Change In Dislocation Density Research Articles

Molecular dynamics simulation of shock-induced plastic deformation and spallation behavior of Cu/Cu64Zr36 crystalline/amorphous composites

Molecular dynamics (MD) simulations were performed to study the shock-induced plastic deformation and spallation failure of Cu/Cu64Zr36 crystalline/amorphous composites with a pre-existing void. The results show that the pre-existing void collapses always perpendicular to the direction of the shock loading, regardless of whether the shock direction starts from the crystalline phase or the amorphous phase. The Cu/Cu64Zr36 composites are more prone to spallation failure when the shock starts from the Cu crystalline phase. The changes of dislocation density and shear transformation zone (STZ) activation are closely related to the magnitude and direction of shock velocity. When the shock velocity reaches 2.0 km/s, dislocations in the crystalline phase disappear, dislocation density is close to zero and STZs activate throughout the entire amorphous phase. In addition, regardless of the shock velocity and direction, no shear bands are generated in the Cu/Cu64Zr36 composites under shock loading, which is significantly different from the case of tensile loading.

Improvement of gradient microstructure and properties of wire-arc directed energy deposition titanium alloy via laser shock peening

Wire-arc directed energy deposition (WADED) technology has been widely used in the remanufacturing of titanium alloy structural components benefited from with the advantages such as high deposition efficiency and low cost. However, due to the coarse and anisotropic microstructure, the complex internal stresses and processing-induced rough surface significantly reduce fatigue performance and reliability of the remanufactured structural components. In this work, surface modification of titanium alloy WADED repair component was carried out via laser shock peening (LSP), and its gradient structure, microhardness, residual stress and fatigue performance and enhancement mechanism were systematically investigated. Results indicated that the different microstructure of each region led to different responses under the action of LSP, which was related to the change of dislocation density. LSP induced crystal defects such as high-density dislocations, twins and stacking faults on the surface. A variety of crystal defects gradually decreased with the depth from the strengthened surface, formed a gradient microstructure and significantly affected the microhardness and residual stress of the repaired components. The surface hardness and compressive residual stress of the repaired components were greatly increased after LSP and the hardened layer and compressive residual stress depth affected layer were 600 μm and 800 μm, respectively. The average fatigue life of the additive repair component increased by 197 % under the synergistic effect of compressive residual stress and gradient microstructure.

Microstructure, mechanical properties, and deformation behaviour of LPBF 316L via post-heat treatment

ABSTRACT The study focused on analysing the changes in dislocation density and elemental segregation at cellular substructures, as well as the transformation of nano-oxide inclusions at different post-treatment temperatures, and their impacts on the strengthening mechanisms of 316L processed by laser powder bed fusion (LPBF). Additionally, through quasi in-situ tensile experiments, the plastic deformation and fracture behaviours of LPBF 316L after annealing at 900 °C were studied, revealing the reasons behind its high ductility. The results indicated that with increasing annealing temperatures, the nano-oxide inclusions coarsened and their density decreased due to the Ostwald ripening mechanism. The coarsened oxide particles act as barriers to moving dislocations and grain boundaries, thereby prolonging the recovery and recrystallization processes. This resulted in cellular substructures exhibiting high thermal stability. Consequently, the ultimate tensile strength of LPBF 316L after annealing at 900 °C is 651 MPa, with a total elongation of 62.3%, surpassing other studies.

Microstructure and properties of Al-4.8Cu-0.45Mn-0.19Cd-0.18Ti-0.17Zr-0.14V aluminum alloy extrusion bar

The mechanical properties, texture transformation, and mechanism of Al–Cu–Mn alloy during annealing were studied using three-dimensional orientation distribution function (ODF), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results showed that the increase in yield strength after annealing at 200 °C is due to the decrease in the proportion of P and M textures and the increase in the proportion of Goss texture. The increase in the change rate of mechanical properties observed during annealing at 250 and 350 °C can be attributed to the higher proportion of M and P textures and the reduced proportion of Goss textures. Among them, the proportion of M texture increased significantly, and the Goss texture decreased significantly after annealing at 350 °C. This is because the structural energy density (Ev) value of the Goss texture is higher than that of the M texture at 250 °C and 350 °C. The Goss texture is in an unstable state of high energy order. The deformed Goss texture is unstable at high temperatures, leading to conversion into M and other textures. The EBSD and TEM analysis revealed that the hardness is mainly related to the change in dislocation density. When the annealing temperature is increased from 250 to 350 °C, the hardness is significantly reduced from 62 HV to 52 HV, and the dislocation density is reduced from 4.0 × 1014 ρ/m−2 to 2.4 × 1014 ρ/m−2.

Investigating the dielectric and ferromagnetic behaviour of Ni and Co dual doped ZnO for diluted magnetic semiconductor

Herein, Ni/Co co-doped ZnO nanoparticles (NPs) were successfully prepared via chemical co-precipitation process. The structural, purity and crystallite size of as-prepared particles were confirmed by using x-ray diffraction (XRD). The results revealed the hexagonal wurtzite structure of ZnO without any impurity phase of as-prepared samples. The crystallite size decreased from 34.9 to 9.6 nm, whereas lattice strain reduced to 0.00086. The change in crystallite size, lattice parameters, strains and dislocation density further indicates the successful incorporation of dopants in ZnO host lattice. The dielectric properties, ac conductivity, and magnetic behaviour were steadily examined. The dielectric constant has been found to be 1070 with 5wt% at low frequency and become constant at higher frequency. It was revealed that dopant additions lead to attain high dielectric constant and low loss possibly attributed to interface polarization, and dipole polarization due to oxygen vacancy defects. M-H measurement observed that ferromagnetism at room temperature pure and doped ZnO samples. The ferromagnetic behaviour of pure ZnO NPs with Ms = 0.17 emu g−1, Mr = 0.11 emu g−1 and Hc = 67 which was increased to Ms = 0.85 emu g−1 and Mr = 0.65 emu/g for 5wt% Co. This improvement with increasing Co concentration indicates that oxygen defects may stabilize the ferromagnetic order. These interesting features encouraging to use this material for ultrahigh dielectric materials and spintronics.

Gradient residual stress evolution and its influence on fatigue life under combined carburising heat treatment and ultrasonic surface rolling process

This study combined carburising heat treatment (CHT) with the ultrasonic surface rolling process (USRP) to improve the rotary bending fatigue properties of 18CrNiMo7-6 alloy steel. Based on the experimentally obtained fatigue limit (σ-1), the evolution of gradient compressive residual stress (CRS) of CHT- and CHT + USRP-treated specimens was analysed below the fatigue limit stress level. Subsequently, the evolution mechanism of gradient CRS of the two reinforced specimens was discussed and analysed from the opposing effects of austenite transformation and dislocation density change on the evolution of CRS. The results demonstrated that the selective bending fatigue performance of the alloy was evidently improved after the introduction of CRS; however, the change in fatigue limit was not evident. The durability under other loads (such as cyclic tension/compression and torsion loads) needs further study. The gradient CRS evolution of CHT specimens was mainly caused by the large residual austenite transformation and small dislocation density change. Therefore, the volume expansion caused by residual austenite transformation gradually increased the gradient compression residual stress of CHT specimens with an increased number of cycles. Contrarily, the degree residual stress evolution of CHT + USRP specimens was more affected by the dislocation density, and the gradient compression residual stress gradually decreased with an increased number of cycles. Finally, the residual stress evolution rate of the two reinforced specimens was divided into two stages: fast and slow.

Dislocation Density-Mediated Functionality in Single-Crystal BaTiO3.

Unlike metals where dislocations carry strain singularity but no charge, dislocations in oxide ceramics are characterized by both a strain field and a local charge with a compensating charge envelope. Oxide ceramics with their deliberate engineering and manipulation are pivotal in numerous modern technologies such as semiconductors, superconductors, solar cells, and ferroics. Dislocations facilitate plastic deformation in metals and lead to a monotonous increase in the strength of metallic materials in accordance with the widely recognized Taylor hardening law. However, achieving the objective of tailoring the functionality of oxide ceramics by dislocation density still remains elusive. Here a strategy to imprint dislocations with {100}<100> slip systems and a tenfold change in dislocation density of BaTiO3 single crystals using high-temperature uniaxial compression are reported. Through a dislocation density-based approach, dielectric permittivity, converse piezoelectric coefficient, and alternating current conductivity are tailored, exhibiting a peak at medium dislocation density. Combined with phase-field simulations and domain wall potential energy analyses, the dislocation-density-based design in bulk ferroelectrics is mechanistically rationalized. These findings may provide a new dimension for employing plastic strain engineering to tune the electrical properties of ferroics, potentially paving the way for advancing dislocation technology in functional ceramics.

Modelling time-dependent relaxation behaviour using physically based constitutive framework

The application of interrupted die motion during the deformation process in a servo press cycle is known to enhance the forming limit of metallic materials. This observation is generally attributed to the transient effects associated with stress relaxation phenomenon. However, attempts to model stress relaxation behaviour using physically based constitutive formulations in the finite element method are scarce. In this work, a modified version of the Kocks–Mecking–Estrin (KME) dislocation density model is used to evaluate the substructural changes during relaxation. An ‘uncoupled’ elasto-viscoplastic approach is employed to accommodate the plastic strain rate effect. This modelling approach is implemented in the commercial finite element software via a user material subroutine. The model is validated through simulations of interrupted uniaxial tensile tests for aluminium alloy AA7075 under two different aging conditions. The implemented model is shown to closely capture the experimental results and account for changes in dislocation density and the athermal component of flow stress during relaxation. Subsequently, the model is applied to simulate monotonic and interrupted hole expansion tests (HET), and the results are discussed qualitatively.

Hardness Prediction of the Heat-Affected Zone in Multilayer Welded SUS316 Stainless Steel Based on Dislocation Density Change Behavior

The effects of strain hardening and recovery/recrystallization on the hardness of the heat-affected zone (HAZ) in multilayer welded austenitic stainless steel SUS316 were investigated in this study. The results revealed that strain hardening due to welding strain and softening due to recovery/recrystallization were the dominant factors affecting the hardness change in the HAZ during multilayer welding process. Furthermore, the relationship between the strain and dislocation density and that between the recovery/recrystallization and dislocation density were quantitatively investigated using positron annihilation lifetime spectroscopy. Based on these results, a new hardness prediction method based on the change in dislocation density in the HAZ during multilayer welding was proposed. The hardness values in the HAZ after the multilayer welding were predicted based on the simulated strain and thermal history, and the calculated hardness values agreed well with the measured results. This indicates that the newly proposed hardness prediction method based on the dislocation density change behavior in the HAZ during multilayer welding is valuable and effective for selecting the appropriate welding conditions before actual welding.

Change in Dislocation Density via Ausforming in Fe-5%Mn-C Alloy with Lath Martensitic Structure

Change in Dislocation Density via Ausforming in Fe-5%Mn-C Alloy with Lath Martensitic Structure

Influence of ultrafine-grained structure parameters on the annealing-induced hardening and deformation-induced softening effects in pure Al

This work investigates the influence of parameters of initial ultrafine-grained (UFG) structure in commercially pure (CP) Al on annealing-induced hardening (AIH) and deformation-induced softening (DIS) effects. UFG structures were formed via processing CP Al by various methods of severe plastic deformation (high pressure torsion (HPT), equal channel angular pressing (ECAP) and combination of ECAP and cold rolling (CR)). AIH and DIS effects are observed in all the studied UFG structures. However, HPT Al demonstrates large increase of strength due to annealing and drastic gain of ductility after subsequent additional deformation whereas in ECAP Al and ECAP + CR Al both effects are much less pronounced. Microstructure characterization by X-ray diffraction (XRD) analysis, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) was performed for HPT Al and ECAP + CR Al in the three studied states: before and after annealing and after annealing and subsequent additional deformation. Analysis of microstructure evolution during annealing and subsequent additional deformation shows that the key microstructure parameter which is responsible for AIH and DIS effect is the change of dislocation density in grain interior in ECAP + CR Al, whereas in HPT Al the effects are related to the change of dislocation density at/near grain boundaries. In addition, outstanding combination of high strength (∼210 MPa), high electrical conductivity (∼62 %IACS) with sufficiently good ductility (7–10 %) and thermal stability (up to 150°С, at least) was achieved for ECAP + CR Al after annealing at 150 °C, 1h.

Effect of Precipitation Size on Dislocation Density Change during Tensile Deformation in Al–Zn–Mg Alloy

Effect of Precipitation Size on Dislocation Density Change during Tensile Deformation in Al–Zn–Mg Alloy

Changes in Grain Shape and Dislocation Density of Carbon Steel during Pendulum SPD

Changes in Grain Shape and Dislocation Density of Carbon Steel during Pendulum SPD

Effect of twin boundary spacing on the mechanical properties of nano-columnar crystalline Cu-Ni alloy

ABSTRACT Nanotwinned exist in crystals as coherent interfaces with low interfacial energy, which can not only improve the strength of metal materials, but also increase the ductility. In this manuscript, we have performed molecular dynamics simulations of the mechanical properties of a nano-columnar crystalline Cu-Ni alloy with different twin boundary spacing. It is found that the model without twin has a stacking fault tetrahedron composed of stair-rod dislocations, which results in a small change in dislocation density at the later stage of deformation, and the average stress after yielding is lower than that of the model with twin. During the deformation process, with the increase of Other atoms, the dislocation slip barrier is enhanced, the tensile strength is increased, and the yield phenomenon is delayed, which is more obvious with the decrease of twin boundary spacing. The dislocation density decreases with the decrease of the spacing of the twin boundary, and the dislocation segments become shorter. When the twin boundary spacing is 0.625 nm, the tensile strength is increased by about 71% compared with the model without twin structure.

Electron Beam-Initiated Grafting of Methyl Methacrylate on Silicon Nanowires: Investigating Optical and Structural Properties

Silicon nanowire (SiNW) is a one-dimensional nanostructured material that had been widely studied due to its potential applications in various fields. Combination of polymers and nanostructured materials offers great potential for enhanced material with many possible applications. The investigation focuses on how this grafting process influences the optical properties of SiNWs, aiming to uncover potential applications for these hybrid materials. This paper comprehensively presents the methodology and characterization of these SiNWs-MMA hybrid materials, exploring their potential applications. The experimental process begins with the preparation of six SiNWs using RF magnetron sputtering, involving the deposition of an Au catalyst and subsequent growth of SiNWs. The radiation-induced grafting involves exposing SiNWs to electron beams and subsequently grafting MMA onto the surface. The outcomes reveal that the grafting percentage of MMA onto SiNWs increases with higher radiation doses, leading to a polymer layer covering the SiNWs. This grafting is confirmed through Fourier-transform infrared (FTIR) spectroscopy, which shows characteristic peaks of MMA on the surface. X-ray diffraction (XRD) analysis demonstrates changes in crystallite size, microstrain, and dislocation density upon grafting, which are attributed to stress relief and the effect of polymer on SiNWs’ lattice. Field emission scanning electron microscopy (FESEM) images exhibit the increasing MMA layer on SiNWs as the grafting percentage increases. UV-visible spectroscopy shows that the introduction of MMA increases the optical band gap of SiNWs, attributed to changes in surface roughness due to the carbon from MMA. This study introduces a novel method of hybridizing SiNWs with MMA through radiation-induced grafting. The detailed characterization of the resulting SiNWs-MMA hybrid materials sheds light on their structural and optical properties. These findings hold the promise of innovative applications in various technological fields, further advancing nanotechnology.

Investigation on microstructure evolution of iron-based alloy via synchronous powder-feeding underwater laser additive

Underwater Laser Additive Manufacturing (ULAM) could further increase the size of synchronous powder-feeding underwater laser cladding (SULC) coatings to achieve underwater preparation devices and emergency repairs. In this paper, the homemade SULC system is used to prepare iron-based planar claddings and bulks, and the grain growth and structural characteristics of underwater additive bulk are obtained by micro-analysis. The grain's growth in the upper part of the bulk is similar to the planar cladding, and the area near the substrate has no obvious columnar grains due to the instability of the temperature gradient for multiple heat input. The EBSD results show that changes in grain size and dislocation density occurred in the additive interface. A periodic fine-grained band in the middle of the bulk appeared, the average hardness of the fine-grained band is 805Hv0.025, and the hardness of the larger equiaxed grain area is 770Hv0.025. Numerical simulation was utilized to obtain temperature field of the SULC laser heating process by introducing the protective airflow and boiling model. The maximum temperature in SULC is about 16% lower than in-air laser cladding, and the boiling model will not affect the temperature distribution. This paper provides theoretical support for the further realization of ULAM.

Pengaruh waktu aging terhadap perubahan sifat fisik paduan ingat bentuk (shape memory alloy) Cu-Zn-Al

Cu-Zn-Al alloy is a shape memory alloy (SMA) that is widely used due to its high transformation temperature. This study aimed to determine the effect of variations in aging time (1.3 and 5 hours) on physical changes (crystal structure and hardness) in Cu-Zn-Al alloys. The method used in this study is powder metallurgy by combining Cu, Zn, and Al powders. Followed by mixing them and then compacting the alloy. The alloy was sintered at 400°C and quenched at room temperature for 24 hours. Then the alloy was given various heat treatments (aging) for 1.3 and 5 hours at 200 °C. Microstructure and hardness tests were carried out on Cu-Zn-Al alloys to determine the properties of the alloy after being treated. From the test, it was found that the hardness value increased from 303 HBN to 375 HBN (1 hour), 351 HBN (3 hours), and 320 HBN (5 hours), and there was a change in crystal size, dislocation density, and lattice strain.

Nonlinear acoustic characterization of heterogeneous plasticity in bent aluminium samples

Knowledge of the state of plastic deformation in metallic structures is vital to prevent failure. This is why non-destructive acoustic tests based on the measurement of first order elastic constants have been developed and intensively used. However, plastic deformations, which are usually heterogeneous in space, may be invisible to these methods if the variation of the elastic constants is too small. In recent years, digital image correlation techniques, based on measurements carried out at the surface of a sample, have been successfully used in conjunction with finite element modeling to gain information about plastic deformation in the sample interior. Acoustic waves can penetrate deep into a sample and offer the possibility of probing into the bulk of a plastically deformed material. Previously, we have demonstrated that nonlinear acoustic methods are far more sensitive to changes in dislocation density than linear ones. Here, we show that the nonlinear Second Harmonic Generation method (SHG) is sensitive enough to detect different zones of von Mises stress as well as effective plastic strain in centimeter-size aluminium pieces. This is achieved by way of ultrasonic measurements on a sample that has undergone a three-point bending test. Because of the relatively low stress and small deformations, the sample undergoes plastic deformation by dislocation proliferation. Thus, we conclude that the nonlinear parameter measured by SHG is also sensitive to dislocation density. Our experimental results agree with numerical results obtained by Finite Element Method (FEM) modeling. We also support the acoustic results by X-ray Diffraction measurements (XRD). Although intrusive and less accurate, they also agree with the acoustic measurements and plastic deformations in finite element simulations.

Effect of Microstructure and Dislocation Density on Material Removal and Surface Finish of Laser Powder Bed Fusion 316L Stainless Steel Subject to a Self-Terminating Etching Process.

Postprocessing of additively manufactured (AM) metal parts to remove support structures or improve the surface condition can be a manually intensive process. One novel solution is a two-step, self-terminating etching process (STEP), which achieves both support removal and surface smoothing. While the STEP has been demonstrated for laser powder bed fusion (L-PBF) 316L stainless steel, this work evaluates the impact of pre-STEP heat treatments and resulting changes in dislocation density and microstructure on the resulting surface roughness and amount of material removed. Two pre-STEP heat treatments were evaluated: stress relief at 470°C for 5 h and recrystallization-solution annealing at 1060°C for 1 h. Additionally, one set of specimens was processed without the pre-STEP heat treatment (as-printed condition). Dislocation density and phase composition were quantified using X-ray diffraction along with standard, metallurgical stain-etching techniques. This work, for the first time, highlights the mechanisms of sensitization of AM L-PBF 316L stainless steel and provides fundamental insights into selective etching of these materials. Results showed that the sensitization depth decreased with increasing dislocation density. For samples etched at a STEP bias of 540 mVSHE, material removal terminated at grain boundaries; therefore, the fine-grained stress-relieved specimen had the lowest post-STEP surface roughness. For surface roughness optimization, parts should be stress relived pre-STEP. However, to achieve more material removal, pre-STEP solution annealing should be performed.

Excellent corrosion resistance of electron beam welded joint and remelted layer of eutectic high-entropy alloy AlCoCrFeNi2.1

Electron beam welding (EBW) and electron beam remelting (EBR) experiments were performed on the eutectic high-entropy alloy (EHEA) AlCoCrFeNi2.1. Electrochemical corrosion experiments of different regions of the joint and the remelted layer were carried out, the subsequent corrosion behavior, the corresponding microscopic morphology, structural characteristics were compared and analyzed in detail. The results show that the corrosion resistance of both the welded joint and the remelted layer is better than that of the cast EHEA. Additionally, the corrosion resistance of the weld center area and the core area of the remelting layer is significantly enhanced. This is mainly because the grain refinement of the weld and the remelted layer, and the redistribution of elements play a key role. Moreover, the effect of large and small angle grain boundaries and the change of dislocation density on the corrosion resistance of different regions cannot be ignored.