Uniform Dislocation Research Articles

The impact of heterogeneous microstructural features on crystal plasticity modeling of plastic anisotropy

Crystal plasticity finite element models typically assume each grain starts with a spatially uniform dislocation distribution, meaning each grain is initialized with a single lattice orientation and all grains share the same slip resistance. This study assessed that assumption by comparing crystal plasticity simulations of tensile tests against experiments on an extruded aluminum 7079 alloy. The simulations utilized computational microstructures with ‘pancaked’ grains and a significant degree of crystallographic texture, consistent with experimental measurements. In one case, the computational microstructures were assigned a single slip resistance and a single lattice orientation for each grain prior to the tension tests. In another case, the extrusion process prior to the tension tests was simulated to cause nonuniform slip resistances, nonuniform crystal orientations within each grain, and nonuniform slip resistances at each material point. Both approaches produced reasonably accurate predictions of the measured yield stress and lateral strain ratio anisotropy, but the predictions considering the initial nonuniformity exhibited less prediction error. Other factors, such as latent vs self hardening of slip systems and fine vs coarse grains, did not have as large an impact. These results suggest that dislocation heterogeneity should be experimentally characterized and fed into future crystal plasticity simulations.

Microscopic mechanism of the combined magnetic-vibration treatment for residual stress reduction

This study proposes a new technology for reducing residual stress using a combined magnetic–vibration treatment. To study the combined reduction effect of magnetic and vibration treatments, the independent reduction effects of vibration and magnetic technology were compared. To study the effect of vibration, magnetic, and combined magnetic-vibration treatments on the residual stress in silicon steel, the change in the mesoscopic structure was studied using electron backscatter diffraction (EBSD). The microstructures of the materials were studied using residual stress measurements, transmission electron microscopy (TEM) observations, X-ray diffraction (XRD) analysis, and magnetic domain observations. The experimental results reveal that all three methods reduced the residual stress, among which the combined magnetic-vibration treatment was the best, and the effect of the composite treatment was greater than the sum of the effects of the individual treatments. All three methods reduced the residual stress by providing energy to increase the movement of dislocations in the material and cause dislocation slip. Compared with the other two methods, the combined magnetic-vibration treatment promoted the transformation of magnetic domains into uniform layered domains and excited dislocation movement in more regions to reduce residual stress. However, after all three types of reduction techniques, the mesoscopic structure, such as grain size and texture, was not significantly affected.

Influence of dislocations on the apparent elastic constants in single metallic crystallites: an analytical approach

Intricate knowledge of dislocation networks in metals has proven paramount in understanding the constitutive behaviour of these materials but current experimental methods yield limited information on the characteristics of these networks. Recently, the isotropic anelastic response of metals has been used to investigate complex dislocation networks through the well-known phenomenon that the observed elastic constants are influenced by dislocations. Considering the dependence of the behaviour of a Frank-Read (FR) source on its initial dislocation character and using discerning characteristics of dislocations, i.e. Burgers vector, line sense and slip system, the present paper takes dislocation character, crystal structure and dislocation network geometry into account and obtains the anisotropic mechanical response for a generic Poisson’s ratio. In this work, the tensile test tangent moduli and yield points are presented for spatially uniform and nonuniform dislocation distributions across slip systems. First, the reversible shear strain of the FR source is derived as a function of initial dislocation character. The area swept by a mobile and initially straight dislocation segment pinned at both ends is given as an explicit function of the line stress. Secondly, the anisotropic anelastic strain contribution of FR sources to the total pre- and at-yield strain in single crystallites is calculated. For a given normal stress and superposition of the principal infinitesimal linear elastic lattice strain and anelastic dislocation strain, the tangent moduli are presented. The moduli and the inception of plastic flow have a notable dependence on initial dislocation character, spatial dislocation distribution and loading direction.

Effect of Rolling Temperature and Subsequence Ageing on the Mechanical Properties and Microstructure Evolution of an Al-Cu-Li Alloy

The mechanical properties and microstructure evolution of an Al-Cu-Li alloy sheet processed via hot rolling (HR) (at 400 °C and 500 °C) or cryorolling (CR) (at −100 °C and −190 °C) and subsequence aging at 160 °C for 10 h were investigated. Before aging, the highest ultimate tensile strength of 502 MPa was achieved when the sheets were cryorolled at −190 °C, while the better ultimate tensile strength of 476 MPa and the best elongation rate of 11.1% was achieved simultaneously when the sheets were cryorolled at −100 °C. The refined grains and numerous uniform deformation-induced dislocations microstructures were responsible for the improved strength and enhanced ductility of the cryorolled sheets compared to that of the alloy processed by hot rolling with a low dislocation density zone (LDDZ) and high dislocation density zone (HDDZ). After aging at 160 °C for 10 h, the ultimate tensile strength further improved resulted from the greater precipitation strengthening, and the increased precipitates provided greater resistance to dislocations movement resulting in the increased ductility although the dislocation density decreased. The uniform dislocation microstructures in the cryorolled sheets provide numerous nucleation sites for the precipitates, leading to higher strength after aging.

Strain Hardening of Low-Carbon Steel in the Area of Jerky Flow

Purpose. The aim of this work is to assess the effect of ferrite grain size of low-carbon steel on the development of strain hardening processes in the area of nucleation and propagation of deformation bands. Methodology. Low-carbon steels with a carbon content of 0.06–0.1% C in various structural states were used as the material for study. The sample for the study was a wire with a diameter of 1mm. The structural studies of the metal were carried out using an Epiquant light microscope. Ferrite grain size was determined using quantitative metallographic techniques. Different ferrite grain size was obtained as a result of combination of thermal and termo mechanical treatment. Vary by heating temperature and the cooling rate, using cold plastic deformation and subsequent annealing, made it possible to change the ferrite grain size at the level of two orders of magnitude. Deformation curves were obtained during stretching the samples on the Instron testing machine. Findings. Based on the analysis of stretching curves of low-carbon steels with different ferrite grain sizes, it has been established that the initiation and propagation of plastic deformation in the jerky flow area is accompanied by the development of strain hardening processes. The study of the nature of increase at dislocation density depending on ferrite grain size of low-carbon steel, starting from the moment of initiation of plastic deformation, confirmed the existence of relationship between the development of strain hardening at the area of jerky flow and the area of parabolic hardening curve. Originality. One of the reasons for decrease in Luders deformation with an increase of ferrite grain size of low-carbon steel is an increase in strain hardening indicator, which accelerates decomposition of uniform dislocations distribution in the front of deformation band. The flow stress during initiation of plastic deformation is determined by the additive contribution from the frictional stress of the crystal lattices, the state of ferrite grain boundaries, and the density of mobile dislocations. It was found that the size of dislocation cell increases in proportion to the diameter of ferrite grain, which facilitates the development of dislocation annihilation during plastic deformation. Practical value. Explanation of qualitative dependence of the influence of ferrite grain size of a low-carbon steel on the strain hardening degree and the magnitude of Luders deformation will make it possible to determine the optimal structural state of steels subjected to cold plastic deformation.

Creep behavior of 316 L stainless steel manufactured by laser powder bed fusion

Additive manufacturing as a new processing technique can produce unique microstructure that is difficult to achieve using conventional techniques. In this study, we have investigated the creep behavior of 316 L stainless steel produced by a laser powder bed fusion process at temperatures of 550, 600 and 650 °C and stresses between 175 and 300 MPa. We found that additively-manufactured 316 L stainless steel had a higher stress dependence of the minimum creep rate than conventionally-made Type 316 SS, which could be attributed to the dislocation cell structure resulting from the printing process. The dislocation cell structure was unstable under creep, evolving into a uniform dislocation structure under the test conditions. While internal porosity in AM 316 L SS may serve as nucleation sites of creep voids and may be responsible for a relatively lower creep life, additively-manufactured 316 SS did not show inferior creep ductility when compared with conventionally-made 316 SS. The creep life of AM 316 L SS could be improved by stabilizing dislocation cell structure and/or reducing internal porosity through an optimized additive manufacturing process.

Co-seismic deformation and slip distribution of 5 April 2017 Mashhad, Iran earthquake using InSAR sentinel-1A image: implication to source characterization and future seismogenesis

We analyzed interferometric synthetic aperture radar sentinel-1A-based observations to characterize the source of 5 April 2017 Mashhad, Iran mainshock (Mw 6.1), for the first time to understand the seismogenic potential of the source area using the estimates of co-seismic displacement and slip distribution on applying the steepest descent method (SDM). SAR pixel offsets (SPO) provided a deep insight into the co-seismic surface deformation of the entire source zone. Based on iterations of a total of 451 models, our analysis of sentinel-1A data from ascending and descending tracks revealed surface deformation occurred in an area of 40 × 30 km with a maximum co-seismic uplift of 10 cm. We estimated geodetic moment of 1.9 × 1020 Nm corresponding to the magnitude (Mw 6.0) of the Mashhad mainshock for which the rupture has a planner geometry with uniform slip dislocation in an elastic half-space with slip of 0.35 ± 0.1 m; strike of N313°E having dip of 48° of the thrust fault associated with oblique motion. The rupture length of 45 ± 3 km along-strike and 30 ± 3 km down dip has been estimated. The best-fit fault model geometry derived from SDM suggests that rupture occurred in the vicinity of the Kashafrud thrust fault, located west to the main Kopeh-Dagh Fault with its strike of 315°E. It is observed that a maximum slip of 0.35 m occurred at a depth of 8 km that extended to 10 km in the crust, which is found to be in unison to the Coulomb stress model that showed low-stressed zone is associated with the majority of events of lower magnitude (M ≤ 4.5) in NE–SW to the mainshock, whilst the EW zone to the mainshock found relatively highly stressed as a probable source for generating relatively higher magnitude earthquakes (M > 4.5) in the future. We infer that the estimates of co-seismic source attributes are essentially important for understanding the nature and extent of earthquake risks for Mashhad, Iran earthquake source area and of the areas of analogous geotectonic settings, elsewhere in the world.

Novel -75°C SEM cooling stage: application for martensitic transformation in steel.

Microstructural changes during the martensitic transformation from face-centred cubic (FCC) to body-centred cubic (BCC) in an Fe-31Ni alloy were observed by scanning electron microscopy (SEM) with a newly developed Peltier stage available at temperatures to −75°C. Electron channelling contrast imaging (ECCI) was utilized for the in situ observation during cooling. Electron backscatter diffraction analysis at ambient temperature (20°C) after the transformation was performed for the crystallographic characterization. A uniform dislocation slip in the FCC matrix associated with the transformation was detected at −57°C. Gradual growth of a BCC martensite was recognized upon cooling from −57°C to −63°C.

Mechanism of low temperature deformation in aluminium alloys

This study investigates differences in the deformation mechanisms between room temperature (296 K) and cryogenic temperatures (77 K) and their advantages for low temperature formability in alloys EN AW 1085, EN AW 5182 and EN AW 6016. Compared to room temperature behaviour, tensile tests showed an overall increase in yield strength, ultimate tensile strength and uniform elongation with differences among the principal alloy types. In general, the improved mechanical properties result from higher strain hardening rates at lower temperatures. The application of an extended Kocks-Mecking approach showed a significant reduction of the dynamic recovery and suggested higher dislocation densities upon cryogenic deformation. This was confirmed via in-situ synchrotron experiments, which also reveal a higher proportion of screw dislocations. Moreover, kernel average misorientation maps from electron backscattered diffraction and in-situ cryogenic deformation in a transmission electron microscope displayed a more uniform dislocation arrangement with a reduction of slip lines and less highly misaligned areas after deformation at lower temperatures. Supported by a detailed characterization of the microstructure and its dislocation structure, the associated fundamental mechanisms we reveal, which are at the origin of the exceptional improvement in mechanical properties, are extensively discussed.

X-Ray Topography Characterization of Large Diameter AlN Single Crystal Substrates

Large diameter aluminum nitride (AlN) substrates, up to 50 mm, were manufactured from single crystal boules grown by physical vapor transport (PVT). Synchrotron-based x-ray topography (XRT) was used to characterize the density, distribution, and type of dislocations. White beam topography images acquired in transmission geometry were used to analyze basal plane dislocations (BPDs) and low angle grain boundaries (LAGBs), while monochromatic beam, grazing incidence images were used to analyze threading dislocations. Boule diameter expansion, without the introduction of LAGBs around the periphery, was shown. A 48 mm substrate with a uniform threading dislocation density below 7.0 x 102 cm-2 and a BPD of 0 cm-2, the lowest dislocation densities reported to date for an AlN single crystal this size, was demonstrated.

Production of high performance multi-crystalline silicon ingot by using composite nucleant

A composite nucleant with Si powder as the main component was introduced to prepare high-performance multicrystalline (HPM) silicon ingot in this work. The effect of different sizes of Si powder on the ingot performance was investigated and compared with the conventional HPM silicon ingot which seeded from poly-Si particles. It is proved that the composite nucleant with Si powder of large particle size (D50 = 355 μm) can induce small and uniform grains, showing uniform minority carrier lifetime distribution and low dislocation cluster area ratio. Meanwhile, the length of red zone at the ingot bottom is significantly shorter than conventional HPM silicon ingot, which significantly increases the yield of the ingot. The ingot grown with suitable composite nucleant can achieve high cell efficiency and the solar cells have low light-induced degradation (LID) value due to low interstitial oxygen concentration which are comparable to the conventional HPM silicon ingot.

Influence factors on the ground surface rupture zone induced by buried normal fault dislocation*

The seismic disaster presents a zonal distribution along the fault strike. In this paper, rupture zone of ground surface soil caused by the uniform dislocation, inclined dislocation and warped dislocation of buried normal fault are studied by constituting a three-dimensional finite element model in Automatic Dynamic Incremental Nonlinear Analysis (ADINA). According to the critical value of surface rupture, the variational features and influencing factors of width and starting position of the “avoiding zone” in engineering construction are analyzed by using 96 model calculations. The main results are as follows: (1) Since the rupture zone of the ground surface soil from the point of mechanics is different from the “avoidance zone” from the point of engineering safety, the equivalent plastic strain and the total displacement ratio should be considered to evaluate the effect of the seismic ground movement on buildings. (2) During fault dislocation, plastic failure firstly occurred on the ground surface soil of the footwall side, and then the larger deformation gradually moved to the side of the hanging wall of the fault with the increase of fault displacement. (3) When the vertical displacement of buried fault reaches 3 m, the width of “avoiding zone” in engineering construction varies within the range of 10–90 m, which is most affected by the thickness of overlying soil and the dip angle of the fault.

Tunable surface boundary conditions in strain gradient crystal plasticity model

The behavior of dislocations in the neighborhood of a metallurgical interface or a free surface can be totally different depending on the boundary conditions. Dislocations cease to move and accumulate around impermeable interfaces, such as grain boundaries or hard (i.e. coated or oxidized) external surfaces. On the contrary, dislocations annihilate on free surfaces, as supported by the image force concept. However the behavior of dislocations depends on the true surface permeability, which falls between these two idealized cases. In this paper, two different numerical methods are applied to model the intermediate surface behaviors: the virtual image geometrically necessary dislocation approach and the generalized elastic foundation approach while a strain gradient crystal plasticity constitutive law simulates the response of a Ni single crystal. It is demonstrated that a uniform dislocation density field on the surface can only be obtained by a variant of the generalized elastic foundation approach.

Revealing the mechanism of extraordinary hardness without compensating the toughness in a low alloyed high carbon steel

There is a continuous quest for discovery of a steel grade which has better properties and lower production cost. To design steel with superior properties for industrial application, it is essential to understand the effect of microstructure and engineer it to fit the purpose. In this study, a counter intuitive strategy has used to reveal the mechanism of high carbon steel with ultrahard structure. High compact force has been used to produce a structure which has ceramic-like hardness without compensating the toughness significantly. The behaviour of high carbon low-alloy steel as the starting material under different stages of deformation has been studied to differentiate various deformation paths and microstructural transformation processes. Microscopy investigation by secondary electron microscopy, high-resolution electron backscattered diffraction (HR-EBSD) analysis and Transmission electron microscopy (TEM) showed that the key point to achieve ~75% increased hardness in this steel is through generation of nano-structured martensite of less than 50 nm grains size which can be formed due to high impact force. In this paper, we reveal a nano grained steel structure with excellent mechanical properties resulting from phase transformation, uniform dislocation distribution, grain refinement and recrystallization.

On the Global Existence of Classical Solutions for Compressible Magnetohydrodynamic Equations

In an interesting recent paper [1] (A. Acharya, Stress of a spatially uniform dislocation density field, J. Elasticity 137 (2019), 151--155), Acharya proved that the stress produced by a spatially uniform dislocation density field in a body comprising a nonlinear elastic material may fail to vanish under no loads. The class of counterexamples constructed in [1] is essentially $2$-dimensional: it works with the subgroup $\mathcal{O}(2) \oplus \langle{\bf Id}\rangle \subset \mathcal{O}(3)$. The objective of this note is to extend Acharya's result in [1] to $\mathcal{O}(3)$, subject to an additional structural assumption and less regularity requirements.

Hyperplasticity mechanism in DP600 sheets during electrohydraulic free forming

Abstract This work discussed the multi-scale mechanisms of hyperplasticity during the high strain rate electrohydraulic forming (EHF) process by exploring the formability of DP600 sheets in a state of uniaxial tensile stress. The experimental results showed that the limit strains and limit dome heights of the deformed specimens obtained by EHF were improved by 15 %–27 % and 22.54 %, respectively, as compared with quasi-static specimens, showing a hyperplasticity characteristic. The inertial effects that occurred during the EHF process were responsible for the macro-scale enhancement in terms of formability, which could generate an additional principal stress along the direction of stretching that slowed the velocity gradient of the necked elements to restrain uneven deformation, resulting in a 60 % broadening of the action zone of maximum Y-displacement. The proportion of inertial effects that contributed to the plastic deformation of the deformed specimens was 87.1 %, indicating that the vast majority of the deformation in the EHF process occurred as a result of inertial effects after the electrical energy was completely discharged. A larger dislocation density and a more uniform dislocation distribution were observed in the EHF specimens, which were regarded as the micro-scale causes of the hyperplasticity in the EHF process. Multiplication and entanglement of dislocations caused by the significant shear stress, together with the extensive nucleation of new dislocations caused by the high strain rates, demonstrated the micro-scale mechanism of hyperplasticity during EHF.

A non-uniform dip slip formula to calculate the coseismic deformation: Case study of Tohoku Mw9.0 Earthquake

AbstractThe distribution of slip faults along the fault plane plays a special role in the kinetic pattern of tectonic deformation. To better understand the coseismic deformation and geodynamics of the earthquake, this paper applied the pile-up theory and derived an analytical formula to describe the non-uniform slip distribution along the fault width. To validate the new formula, it was tested with the coseismic displacements at the global positioning system (GPS) stations for the Tohoku earthquake in 11 March, 2011. Then, the computed horizontal and vertical displacements calculated using NDSM were compared to back-slip model (BSM) using GPS data obtained from the Jet Propulsion Laboratory (JPL). Finally, the theoretical analysis revealed that the analytical formulas derived here can be perceived as the expansion and perfection of the uniform dislocation model. Meanwhile, our results showed that the characteristics of the spatial distribution deformation from NDSM are similar to those derived by GPS measurements. Furthermore, the near-field RMS errors indicated that the horizontal displacements estimated using NDSM is 27.5%, and 35.6% for the vertical components. Our new formulas and findings could assist better portray the crustal deformation in some region and geodynamics in specific earthquake.

Microstructural homogeneity, mechanical properties, and wear behavior of in situ Mg2Si particles reinforced Al–matrix composites fabricated by hot rolling

In situ Mg2Si particles often show coarse and irregular morphologies in Al–matrix composites, which results in inhomogeneous microstructures and poor mechanical properties. This leads to poor deformation ability in Mg2Si particles reinforced Al–matrix composites. Herein, a refined process exploiting refined Mg2Si particles was developed to enhance microstructure homogeneity after hot rolling in an Al-11.73%Mg-6.63%Si composite. The hot-rolled sheet reduces the macroscopic cracking on the edge when the Mg2Si particles of the as-cast microstructure are refined by the addition of phosphorus. The refined Mg2Si particles in the as-cast state shows further size reduction and exhibits homogeneous distribution by hot rolling (reduction of 76%). The microstructure also shows uniform deformation zones, subgrains and high-density dislocation regions via hot rolling. The homogeneous deformation microstructure results in needle-like, high-density precipitates after artificial aging and decreases the stress concentration of load bearing, which is beneficial to the tensile properties and wear resistance of the composite. The wear behavior of the composites improves with Mg2Si-particle refinement and microstructural homogeneity.

Mechanics of Inflationary Deformation During Caldera Collapse: Evidence From the 2018 Kīlauea Eruption

AbstractDuring the 2018 Kīlauea eruption the caldera floor dropped 500 m in 62 nearly periodic events of up to 8 m. Caldera collapse maintains pressure in the magma reservoir necessary to sustain high‐rate eruptions. The 2018 collapses were accompanied by inflationary tilts and displacements, similar to observations at other basaltic calderas. Collapse is modeled in 2‐D by uniform slip dislocations intersecting a magma chamber. Collapses occurred rapidly, such that mass within the chamber was constant. For vertical faults surface deformation outside the collapse results only from chamber pressurization with displacements antiparallel to precollapse deflation, similar to observations. For inward dipping faults the predicted ratio of horizontal to vertical displacements during collapse is greater than for the precollapse deflation, as observed. For outward dipping faults the horizontal displacements are inward, contrary to data. The average deformation trends during the eruption depend on fault dip and whether stress required to trigger collapses was constant or changed with time.

Cold Cracking Resistance of Butt Joints in High-Strength Steels with Different Welding Techniques

Investigation results are presented to improve the structural strength (cold crocking resistance) of weld joints in high-strength steels with the yield limit over 600 MPa. The effect of arc, laser, and hybrid laser-arc welding conditions on the weld metal structure and diffusion hydrogen saturation of build-up and fused base metals was experimentally studied. The diffusion hydrogen saturation of build-up (arc and hybrid welding) and fused (laser welding) metals was chromatographically examined. In gas-shielded arc welding, the diffusion hydrogen content in the fused base metal was shown to be limited to the concentration that does not exceed 0.4 ml/100 g due to an increase in the welding speed from 18 to 50 m/h. In laser and hybrid laser-arc welding of high-strength steels with the yield limit over 600 MPa, the diffusion hydrogen content in the fused metal makes up 0.07 and 0.2–0.3 ml/100 g, respectively, regardless of the welding speed. The cold cracking resistance was evaluated by a commonly accepted procedure of special reference butt samples. Optical and transmission microscopic studies permitted of revealing the effect of arc, laser, and hybrid laser-arc welding conditions on the weld metal structure and gaining detailed information on the dislocation density distribution. The relation between the level of local internal stresses and the structural factors of dislocation density distribution in the weld metal was established. In arc and laser welding, local internal stresses were shown to be reduced to the values that do not exceed 0.22 of the theoretical metal strength if the welding speed would make up 50 m/h. In hybrid laser-arc welding at 72–110 m/h, maximum local internal stresses are also less than 0.22 of the theoretical metal strength. An increase in the cold cracking resistance in butt weld joints of high-strength 14KhGN2MD and N-A-XTRA-70 steels was established to be reached due to a low concentration of diffusion hydrogen in fused metal and formation of the fine-grained structure of lower bainite with the uniform dislocation density distribution.