Evolution Of Grains Research Articles

Microstructural evolution and dynamic mechanical behavior of PM 2195 Al-Li alloy with resistance to shear instability in high strain rate deformation

The risk of adiabatic shear instability is common in traditional alloys covering high-strength steels, copper alloys and aluminum alloys during high strain rate deformation. However, the research on shear instability of 2195 Al-Li alloy prepared by powder metallurgy (PM) at high strain rates is still blank. The purpose is to explore its microstructure evolution and dynamic compression behavior. In this work, the 2195 Al-Li alloy prepared by PM amazingly demonstrates excellent resistance to shear instability at high strain rates during Split-Hopkinson pressure bar (SHPB) experiment. The microstructural evolution of grains and T1 (Al2CuLi) precipitate phase, and dynamic compression behavior of PM 2195 Al-Li alloy at various strain rates from 3000 s−1 to 9000 s−1 are investigated in detail employing correlative electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM). During high strain rate deformation, the alloy undergoes grain fragmentation and rotation, increasing dislocation density around the grains. When the strain rate is from 3000 s−1 to 7500 s−1, the Kernel Average Misorientation (KAM) value increases, indicating intensified localized strain. At a strain rate of 9000 s−1, a small number of grains are surrounded by nanocrystals (around 200 nm) that undergo stress release, while the average size of micrometer-sized grains is 0.81 μm. As the strain rate increases, the T1 phases in the alloy extends, vibrates, and fractures before dissolving in the matrix, while high-rate deformation promotes dislocation entanglement and defects, leading to the decomposition or ordering of precipitated phases through rapid plastic deformation. By virtue of energy dissipation strategy via the formation of nanocrystallines and dislocation behavior induced by the dissolution of T1 phases, the "positive-positive" effect is observed. The PM 2195 Al-Li alloy undergoes no shear failure even at the strain rate of 9000 s−1, showing great prospects in future applications in dynamic-loading conditions.

Microstructural evolution of GH4079 superalloy during hot deformation and heat treatment

Through hot compression and subsequent heat treatment experiments on GH4079 superalloy, the influence of different hot deformation and heat treatment conditions on the evolution of GH4079 superalloy microstructure was studied. The evolution of grains and γ′ phases under different thermomechanical conditions was investigated. The results indicate that at deformation temperatures above 1100 °C and strain rates ranging from 0.01 to 0.1 s−1, the primary mechanism of dynamic softening in the alloy is discontinuous dynamic recrystallization (DDRX). When the deformation temperature is below 1100 °C and strain rates range from 0.01 to 0.1 s−1, both continuous dynamic recrystallization (CDRX) and DDRX are the main dynamic softening mechanisms. Due to high strain energy accumulation, the samples deformed at lower temperatures (below 1100 °C) and then solution-treated at 1120 °C for 8 h exhibit significant increases in recrystallization volume fraction and recrystallization grain size. Conversely, the samples deformed at higher temperatures (above 1100 °C) and then solution-treated at 1120 °C for 8 h show minimal changes in recrystallization volume fraction and recrystallization grain size. After solution treatment at 1140 °C, the grain size of the alloy significantly increases. The samples deformed at higher strain rates exhibit the evolution of fine γ′ phases into elongated rod shapes during solution treatment, while larger γ′ phases undergo splitting processes.

Evolution of grain and ripple structures on the surface of aluminum parts fabricated by laser foil printing process at preheat temperatures

Evolution of grain and ripple structures on the surface of aluminum parts fabricated by laser foil printing process at preheat temperatures

Influence of particle crushing on the critical state line of rockfill materials

The mechanical behaviour of rockfill materials is strongly affected by particle breakage, which causes a continuous variation of the grain size distribution (GSD) until a stationary condition. Although the evolution of grain crushing caused among others by the initial grading, the relative density and stress level has been extensively studied, the relationship describing the influence of the current GSD on other soil properties such as the critical state line is not uniquely defined. In this study, a series of triaxial compression tests with various stress paths under monotonic loading were performed on a rockfill to examine the effect of particle breakage on the position of the critical state lines (CSLs) in the compression plane. Specimens were prepared with the same initial grading and relative density to investigate the evolution of the GSD as a result of grain breakage determined by different stress paths applied in a large triaxial apparatus. In addition, a recently proposed simplified procedure is employed to capture the evolution of the CSLs of the tested crushable rockfill as a function of a breakage parameter related to the breakage index Bg. Experimental results obtained for the tested rockfill demonstrate that the abovementioned procedure is capable to predict the position of the CSL linked to the GSD reached at the end of the triaxial test, also when the critical state condition, defined as the ultimate condition in which shearing could continue indefinitely without changes in volume or effective stresses, is not reached.

Microstructural evolution during superplastic deformation of commercial Al5083 alloy

The aluminum alloy SPF process has found extensive application in the aviation and rail transportation industries. The commercial Al5083 alloy presents the advantages of low density, high specific strength, a simplified manufacturing technique, and low cost, thus making it an appealing option for producing components with minor strain and complex configurations. This study focused on examining the mechanical properties of the commercially available Al5083 alloy. Different temperatures (450 °C, 480 °C, 510 °C) and strain rates (0.01 s−1, 0.005 s−1, 0.001 s−1, 0.0005 s−1) were employed to investigate the behavior of the material. Furthermore, a constitutive equation was developed to describe the alloy's response under superplastic conditions. The utilization of EBSD allowed for the investigation of the evolution of grain and dislocation density, while XRD was employed to determine the development of macroscopic texture strength and texture type. The dominant mechanism of superplastic deformation in the commercial Al5083 alloy was identified as dynamic recrystallization and intra-crystalline slip, which distinguishes it from conventional fine-grained aluminum alloys. SEM was used to establish the exponential relationship between void volume fraction and true strain. The results provide valuable insights into the evolution of the macro-mechanical properties and microstructure of commercial Al5083 alloy during superplastic forming.

Effects of cooling rate and cryogenic temperature on the mechanical properties and deformation characteristics of an Al-Mg-Si-Fe-Cr alloy

The Al-Mg-Si-Fe-Cr alloy was subjected to water quenching (WQ) and air quenching (AQ) after solution treatment for 30 min at 843 K followed by a peak artificial aging treatment at 453 K for 8 h. Precipitates of different sizes and precipitate free zone (PFZ) of different widths were obtained. Uniaxial tensile tests were conducted to investigate the effect of the microstructure and deformation temperature on the mechanical properties and deformation behavior of the WQ and AQ samples at 298 K and 77 K. The results showed that the yield strength and work hardening rate of the AQ samples were lower than those of the WQ sample. In addition, the strength and ductility of the two samples are significantly improved at cryogenic temperatures. Furthermore, quasi-in situ electron back scatter diffraction (EBSD) was conducted to investigate the orientation evolution of grains with strains of 1%, 3%, 7% and 10% for the two samples at 298 K and 77 K. The different orientation evolution of the two samples was caused by the nonuniform deformation in the PFZ and grain interior. The dislocation characteristics and fracture morphology were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Because the rod-shaped precipitates hinder the dislocation movement, no obvious slip traces could be observed on the deformed surface of the AQ sample. In addition, two slip systems are activated for most grains of the WQ sample at 77 K, but only one slip system is activated at 298 K. Due to the weak grain boundaries deformed at 298 K, the stress applied to grains is consistent with the tensile direction. However, the grain boundaries are strengthened when tested at 77 K, and the stress field of the grain boundaries is more complex, resulting in the stress not only in the tensile direction. More slip systems are activated to coordinate the deformation, improving the ductility. TEM images reveal that the deformation of the PFZ is more severe for the AQ sample at 298 K than at 77 K. The deformation in the precipitate-free zone and within the grain was more homogeneous in the AQ sample at 77 K, which increased its ductility compared to the AQ sample at 298 K.

Phase-field simulation of grain nucleation, growth, and Rayleigh distribution of U3Si2 nuclear fuel

U3Si2 is a potential accident-tolerant fuel (ATF) due to its high thermal conductivity and uranium density relative to UO2. The grain size and distribution play an essential role in the service performance of U3Si2. However, the grain evolution is quite complicated and remains unclear, which limits further application of U3Si2 in the ATF assembly. In the present work, a phase-field model is employed to investigate the nucleation and growth of grains in U3Si2. Our results show that the number of grains rises rapidly at the nucleation stage until they occupy the whole system. After that, the grain radius and area continue to grow, and the grain number decays. The grain area increases in time according to the linear law, while the mean grain radius increases with time in a power law form with the scaling growth exponent z = 0.42, which is quite close to the theoretically predicted value. Finally, we performed statistical analysis and found that the grain size evolution of U3Si2 obeys Rayleigh distribution. Our simulation not only elucidates the nucleation and evolution of grains in U3Si2 during the thermal treatment process unambiguously but also provides a fundamental study on the investigation of grain growth, subdivision, and even amorphization in the irradiated condition, which is very important for U3Si2 used as ATF in the light water reactor.

Application of a new grain boundary technology for quasi-single crystalline silicon ingots

A new functional grain boundary group (FGBG) technology was employed to generate quasi-single crystalline silicon ingots with a high single crystalline area ratio. Fabrication of the FGBG structure was achieved and special grain boundaries were formed between adjacent seed crystal strips with 〈100〉 crystal orientation, with a 30° crystal orientation angle between the special grain boundaries. The influence of FGBG on the evolution of grains, dislocation, and surface morphology was analyzed by photoluminescence and electron back-scattered diffraction. FGBG effectively suppressed the side polycrystalline grains (spontaneous nucleation on the crucible wall) intrusion at the early stage of solidification. Additionally, when the crystal growth height was increased, FGBG acted as a barrier layer for the external polycrystalline grains under the control of an appropriate horizontal temperature gradient and slightly convex solid–liquid interface shape, preventing most of the polycrystalline grains around the ingot from growing into the crystal, and blocking the dislocations generated by polycrystalline grains near the grain boundary. The application of this FGBG technology will greatly improve the quality of quasi-single crystalline silicon ingots with a weight of 1400 kg (G7).

Influence of grain boundary energy anisotropy on the evolution of grain boundary network structure during 3D anisotropic grain growth

In this paper, we present the results of grain growth simulations in three-dimensions using an existing level set method. Most of the previous grain growth studies have been either isotropic or, if they are not isotropic, consider simplified models of anisotropy. We perform these simulations using three different grain boundary (GB) energy functions (a realistic 5D GB energy function, a Read–Shockley energy model that depends only on disorientation angle, and an isotropic GB energy model). We compare the results in terms of several statistical microstructural descriptors (crystallographic texture, GB character distribution, triple junction distribution (TJD), etc.). In addition to considering different energy models, we also compare the evolution of microstructures that have different initial crystallographic textures (fiber texture and random texture). We find that the morphological evolution of individual grains can be completely different depending on the GB energy function employed. However, we also find that certain spatially independent microstructural statistics (e.g. orientation distribution function, misorientation distribution function, and TJD) are similar at steady state for all of the tested GB energy functions.

Dust grain shattering in protoplanetary discs: collisional fragmentation or rotational disruption?

ABSTRACT In protoplanetary discs, the coagulation of dust grains into large aggregates still remains poorly understood. Grain porosity appears to be a promising solution to allow the grains to survive and form planetesimals. Furthermore, dust shattering has generally been considered to come only from collisional fragmentation; however, a new process was recently introduced, rotational disruption. We wrote a one-dimensional code that models the growth and porosity evolution of grains as they drift to study their final outcome when the two shattering processes are included. When simulating the evolution of grains in a disc model that reproduces observations, we find that rotational disruption is not negligible compared to the fragmentation and radial drift. Disruption becomes dominant when the turbulence parameter α ≲ 5 × 10−4, if the radial drift is slow enough. We show that the importance of disruption in the growth history of grains strongly depends on their tensile strength.

Subgrain Coalescence Simulation by Means of an Advanced Statistical Model of Inelastic Deformation

The development of technological methods for processing and manufacturing of functional (with a priori targeted properties) polycrystalline materials and products made of these materials still remains an acute problem. A multilevel modeling approach offers researchers the opportunity to describe inelastic deformation by applying internal variables that give an effective characterization of the material structure at different structural scale levels. High temperature plastic deformation is accompanied by these processes, which leads to a significant rearrangement of the meso- and microstructure of the material. The most substantial contribution to changing the properties of polycrystals is made by the evolution of grain and defect structures at the expense of dynamic recrystallization, which significantly depends on dynamic recovery. In this paper, we consider the problem of the coalescence of subgrains undergoing rotation during inelastic hot deformation. This process is called subgrain coalescence, and it is one of the dynamic recovery mechanisms responsible for changes in the fine subgrain structure. Under applied thermomechanical loads, the coalescence process promotes the formation of recrystallization nuclei and their subsequent growth, which can greatly change the grain structure of a polycrystal. The problem was solved in terms of the advanced statistical model of inelastic deformation, modified to describe the subgrain coalescence process. The model takes into account the local interactions between contacting structural elements (subgrains). These have to be considered so that the grain coalescence caused by a decrease in subboundary energies during their progressive merging can be adequately analyzed. For this purpose, a subgrain structure quite similar to the real structure was modeled using Laguerre polyhedra. Subgrain rotations were investigated using the developed model, which relies on the consideration of the excess density edge component of the same sign dislocations on incidental subgrain boundaries. The results of modeling of a copper polycrystal are presented, and the effects of temperature and strain rate on the subgrain coalescence process is demonstrated.

Research of rolling-drawing coupled deformation on the microstructure-property evolution and strengthening mechanism of 6201 conductive tubes

To meet the major needs of national defense, transportation, aerospace, and other fields, aluminum alloys with excellent comprehensive performance are continually developed. To reduce the high cost of preparing high-performance aluminum alloys and alleviate the limitations of existing research, the effects of a rolling-drawing coupled deformation on the microstructure-property evolution and strengthening mechanism of 6201 conductive tubes developed by using two different processes were investigated. Subsequently, the evolution of grains, precipitates, texture, and dislocation were examined by performing optical microscopy, scanning electron microscopy, X-ray diffraction, and electron backscattered diffraction. The results showed that the main phases of the investigated tubes were the α-Al matrix, Mg2Si, and Al0.5Fe3Si0.5 second phase. The Al0.5Fe3Si0.5 phase with a face-centered cubic structure functioned as the inhomogeneous nucleation core of Mg2Si. In the rolling process, severe plastic-deformation-induced re-dissolution of the precipitated phases was observed, improving the effect of the subsequent solution treatment and aging treatment. An optimal ultimate tensile strength (UTS) of 341 MPa and electrical conductivity (EC) of 54.12% IACS were successfully obtained in the “extrusion-rolling-solution-aging-drawing” process. Compared with those of the initial tube, these values increased by 136.81% and 41.60%, respectively. However, after the order of rolling and solution treatments was reversed, the UTS and EC of the final tube decreased to 325 MPa and 52.91% IACS, respectively. The microstructure and performance evolution revealed that the uniformly distributed fine Mg2Si phase resulting from the good fit of the processes plays an important role in achieving a higher UTS and EC.

Effect of heat treatment temperature on the microstructural evolution of CM247LC superalloy by laser powder bed fusion

To attain the desired mechanical properties of an additively manufactured component, robust post-processing in term of thermal treatment is highly required to reduce the crystallographic anisotropy. However, the microstructure appearance with respect to the post-treatment temperature is not well understood mechanistically. In this study, the microstructural evolution of grains of a laser-powder bed fused (L-PBF) nickel-base superalloy, CM247LC, during post-processing heat treatment is investigated systematically. Recrystallization barely happens below the γ' solvus temperature leading to a remaining unhomogenized dendritic(cellular) structure. However, recrystallization is introduced above the γ' solvus temperature. By considering the grain boundary (GB) migration mechanisms and supported by experimental observations, the sluggish recrystallization behavior of this γ'-strengthened nickel-based superalloy has been understood. Owing to lack of the difference in stored energy between adjacent grains, this primary driving force is constrained. The GB migration is majorly driven by capillarity force (1–10 MPa) before the recrystallization occurrence, which is evident by the evolution of GB curvatures. On the other hand, the Zener pinning force generated from GB precipitates including carbides and γ' precipitates provides the dragging force in the comparable scale (1–10 MPa) against the GB migration.

Experimental Simulation of Burial Diagenesis and Subsequent 2D-3D Characterization of Sandstone Reservoir Quality

Characterization of deeply buried sandstones and their reservoir quality is of paramount importance for exploring, developing, and subsurface storage of energy resources. High reservoir quality in deeply buried sandstones is commonly correlated with the occurrence of grain coatings that inhibit quartz cementation. The development of reliable models that can predict reservoir quality relies on incorporating quantitative understanding of these diagenetic processes. Hydrothermal experiments simulating burial diagenesis were integrated with multi-scale X-ray tomography to quantify the 3-dimensional evolution of grain coating volume and porosity with increasing temperature; while microscopic and automated quantitative mineralogy analysis were used to track the associated mineralogical alterations. To simulate reservoir evolution, sandstone samples from the Lower Jurassic Cook Formation (Oseberg Field, 30/6-17R, Norway) were exposed to a silica supersaturated Na2CO3 (0.1 M) solution for up to 360 h at temperatures of 100–250°C. The experimental results show the main porosity and permeability reduction window is associated with pore-filling kaolinite, and lies between 150 and 200°C, above which little change occurs. Volumetric increases in grain coating start to occur at ∼150°C through precipitation of authigenic chlorite, and continue to 250°C, irrespective of the experimental duration. Together with preexisting siderite coatings, the newly precipitated chlorite prevents the loss of reservoir quality by inhibiting quartz overgrowth development. Pore flow simulations based on the observed temperature-dependent 3-dimensional pore networks allow us to characterize pore-throat and permeability evolution and gain quantitative understanding of the impact of diagenetic overprinting on deeply buried sandstone reservoirs.

Extrinsic Anisotropy of Two-Phase Newtonian Aggregates: Fabric Characterization and Parameterization.

Rocks of the Earth's crust and mantle commonly consist of different minerals with contrasting mechanical properties. During progressive, high‐temperature (ductile) deformation, these rocks develop extrinsic mechanical anisotropy linked to strain partitioning between different minerals, amount of accumulated strain, and bulk strain geometry. Extrinsic anisotropy plays an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain‐ and rock‐scale fabrics causing this anisotropy cannot be directly simulated in large‐scale numerical simulations. For two‐phase aggregates–a good rheological approximation of most Earth's rocks–we propose a method to indirectly approximate the extrinsic viscous anisotropy by combining (a) 3D mechanical models of rock fabrics, and (b) analytical effective medium theories. Our results confirm that weak inclusions induce substantial weakening by forming a network of weak thin layers with limited lateral connectivity. Consequently, even when the inclusion phase is extremely weak, structural weakening is not larger than 30–60%, less than in previous estimates. On the other hand, the presence of strong inclusions does not have a profound impact on the effective strength of the aggregate, and lineated fabrics only develop at relatively low viscosity contrasts. When rigid inclusions become clogged, however, the aggregate viscosity can increase over the theoretical upper bound. We show that the modeled grain‐scale fabrics can be parameterized as a function of the bulk deformation and material phase properties and combined with analytical solutions to approximate the anisotropic viscous tensor.

Grain-growth mediated hydrogen sorption kinetics and compensation effect in single Pd nanoparticles

Grains constitute the building blocks of polycrystalline materials and their boundaries determine bulk physical properties like electrical conductivity, diffusivity and ductility. However, the structure and evolution of grains in nanostructured materials and the role of grain boundaries in reaction or phase transformation kinetics are poorly understood, despite likely importance in catalysis, batteries and hydrogen energy technology applications. Here we report an investigation of the kinetics of (de)hydriding phase transformations in individual Pd nanoparticles. We find dramatic evolution of single particle grain morphology upon cyclic exposure to hydrogen, which we identify as the reason for the observed rapidly slowing sorption kinetics, and as the origin of the observed kinetic compensation effect. These results shed light on the impact of grain growth on kinetic processes occurring inside nanoparticles, and provide mechanistic insight in the observed kinetic compensation effect.

Anomalous strain-energy-driven macroscale translation of grains during nonisothermal annealing

We report a mode of grain growth, involving the macroscopic translation of grain centers during nonisothermal annealing. Through synchrotron high-energy x-ray diffraction microscopy, we find dissolution of semicoherent precipitates generates dislocations, thereby raising the stored strain energy within grains. The subsequent evolution of grains shows unexpected grain translations over length scales of 10--100 \ensuremath{\mu}m. Phase-field simulations reveal that such translations are not uncommon in strain-energy-driven grain growth, wherein different regions of a grain may grow and shrink simultaneously.

Evolution of Grain and Seed Cleaning Equipment in Russia

The authors determined that in the development of grain production technical support, the main role was played by such a system-forming factor as the transition of the country’s grain enterprises to private ownership. Over the past 30 years, private companies and individuals became grain owners (with the exception of a small share). The authors substantiated the necessity for the development of grain-seed cleaning equipment in Russia. They showed the relevance of developing a new scientific and technical policy in the field of machine development support for grain production and its optimal functioning.(Research purpose) To carry out an evolutionary analysis of grain and seed cleaning equipment development and functioning, to determine the main periods and system-forming factors of its development.(Materials and methods) The authors applied the historical-analytical method in addition to technical systems. As research objects, they studied the original works of domestic and foreign authors for more than 100 years: monographs, dissertations, reports of research institutions, machine testing stations protocols, scientific journals, conference materials, as well as descriptions for domestic and foreign patent documentation.(Results and discussion) They described the evolution of grain-seed cleaning equipment development over the past 150 years: from the simplest tools to complex machine systems of industrial flow technologies. The authors presented the main characteristics of grain-seed cleaning machines and indicators of grain mixtures separation by sieves processes at various stages.(Conclusions) The main system-forming factors influencing the technical support of grain production such as: socio-economic conditions; soil and climatic conditions; science and technology policy; organizational factors were revealed. They substantiated four stages of its development: the first (1870-1930) – the use of the simplest manual machines of foreign production; the second (1930-1950) – the birth of domestic production of grain-seed cleaning equipment; the third (1950-1991) – the transition from the use of separate machines to industrial flow technologies; the fourth (1991 – to the present) – the transition from traditional flow technology to a variety of technologies and machines.

Hydride growth mechanism in zircaloy-4: Investigation of the partitioning of alloying elements

The long-term safety of water-based nuclear reactors relies in part on the reliability of zirconium-based nuclear fuel cladding. Yet the progressive ingress of hydrogen during service makes zirconium alloys subject to delayed hydride cracking. Here, we use a combination of electron back-scattered diffraction and atom probe tomography to investigate specific microstructural features from the as-received sample and in the blocky-α microstructure, before and after electrochemical charging with hydrogen/deuterium followed by a low temperature heat treatment at 400 °C for 5 h followed by furnace cooling at a rate of 0.5 °C/min. Specimens for atom probe were prepared at cryogenic temperature to avoid the formation of spurious hydrides. We report on the compositional evolution of grains and grain boundaries over the course of the sample's thermal history, as well as the ways the growth of the hydrides modifies locally the composition and the structure of the alloy. We observe a significant amount of deuterium left in the matrix, even after the slow cooling and growth of the hydrides. Stacking faults form ahead of the growth front and the segregation of Sn at the hydride/matrix interface and on these faults. We propose that this segregation may facilitate further growth of the hydride. Our systematic investigation enables us discuss how the solute distribution affects the evolution of the alloy's properties during its service lifetime.

Microscale Observation via High-Speed X-ray Diffraction of Alloy 718 During In Situ Laser Melting

The laser melting process is accompanied by rapid evolution in temperature, phase, structure, and strain because of its high heating and cooling rates. In this study, the evolution of grains within a thin solid plate of Ni alloy 718 during laser processing was probed with in situ high-energy x-ray diffraction experiments. The high temporal and spatial resolution available in the measurement allowed us to study the rapid evolution of the melted region beneath the surface of the sample. The characterization of the evolution of secondary phases, i.e., Laves and carbide, was captured despite the weak diffracted peaks caused by small volume fractions. Thermal history was estimated based on changes in the lattice spacing from the thermal contraction upon cooling. The temporal variation in 2θ with azimuthal direction revealed the evolution in anisotropy of lattice spacing and thus of the mechanical state during laser processing.