Nanostructure and related mechanical properties of an Al–Mg–Si alloy processed by severe plastic deformation

Microstructural features and mechanical properties of an Al–Mg–Si alloy processed by high-pressure torsion (HPT) have been investigated using transmission electron microscopy, X-ray diffraction, three-dimensional atom probe, tensile tests and micro-hardness measurements. It is shown that HPT processing of the Al–Mg–Si alloy leads to a much stronger grain size refinement than of pure aluminium (down to 100 nm). Moreover, massive segregation of alloying elements along grain boundaries is observed. This nanostructure exhibits a yield stress even two times higher than that after a standard T6 heat treatment of the coarse-grained alloy.     

Dynamic balance between grain refinement and grain growth during high-pressure torsion of Cu powders

High-pressure torsion (HPT) was applied to unmilled coarse-grained (CG) Cu powders with low initial dislocation density and cryomilled nanocrystalline (nc) Cu powders with high initial dislocation density, with identical processing parameters. HPT of unmilled CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between deformation-induced dislocation accumulation and dislocation annihilation. The equilibrium structures are governed by these two dynamic balances.     

Tensile behaviour of submicrocrystalline ferritic steel processed by large-strain deformation

Mechanical tests have been carried out on Fe–15%Cr ferritic stainless steel with various microstructures. Ultrafine-grained microstructures with grain sizes of 0.2–0.3 µm were developed by large-strain cold-working and light annealing. The effects of severe deformation on the mechanical behaviour of as-processed and recovered steel were evaluated with reference to the same material having conventional work-hardened and recrystallised microstructures. Despite the low dislocation density in the fine grain interiors in the as-processed state, the samples with strain-induced submicrocrystalline structure were characterised by high internal stresses that resulted in a higher strength than could be expected from simple grain-size strengthening. These internal stresses were associated with a non-equilibrium state of strain-induced grain boundaries after severe deformation.     

High-pressure torsion of metastable austenitic stainless steel at moderate temperatures

We subjected samples of a 304 metastable austenitic stainless steel to high-pressure torsion (HPT) in the temperature range of 303–573 K, (i.e. at different austenite stabilities), to examine their microstructures and mechanical properties. HPT processing at room temperature led to the formation of a lamellar microstructure with austenitic and martensitic phases, of which sizes were characterised by prior austenite grains, whereas HPT processing at moderate temperatures produced nanostructured austenite grains through mechanical twinning. The nanostructured 304 steel with an average grain size of ~70 nm exhibited a fine balance between tensile strength (~1.7 GPa) and reduction of area (~55%).     

Effects of cementite morphology on short-fatigue-crack propagation in binary Fe–C steel

The effects of cementite morphology on short-fatigue-crack propagation were investigated by subjecting smooth steel specimens to rotating-bending fatigue tests. Intergranular and transgranular cementite precipitation steels free of other phases were prepared through heat treatments. In the intergranular cementite precipitation steel, short-fatigue-crack propagation was decelerated at the grain boundaries, and accelerated in the grain interiors. In contrast, the transgranular precipitation steel showed the deceleration of the crack propagation in the grain interiors but not in the vicinity of the grain boundaries.     

Diffusion and induced magnetic anisotropy in nanocrystalline Fe73.5Si13.5B9Nb3Cu1

⁷¹Ge tracer diffusion was studied to gain insight into the atomistic transport processes underlying the formation of magnetic anisotropy in nanocrystalline soft-magnetic Fe_73.5Si_13.5B₉Nb₃Cu₁. The interfacial diffusion characteristic was determined by the residual intergranular amorphous phase, which gives rise to a strongly reduced interface diffusivity compared with grain boundaries in metals. Ge diffusion in the nanocrystallites, which is considered to characterize Si self-diffusion, is much slower than Fe diffusion owing to the D0₃ order of the Fe₃Si nanocrystallites. Slow Si diffusion in the nanocrystallites is identified as the rate-controlling process for the generation of the field-induced magnetic anisotropy.     

Kinetics of Ostwald ripening in the presence of monomer diffusion along grain boundaries

The kinetics of an Ostwald ripening is studied where the prevailing mechanism of cluster growth is monomer diffusion along grain boundaries. The model describes self-consistently the situation when the delivery of matter to a growing cluster is carried out by monomer diffusion along grain boundaries. An analytical and numerical study of Ostwald ripening kinetics in a homogeneous supersaturated solution is performed. The possible transition modes arising during the Ostwald ripening process are discussed with reference to changes in the dominant monomer delivery mechanism.     

Texture and grain boundary character distribution (GBCD) in rapidly solidified and annealed Fe–6·5 mass% Si ribbons

Abstract

The type and frequency of grain boundaries, the so-called grain boundary character distribution (GBCD), has been determined in rapidly solidified and subsequently annealed Fe-6·5 mass% Si alloy ribbon by the scanning electron microscopy-electron channelling pattern (SEM-ECP) technique. High frequencies of low-angle boundaries and coincidence boundaries with Σ3, Σ9, Σ11, Σ17 and Σ19 were observed in a fully annealed ribbon with well defined {110} texture. The total frequency of low-angle boundaries and coincidence boundaries is almost one-half of all grain boundaries. The coincidence boundaries which occurred more frequently are exactly those predicted theoretically from the coincidence orientations for 〈110〉 rotation in cubic crystals, similar to those observed previously in {100} textured ribbons of the same alloy produced by the same processing method. The presence of a close relationship between the type of texture and GBCD has been confirmed by experiment on differently textured ribbons of the same material.     

Equal channel angular pressing of magnesium at room temperature: the effect of processing route on microstructure and texture

Various processing routes were applied to as-cast magnesium during equal channel angular pressing with back-pressure (BP) at room temperature (RT). Multiple passes with BP resulted in crack-free microstructures independent of the processing route. Microstructure and texture examination by electron backscatter diffraction and X-ray diffraction techniques after four passes revealed largely recrystallised grains. The most profound finding was the formation of special grain boundaries Σ13a and {10-12} twin boundaries during recrystallisation at RT.     

Nanoindentation creep behavior of grain boundary in pure magnesium

Nanoindentation creep tests were performed at the grain boundary and grain interior in pure magnesium. The grain boundary showed a high strain rate sensitivity exponent and was dominated by grain boundary sliding due to the high diffusion rate at the grain boundary. The grain boundary affected the deformation behavior of the area at a distance of 2 µm into the grain interior. On the other hand, the grain interior had a low strain rate sensitivity exponent, because its matrix was too large to be influenced by the grain boundary. The deformation mechanism in the grain interior was determined to be dislocation slip.     

Kinetics of interstitial segregation in Cottrell atmospheres and grain boundaries

Trapping of interstitial (e.g. carbon) atoms is driven by the reduction in energy in the system. Diffusion of interstitials, together with their trapping in dislocation cores and/or grain boundaries, is studied by the thermodynamic extremal principle. In addition to the total Gibbs energy, a well-established formulation of the total dissipation is applied. Dimension-free evolution equations are derived, whose solution is well approximated by an easy to handle kinetic equation. Cottrell’s power law can be verified in the initial stage.     

Phase transformation of germanium by processing through high-pressure torsion: strain and temperature effects

Germanium has been processed by high-pressure torsion (HPT) under a nominal pressure of 24 GPa at room temperature and cryogenic temperature. The samples processed at room temperature were composed of a diamond-cubic Ge-I phase and a metastable tetragonal Ge-III phase. The formation of Ge-III was significantly suppressed and Ge-I and an amorphous phase, in addition to a small amount of body-centred-cubic Ge-IV, appeared in the case of cryogenic HPT processing at 100 K. The Ge-IV phase gradually disappeared at room temperature. These results indicated that shear strain and thermal energy are important for promoting the formation of Ge-III.     

Formation of FeNi with L10-ordered structure using high-pressure torsion

FeNi with the L1₀-ordered structure is formed over an astronomical timescale in meteorites. In this study, severe plastic deformation using high-pressure torsion (HPT) is employed for the production of the L1₀ structure in the laboratory. Its formation is confirmed by X-ray diffraction analysis and transmission electron microscopy. Processing of elemental nanopowders by HPT is an effective method for the formation of the L1₀ phase, which is enhanced by the addition of Co to FeNi or annealing after HPT. The formation of the phase must be associated with enhanced diffusion through HPT.     

Stacking-fault formation in [001] small-angle symmetric tilt grain boundaries in cubic zirconia bicrystals

Small-angle symmetric [001] tilt grain boundaries in cubic zirconia bicrystals with misorientation angles 2θ =1.0° and 2θ =5.0° have been fabricated by diffusion bonding. High-resolution electron microscopy observations revealed that the 1.0° boundary consists of a periodic array of mixed dislocations with Burgers vector b =( a /2)[101] or b = ( a /2)[101], while the 5.0° boundary consists of a periodic array of edge dislocations with Burgers vector b = ( a /2)[100], associated with stacking faults at alternate intervals. This suggests that there is a critical angle for structural transitions in the series of the [001] small-angle tilt grain boundaries.     

Energetics and kinetics of surfaces and interfaces in the Fe-Pb system

The energy and the diffusivity of interfaces in the solid Fe-liquid Pb system have been investigated in the temperature range 650-900°C. Grain-boundary grooves are formed at the solid Fe-liquid Pb interface and these have been studied by atomic force microscopy. From the topography of the grooves the relative interfacial energies and interfacial diffusivities are obtained. It is found that liquid Pb does not wet the grain boundaries in Fe. Possible mechanisms for the growth of the grain-boundary grooves are discussed.     

Internal stress and defect-related free volume in submicrocrystalline Ni studied by neutron diffraction and difference dilatometry

A combined study of neutron diffraction and difference dilatometry on submicrocrystalline Ni prepared by high pressure torsion aims at studying the anisotropic behaviour during dilatometry and its relation to internal stress and structural anisotropy. Macroscopic stresses were undetectable in the dilatometer samples. Along with specific tests such as post cold-rolling, this shows that an observed anisotropic length change upon annealing is not caused by internal stress, but can be explained by the inherent microstructure, i.e. the anisotropic annealing of relaxed vacancies at grain boundaries of shape-anisotropic crystallites.     

Turning memory development inside out

These papers provide a useful progress report on how the mature and successful field of memory development is transcending traditional boundaries of populations, content, context, and design. Examining children’s memory for distant as well as recent occurrences, for social interactions as well as individual experiences, for meaningful as well as arbitrary information, and for emotion-laden as well as neutral experiences is creating a broader and more vigorous field. Even greater progress can be made by measuring at a fine grain level the processing activities that experiences elicit, how such immediate processing activities shape later memory, and how changes in processing with age and experience produce memory development.     

Effect of grain orientation and local strain on the quality of polycrystalline YBa 2 Cu 3 O 7 superconductive films

The critical current densities of superconducting thin films and their dependence on the film structural characteristics has been a major research interest for more than a decade. Controlling this relationship is crucial if large-scale high-quality YBa 2 Cu 3 O 7 (YBCO) tapes are to be produced. Two major keystones of information have been established in this field. Firstly, there is a direct relationship between the critical current density and the grain-boundary angle in polycrystalline YBCO films. Grain boundaries with a mismatch angle higher than 5° usually result in reduced critical current densities. This detrimental effect of large-angle grain boundaries to the quality of YBCO films has been attributed to strain fields resulting from such grain boundaries. Secondly, the quality of the YBCO film can be enhanced by straining its lattice in specific direction. Here, we report, for the first time, direct experimental results coupling local grain orientation and local strain maps of thin YBCO films deposited on a (001) biaxially textured nickel substrate. These results were correlated to the quality of the film and showed how grain structure in the nickel substrate affects the grain structure in the YBCO films even in the presence of several buffer layers. More importantly, the data show that highquality films with high critical current densities can be produced, in spite of large-angle grain boundaries, if the film is compressed in the range of 0.5% strain normal to the a axis.     

Detrimental effect of cellular precipitation on the creep strength of Inconel740H

Cellular precipitation (also known as discontinuous precipitation) has been observed at the grain boundaries of a newly developed nickel-based Inconel740H alloy designed for use at 700?°C in advanced ultrasupercritical coal-fired power plants. By means of element mapping and selected area diffraction, the cellular precipitates were identified as Cr-rich M₂₃C₆ carbides. The onset of cellular precipitation was found to follow a pucker mechanism in Inconel740H. The cellular precipitates at the grain boundaries, even at low volume fractions, were severely detrimental to the creep strength at 750?°C. The creep rupture life of Inconel740H containing cellular precipitates at grain boundaries was only one-tenth of that for the alloy without cellular precipitates. The reason for the drastically decreased creep rupture life is attributed to the poor resistance of cellular precipitates to crack propagation during creep.     

Effect of initial texture on deformation-induced grain growth in pulsed electrodeposited microcrystalline copper

Owing to the presence of large fraction of grain boundaries, deformation-induced grain growth is commonly observed in fine-grained electrodeposited metals. Here we demonstrate that microcrystalline copper (d～1–10?µm) with different textures produced by electrodeposition exhibit significant deformation-induced grain growth in tension by coalescence along with twin boundary migration and detwinning. Oriented growth with the formation of a cube texture was noted in the deformed samples. There was an increased fraction of twin boundaries with large angular deviation from Brandon's criterion during deformation.