Non-equilibrium Grain Research Articles

Acceleration of grain boundary diffusion during ultra-fast firing (UHS) of alumina powder compacts

The sintering rate of a ceramic at a given temperature and relative density is often higher if the heating to that sintering temperature is more rapid. This has previously been explained in terms of microstructural coarsening during slow heating, which degrades the sinterability of the body before it reaches the sintering temperature. In this work the sintering of pure alumina with conventional heating rates (CS, 100–900 °C/hour) was compared with ultra-fast firing, using the method known as ultrafast high-temperature sintering (UHS, 120–140 °C/s). Despite the wide difference in heating rate, the SEM microstructures for a given density were similar and a modified Master Sintering Curve analysis of the results showed that the small pore size reduction observed was not sufficient to account for the acceleration in sintering of UHS samples compared with CS. There remained an additional acceleration by a factor of up to ∼20 that was attributed to a higher grain boundary diffusion coefficient in UHS. It is suggested that this is the result of the boundaries forming between powder particles not having time to relax to their lowest energy, most compact structures during UHS, i.e. non-equilibrium grain boundaries. In support of this hypothesis, it is shown that powder compacts that were first partially pre-sintered with conventional heating rates and then subjected to UHS did not show the same acceleration of sintering displayed by UHS of green bodies. The implication is that the slow pre-sintering allowed relaxation of the grain boundaries to low energy structures with low diffusion coefficients.

The capacity of disclinated non-equilibrium GBs to accommodate point defects in tungsten

Experimental observations reveal that disclinated non-equilibrium grain boundaries (GBs) exist extensively in polycrystalline metals; however, the interaction between these GBs and irradiation-induced point defects is rarely reported. In the present work, Molecular statics simulation was used to evaluate the capacity of disclinated non-equilibrium GBs to accommodate point defects in tungsten. Simulation results showed that disclinated non-equilibrium GBs are more efficient sinks for point defects than their equilibrium counterparts because of long-range stress field around them. Continuous segregation of point defects will change the structure of the disclinated non-equilibrium GBs and leads to the GBs tending to relax to equilibrium state. According to theoretical calculation, the disclinated non-equilibrium GBs can absorb a large number of point defects before transforming into equilibrium state; therefore, disclinated non-equilibrium GBs have very strong capacity to accommodate point defects and can be used as strong defect sinks for developing radiation resistance materials.

Tailoring surface properties and corrosion resistance of laser shock peened Ti6Al4V alloy by low-temperature annealing

The present study examined the effects of laser shock peening (LSP) and low-temperature annealing after LSP (LSPA) on the corrosion resistance of the Ti6Al4V alloy. The high density of dislocations in the LSP samples contributes to the rapid formation of a TiO₂ passive film during the electrochemical corrosion process. In this study, low temperature annealing was employed to further improve the corrosion resistance. The results indicated that the corrosion resistance of the LSPA samples was significantly improved compared to that of the LSP-treated samples. The LSPA treatment facilitated the pre-generation of a dense TiO2 passive film on the alloy surface via oxidation, reduced corrosion rate, inhibited the penetration of dissolved oxygen and ions present in the sodium chloride solution into the alloy surface. Meanwhile, low-temperature annealing could rearrange and annihilate dislocations, creating subgrains and relaxing non-equilibrium grain boundaries induced by deformation. This process reduced active sites and grain boundary energy, ultimately improving the corrosion resistance of the Ti6Al4V alloy.

Calorimetric and Volumetric Studies of Dislocations During Martensitic Transformations in Shape Memory TiNi Alloy

Based on the use of appropriate approaches, methods and results of the analysis of a number of topical problems of physical materials science, an in-depth analysis has been done of calorimetric and volumetric data for direct, reverse and deformation martensitic transformations in a nanostructured Ti49.3Ni50.7 shape memory alloy obtained by cold rolling with simultaneous action of pulsed high-density current. For the first time, by applying a new technique for processing calorimetric spectra (peaks), the staging and “kinetics” of changes in heat content, as well as thermal effects (enthalpies of individual stages) during direct and reverse martensitic transformations during cooling or heating of samples at a constant rate (3 × K/ min) in the range 170–370 K has been done. It is shown, by processing volumetric data, using theoretical values of the dislocation density and elements of the classical theory of dislocations, that in the Ti49.3Ni50.7 shape memory alloy subjected to cold deformation accompanied by the action of a pulsed current, a deformation martensitic transformation occurs, leading to a positive volume effect (∆V/V) ≈ 3 × 10–3), which can be largely due to dislocations. It is shown, by applying the theoretical values of the dislocation density and elements of the classical theory of dislocations, that the possible contributions of dislocations to the enthalpies of direct and reverse martensitic transformations (in the Ti49.3Ni50.7 alloy) can and should be significantly lower in absolute value, but opposite in sign to the observed enthalpies of direct and reverse martensitic transformation in a given alloy. It is shown that the physics of the processes under consideration is contained to a certain extent in scientific discoveries No. 239 “The phenomenon of thermoelastic equilibrium during phase transformations of the martensitic type – the Kurdyumov effect” and No. 339 “The phenomenon of the formation of non-equilibrium grain boundaries in polycrystals when they absorb lattice dislocations”.

Revealing the high strength and high thermal stability of a nano-lamellar Cu-0.1 at.% Zr alloy

Nano-lamellar structured Cu-0.1 at.% Zr alloys are prepared by the combination of equal channel angular pressing and subsequent cryogenic rolling. The advanced processing route enables a systematic reduction of the lamellae boundary spacing down to 45 nm and a concurrent increase of yield strength up to 626 MPa. Subsequent annealing at temperatures up to 673 K induced recovery processes and uniform coarsening through triple junction motion, avoiding a catastrophic degradation of the microstructure and performance and preserving the yield strength at a level comparable to ultrafine-grained copper. Simultaneously, the tensile uniform elongation experienced a substantial recovery up to 11.9 %. The high thermal stability is mainly attributed to the low mobility of grain boundary and the relaxation of non-equilibrium grain boundaries, both of which are facilitated by the microalloying of Cu with Zr. Based on the theory of dislocation pile-up at boundaries, the Hall-Petch equation is adopted and extended to the lamellar structured Cu-0.1 at.% Zr alloy, revealing a lower-critical bound of 90 nm for lamellae boundary spacing, corresponding to an effective boundary spacing of about 150 nm.

High corrosion resistance of nanograined and nanotwinned Al0.1CoCrFeNi high-entropy alloy processed by high-pressure torsion

This study examines the corrosion resistance of ultrafine-grained Al0.1CoCrFeNi high-entropy alloy processed by high-pressure torsion (HPT), focusing on stored energy, diffusion, and the high-entropy effect. Microstructural analysis revealed a single-phase FCC structure with numerous defects and reduced crystallite size following the HPT process. Samples with higher HPT turns formed high-angle grain boundaries, equiaxed nanograins, and numerous nanotwins, contributing to a notable increase in Vickers microhardness from 159 Hv for the initial sample to 550 Hv after 5 turns of HPT. Corrosion behavior is found to be influenced by stored energy and chromium diffusion. Low HPT turns increased the corrosion rate due to sluggish diffusion of chromium and high stored energy in the form of dislocations. Conversely, corrosion resistance improved with increased HPT turns due to fast diffusion through non-equilibrium grain boundaries and the formation of nanotwins with lower stored energy. These findings suggest that a relatively low entropy plays a critical role in the chromium segregation from the solid solution for forming a passive film and enhancing corrosion resistance.

A multiscale FEM-MD coupling method for investigation into atomistic-scale deformation mechanisms of nanocrystalline metals under continuum-scale deformation

This study aims to develop a multiscale bridging method for investigating nanocrystalline metals based on macro-scale deformation. For this purpose, we propose a hierarchical multiscale computational method that can focus on some of the elements in a finite element model for scale bridging to atomistic-scale models. This method assumes that atomistic-scale nanocrystalline models are related to the integration points in a finite element and deform based on the macro-scale deformation. Nanocrystalline aluminum was chosen for the validation of the multiscale method. The finite element method (FEM) and the molecular dynamics (MD) method were used for continuum-scale and atomistic-scale simulations, respectively. We utilized the notion of the CauchyBorn rule (CBR) for communicating deformation information from the continuum scale to the atomistic scale. We studied three different cases with two nanocrystalline models and two loading cases to compare differences resulting from crystal structures and loading. Based on the crystal structure change during relaxation, nonequilibrium grain boundaries (NEGBs) were shown to play a role as deformation mechanisms in the plastic regime and induce the onset and migration of crystal defects, including deformation twins, as reported in the experiment. Furthermore, the crystal orientation dependence of the onset of crystal defects was confirmed by the comparison of the results from the two different nanocrystalline models. The qualitative agreement of the results with experimental observations is also confirmed. The proposed ‘FEM-MD’ method can bridge a large-scale gap, for example, from a nano-scale to a continuum-scale such that an MD model can be coupled to a millimeter or centimeter scale compared to other embedding methods. The present method is ideal for investigating the dislocation behavior of nanocrystalline materials, which contain multi-grained nanostructure at finite temperature, undergoing various loading scenarios at the macro-scale.

Resolving the dynamic properties of entangled linear polymers in non-equilibrium coarse grain simulation with a priori scaling factors.

The molecular weight of polymers can influence the material properties, but the molecular weight at the experiment level sometimes can be a huge burden for property prediction with full-atomic simulations. The traditional bottom-up coarse grain (CG) simulation can reduce the computation cost. However, the dynamic properties predicted by the CG simulation can deviate from the full-atomic simulation result. Usually, in CG simulations, the diffusion is faster and the viscosity and modulus are much lower. The fast dynamics in CG are usually solved by a posteriori scaling on time, temperature, or potential modifications, which usually have poor transferability to other non-fitted physical properties because of a lack of fundamental physics. In this work, a priori scaling factors were calculated by the loss of degrees of freedom and implemented in the iterative Boltzmann inversion. According to the simulation results on 3 different CG levels at different temperatures and loading rates, such a priori scaling factors can help in reproducing some dynamic properties of polycaprolactone in CG simulation more accurately, such as heat capacity, Young's modulus, and viscosity, while maintaining the accuracy in the structural distribution prediction. The transferability of entropy-enthalpy compensation and a dissipative particle dynamics thermostat is also presented for comparison. The proposed method reveals the huge potential for developing customized CG thermostats and offers a simple way to rebuild multiphysics CG models for polymers with good transferability.

Microstructure evolution of ultra-fine grained HfC ceramics with high hardness sintered under ultrahigh pressure by plastic deformation densification mechanism

HfC ceramics has potential for serving in extreme environment but it is impossible to achieve full densification with fine grains. Currently, ultra-high pressure sintering is an effective method to promote densification of ultra-high temperature ceramics and suppress grain growth. In this study, fully dense HfC ceramics with high hardness and ultrafine grains was fabricated under 15 GPa at relative low temperature. The nanocrystalline HfC ceramic with grain size of 130 nm, high Vickers hardness of 23.5 GPa and fracture toughness of 2.54 MPa·m1/2 was sintered at 1500 °C. The microstructure characterization identified that HfC ceramics sintered at 1500 °C formed low angle subgrains, twin boundaries and stacking faults, which enhanced the hardness and refined grains. However, the low angle subgrain boundaries transformed to high angle grain boundaries and grains grow up with temperature increasing while the dislocation density decreased. These experimental phenomenon and calculation model demonstrate that plastic deformation was the dominating densification mechanism. The nonequilibrium grain boundary and lattice strain field results proved that the plastic deformation densification mechanism exhibited positive influence on the mechanical properties.

Specific Features of Grain Boundaries in Nickel Processed by High-Pressure Torsion

Publications on obtaining bulk nanostructured materials with special properties by various modes of severe plastic deformation, using nickel as an example, are briefly reviewed. Particular attention is paid to the state of grain boundaries, commonly referred to as nonequilibrium, or deformation-modified boundaries, revealed by electron microscopy, including scanning tunneling, Mossbauer spectroscopy, and diffusion studies. It is shown that contribution of specific state of grain boundaries to additional strengthening is often overestimated. In particular, in Ni processed by high pressure torsion, nonequilibrium grain boundaries are formed, which have increased energy and are ultrafast diffusion paths, but they contribute relatively little to the total strengthening.

Effect of low-temperature annealing on microstructure and mechanical properties of Ti6Al4V produced by laser shock peening

In this study, a combination of laser shock peening (LSP) and low-temperature annealing (LTA) was used to modulate the microstructures of Ti6Al4V, which can achieve an optimal combination of strength and plasticity. Microstructure observations showed that after LSP treatment, serious plastic deformation and grain refinement occurred. This resulted in the formation of non-equilibrium grain boundaries (GBs) composed of dislocations, twins, and martensite transformation. After LTA treatment, the dislocation density decreased as it rearranged, resulting in GB relaxation. The GB state can change from non-equilibrium to equilibrium. Additionally, LTA treatment can increase the activation energy and facilitate element diffusion from high to low concentration. The decrease in dislocation density and element diffusion results in GB with low excess energy, increasing the stability of the microstructural. The LSP composite LTA treatment showed a high tensile strength of 994 MPa and an excellent elongation of 17.4% compared to the untreated specimen. Excellent microstructure stability and mechanical properties are achieved in Ti6Al4V after combined post-treatment of LSP and LTA.

Understanding the recrystallization and phase transformation of gradient nanostructured M50 steel under the athermal effect of electrical pulses

M50 steel is a commonly used material for aeroengine main shaft bearings. In this study, a gradient nanostructured M50 steel was fabricated by an efficient surface nanocrystallization method, i.e., ultrasonic shot peening (USP) technology. The recrystallization behavior and phase transformation of gradient nanostructured M50 steel under the athermal effect of electrical pulses were investigated. Compared with the USPed sample, the ferrite structure size was 32.5% smaller, and the structure size uniformity was 51.5% greater in the 0–50 μm region after USP treatment and electrical pulsing treatment (EPT) with 38 A/mm2–1 s. Athermally induced ferrite recrystallization by EPT accompanied with obvious precipitation of VC and Mo2C carbides, and decomposition of austenite occurred during ferrite structure coarsening. EPT has different responses to different depths of gradient M50 steel, and there is an optimal current density to achieve structure refinement in different depths. However, the coarse-grain tempered M50 steel did not undergo obvious ferrite structure refinement under the same EPT parameters. The results revealed that the EPT promoted static recrystallization of incompletely recrystallized structures after USP processing by activating the release of stored deformation energy. Static recrystallization under the athermal effect of EPT was achieved by promoting the migration of the subgrain boundary to absorb the surrounding dislocations and increase the misorientation between itself and the surrounding ferrite structure. The mechanism of the athermal effect of EPT was based on electron-defect interactions, such as dislocations and lattice distortions. High-density nonequilibrium grain boundaries, acting as “active sites”, accelerating the process of recrystallization under the effect of electric pulses. This study proposed a method for optimizing EPT parameters through the change in instantaneous resistance to evaluate the degree of recrystallization of the gradient structure.

Grain Boundary Diffusion of 57Co in Ultrafine Grained Niobium Processed by Severe Plastic Deformation

Grain-boundary diffusion of Co in ultrafine-grained Nb processed by severe plastic deformation by high-pressure torsion has been studied by layer-by-layer radiometric analysis. Coefficients of grain-boundary diffusion for several temperatures have been determined. It is shown that diffusion along grain boundaries proceeds much faster than in coarse-grained niobium with relaxed boundaries, which is explained by the formation of "non-equilibrium" grain boundaries under the severe plastic deformation, which are the paths of ultrafast diffusion. Recovery processes occur in non-equilibrium grain boundaries under heating, due to which their properties approach to those of conventional high-angle boundaries in the coarse-grained material.

Microstructural evolution and oxidation in α/β titanium alloy under fretting fatigue loading

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation, thus potentially causing unanticipated catastrophic fractures. The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous, which obstructs the understanding of fretting fatigue issues, and is unable to guarantee the reliability of structures for long-term operation. Here, fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of an α/β titanium alloy up to 108 cycles. Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone. The grain size in the topmost nanostructured layer could be refined to ∼40 nm. The grain refinement process involves the initial grain rotation, the formation of low angle grain boundary (LAGB; 2°–5°), the in-situ increments of the misorientation angle, and the final subdivision, which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays. The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary (HAGB) and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer, thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress. Eventually, it facilitates surface degradation and the formation of catastrophic fractures.

Влияние состояния границ зерен на эффект пластификации в ультрамелкозернистом сплаве Al-0.4Zr

The influence of small additional deformation by cold rolling (CR) on the microstructure and mechanical properties of ultrafine-grained (UFG) Al–0.4Zr alloy structured by high pressure torsion (HPT) has been studied. The results are compared with the application of small additional deformation by HPT, which, after low-temperature annealing, leads to a substantial increase in ductility (plasticization effect) with small decrease in strength. It is shown that, in contrast to the additional deformation of HPT, the deformation of CR after intermediate low-temperature annealing leads to a sharp drop in plasticity to ~2%, while the strength increases to ~275 MPa. The key role of the nonequilibrium state of grain boundaries in the manifestation of the plasticization effect in the UFG Al–0.4Zr alloy is revealed. A new approach is proposed for simultaneously increasing the strength and ductility of the UFG Al–0.4Zr alloy due to a small additional deformation by CR without intermediate annealing. As a result of this approach, a significant increase in strength by ~30% (ultimate tensile strength ~223 MPa) was achieved with a simultaneous increase in ductility up to ~26%, which is associated with an increase in the strain hardening rate due to an increase in the density of lattice dislocations in the UFG structure with nonequilibrium grain boundaries. The strain rate sensitivity and strain hardening coefficients have been determined for the UFG Al–0.4Zr alloy in various states.

A MODEL OF MICROSTRUCTURE EVOLUTIONUNDER HIGH- STRAIN RATE DEFORMATION OF COPPER

A physical model of high-strain rate deformation of metallic materials has been developed to describe the evolution of microstructure during the process. The model is based on representations of the mechanisms of grain boundary sliding, strain-induced grain growth and dynamic fragmentation of coarse grains developed in the framework of the theory of nonequilibrium grain boundaries. These representations are modified for the description of high-speed flow and supplemented by a model of dynamic recrystallization. It is shown that during high-strain rate deformation of metallic materials, grain boundary sliding dominates in the material regions with a fine-grained submicron structure and the material deforms in the superplasticity mode. In regions of material with larger grain size, grain volume deformation dominates, accompanied by fragmentation and formation of fine-grained structure with submicron grain size. The transformation of the structure from fine-grained to coarse-grained occurs due to strain-induced grain growth. Transformation of structure from coarse grain to fine grain occurs due to dynamic fragmentation. The transition from grain boundary sliding to grain volume deformation occurs at grain size corresponding to the optimum value for the course of dynamic recrystallization. The cyclic change of deformation mechanisms (from grain boundary strain-induced to grain volume deformation) is caused by alternation of the processes of deformation-stimulated grain growth and dynamic fragmentation. The alternation of the above cycles of grain boundary and intragranular deformation in different regions of the material makes it possible to maintain a high strain rate and suppress the processes of both local hardening and local softening. The high strain rate is achieved at the expense of a high strain of the regions in which grain sliding dominates. In the regions dominated by intragranular deformation, the structure is “prepared” for superplastic flow.

Effect of σ-Phase on the Strength, Stress Relaxation Behavior, and Corrosion Resistance of an Ultrafine-Grained Austenitic Steel AISI 321

This paper reported the results of research into the effect of Equal Channel Angular Pressing (ECAP) temperature and 1-h annealing temperature on mechanical properties, stress-relaxation resistance, and corrosion resistance of austenitic steel AISI 321L with strongly elongated thin δ-ferrite particles in its microstructure. The formation of α′-martensite and fragmentation of austenite grains takes place during ECAP. Ultrafine-grained (UFG) steels demonstrate increased strength. However, we observed a reduced Hall–Petch coefficient as compared with coarse-grained (CG) steels due to the fragmentation of δ-ferrite particles. UFG steel specimens were found to have 2–3 times higher stress-relaxation resistance as compared with CG steels. For the first time, the high stress-relaxation resistance of UFG steels was shown to stem from a internal stress-relaxation mechanism, i.e., the interaction of lattice dislocations with non-equilibrium grain boundaries. Short-time 1-h annealing of UFG steel specimens at 600–800 °C was found to result in the nucleation of σ-phase nanoparticles. These nanoparticles affect the grain boundary migration, raise strength, and stress-relaxation resistance of steel but reduce the corrosion resistance of UFG steel. Lower corrosion resistance of UFG steel was shown to be related to the formation of α′-martensite during ECAP and the nucleation of σ-phase particles during annealing.

An OpenMP GPU-offload implementation of a non-equilibrium solidification cellular automata model for additive manufacturing

In this paper, performance strategies on GPU-based HPC platforms of a cellular automata (CA) simulation code for non-equilibrium solidification, including nucleation, grain growth, solute partitioning and transport for the metal additive manufacturing (AM) process are investigated using OpenMP 4.5. To accurately report the speed-up for multicore CPUs and GPUs, a rigorous performance analysis employed optimizations appropriate for both CPU-only code (baseline) and GPU offload codes for an isothermal test problem. The performance results on Summit at the Oak Ridge Leadership Computing Facility indicate that using a precomputed list of interface cells significantly decreased the wall-clock time on GPUs. The speedup due to GPU acceleration was evaluated for a full Summit node and measured to be 1.8X when comparing a 6 MPI tasks run with 6 GPUs versus 36 MPI tasks on the CPU only. That speed-up was found to be 7.9X when comparing 6 MPI tasks with 6 GPUs versus the 6 MPI tasks running on the CPU only. Performance measurements showed that system total time is almost constant for runs with more than 96 MPI tasks (or GPUs), indicating that the GPU-accelerated code showed an excellent weak scaling performance.Finally, a rapid directional solidification problem was considered to demonstrate the CA code capability on Summit. It was found that a mesh size of at least 0.05 μm is recommended for the AM-like simulations in order to obtain accurate elongated grain microstructure and elongated subgrain features, which are in qualitative good agreement with experimental data. The results presented in this study indicate that the performance strategies on GPU-based HPC platforms for the CA code are appropriate for novel HPC exascale platforms.

Interaction between disclinated non-equilibrium grain boundaries and radiation-induced interstitial/vacancy in tungsten

Experimental works show that there are plenty of disclinated non-equilibrium grain boundaries (GBs) in polycrystalline materials obtained by the severe plastic deformation method. How these GBs affect the irradiation-induced defects is still an open question. In the present work, molecular dynamics simulation was used to investigate the interaction between disclinated non-equilibrium GBs and irradiation-induced interstitial/vacancy in tungsten. There exists a long-range stress field around the disclinated non-equilibrium GBs. Such a long-range stress field leads to strong interaction between interstitial/vacancy and the GB. The interaction energy calculations suggest that interstitial and vacancy can be attracted strongly by non-equilibrium GBs containing negative and positive disclinations, respectively. This unique interaction behavior is further confirmed by diffusion of interstitials/vacancies near these GBs. The present work clearly demonstrates that disclinated non-equilibrium GBs are stronger irradiation-induced defect sinks than their equilibrium counterparts. So increasing the proportion of disclinated non-equilibrium GBs may be an effective way to develop new-generation irradiation-resistant materials.

Additive manufacturing of 15-5PH/WC composites with the synergistic enhancement of strength and ductility

Directed energy deposition of 15-5PH/WC composites with tailored reinforcement architecture was carried out by mechanical mixing of 15-5 PH powders and WC particles. The 15-5PH/WC composites exhibit a synergistic enhancement of strength and ductility than that without WC addition, due to the combined effects of grain refinement, non-equilibrium grain boundary strengthening, solution strengthening, and transformation-induced plasticity.