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So-called strength-ductility trade-off is usually an inevitable scenario in precipitation-strengthened alloys. To address this challenge, high-density coherent nanoprecipitates (CNPs) as a microstructure effectively promote ductility though multiple interactions between CNPs and dislocations (i.e., coherency, order, or Orowan mechanism). Although some strain hardening theories have been reported for individual strengthening, how to increase, artificially and quantitatively, the ductility arising from cooperative strengthening due to the multiple interactions has not been realized. Accordingly, a dislocation-based theoretical framework for strain hardening is constructed in terms of irreversible thermodynamics, where nucleation, gliding, and annihilation arising from dislocations have been integrated, so that the cooperative strengthening can be treated through thermodynamic driving force ∆G and the kinetic energy barrier. Further combined with synchrotron high-energy X-ray diffraction, the current model is verified. Following the modeling, the yield stress σy is proved to be correlated with the modified strengthening mechanism, whereas the necking strain εn is shown to depend on the evolving dislocation density and, essentially, the enhanced activation volume. A criterion of high ∆G-high generalized stability is proposed to guarantee the volume fraction of CNPs improving σy and the radius of CNPs accelerating εn. This strategy of breaking the strength-ductility trade-off phenomena by controlling the cooperative strengthening can be generalized to designing metallic structured materials.
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Materials with enhanced electron and reduced phonon transport properties are preferred for thermoelectric applications. The defect engineering process can optimize the interrelated electron and phonon transport properties to enhance thermoelectric performance. As the influence of various crystalline defects on the functional properties of materials is diverse, it is crucial to scale, optimize, and understand them experimentally. With this perspective, crystalline defects in InGaSb ternary alloys were engineered and their influence on the thermoelectric properties was studied experimentally. Crystalline defects such as point defects, dislocations, and compositional segregations were induced in In0.95Ga0.05Sb crystals by the addition of excess constituent elements, In, Ga, or Sb. The addition of excess Ga increased point defects, whereas excess Sb reduced dislocation densities. The thermoelectric figure of merit value (ZT) of In0.95Ga0.05Sb+Ga0.02 was recorded to be 0.87 at 573 K, which is the highest among other reported values of III-V semiconductors. The collective interactions of compositional segregations, point defects, and dislocations with electrons and phonons enhanced the ZT in this study.
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High entropy alloys have excellent wear resistance, so they have great application prospects in the fields of wear resistance and surface protection. In this study, the wear resistance of the FeNiCrCoCu high entropy alloy coating was systematically analyzed by the molecular dynamics method. FeNiCrCoCu high entropy alloy was used as a coating material to adhere to the surface of a Cu matrix. The friction and nanoindentation simulation of this coating material were carried out by controlling the ambient temperature. The influence of temperature on its friction properties was analyzed on five aspects: lattice structure, dislocation evolution, friction coefficient, hardness, and elastic modulus. The results show that with the increase of temperature, the disorder of the lattice structure increases, which leads to an increase of the tangential force and friction coefficient in the friction process. At 300 K and 600 K, the ordered lattice structure of the high entropy alloy coating material is basically the same, and thus its hardness is basically the same. However, the dislocation density at 600 K is significantly reduced compared with that at 300 K, resulting in an increase of the elastic modulus of the material from 173 GPa to 219 GPa. At temperatures of 900 K and 1200 K, lattice disorder takes place rapidly, and dislocation density also decreases significantly, resulting in a significant decrease in the hardness and elastic modulus of the material. When the temperature reaches 900 K, the wear resistance of the FeNiCrCoCu high entropy alloy coating decreases sharply. This work is of great value in the analysis of wear resistance of high entropy alloys at high temperature.
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A series of characterization methods involving high-resolution X-ray diffraction (HR-XRD), electron channel contrast imaging (ECCI), cathodoluminescence microscopy (CL), and atomic force microscopy (AFM) were applied to calculate the dislocation density of GaN-on-Si epitaxial wafers, and their performance was analyzed and evaluated. The ECCI technique, owing to its high lateral resolution, reveals dislocation distributions on material surfaces, which can visually characterize the dislocation density. While the CL technique is effective for low-density dislocations, it is difficult to accurately identify the number of dislocation clusters in CL images as the density increases. The AFM technique analyzes surface dislocation characteristics by detecting surface pits caused by dislocations, which are easily affected by sample and probe conditions. A prevalent method for assessing the crystal quality of GaN is the rocking curve of HR-XRD (ω-scan), which calculates the dislocation density based on the FWHM value of the curves. By comparing the above four dislocation characterization methods, the advantages and limitations of each method are clarified, which also verifies the applicability of DB=ß29b2 for GaN-on-Si epitaxial wafers. This provides an important reference value for dislocation characterization in GaN-on-Si materials. The accuracy evaluation of dislocation density can truly and reliably reflect crystal quality, which is conducive to further optimization. Furthermore, this study can also be applied to other heterogeneous or homogeneous epitaxial materials.
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Ni-base superalloys operate in harsh service conditions where cyclic heating and cooling introduce deformation fields that need to be investigated in detail. We used the high-angular-resolution electron backscatter diffraction method to study the evolution of internal stress fields and dislocation density distributions in carbides, dendrites, and notch tips. The results indicate that the stress concentrations decay exponentially away from the notch, and this pattern of distribution was modified by the growth of cracks and the emission of dislocations from the crack tip. Crack initiation follows crystallographic traces and is weakly correlated with carbides and dendrites. Thermal cycles introduce local plasticity around carbides, the dendrite boundary, and cracks. The dislocations lead to higher local stored energy than the critical value that is often cited to induce recrystallization. No large-scale onset of recrystallization was detected, possibly due to the mild temperature (800 °C); however, numerous recrystallized grains were detected in carbides after 50 and 80 cycles. The results call for a detailed investigation of the microstructure-related, thermally assisted recrystallization phenomenon and may assist in the microstructure control and cooling channel design of turbine blades.
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The creep deformation behavior and age strengthening behavior of 304 stainless steel under high stress levels were systematically studied by uniaxial creep test, tensile test, XRD diffraction test and transmission electron microscopy. The results show that the total creep strain and the initial creep strain rate increase with the increase in stress level, and the creep strain in the whole aging process is mainly produced in the initial creep stage. The calculated stress exponent shows that the main mechanism of creep deformation of 304 stainless steel at 453 K is dislocation slip. The strength and plasticity of 304 stainless steel after creep aging are improved simultaneously. Microstructural observations indicate an increase in dislocation density and martensite content, as well as austenite and twins, leading to an improvement in strength and plasticity, respectively. In addition, considering the influence of dislocation density on creep behavior, the relative dislocation density increase is introduced into the hyperbolic sine creep model, and a simple mechanism-based creep aging constitutive model is established. The creep strain predicted by the model is in good agreement with the experimental data of 304 stainless steel. The findings can provide theoretical support for the application of creep age forming in 304 stainless steel parts.
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The microstructure evolution associated with the cold forming sequence of an Fe-14Cr-1W-0.3Ti-0.3Y2O3 grade ferritic stainless steel strengthened by dispersion of nano oxides (ODS) was investigated. The material, initially hot extruded at 1100 °C and then shaped into cladding tube geometry via HPTR cold pilgering, shows a high microstructure stability that affects stress release heat treatment efficiency. Each step of the process was analyzed to better understand the microstructure stability of the material. Despite high levels of stored energy, heat treatments, up to 1350 °C, do not allow for recrystallization of the material. The Vickers hardness shows significant variations along the manufacturing steps. Thanks to a combination of EBSD and X-ray diffraction measurements, this study gives a new insight into the contribution of statistically stored dislocation (SSD) recovery on the hardness evolution during an ODS steel cold forming sequence. SSD density, close to 4.1015 m-2 after cold rolling, drops by only an order of magnitude during heat treatment, while geometrically necessary dislocation (GND) density, close to 1.1015 m-2, remains stable. Hardness decrease during heat treatments appears to be controlled only by the evolution of SSD.
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Experiments were conducted to reveal the nanostructure evolution in additively manufactured (AMed) 316L stainless steel due to severe plastic deformation (SPD). SPD-processing was carried out using the high-pressure torsion (HPT) technique. HPT was performed on four different states of 316L: the as-built material and specimens heat-treated at 400, 800 and 1100 °C after AM-processing. The motivation for the extension of this research to the annealed states is that heat treatment is a usual step after 3D printing in order to reduce the internal stresses formed during AM-processing. The nanostructure was studied by X-ray line profile analysis (XLPA), which was completed by crystallographic texture measurements. It was found that the as-built 316L sample contained a considerable density of dislocations (1015 m-2), which decreased to about half the original density due to the heat treatments at 800 and 1100 °C. The hardness varied accordingly during annealing. Despite this difference caused by annealing, HPT processing led to a similar evolution of the microstructure by increasing the strain for the samples with and without annealing. The saturation values of the crystallite size, dislocation density and twin fault probability were about 20 nm, 3 × 1016 m-2 and 3%, respectively, while the maximum achievable hardness was ~6000 MPa. The initial <100> and <110> textures for the as-built and the annealed samples were changed to <111> due to HPT processing.
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Micron-sized coatings prepared using physical vapor deposition (PVD) technology can peel off in extreme environments because of their low adhesion. Laser micro-melting (LMM) technology can improve the properties of the fabricated integrated material due to its metallurgical combinations. However, the microstructural changes induced by the high-energy laser beam during the LMM process have not been investigated. In this study, we used the PVD-LMM technique to prepare NiCr coatings with a controlled thickness. The microstructural changes in the NiCr alloy coatings during melting and cooling crystallization were analyzed using molecular dynamics simulations. The simulation results demonstrated that the transition range of the atoms in the LMM process fluctuated synchronously with the temperature, and the hexagonal close-packed (HCP) structure increased. After the cooling crystallization, the perfect dislocations of the face-centered cubic (FCC) structure decreased significantly. The dislocation lines were mainly 1/6 <112> imperfect dislocations, and the dislocation density increased by 107.7%. The dislocations in the twinning region were affected by the twin boundaries and slip surfaces. They were plugged in their vicinity, resulting in a considerably higher dislocation density than in the other regions, and the material hardness increased significantly. This new technique may be important for the technological improvement of protective coatings on Zr alloy surfaces.
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The microstructure and texture of the intermetallics in Al/Mg/Al multi-layer composite fabricated by Accumulative Roll Bonding (ARB) at 400 °C up to 6 cycles were investigated using Electron BackScatter Diffraction (EBSD) and Synchrotron X-ray Diffraction (SXRD). EBSD and SXRD analysis have shown that ARB processing leads to the formation of Al3Mg2 and Mg17Al12 intermetallics soon after the second ARB cycle with a global thickness of 12 (N = 2) to 22 µm (N = 6). The polycrystalline intermetallics plates growth was columnar and normal to the bonding interface. A constitutional liquefaction region was depicted ahead of the plates with an unusual rugged migration front. The Al3Mg2 and Mg17Al12 intermetallic compounds which formed after 2 ARB cycles have approximately the same average grain size (1.0 µm) at this cycle. After 4 ARB cycles, the grain refinement of Al3Mg2 is more than 4 times higher than in Mg17Al12. The average grain size of Al3Mg2 and Mg17Al12 reach 0.2 and 0.9 µm, respectively. After 6 cycles of ARB, the average grain size of both Al3Mg2 and Mg17Al12 increased to 1.5 µm and 2.8 µm, respectively. The dislocation density obeyed a ρAl3Mg2 > ρAZ31 > ρAl 1050 â¼ ρMg17Al12 hierarchy after N = 4 and 6 ARB cycles and the Al3Mg2 was shown to store more dislocations. Through the ARB processing, a usual strong basal (0002) texture was depicted in AZ31 layers and a weak rolling texture was shown in Al 1050 layers with a dominant Rotated Cube (001) 110 > component that vanished after upon increasing ARB cycles. The Al3Mg2 and Mg17Al12 intermetallics were characterized by a random texture.
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Compared to Cr-Ni stainless steel, nickel-saving stainless steel is a low-cost austenitic stainless steel. We studied the deformation mechanism of stainless steel at various annealing temperatures (850 °C, 950 °C, and 1050 °C). The grain size of the specimen increases with increasing annealing temperature while the yield strength decreases, which follows the Hall-Petch equation. When plastic deformation occurs, dislocation increases. However, the deformation mechanisms can vary between different specimens. Stainless steel with smaller grains is more likely to transform into martensite when deformed. While twinning occurs when the grains are more prominent, the deformation results in twinning. The phase transformation during plastic deformation relies on the shear, so the orientation of the grains is relevant before and after plastic deformation.
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Modern construction materials, including steels, have to combine strength with good formability. In metallic materials, these features are obtained for heterogeneous multiphase microstructures. Design of such microstructures requires advanced numerical models. It has been shown in our earlier works that models based on stochastic internal variables meet this requirement. The focus of the present paper is on deterministic and stochastic approaches to modelling hot deformation of multiphase steels. The main aim was to survey recent advances in describing the evolution of dislocations and grain size accounting for the stochastic character of the recrystallization. To present a path leading to this objective, we reviewed several papers dedicated to the application of internal variables and statistical approaches to modelling recrystallization. Following this, the idea of the model with dislocation density and grain size being the stochastic internal variables is described. Experiments composed of hot compression of cylindrical samples are also included for better presentation of the utility of this approach. Firstly, an empirical data describing the loads as a function of time during compression and data needed to create histograms of the austenite grain size after the tests were collected. Using the measured data, identification and validation of the models were performed. To present possible applications of the model, it was used to produce a simulation imitating industrial hot-forming processes. Finally, calculations of the dislocation density and the grain size distribution were utilized as inputs in simulations of phase transformations during cooling. Distributions of the ferrite volume fraction and the ferrite grain size after cooling recapitulate the paper. This should give readers good overview on the application of collected equations in practice.
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The influence of rare earth (RE) elements on the microstructure and mechanical performance of an extruded ZK60 Mg alloy was studied. Two types of RE elements were added to a ZK60 material and then extruded at a ratio of 18:1. The first new alloy contained 2 wt% Y while the second one was produced using 2 wt% Ce-rich mischmetal. The microstructure, the texture, and the dislocation density in a base ZK60 alloy and two materials with RE additives were studied by scanning electron microscopy, electron backscattered diffraction, and X-ray line profile analysis, respectively. It was found that the addition of RE elements caused a finer grain size, the formation of new precipitates, and changes in the initial fiber texture. As a consequence, Y and Ce-rich RE elements increased the strength and reduced the ductility. The addition of these two types of RE elements to the ZK60 alloy decreased the work hardening capacity and the hardening exponent mainly due to grain refinement.
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In applications critical to the geological, materials, and engineering sciences, deformation occurs at strain rates too small to be accessible experimentally. Instead, extrapolations of empirical relationships are used, leading to epistemic uncertainties in predictions. To address these problems, we construct a theory of the fundamental processes affecting dislocations: storage and recovery. We then validate our theory for olivine deformation. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. It predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for Earth conditions differ from extrapolated empirical relationships. For example, it predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of postseismic creep.
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The effect of a pulsed magnetic field on the microstructure of a QAl9-4 aluminium bronze alloy was studied in this work. It was found that the dislocation density, grain boundary angle, and microhardness of the alloy significantly changed after the magnetic field treatment with a peak magnetic induction intensity of 3T, pulse duration of about 100 us, pulse interval of 10 s, and pulse time of 360. EBSD was used to test the KAM maps of the alloy microzone. It was found that the alloy's dislocation density decreased by 10.88% after the pulsed magnetic field treatment; in particular, the dislocation in the deformed grains decreased significantly. The quantity of dislocation pile-up and the degree of distortion around the dislocation were reduced, which decreased the residual compressive stress on the alloy. Dislocation motion caused LAGB rotation, which reduced the misorientation of adjacent points inside the grain. The magnetic field induced the disappearance of deformation twins and weakened the strengthening effect of twins. The microhardness test results show that the alloy's microhardness decreased by 8.06% after pulsed magnetic field treatment. The possible reasons for the magnetic field effect on dislocation were briefly discussed. The pulsed magnetic field might have caused the transition to the electronic energy state at the site of dislocation pinning, which led to free movement of the vacancy or impurity atom. The dislocation was easier to depin under the action of internal stress in the alloy, changing the dislocation distribution and alloy microstructure.
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Laser shock peening (LSP) is an innovative and promising surface strengthening technique of metallic materials. The LSP-induced plastic deformation, the compressive residual stresses and the microstructure evolution are essentially attributed to the laser plasma-induced shock wave. A three-dimensional finite element model in conjunction with the dislocation density-based constitutive model was developed to simulate the LSP of pure Al correlating with the LSP-induced shock wave, and the predicted in-depth residual stresses are in reasonable agreement with the experiment results. The LSP-induced shock wave associated with the laser spot diameter of 8.0 mm propagates in the form of the plane wave, and attenuates exponentially. At the same time, the propagation and attenuation of the LSP-induced shock wave associated with the laser spot diameter of 0.8 mm are in the form of the spherical wave. The reflection of the LSP-induced shock wave at the bottom surface of the target model increases the plastic deformation of the target bottom, resulting in the increase of dislocation density and the decrease of dislocation cell size accordingly. Reducing the target thickness can significantly increase the reflection times of the LSP-induced shock wave at the bottom and top surfaces of the target model, which is considered to be conductive to the generation of the compressive residual stress field and grain refinement.
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Microalloyed steels offer a good combination of desirable mechanical properties by fine-tuning grain growth and recrystallization dynamics while keeping the carbon content low for good weldability. In this work, the dislocation density evolution during hot rolling was correlated by materials modeling with flow curves. Single-hit compression tests at different temperatures and strain rates were performed with varying isothermal holding times prior to deformation to achieve different precipitation stages. On the basis of these experimental results, the dislocation density evolution was evaluated using a recently developed semi-empirical state-parameter model implemented in the software MatCalc. The yield stress at the beginning of the deformation σ0, the initial strain hardening rate θ0, and the saturation stress σ∞-as derived from the experimental flow curves and corresponding Kocks plots-were used for the calibration of the model. The applicability for industrial processing of many microalloyed steels was assured by calibration of the model parameters as a function of temperature and strain rate. As a result, it turned out that a single set of empirical equations was sufficient to model all investigated microalloyed steels since the plastic stresses at high temperatures did not depend on the precipitation state.
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The single-crystal Ni-based superalloys, which have excellent mechanical properties at high temperatures, are commonly used for turbine blades in a variety of aero engines and industrial gas turbines. Focusing on the phase interface of a second-generation single-crystal Ni-based superalloy, in-situ TEM observation was conducted at room temperature and high temperatures. Intensity ratio analysis was conducted for the measurement of two-phase interface width. The improved geometric phase analysis method, where the adaptive mask selection method is introduced, was used for the measurement of the strain field near the phase interface. The strained irregular transition region is consistent with the calculated interface width using intensity ratio analysis. An intensity ratio analysis and strain measurement near the interface can corroborate and complement each other, contributing to the interface structure evaluation. Using TEM in-situ heating and Fourier transform, the change of dislocation density in the γ phase near the two-phase interface of the single-crystal Ni-based superalloy was analyzed. The dislocation density decreases first with the increase in temperature, consistent with the characteristics of metal quenching, and increases sharply at 450 °C. The correlation between the variation of dislocation density at high temperatures and the intermediate temperature brittleness was also investigated.
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The development of aluminium composite with the inclusion of advanced materials is a continuous research process due to the increasing industrial demand for advanced hybrid materials. To cater for this need, this research work focuses on the development of Al 7075 alloy reinforced with TiB2 and graphene and on the evaluation of its strengthening mechanism. Two different modes of improving the strength of the hybrid composite have been followed; one is by the inclusion of graphene at three levels of 0.1, 0.2 and 0.3%, and another by the processing route, squeeze casting technique by compression of the molten hybrid composite slurry before casting. The microstructure and characterisation of the composite material are examined and analysed with the help of XRD, SEM, EDAX and chemical spectroscopy. A microstructure evaluation is employed to justify the homogenous dispersal and the existence of reinforced particles. A tensile test is conducted at room temperature and high temperature environments to assess the tensile strength. The research outcome affirms that a significant improvement in tensile and hardness has been noted in comparison with base alloy. The fracture-morphology results affirm the change in fracture mode from brittle to ductile when the tensile testing environment changes from room temperature to high temperature. Finally, the dispersion strengthening mechanism is validated with an empirical approach.
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To relate the bainitic microstructures to the mechanical properties of steel, the average dislocation density needs to be determined. Using X-ray diffraction and diffraction line broadening analysis, this research quantifies the average dislocation density in the four bainite phase matrices, (upper bainite, upper and lower bainite mixture, lower bainite, lower bainite and martensite mixture), which are transformed in a wide range of isothermal temperatures. The effects of isothermal temperatures on the average dislocation density are assessed for different thermal dynamic driving forces in terms of activation energy and cooling rate. It is found that as isothermal holding temperature is increased, the dislocation density in the bainite matrix decreases from 1.55 × 1017 to 8.33 × 1015 (m-2) due to the reduction in the plastic deformation in the austenite in the transformation. At the same time, the activation energy required decreases only after passing the martensite and lower bainite mixed phase. A new method for better estimating the average dislocation density in bainitic steel is also proposed.