Non-equilibrium State Of Grain Boundaries Research Articles

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.

“Non-equilibrium” grain boundaries in additively manufactured CoCrFeMnNi high-entropy alloy: Enhanced diffusion and strong segregation

Grain boundary diffusion in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy is systematically investigated at 500 K under the so-called C-type kinetic conditions when bulk diffusion is completely frozen. In the as-manufactured state, general (random) grain boundaries are found to be characterized by orders-of-magnitude enhanced diffusivities and a non-equilibrium segregation of (dominantly) Mn atoms. These features are explained in terms of a non-equilibrium state of grain boundaries after rapid solidification. The grain boundary diffusion rates are found to be almost independent on the scanning/building strategy used for the specimen’s manufacturing, despite pronounced microstructure differences. Grain boundary migration during diffusion annealing turned out to preserve the non-equilibrium state of the interfaces due to continuous consumption of the processing-induced defects by moving boundaries. Whereas the kinetic “non-equilibrium” state of the interfaces relaxes after annealing at 773 K, the non-equilibrium segregation is retained, being further accompanied by a nano-scale phase decomposition at the grain boundaries. The generality of the findings for additively manufactured materials is discussed.

Application of Nuclear Gamma-Resonance Spectroscopy for Determination of Grain-Boundary Diffusion Characteristics and the State of Grain Boundaries

Capabilities of application of Mössbauer spectroscopy for determination of grain-boundary diffusion parameters in coarse-grained and ultrafine-grained materials have been analyzed. Application of this method for revealing of non-equilibrium state of grain boundaries in ultrafine-grained materials obtained by severe plastic deformation is demonstrated.

Simulation of the Effect of Ultrasound on the Dislocation Structure of Deformed Polycrystals

Computer simulation was used to study the relaxation of disordered systems of dislocations in the stress field of nonequilibrium grain boundaries upon ultrasonic treatment (UST). The effect of ultrasound is simulated by an oscillatory shear stress applied to the crystal. Edge dislocations in a model grain with three nonparallel slip systems located at an angle of 60° to each other were examined. The nonequilibrium state of grain boundaries is simulated with the aid of a quadrupole of wedge disclinations located at its junctions. This study showed that the UST caused a rearrangement of the dislocation structure and led to a reduction of internal stresses. The amplitude of ultrasound and the degree of the nonequilibrium state of grain boundaries (strength of the disclination quadrupole) significantly affect the relaxation of the dislocation structure. There are optimum values of the UST amplitude, at which a maximum reduction of the internal stresses is achieved. This study also investigated the dependence of the degree of relaxation of internal stresses on the amount of dislocations in the grain.

Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters in Submicrocrystalline Metals: III. Model for the Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters

A theoretical model is developed to describe the influence of recovery and recrystallization on the macroelastic limit, the yield strength, and the grain-boundary hardening coefficient of the submicrocrystalline (SMC) metals fabricated by severe plastic deformation. The anomalous hardening and the increase in the grain-boundary hardening coefficient upon annealing of the SMC metals are shown to be related to defect accumulation in migrating grain boundaries in them. Equations are derived to relate the Hall–Petch relation parameters to the grain-boundary migration velocity, the nonequilibrium state of grain boundaries, the lattice dislocation density, and the annealing temperature and time. The results of the numerical calculations performed using the developed model are compared with the experimental results obtained in part I of this work.

Effect of Equal Channel and Dynamic Channel Angular Pressing of Ni and Further Heat Treatment on its Structure and Grain Boundary Diffusion

Formation of microstructure in Ni under equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), its thermal stability and diffusion properties of grain boundaries are investigated. Grain boundary diffusion in the ultrafine-grained Ni is found to be significantly faster than in the coarse-grained Ni, which indicates a 'non-equilibrium' (deformation-modified) state of grain boundaries in the former. The effect of non-equilibrium state of grain boundaries on the level of internal stresses is analyzed.

Effect of Grain Boundary State on Diffusion and Diffusion-Controlled Processes in Ultrafine-Grained Materials Processed by Severe Plastic Deformation

Experimental studies on the grain boundary diffusion and processes controlled by it in the ultrafine-grained metallic materials produced by various methods of severe plastic deformation are reviewed. Correlation between the increased diffusion permeability of grain boundaries and features of recrystallization and deformation development in these materials possessing the non-equilibrium state of grain boundaries formed during severe plastic deformation in the temperature range of T < 0.35Tm is demonstrated and analyzed.

Effect of grain boundary diffusion acceleration during structural superplasticity of nano- and microcrystalline materials

It is known that the superplasticity phenomenon isassociated with the development of grainboundarysliding (GBS). This is a special deformation mode,which acts at enhanced temperatures and especiallyeffectively operates in finegrained materials. It wasshown in [1–5] that the effective accommodation ofsliding in the grain joints and a special nonequilibriumstate of grain boundaries are necessary for GBS development. The expression for strain rate in superplasticmaterials has the form [2](1)where

Ultra-fast diffusion channels in pure Ni severely deformed by equal-channel angular pressing

Grain boundary self-diffusion in Ni severely deformed by equal-channel angular pressing has been investigated. The radiotracer technique was applied by using the 63Ni isotope and high-precision mechanical grinding. Ultra-fast diffusion rates, which exceed those along general high-angle grain boundaries in annealed coarse-grained Ni by orders of magnitude, were observed. Such high diffusivities were attributed to a non-equilibrium state of grain boundaries produced by severe plastic deformation. A significant downward deviation from an Arrhenius plot for ultra-fast diffusivity for temperatures above 400 K was found. This was interpreted in terms of a partial relaxation of the non-equilibrium grain boundary structure. Additionally, the formation of ultra-fast diffusion channels, presumably in the form of percolating porosity as a result of stepwise annealing of the material, was observed. This phenomenon is discussed in terms of the free volume redistribution in grain boundaries. It is proposed to quantify the excess energy of interfaces in terms of the observed enhancement of diffusivity. This approach allows the excess energy of non-equilibrium grain boundaries to be determined from diffusion measurements.

10.1007/s11455-008-2015-5

The depth-concentration profiles of nickel upon diffusion in submicrocrystalline (SMC) molybdenum processed by severe plastic deformation (SPD) have been studied by Auger electron spectroscopy. The coefficients (D b) and activation energies of the grain-boundary diffusion of nickel in SMC molybdenum were determined in a 973–1123 K temperature interval. The results indicate that a difference between the D b values in SMC and coarse-grained molybdenum is related to a nonequilibrium state of grain boundaries in the SPD-processed metal.

Nonequilibrium state of grain boundaries and spontaneous grain-boundary slippage in bicrystals

Variables (order parameters) that are related to plastic deformation sites and complement the local densities of grain-boundary defects are separated out in a bicrystal being deformed under creep. By solving an evolutionary equation for the order parameters, it is shown that the nonuniformity and periodicity of spontaneous grain-boundary slippage in bicrystals and an increase in the grain-boundary strain rate under the conditions of boundary-lattice dislocation interaction may be attributed to the occurrence of local nonequilibrium regions (autosolitons).

Grain-boundary diffusion of nickel in submicrocrystalline molybdenum processed by severe plastic deformation

The depth-concentration profiles of nickel upon diffusion in submicrocrystalline (SMC) molybdenum processed by severe plastic deformation (SPD) have been studied by Auger electron spectroscopy. The coefficients (Db) and activation energies of the grain-boundary diffusion of nickel in SMC molybdenum were determined in a 973–1123 K temperature interval. The results indicate that a difference between the Db values in SMC and coarse-grained molybdenum is related to a nonequilibrium state of grain boundaries in the SPD-processed metal.

Thermal stability of ultra fine-grained steel containing dispersed oxides

Annealing behaviour of an ultra fine-grained steel was studied at temperatures of T=600–900 ° C. At T⩽800 ° C, recovery develops more readily at grain boundaries as compared to grain interiors, leading to a fast release of lattice distortions associated with the non-equilibrium state of grain boundaries.

Enhancement of diffusion in deformation-induced non-equilibrium grain boundaries

Abstract The nature of the non-equilibrium state of grain boundaries (GBs) is investigated using a Ni-based superalloy with a microduplex γ + γ′ microstructure which is stable against the grain growth at low temperatures by means of a direct diffusion experiment. The diffusion penetration profiles (63Ni) in the as-annealed and pre-cold-deformed (10%) states of the alloy have been measured by the residual activity techniques. The influence of a number of factors (temperatre, time of diffusion anneal and time of preliminary anneal) on the shape of the penetration profiles has been investigated. The profiles for the temperatures above 923 K exhibit a single part associated with GB diffusion with an almost constant slope—∂(Inc)/∂y 6/5. Below 923 K the GB-related region of the profiles for the pre-cold-deformed samples exhibits two distinct parts with different slopes. The deeply penetrating parts of these profiles are associated with diffusion along non-equilibrium GBs, which disappear after pre-annealing at higher temperatures. On the basis of the diffusion data obtained, the non-equilibrium state of GBs is identified as a metastable state and the process of relaxation of the non-equilibrium GB structure is interpreted as a structural phase transformation.

Enhancement of diffusion in deformation-induced non-equilibrium grain boundaries

Abstract The nature of the non-equilibrium state of grain boundaries (GBs) is investigated using a Ni-based superalloy with a microduplex γ + γ′ microstructure which is stable against the grain growth at low temperatures by means of a direct diffusion experiment. The diffusion penetration profiles (63Ni) in the as-annealed and pre-cold-deformed (10%) states of the alloy have been measured by the residual activity techniques. The influence of a number of factors (temperatre, time of diffusion anneal and time of preliminary anneal) on the shape of the penetration profiles has been investigated. The profiles for the temperatures above 923 K exhibit a single part associated with GB diffusion with an almost constant slope—∂(Inc)/∂y 6/5. Below 923 K the GB-related region of the profiles for the pre-cold-deformed samples exhibits two distinct parts with different slopes. The deeply penetrating parts of these profiles are associated with diffusion along non-equilibrium GBs, which disappear after pre-annealing at ...

Damping properties of 18Cr10Ni stainless steel with submicrocrystalline structure

The samples of 18Cr10Ni stainless steel containing a submicrocrystalline austenitic phase were processed by deformation-heat treatment. The investigations show that the internal friction and yield strength values of these samples are higher than in the steel with a coarse-grained austenitic structure, by a factor of 4 and of 5, respectively. The effect revealed is connected with both the small grain size and the non-equilibrium state of grain boundaries in the submicrocrystalline austenitic phase of the samples.

Diffusion along nonequilibrium grain boundaries in a nickel-base superalloy

The diffusivity D′ δ and microstructure of grain boundaries in a nickel-base superalloy (Ni + 9Cr + 6.5W + 4Mo + 16Co + 4.5Al + 2Ti + 0.12C, in wt.%) were studied in the as-annealed and 10% cold pre-deformed states after subsequent anneals at 600 < T < 1000 °C. At temperatures of 600 and 700 °C the process of grain boundary structure recovery with an enhanced grain boundary self-diffusion was observed. For the estimation of the “time of life” τ of the nonequilibrium state of grain boundaries, a model of their structural relaxation is proposed. It is shown that the calculated period of time τ is in the order of the diffusion anneal time t at low temperatures. This result leads to the conclusion that the enhanced grain boundary diffusion is connected with the nonequilibrium grain boundary structure.

The non-equilibrium state of grain boundaries and the grain boundary precipitations in aluminium alloys

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