Saturated Grain Research Articles

Erratum for the Research Article: "Complex dislocation loop networks as natural extensions of the sink efficiency of saturated grain boundaries in irradiated metals," by He et al.

Erratum for the Research Article: "Complex dislocation loop networks as natural extensions of the sink efficiency of saturated grain boundaries in irradiated metals," by He et al.

Complex dislocation loop networks as natural extensions of the sink efficiency of saturated grain boundaries in irradiated metals.

The development of radiation-tolerant structural materials is an essential element for the success of advanced nuclear energy concepts. A proven strategy to increase radiation resistance is to create microstructures with a high density of internal defect sinks, such as grain boundaries (GBs). However, as GBs absorb defects, they undergo internal transformations that limit their ability to capture defects indefinitely. Here, we show that, as the sink efficiency of GBs becomes exhausted with increasing irradiation dose, networks of irradiation loops form in the vicinity of saturated or near-saturated GB, maintaining and even increasing their capacity to continue absorbing defects. The formation of these networks fundamentally changes the driving force for defect absorption at GB, from "chemical" to "elastic." Using thermally-activated dislocation dynamics simulations, we show that these networks are consistent with experimental measurements of defect densities near GB. Our results point to these networks as a natural continuation of the GB once they exhaust their internal defect absorption capacity.

Effect of Si on the evolution of plasticity mechanisms, grain refinement and hardness during high-pressure torsion of a non-equiatomic CoCrMnNi multi-principal element alloy

The present study unraveled the defining role of small silicon (Si) addition (5 atomic %) in dramatically altering the plasticity mechanisms, grain refinement and hardening response of a non-equiatomic CoCrMnNi multi-principal element alloy (MPEA) during high-pressure torsion (HPT) processing. Both the Si-free and the Si-added MPEAs had a face-centered cubic (FCC) structure and were subjected to a quasi-constrained HPT processing at 6 GPa pressure to different number of turns (0.5 and 5). Microstructure evolution was studied at the center and edge of the HPT-processed discs using X-ray diffraction line profile analysis (XLPA) and transmission electron microscopy (TEM). Si addition altered the predominant plasticity mechanism from micro-band formation to extensive occurrence of nano-twinning at the early stage of HPT processing. At later stages of HPT processing, both alloys exhibited deformation twinning but its propensity was considerably higher for the Si-added MPEA, as revealed by ∼50% higher twin fault probability. Additionally, the Si-added MPEA showed ∼30% higher dislocation density at any given stage of HPT processing compared to the Si-free MPEA. A significantly accelerated nano-structuring coupled with a finer saturation grain size was observed in the Si-added MPEA (34 nm for Si-free versus 23 nm for Si-added). These effects can be explained by the influence of Si addition on lowering the stacking fault energy (SFE) (from ∼40 mJ/m2 in Si-free to ∼20 mJ/m2 in Si-added MPEA) and increasing the solute pinning effect of Si on lattice defects. The plasticity mechanisms at nano-scale were also influenced by the presence of Si as confirmed by the formation of nano-twins and stacking faults inside the nano-grains for the Si-added and Si-free MPEAs, respectively. The differences in plasticity mechanisms and microstructure evolution resulted in enhanced hardness in the early stages of HPT processing for the Si-added MPEA, but the difference in hardness between the two alloys tended to be reduced at higher strains.

Scaling effects on the seismic response of a closed-end pipe pile embedded in dry and saturated coarse grain soils

Foundations can be subjected to dynamic or seismic loads depending on their applications and the site being constructed in. The researchers concentrated their works on investigating the reasons of the significant damage of piles during seismic excitation. Based on the findings of laboratory experiments and other numerical analyses, such failures were referred to as the kinematic impact of the earthquake on piles since they were associated with discontinuities in the subsoil because of sudden changes in soil stiffness. The current work investigates the seismic response of closed-end (CE) pipe pile using three-dimensional finite element analysis, including the impact of the scaling-up model, acceleration-time history of the ground motion, and ground conditions. The numerical model is developed using a variety of scaling rules and the outputs of the available laboratory tests. The current results showed that the saturated sand models have larger pile deformation factors than dry sand models. Pile frictional resistance was evaluated numerically, and the entire findings were evaluated against the earlier work. Mainly, the frictional resistance around the pile shaft was lower than that at the pile tip, and the frictional resistance factor on the soil surface of dry soil models was larger than that of saturated soil models. Owing to the acceleration amplifications, the pile and soil suffered cycles of compression and tension stresses. A hysteresis loop is broader and flatter on the x-axis as the shear strain increases serve to identify the shear stress–strain plane behavior. The main outputs of the scaled models were normalized to provide a deep insight of model to prototype scaling effects.

Solid solution hardening effects on structure evolution and mechanical properties of nanostructured binary and high entropy alloys after high pressure torsion

Two different alloy series (Cu-X, Ni-X) have been selected to investigate the effects of solutes on the saturation grain size, the thermal stability and mechanical properties after high pressure torsion. The results of the Cu-X series indicate that the saturation grain size does not correlate with the stacking fault energy but shows good agreement with solid solution hardening according to the Labusch model. This correlation does not only hold for binaries, but also for chemically complex high entropy alloys (Ni-X) in the form of (CrMnFeCo)xNi1-x, where the Varvenne model is used to describe solid solution hardening. The alloy series exhibit a grain size in the range of 50 – 425 nm after high pressure torsion and the solutes increase the strength as well as the thermal stability of the alloys after annealing. The nanostructured alloys exhibit an enhanced strain rate sensitivity exponent, as determined from nanoindentation strain rate jump and constant contact pressure creep testing, whereas an enhanced rate sensitivity is found at low strain rates. The relatively lower rate sensitivity of the alloys as well as their higher thermal stability indicate, that defect storage and annihilation is strongly influenced by a complex interaction of solutes, dislocations and grain boundaries.

Insight into the creep-fatigue interaction and remaining creep damage mechanisms in different micro-regions of 9%Cr steel welded joints

The deformation and damage mechanisms of welded joints have gained high interest for their widespread use in power plants. The inter-critical heat-affected zone (ICHAZ) is generally identified as the weakest region in the whole welded joint. However, the micro-regions' nanoindentation and creep results in the present work reveal that the weakest position moves to base material after fatigue loading. It is found from electron backscatter diffraction (EBSD) analysis that the early saturated grain growth in the ICHAZ contribute to the movement of the weakest region. It is important to note that compared with the creep strength in the as-received condition, prior fatigue loading enhances the creep deformation. It is also observed from three-dimensional X-ray tests that large fatigue cycles and long hold time of fatigue loading accelerate the nucleation of creep voids. Finally, a fatigue damage parameter based on the nanoindentation microhardness is proposed, which is validated to be capable of capturing the effect of various loadings on the subsequent creep life and creep fracture location. These results benefit the understanding of the creep behaviour of welded joints after fatigue damage and provide a guide for the design and damage evaluation of welded components.

Mg Defect Induced Ferromagnetic Ordering in Li-Doped MgO Nanostructures

Magnesium deficient Li-doped MgO nanostructures were fabricated by hydrothermal method to investigate their structural, electronic, and magnetic properties. X-ray diffraction study confirms the single-phase crystallinity and changing the Li concentration (2%, 4% & 6%) in the host matrix of MgO induced strain and increased the crystallite size. Field-effect scanning electron microscopy images show the change in morphology from flakes to prismatic type nanostructures with the variation in Li content. The increase in Mg defects with an increase in Li doping, elucidated by X-ray photoelectron spectroscopy, would be the plausible reason for origin of d0 ferromagnetism and can be explained through (Li1+–VMg–Li1+) exchange interaction mechanism. Also, this increase in Mg defects leads to an increase in the value of saturation magnetization and grain size as well. Furthermore, electron paramagnetic resonance measurements explored the concepts of g-value and line width which are related with Mg defects and super exchange interaction of paramagnetic ions.

From diluted solid solutions to high entropy alloys: Saturation grain size and mechanical properties after high pressure torsion

Effects of solutes on saturation grain size and mechanical properties are investigated for the Cantor alloy and Ni-enriched variations ((CrMnFeCo)xNi1-x) with x=0.8, 0.4, 0.08 and 0. Indentation on coarse-grained and severely deformed states shows increasing hardness with increasing alloying content due to higher solid solution strengthening and Hall-Petch contributions. Nanoindentation strain rate jump tests reveal similar rate sensitivities of the deformed states without pronounced transient regimes. All compositions exhibit a history dependent softening indicating an unstable microstructure. The saturation grain size ds after HPT deformation inversely correlates with the solid solution strengthening contribution, i.e. the higher Δτ the lower ds.

Influence of severe plastic deformation on the microstructure and hardness of a CoCrFeNi high-entropy alloy: A comparison with CoCrFeNiMn

The evolution of microstructure and hardness in a CoCrFeNi high-entropy alloy (HEA) processed by severe plastic deformation (SPD) was studied. SPD-processing was carried out using a high-pressure torsion (HPT) technique up to twenty turns at room temperature that introduces a highest nominal shear strain of about 800. It was found that most of grain refinement and an increase of lattice defect (dislocations and twin faults) density occurred up to the shear strain of ~10. The saturation grain size was about 80 nm, while the maximum values of the dislocation density and the twin fault probability were ~150 × 1014 m−2 and ~3%, respectively. In addition, a 111 texture was formed during HPT-processing. The evolution of hardness with strain followed a trend suggested by the changes in the microstructure. The saturation hardness was as high as ~5100 MPa. The microstructure and hardness obtained for the HPT-processed CoCrFeNi were compared with the values determined formerly for CoCrFeNiMn. It was found that, although the saturation grain size in the CoCrFeNi was much higher than that for the CoCrFeNiMn HEA, the hardness was similar for the two alloys due to the close values of the twin fault probability which can be explained by the similar stacking fault energies.

Evolution of microstructure and hardness in Hf25Nb25Ti25Zr25 high-entropy alloy during high-pressure torsion

A four-component equimolar high-entropy alloy (HEA) with the composition of HfNbTiZr and body-centered cubic (bcc) structure was processed by HPT at RT. The evolution of the dislocation density, the grain size and the hardness was monitored along the HPT-processed disk radius for different numbers of turns between ¼ and 20. It was found that most of the increase of the dislocation density and the refinement of the grain structure occurred up to the shear strain of ∼40. Between the strains of ∼40 and ∼700, only a slight grain size reduction was observed. The saturated dislocation density and grain size were ∼2.1 × 1016 m−2 and ∼30 nm, respectively. The saturation in hardness was obtained at ∼4450 MPa. These values were similar to the parameters determined in the literature for five-component HEAs processed by HPT. The analysis confirmed that the main component in the strength was given by the friction stress in the HPT-processed bcc HfNbTiZr HEA. It was also revealed that the contribution of the high dislocation density to the strength was significantly higher than the effect of the small grain size.

High resolution imaging of the magnetic field in the central parsec of the Galaxy

We discuss a high resolution (FWHM∼0.45 arcsec) image of the emissive polarization from warm dust in the minispiral in the Galactic Centre and discuss the implications for the magnetic field in the dusty filaments. The image was obtained at a wavelength of 12.5 μm with the CanariCam multimode mid-infrared imager on the Gran Telescopio Canarias. It confirms the results obtained from previous observations but also reveals new details of the polarization structures. In particular, we identify regions of coherent magnetic field emission at position angles of ∼45o to the predominantly north–south run of field lines in the Northern Arm which may be related to orbital motions inclined to the general flow of the Northern Arm. The luminous stars that have been identified as bow-shock sources in the Northern Arm do not disrupt or dilute the field but are linked by a coherent field structure, implying that the winds from these objects may push and compress the field but do not overwhelm it. The magnetic field in the low surface brightness regions in the East-West Bar to the south of SgrA* lies along the Bar, but the brighter regions generally have different polarization position angles, suggesting that they are distinct structures. In the region of the Northern Arm sampled here, there is only a weak correlation between the intensity of the emission and the degree of polarization. This is consistent with saturated grain alignment where the degree of polarization depends on geometric effects, including the angle of inclination of the field to the line of sight and superposition of filaments with different field directions, rather than the alignment efficiency.

Grain boundary engineering parameters for ultrafine grained microstructures: Proof of principles by a systematic composition variation in the Cu-Ni system

The main principles of grain boundary engineering of an ultrafine grained microstructure are studied via a systematic variation of the stacking fault energy (γSFE), solid solution effects (SSEs) and the homologous deformation/annealing temperatures choosing the Cu-Ni system as a model case for non-segregating alloys. Cu and Ni are completely miscible and the γSFE varies strongly with the alloy composition. Ultrafine grained microstructures are produced by high-pressure torsion and their evolution upon annealing is investigated. The thermal stability and the saturation grain sizes after deformation are determined by SSEs. The fraction of deformation twins varies in accordance with the γSFE. The increase of the length fraction of Σ3 grain boundaries versus the grain size is found to be governed by the SSEs, whereas the increase of the length fraction of Σ9 grain boundaries versus the grain size depends on the γSFE. The results indicate that grain boundary engineering may lead to an optimum ultrafine grained structure, i.e. with a high length fraction of Σ3 and Σ9 grain boundaries of approximately 40%. This is a significantly large fraction in comparison to a not-engineered microstructure and achieved for grain sizes below 1000 nm in alloys close to the equiatomic composition having a low γSFE and a high melting temperature. This high fraction of Σ3n grain boundaries (including their conjunctions) proved itself most effective for microstructure stabilization in this virtually non-segregating system. Thus, for Cu50Ni50 and Cu65Ni35, a narrow grain size distribution of small grains, a high hardness and a high fraction of special grain boundaries can be adjusted.

A Simple and Improved Model for Describing Soil Hydraulic Properties from Saturation to Oven Dryness

Core Ideas Soil hydraulic properties from saturation to oven dryness are needed to simulate soil water movement. A new soil hydraulic model was compared with the classic van Genuchten–Mualem models. The new model with different fitted parameters was more efficient than the van Genuchten–Mualem models. Soil water retention characteristics and hydraulic conductivity from saturation to oven dryness are needed for simulating soil water movement and solute transport, especially in arid and semiarid regions. The aim of this study was to derive a new soil hydraulic model and compare this model with the classic van Genuchten–Mualem (VGM) models (M1). A total of 12 soils with varying basic properties were selected from the Unsaturated Soil Hydraulic Database (UNSODA) for the evaluation of the new model. Five pairs of soil hydraulic models were considered, including M1 and the new model with different parameters fitted (M2–M5). Correlation analysis was conducted to analyze the relationships between the new model parameters and basic soil properties. In addition, the Morris method was applied to quantify the parameter sensitivity of the new model. Results showed that M2 to M5 gave slightly better performance than M1 in predicting soil water retention characteristics, with a decrease of ∼15% in the root mean squared error (RMSE). However, they produced substantially better performance than M1 in estimating soil hydraulic conductivities, with a decrease of ∼40% in the RMSE. The new model with all of the model parameters being fitted (M2) was more efficient than those with one or two model parameters not fitted. However, an over‐parameterization problem was detected with M2. Physically unrealistic parameters (e.g., the saturated soil water content and effective grain diameter) exist in M2. These parameters were found to have important influences on the results of the soil hydraulic properties prediction based on the Morris‐based sensitivity analysis. Overall, the new model with two parameters not fitted (M5) can best describe the soil hydraulic properties from saturation to oven dryness and had the potential to simulate water and solute transport in the vadose zone.

Influence of solute effects on the saturation grain size and rate sensitivity in Cu-X alloys

Three Cu-alloys (CuSn5, CuZn5, CuZn30) have been selected, to study the influence of solutes on the saturation grain size after high pressure torsion as well as the resulting mechanical properties with an emphasis on strain rate sensitivity. Compared to each other, the alloys exhibit either similar stacking fault energies or similar solid solution strengthening, allowing a separation of both parameters. The results indicate that the saturation grain size correlates with the solid solution strengthening but not with the stacking fault energy. Moreover, a reduced strain rate sensitivity with reduced grain size is observed, which is attributed to the microstructural stability.

Sensitivity analysis of Immersed Boundary Method simulations of fluid flow in dense polydisperse random grain packings

Polydisperse granular materials are ubiquitous in nature and industry. Despite this, knowledge of the momentum coupling between the fluid and solid phases in dense saturated grain packings comes almost exclusively from empirical correlations [2–4, 8] with monosized media. The Immersed Boundary Method (IBM) is a Computational Fluid Dynamics (CFD) modelling technique capable of resolving pore scale fluid flow and fluid-particle interaction forces in polydisperse media at the grain scale. Validation of the IBM in the low Reynolds number, high concentration limit was performed by comparing simulations of flow through ordered arrays of spheres with the boundary integral results of Zick and Homsy [10]. Random grain packings were studied with linearly graded particle size distributions with a range of coefficient of uniformity values (C u = 1.01, 1.50, and 2.00) at a range of concentrations (ϕ ∈ [0.396; 0.681]) in order to investigate the influence of polydispersity on drag and permeability. The sensitivity of the IBM results to the choice of radius retraction parameter [1] was investigated and a comparison was made between the predicted forces and the widely used Ergun correlation [3].

Microtexture analysis of restoration mechanisms during high pressure torsion of pure nickel

The grain refinement in severe plastically deformed pure metals is limited to few 100nm and the saturated microstructure primarily depends on deformation temperature and impurities. In present study high pressure torsion (HPT) technique is used for understanding the mechanism of microstructure restoration processes at saturation state for 99.99% pure nickel between 77 and 673K. The saturation grain size increased with deformation temperature. Electron back scatter diffraction (EBSD) measurements along with microtexture evaluation show that the grain boundary (GB) misorientation distribution remains similar for all the temperatures; however, new texture components and special GB characteristics start to develop at higher temperatures. The differences in the GB migration characteristics and the resulting microtexture changes suggest that at low temperatures GB migration is facilitated by the variations in elastic strain energy density among neighboring grains; however, with increasing temperature thermally driven migration processes induced by plastic stored energy become important.

Stress-driven migration, convergence and splitting transformations of grain boundaries in nanomaterials

Convergence and splitting transformations of grain boundaries (GBs) migrating under stress in nanocrystalline and ultrafine-grained materials are theoretically described. With the disclination model of GB junctions, the elastic interaction between migrating GBs that mediate plastic deformation and their response to the external stress are examined. A special attention is devoted to convergence of migrating GBs with their immobile counterparts as well as to their following splitting transformations. Equilibrium migration distances, energy and critical stress characteristics for migration of GBs and their various transformations are calculated. In particular, it is theoretically revealed that a GB migrating under a comparatively high shear stress tends to converge with its immobile counterpart and then splits into two new GBs that migrate in opposite directions. We estimated the critical stress for convergence and splitting transformations of GBs and the saturation grain size in metals (Cu and Ni) processed by severe plastic deformation. Our estimates are consistent with the corresponding experimental data reported in the literature.

Controlling the high temperature mechanical behavior of Al alloys by precipitation and severe straining

The aim of this work was to investigate the influence of the precipitate distribution on the microstructure and on the room and high temperature mechanical properties of an age-hardenable aluminium AA6082 alloy following severe straining by high-pressure torsion (HPT). With this goal, specimens in the as-cast and T6 peak-aged conditions were processed by HPT using 0.5, 1 and 5 turns at room temperature. At high strain levels (γ>100), similar saturation grain sizes (~250nm), high-angle boundary fractions (~80%) and hardness values (Hv~160) were obtained for both initial conditions. Grain refinement led to significant strengthening and to good ductility values at room temperature. Analysis by TEM and EDS elemental mapping revealed that HPT processing of the as-cast condition led to fracture of the stable β-phase into many small precipitates located preferentially along grain boundaries and triple junctions. By contrast, HPT processing of the T6 peak-aged specimens revealed a partial dissolution of the needle-shaped nanoprecipitates. The different evolutions of the precipitate distributions following straining in the as-cast and peak-aged conditions gave rise to dramatic differences in the mechanical properties and the operative deformation mechanisms at warm temperatures. These results provide evidence that the high temperature mechanical behavior of age-hardenable Al alloys may be conveniently controlled by altering the precipitate distribution followed by severe straining.

Ultra-High-Strength Interstitial-Free Steel Processed by Equal-Channel Angular Pressing at Large Equivalent Strain

The billets of interstitial-free (IF) steel are deformed by equal-channel angular pressing (ECAP) at 298 K (25 °C) adopting the route BC up to an equivalent strain (e vm) of 24. The evolution of microstructures and their effects on the mechanical properties are examined. The microstructural refinement involves the elongation of grains, the subdivision of grains to the bands with high dislocation density, and the splitting of bands into the cell blocks and then cell blocks into the cells. The widths of the bands and the size of cells decrease with strain. The degree of reduction in the grain size is highest at the low strain level. However, most of the boundaries at this stage are of low-angle boundaries (at e vm = 3). Thereafter, the misorientation angle increases by progressive lattice rotation with strain. The coarse bands transform step by step from the lamellar structure to the ribbon-shaped grains and finally to the near-equiaxed grain structures with the subgrains of a saturated low-angle grain boundary fraction of 0.34 at very large strain >15. The as-received coarse-grained microstructure (grain size of 57.6 ± 21 µm) has been refined to 257 ± 48 nm at an equivalent strain of 24. The strength increases considerably up to e vm = 3 due to grain refinement and high dislocation density. However, the strengthening at later stages is mainly due to the increase in misorientation angle and refinement. Initial yield strength of 227 MPa is increased to a record value of 895 MPa on straining up to e vm = 24 at 298 K (25 °C). Uniform elongation decreases drastically at low equivalent strain but it regains marginally later. The ECAPed sample fails by a ductile fracture at e vm = 0.6 to 6 but by a mixed mode of ductile–brittle fracture at larger strain of 9 to 24.

Tension-induced softening and hardening in gradient nanograined surface layer in copper

With surface mechanical grinding treatment, a gradient nanograined (GNG) surface layer is produced on a bulk coarse-grained (CG) pure Cu, where the grain size increases gradually from 20 nm (topmost surface) to micrometer scale. Microhardness measurements of the GNG/CG sample after tension revealed that tension induces softening for grains smaller than 165 nm and induces hardening above this size. This critical size agrees with the strain-induced saturation grain size of Cu subjected to severe plastic deformation.