Special features of grain-boundary diffusion in nanostructural and submicrocrystalline materials caused by structural heterogeneity of grain boundaries

Mechanisms of grain-boundary diffusion are analyzed based on models of structurally heterogeneous grain-boundaries and multipath diffusion in a system of structural elements responsible for accelerated mass transfer. The effective (averaged) coefficients of grain-boundary diffusion are obtained in models of continuous and discrete distribution of partial activation energies. The results of originally processed diffusion data in some submicrocrystalline materials are presented. The conclusion is drawn that, at low temperatures, the mass transfer in these materials is achieved through a small portion of active diffusion paths, presumably via grain triple junctions.     

Mössbauer spectroscopy of grain boundaries in submicrocrystalline molybdenum obtained by severe plastic deformation

Electron microscopy and emission Mössbauer spectroscopy have been used to study grain boundaries in submicrocrystalline molybdenum obtained by high-pressure torsion.     

Structure and properties of grain boundaries in submicrocrystalline molybdenum prepared by high-pressure torsion

Transmission electron microscopy was used to study submicrocrystalline structure and thermal stability of molybdenum prepared by severe plastic deformation at 400°C by high-pressure (6 GPa) torsion using 5 revolutions of anvils. Emission Mössbauer spectroscopy was used to study the state of grain boundaries in submicrocrystalline Mo annealed at temperatures of 350–800°C.     

A grain-boundary diffusion model of dynamic grain growth during superplastic deformation

Dynamic grain growth during superplastic deformation is modelled on the basis of a grain-boundary diffusion mechanism. On the grain boundary where a static and a dynamic potential difference coexist, matter transport along the boundary is assumed to contribute to dynamic grain growth through depositing the matter on the grain surface located opposite to the direction of grain-boundary migration. The amount of the diffusive matter during deformation is calculated for an aggregate of spherical grains and is converted to the increment of mean boundary migration velocity. The obtained relationship between the strain rate and the dynamic grain growth rate is shown to be independent of deformation mechanisms, provided that the grain growth is controlled by grain-boundary diffusion. The strain dependence, strain-rate dependence and temperature dependence of grain growth predicted from this model are consistent with those observed in superplastic ZrO₂-dispersed Al₂O₃.     

Evolution of the structure upon heating of submicrocrystalline and nanocrystalline copper produced by high-rate deformation

Methods of electron microscopy, dilatometry, and microhardness and resistivity measurements have been used to study the effect of annealing on the process of recrystallization of a mixed submicrocrys-talline+nanocrystalline (SMC+NC) structure of 99.8% copper produced by high-rate (∼10⁵ s⁻¹) deformation using dynamic channel angular pressing (DCAP). It has been shown that the SMC+NC structure of copper is thermally stable upon heating to a temperature of 150°C. It has been found that the ρ/ρ⁰ ratio of copper with an SMC+NC structure at a temperature of 4.2 K is considerably (by 5 times) higher than ρ/ρ⁰ of copper in the annealed coarse-grained state. This effect is due to a high concentration of defects and a high degree of dispersity of the copper structure after DCAP. Changes in the microhardness and in the resistivity (at a temperature of 4.2 K) of the SMC+NC copper after annealing characterize the level of relaxation processes.     

Grain-boundary diffusion of cobalt in submicrocrystalline molybdenum obtained by high-pressure torsion

Radiometric layer-by-layer analysis has been used to study the grain-boundary diffusion of Co in submicrocrystalline Mo produced by severe plastic deformation using high-pressure torsion. It has been found that, under severe plastic deformation, nonequilibrium grain boundaries have been formed, which are ultrafast diffusion paths. During annealing, the recovery of nonequilibrium grain boundaries takes place. The properties of boundaries that underwent the recovery were shown to be close to those of boundaries originated by recrystallization in coarse-grained Mo. As the annealing temperature increases, the fraction of nonequilibrium boundaries decreases; and beginning from 823 K, there are no more nonequilibrium boundaries in the structure.     

Evolution of the structure and hardness of nickel upon cold and low-temperature deformation under pressure

The effect of the deformation temperature ensuring different mobility of dislocations on the change of stages of the structural state of single-crystal nickel has been studied. It has been shown that the deformation temperature affects the type of arising boundaries and the degree of deformation that corresponds to the transition of the material to a new structural state. The formation of microtwins and deformation bands in the structure at the liquid-nitrogen temperature not only retards the formation of a homogeneous submicrocrystalline structure but also leads to a lesser strain hardening.     

Wetting of molybdenum grain boundaries by nickel: effect of the boundary structure and energy

The structural effect of the penetration of nickel along symmetrical [101] tilt grain boundaries (GBs) in two different molybdenum bicrystals is investigated. The selection of GBs (Σ=3{121} and Σ=11{323}) is governed by their different energy so that a different penetration behaviour is expected. The temperature of treatment is 1350°C, i.e. above the eutectic temperature. The analysis of the Mo–Ni phase formed on the surface of the bicrystal, the concentration profile along the GB and the identification of the nanophases present at the GB is performed by using several experimental techniques from microscopic to nanoscopic scales. Important differences in the penetration of nickel are found for the two investigated GBs.     

Evolution of microstructure and microtexture upon recrystallization of submicrocrystalline niobium

Laws of recrystallization in niobium after a submicrocrystalline structure was formed in it by high pressure torsion at room temperature are studied by transmission and scanning electron microscopy. Recrystallization is shown to begin at 300 °C due to the growth of individual microcrystallites. Continuous recrystallization develops in the temperature range 300–800°С. As isothermal annealing temperature is increased, the area occupied by recrystallized grains, which (110) planes are parallel to the sample surface, increases to 90%. Discontinuous and continuous recrystallization that takes place simultaneously upon annealing at 900°С results in grain refinement and more pronounced size inhomogeneity in the structure. The grain refinement is accompanied by a smearing of the recrystallization texture.     

Thermal stability of grain structure and defects in submicrocrystalline and nanocrystalline nickel

The grain and defect structure of submicrocrystalline and nanocrystalline nickel produced by equichannel angular pressing or by ball milling, respectively, has been studied by isochronal and isothermal annealing experiments, and X-ray investigations. A detailed analysis of the thermal stability of the different types of microstructures will be presented.     

Equations of diffusion in nonequilibrium grain boundaries

Equations are derived which describe self-diffusion and interdiffusion in nonequilibrium grain boundaries. The influence of the degree of the nonequilibrium state of the boundaries on the development of the process of grain-boundary diffusion is examined. The kinetics of the variation of the nonequilibrium excess volume due to the diffusional mass transfer along grain boundaries is analyzed.     

Effects of grain growth on grain-boundary diffusion creep by molecular-dynamics simulation

Molecular-dynamics simulations are used to elucidate the effects of grain growth on grain-boundary diffusion creep and grain-boundary sliding during high-temperature deformation of a nanocrystalline Pd model microstructure. The initial microstructure consists of a 25-grain polycrystal with an average grain size of about 15 nm and a columnar grain shape. Prior to the onset of significant grain growth, the deformation proceeds via the mechanism of Coble creep accompanied by grain-boundary sliding. While grain growth is generally known to decrease the creep rate due to the increase of the average grain size, the results obtained in this study reveal an enhanced creep rate at the onset of the grain growth, when rapid grain-boundary migration occurs. The enhanced creep rate is shown to arise from topological changes during the initial growth phases, which enhance both the stress-induced grain-boundary diffusive fluxes and grain-boundary sliding. Dislocations generated as a result of grain-rotation-induced grain coalescence and grain-boundary decomposition in the vicinity of certain triple junctions also contribute to the deformation.     

Effect of grain refinement by severe plastic deformation on the next-neighbor misorientation distribution

Next-neighbor misorientation distributions (NNMD) in severely deformed polycrystalline materials are commonly measured by orientation imaging. A procedure is proposed which enables the separation of NNMD of ultrafine-grained materials into two parts: the distribution of misorientations between newly emerged grains within the original (“parent”) grain interior (“internal daughter grains”) and the distribution of misorientations between grains adjacent to an original grain boundary on its opposite sides (“grain boundary daughter grains”). The procedure is based on electron backscatter diffraction orientation map analyses carried out on different planes of deformed samples considering the evolution of the grain size and shape during severe plastic deformation. It was applied to copper processed by up to three passes of equal-channel angular pressing. A characteristic feature of the measured NNMD is the occurrence of a double peak, which is clearly due to the differences between the NNMD of the two distinct populations of new grains defined above. The peak at low angles represents mainly the continual grain subdivision process in the interior of a parent grain (and is associated with internal daughter grains), while the peak at large angles is due to the high angle misorientations of the grain boundary daughter grains.     

The theory of anomalous grain growth in submicrocrystalline materials produced by severe plastic deformation

A new approach to describing anomalous grain growth based on the idea of transition of migrating grain boundaries into a highly nonequilibrium state is proposed. The effect of the grain-size scatter and the initial dislocation density on the evolution of this process has been considered. Conditions that are required for the realization of an anomalously rapid grain growth have been analyzed.     

Effect of deformation mode and grain orientation on misorientation development in a body-centered cubic steel

Strain-induced misorientation development was studied in an IF steel as a function of strain for two deformation modes, plane strain compression and simple shear. Using electron back-scattered diffraction, orientation maps of “large” areas were obtained, from which several individual grains associated with the principal texture components could be extracted so that only intragranular misorientations could be estimated for these orientations. It was observed that the increase of the misorientation angle was more prominent in simple shear than in plane strain compression and that the orientation influence was different for each mode. Considering texture evolution as a possible source of misorientation development, the lattice spin tensor was estimated with the Taylor model for the two deformation modes; both reorientation axis and angle were compared with misorientation angle and axis. The striking concordance of both quantities allows us to conclude that there is a direct contribution of texture evolution to misorientation accumulation with strain.     

Nanohardness of molybdenum in the vicinity of grain boundaries and triple junctions

We performed a nanoindentation study of the inherent mechanical properties of grain boundaries and their triple junctions in molybdenum tricrystals of controlled geometry grown from the melt. A region with increased hardness is revealed in the vicinity of all grain boundaries studied. The width of this region is about 2 μm and the maximum relative increase of hardness at the grain boundary is about 30%. The local nanohardness at the triple junctions was found to be higher than that at the grain boundaries. This effect is analyzed in terms of the indentation size effect model of Nix and Gao, assuming that the grain boundaries represent ideal barriers for the first nucleated dislocation loops. We calibrated the parameters of the developed model using the data for nanohardness of grain boundaries. The model predicts values of nanohardness at the triple junctions which are in good agreement with the experimental results.     

Evolution of grain structure in nickel oxide scales

In systems such as the oxidation of nickel, in which grain-boundary diffusion in the oxide can control the rate of oxidation, understanding of the factors governing the grain structure is of importance. High-purity mechanically polished polycrystalline nickel was oxidized at 700°C, 800°C, and 1000°C for times up to 20 hr in 1 atm O₂. The scale microstructures were examined by parallel and transverse cross section transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Texture coefficients were found by X-ray diffraction (XRD). Each grain in the transverse section grain boundary networks was systematically analyzed for width parallel to the Ni-NiO interface and perpendicular length, for boundary radius of curvature and for number of sides. The variation of these parameters with depth in the scale was examined. In particular, grains were increasingly columnar (i.e., with ratio of grain length to width >1) at higher temperatures and longer times. Columnar grain boundaries tended to be fairly static; the columnar grain width was less than the rate controlling grain size predicted from the oxidation rate. The mean boundary curvature per grain provided a guide to the tendency for grain growth, except in the region of the Ni-NiO interface, where the boundaries were thought to be pinned.     

On the influence of the grain boundary misorientation on the plastic deformation of aluminum bicrystals

Aluminum bicrystals with symmetric 〈1 1 2〉 tilt boundaries and misorientations of 8.7° (small angle), 15.4° (transition), and 31.5° (large angle) were deformed in a channel die experiment in order to study the influence of misorientation on the deformation at grain boundaries. Samples were characterized by strain measurements and microtexture mappings. The experiments were compared to crystal plasticity finite element simulations. We studied strain heterogeneity at the macroscopic and at the microscopic level. Even macroscopically homogeneous areas showed microscopic heterogeneity in the form of bands of different sets of glide systems. We observed clear effects of the grain boundary misorientation on the deformation kinematics close to the boundaries. The 8.7° grain boundary did not show any orientation change which was interpreted in terms of free dislocation penetration. In contrast, the 15.4° and 31.5° bicrystals showed orientation changes which were attributed to dislocation pile-ups.