows the conventional Hall–Petch relationship. These findings will shed light on the understanding of the plastic deformation mechanisms and the fabrication of high strength nano-twinned multilayered metallic materials.

Shock Response of Commercial Purity Polycrystalline Magnesium Under Uniaxial Strain at Elevated Temperatures

In the present paper, results of plate impact experiments designed to investigate the onset of incipient plasticity in commercial purity polycrystalline magnesium (99.9%) under weak uniaxial strain compression and elevated temperatures up to melt are presented. The dynamic stress at yield and post yield of magnesium, as inferred from the measured normal component of the particle velocity histories at the free (rear) surface of the target plate, are observed to decrease progressively with increasing test temperatures in the range from 23 to 500 °C. At (higher) test temperatures in the range 500–610 °C, the rate of decrease of dynamic stress with temperature at yield and post-yield in the sample is observed to weaken. At still higher test temperatures (617 and 630 °C), a dramatic increase in dynamic yield as well as flow stress is observed indicating a change in dominant mechanism of plastic deformation as the sample approaches the melt point of magnesium at strain rates of ~105/s. In addition to these measurements at the wavefront, the plateau region of the free surface particle velocity profiles indicates that the longitudinal (plastic) impedance of the magnesium samples decreases continuously as the sample temperatures are increased from room to 610 °C, and then reverses trend (indicating increasing material longitudinal impedance/strength) as the sample temperatures are increased to 617 and 630 °C. Electron back scattered diffraction analysis of the as-received and annealed pre-test magnesium samples reveal grain coarsening as well as grain re-orientation to a different texture during the heating process of the samples.