Fibrous Grains Research Articles

A Study on Microstructure Evolution of 7055 Aluminum Alloy Based Under Extreme Environment

In order to fill the mechanism of microstructure evolution and corrosion resistance of 7055 aluminum alloy in extreme environment, the 7055 aluminum alloy Which was subjected to T6I4 heat treatment was selected as the research object. High speed machining experiments were performed at low temperature (-60°C) and high temperature (350°C). The dislocation density, grain size, TEM microstructure and corrosion resistance of the cutting layer of 7055 aluminum alloy were investigated at low temperature (-60°C) and high temperature (350°C). The experimental results show that the dislocation density at low temperature is higher than that at high temperature under the same cutting speed and feed rate, and the grain size at low temperature is higher than that at high temperature too. In the low temperature environment, the continuous of the grain boundary is enhanced with the cutting speed increases, and the precipitation phase in the crystal is reduced and the precipitation phase on the grain boundary is obviously increased. Under the high temperature environment, with the increasing cutting speed, the degree of fibrosis of the grains is weakened and the dynamic recrystallization is obviously enhanced. Microstructures show significant dynamic recrystallization after high temperature cutting. And when the feed rate is 0.05 mm, the microstructure of 7055 aluminum alloy exists part of the sub-crystal; The larger the feed amount, the finer and longer the fibrous grains of the metallographic structure of cutting layer metal of 7055 aluminum alloy. There is a lot of dislocations in the cutting layer of 7055 aluminum alloy in low temperature environment. The higher the cutting speed, the smaller the dislocation density. No dislocations were found in the microstructure of the 7055 aluminum alloy cutting layer in the high temperature environment,and, the higher the cutting speed is, the less the number of precipitated phases is, and the GP region and η ‘phase were transformed into η phase.

Effect of cryorolling on microstructure and mechanical properties of a peak-aged AA6082 extrusion

Peak-aged AA6082 flat extrusions were cryorolled at liquid nitrogen temperature to three different thickness reductions, the reductions were 21%, 42% and 85%, respectively. Microstructure and plastic deformation behaviors of the initial as well as the rolled materials were studied by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests at room temperature. The initial material showed fibrous grain structure. After cryorolling to 21% thickness reduction, grains in the alloy were obviously elongated and importantly, microband traces were detected in selected grains. More detailed information was obtained in the alloy after 42% thickness reduction. Here, numerous microbands aligned along slip planes with the highest resolved shear stress, producing fragmentation of grains, while rests of elongated grains were microband-free. A cryorolling strain of 85% has been found to be desirable for producing a hierarchical microstructure in the materials volume, featuring nano- and ultrafine grains with size <100 nm coexisted with micro-sized grains. The development of hierarchical microstructure could be considered as a consequence of the intersection between families of microbands. As a result, both the strength and uniform elongation of the severely cryorolled alloy, i.e. 85% thickness reduction, were more superior to other rolled alloys. Therefore, this study revealed that the single processing procedure, i.e., cryorolling, can be utilized to tailor for desired properties in favor of both strength and ductility for peak-aged AA6082 extrusions.

Effects of heating rate during solid-solution treatment on microstructure and fatigue properties of AA2524 T3 Al–Cu–Mg sheet

T3 AA2524 (Al–Cu–Mg) aluminum alloy sheets were fabricated using two different heating rates (air furnace and salt bath) during the solid-solution treatment. The corresponding microstructure, recrystallization mechanisms, and fatigue behavior were investigated by means of transmission electron microscopy, scanning electron microscopy, electron back scattering diffraction, and optical microscopy. The results showed that the heating rate had a significant effect on the microstructure, owing to the different recrystallization mechanisms induced, namely, strain-induced boundary migration and particle-stimulated nucleation. Elongated fibrous grains (approximately 75μm) with Cube, Goss, and S orientations were obtained for low heating rate, while fine equiaxed grains structure (approximately 13μm) with a P-type orientation were obtained for high heating rate. The rate of fatigue crack growth in the case of former was lower than that in the latter during the stable stage. The effects of crack closure, induced by the large grain size and a high Schmid factor, would be responsible for such difference in fatigue property.

Synthesis of honeycomb-like structured porous Si3N4 ceramics with exceptionally high number of cells per square inch

Honeycomb-like structured porous Si3N4 ceramics with exceptionally high number of cells per square inch were prepared by the self-organization method of alginate. The obtained samples have highly ordered channels, high porosity and well mechanical strength. It is found that the microstructure and properties of obtained porous Si3N4 ceramics can be effectively controlled by choosing different kinds of alginates. And it is worth to note that the obtained ceramics possess interesting hierarchical pore structure, including the unidirectionally aligned channels with pore size of 50–120µm (first pore structure) and the interlocked microstructure of fibrous β-Si3N4 grains within channel walls with pore size of 0.8–2.6µm (secondary pore structure), making it desirable for filter applications.

Preparation of porous Si3N4 ceramics with unidirectionally aligned channels

We prepared porous Si3N4 ceramics with highly ordered and unidirectionally aligned channels by the self-organization method using an alginate template. The obtained samples show uniform pore distribution, high cell density, good mechanical strength and hierarchical pore structure. A great number of long fibrous β-Si3N4 grains protrude into channels, and other short length fibrous β-Si3N4 grains interlock within channel walls forming a peculiar hierarchical pore structure. The pore size, thickness of channel walls, cell density, porosity and mechanical properties of the sintered porous Si3N4 ceramics could be controlled by choosing different solid loadings, whereas β-Si3N4 grains growth could be controlled by adopting different sinter temperatures.

Effect of argon gas pressure on residual stress, microstructure evolution and electrical resistivity of beryllium films

The residual stress in beryllium films fabricated on K9 substrates by using magnetron sputtering deposition is measured by using a curvature method and is theoretically estimated by using the Nix and Clemens (NC) model. The experimental results indicate that the 1.3-μm-thick film is always in a tensile state for pressure variations in the range from 0.4 to 1.2 Pa. When the sputtering gas pressure is increased, the average stress increases at first, after which it decreases by a remarkable amount. The observed descending trend of the tensile stress when the sputtering gas pressure is beyond 0.6 Pa is mainly attributed to the grain size in the film being larger than that in the film when the pressure is below 0.6 Pa. The maximal residual stress of 552 MPa at a sputtering gas pressure of 0.6 Pa is close to the tensile strength (550 MPa) of the corresponding beryllium bulk material and is about 8 times smaller than that calculated by using the N-C model. In addition, the surface morphologies of the as-fabricated films reveal fibrous grains while the cross-sectional morphologies are characterized by a coarsening of columnar grains. The measured electric resistivity of each film strongly depends on its porosity and the sizes of its grains.

A novel processing route for the manufacture of EN AW 6082 forged components

An attractive combination of strength and formability, corrosion resistance and recyclability has made the age-hardening EN AW 6082 one of the most popular aluminium forging alloys. 6082 alloy forgings are thus used extensively in automotive suspension components. The conventional manufacturing cycle starts with the casting of 6082 billets that are subsequently extruded into smaller diameter forging stock. The relatively higher temperatures generated by frictional heating and high strains introduced at the surface during extrusion encourage recrystallisation and produces a heterogeneous structure with fully recrystallised small grains at the surface, fibrous grains at the centre. Hence, the surface of the forged 6082 components inevitably undergoes grain growth once exposed to high temperatures during the solution heat treatment. An attempt was made in the present work to identify an alternative process to achieve a homogeneous section structure with no evidence of coarse grains across the section, which in turn will ensure a better fatigue performance and a longer service life.

Microstructural, nanomechanical, and microtribological properties of Pb thin films prepared by pulsed laser deposition and thermal evaporation techniques

In this work, the authors compare the morphological, structural, nanomechanical, and microtribological properties of Pb films deposited by thermal evaporation (TE) and pulsed laser deposition (PLD) techniques onto Si (111) substrates. Films were investigated by scanning electron microscopy, surface probe microscopy, and x-ray diffraction in θ-2θ geometry to determine their morphology, root-mean-square (RMS) roughness, and microstructure, respectively. TE films showed a percolated morphology with densely packed fibrous grains while PLD films had a granular morphology with a columnar and tightly packed structure in accordance with the zone growth model of Thornton. Moreover, PLD films presented a more polycrystalline structure with respect to TE films, with RMS roughness of 14 and 10 nm, respectively. Hardness and elastic modulus vary from 2.1 to 0.8 GPa and from 14 to 10 GPa for PLD and TE films, respectively. A reciprocal friction test has shown that PLD films have lower friction coefficient and wear rate than TE films. Our study has demonstrated for first time that, at the microscale, Pb films do not show the same simple lubricious properties measured at the macroscale.

Microstructural evolution of indirectly extruded seamless 6xxx aluminum tubes with axial variable wall thickness

For this study, seamless square tubes of aluminum alloys EN AW-6060 and EN AW-6082 were extruded with axial variable wall thickness by applying an axial moveable stepped mandrel. The microstructures in thick-walled and thin-walled sections of these tubes were investigated and correlated to the extrusion conditions. The results for EN AW-6060 revealed fibrous grains at low ram velocity (vram=0.8mm/s) and thus low strain rate. In these cases, peripheral coarse grains were also observed on the tube surfaces. Applying a higher vram=3.3mm/s lead to a fully recrystallized microstructure. For EN AW-6082 the fibrous microstructure was predominant. This was attributed to the Mn-content of this alloy, which acts as an inhibitor for recrystallization. Only at the highest values of strain, strain rate and temperature a recrystallized microstructure was observed for this alloy. EBSD analyzes of boundary misorientation angles indicate that geometric dynamic recrystallization seems to be the predominant mechanism at low strain rates. The facts that the amount of low angle boundaries significantly decreased as subgrains were removed and the amount of high angle grain boundaries is increased revealed that recrystallization occurred at higher strain rates and strains. Furthermore, the transition areas, where the wall thickness was gradually reduced were investigated. Due to increasing strain and thus increasing strain rate, thin-walled tube segments show lower grain size than thick-walled sections. It was also revealed that the PCG layer thickness remained about constant with increased strain, but since the tube wall thickness was reduced, the recrystallized fraction increased for higher strains.

Study of tensile properties, fractography and morphology of aluminium (1xxx)/coconut shell micro particle composites

Aluminium (1xxx)/coconut shell micro particle (Al/CMP) composites have been developed using a compo cast technique. Coconut shells (CSs) were processed with the aid of mortar/pestle and disc grinder and then classified with a set of sieves vibrated with a sine shaker. The CMP additions increased from 2% to 10% at 2% interval. Microstructural/chemical composition analyses were carried out with the aid of scanning electron microscopes (ASPEX 3020) with attached energy dispersive X-ray spectroscopy. Phases were identified using an X-ray diffractometer (XRD). The tensile properties and mode of fracture were studied using Instron extensometer and Avery Denison Impact Testing Machine respectively. Results revealed 99.3% purity of aluminium matrix. The presence of new phases in the aluminium matrix is attributable to chemical interaction between Al and CMPs. The fine grained structure of Al/CMPs composites was confirmed by SEM and optical micrographs. The enhancement in the tensile properties is attributable to the presence of hard phases in the Al matrix and good interfacing bonding between Al matrix and CMP reinforcements. The cone and cup surface appearance with fibrous, dull, dimple and goose grain microstructure of the fracture surfaces of the composites is an indication of ductile fracture. Hence low cost metal matrix aluminium based composites have been developed.

Fabrication and Property of Porous Silicon Nitride Ceramics with Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-Yb&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Additions

Wave-transparent porous Si3N4 ceramics were fabricated using 9wt% multiple sintering additives of Y2O3 and Yb2O3. The influence of sintering additive composition on the porosity, mechanical properties and microstructure of porous Si3N4 ceramics was investigated. The results showed that the porous Si3N4 ceramics with porosity ranging from 32% to 43% were prepared. For the porous Si3N4 ceramics sintered at 1690°C, flexural strength, fracture toughness and the β-Si3N4 phase content increased as the Yb2O3 content of multiple sintering additives increased from 0 to 9 wt%. In most specimens, microstructures with interlocked fibrous β-Si3N4 grains and uniformly distributed fine pores were obtained with employing scanning electron microscopic examination and X ray diffraction analysis. Sinterability and phase transformation of Si3N4 with more Yb2O3 content were more pronounced than others.

Microstructural Characterization of Indirectly Extruded EN AW-6060 Square Tubes with Axial Variable Wall Thickness

Today, innovative lightweight constructions increasingly demand for profiles with higher strength and stiffness. In this investigation an axially moveable stepped mandrel allowed the manufacturing of load adapted (tailored) aluminum tubes with axial variable wall thicknesses by the extrusion process. Thus, on the one hand it is possible to produce thick-walled sections for highly stressed areas and on the other hand save profile weight by applying reduced wall thicknesses in areas with lower loads. Varying the extrusion ratio along tube direction affects the product velocity as well as the profile exit temperature und thus the microstructure in different tube sections. At high temperatures and high strain rates the microstructures revealed very large grains due to static recrystallization. However at low temperatures and low strain rates dynamic recovery lead to a microstructure dominated by fibrous grains. Small equiaxed grains were also found indicating geometric dynamic recrystallization.

Recrystallization behaviour of AA6063 extrusions

Cylindrical profiles of an AA6063 aluminium alloy were produced in a lab-scale direct extrusion set-up. The extrusion was performed at 300 °C, 450 °C and 550 °C, respectively, with the same ram speed. Immediate water quenching was applied to the profiles and the end of billet (butt-end) after extrusion. Microstructure and texture of the material in different states were measured by electron back-scattered diffraction. Only the profile extruded at 300 °C, was found in the deformed state after extrusion, featuring a fibrous grain structure and a strong <111> and weak <100> double fibre texture. Post-extrusion annealing of this profile at 450 °C resulted in an almost fully recrystallized structure (recrystallized fraction of 87%) and with a texture similar to that of the as-deformed state. The profile extruded at 450 °C was almost fully recrystallized (recrystallization fraction 91%) already after quenching, and with a texture characterized by a weak <111> and strong <100> double fibre. The profile extruded at 550 °C showed a partially recrystallized grain structure with recrystallization fraction of 71%, and with a texture dominated by a <100> fibre. The influence of the deformation conditions on the recrystallization behaviour, in terms of recrystallization kinetics and mechanisms, are discussed in view of these results.

Effect of extrusion press exit temperature and chromium on grain structure of EN AW 6082 alloy forgings

Extruded bar is the typical forging stock in the manufacture of EN AW 6082 forged suspension components. The frictional heating and high strains introduced during extrusion leads to fine equiaxed surface grains that inevitably grow during forging owing to a very small initial grain size and a strain-free state. Higher extrusion temperatures as well as Cr addition have a favourable impact on the depth of the coarse grain zone in 6082 forgings. The interior fibrous grains may recrystallise to produce small equiaxed grains during forging and suffer abnormal growth during the solution heat treatment. Sections with entirely coarse grains appear to be the standard after the T6 heat treatment when the 6082 alloy is Cr-free regardless of the press exit temperature of the extrusion step in the production cycle. However, coarse grains are confined to the surface regions in forgings produced from 6082 alloy with as much as 0·13 wt-%Cr regardless of the press exit temperature. High extrusion temperatures reduce the strain energy while Cr forms dispersoids that retard recrystallisation and grain growth during subsequent processing.

Optimization of Parameters and Microstructural Properties of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Thin Films Grown by Pulsed Laser Deposition (PLD)

Ba0.5Sr0.5Co0.8Fe0.2O3 − δ thin films were grown by pulsed laser deposition (PLD) in the temperature range from room temperature (RT) to 1073 K and at oxygen pressures from 6.66 to 39.99 Pa in order to produce dense defect-free thin films. Si with a native oxide layer and MgO were used as the substrate materials. The structure of the thin films was highly dependent on substrate temperature, material and oxygen partial pressure, leading to formation of different microstructures – pores, cracks, columnar and fibrous grains. Cracks and delamination of the thin films were observed in dense layers at higher temperatures, while this was not the case with the columnar thin films. Differences in thermal expansion coefficient, phase transformation and oxygen non-stoichiometry of BSCF are possible explanations for the cracking of the dense thin films. Thin films with a columnar structure are positively influenced by annealing inducing grain growth and densification.

The Effect of Intermetallic Phases on Ductile to Brittle Transition of Aluminium-Iron Alloy

The effect of intermetallic phases and grain size on ductile to brittle transition temperature of Aluminium-Iron alloy (Al–11% Fe) was investigated in this research work. An Izod impact testing method was adopted to study the DBTT in the temperature interval of 77 K to 373 K. The ductile-brittle transition points: fracture transition plastic (FTP), fracture-appearance transition temperature (FATT), impact energy transition temperature (IETT), fractional surface area of cleavage (brittle) and fibrous (ductile) fractures and grain size of the samples were also determined. The fracture toughness of Al-Fe alloy found decreasing with temperature in contrast to conventional materials. The fractographic investigation revealed that the microstructural changes play a major role in determining the fracture toughness of these alloys. Annealing of these samples slightly improved the fracture toughness as the spherical morphology of intermetallic particles resists the crack propagation.

Comparison of Cast and Extruded Stock for the Forging of AA6082 Alloy Suspension Parts

High-precision near-net shape parts with excellent surface qualities can be produced with the forging process with a minimum of finishing operations thanks to the good formability of aluminium alloys. There has been a rapid increase in the use of aluminium forgings predominantly in the automotive industry, where weight savings for reduced fuel consumption and exhaust emissions is mandated by legislation. Aluminium forgings provide, in addition to low weight, high strength, good corrosion resistance and a fibrous grain structure to improve fatigue resistance. Typical commercial forging stock is the round bars produced by the extrusion of cast billets. An alternative process route that has received increasing attention in recent years is the casting of forging stock by a horizontal direct chill casting technique to make smaller billets without the need for extrusion to reduce their diameter. The anisotropy imparted to the forging stock via extrusion, often regarded as useful for the forging, is certainly missing in the former. However, cast stock has been reported to be more resistant to the formation of coarse surface grains than the extruded counterpart. The present work was undertaken to compare the casting and extrusion routes for the manufacture of 6082 alloy forging stock.

Sputtering pressure influence on growth morphology, surface roughness, and electrical resistivity for strong anisotropy beryllium film

The strong anisotropy beryllium (Be) films are fabricated at different sputtering pressures by direct current magnetron sputtering. With the increase of pressure, the deposition rate of Be film first increases, and when the pressure exceeds 0.8 Pa, it gradually descends. The X-ray diffraction analysis indicates that Be film is of α-Be phase, its surface always reveals the (101) crystal plane possessing the low surface energy. As for the growth morphology of Be film, the surface is mainly characterized by the fibrous grains, while the cross section shows a transition from a columnar grain to a mixed grain consisting of a cone-shaped grain and a columnar grain as the sputtering pressure increases. The large grain fraction decays exponentially from 75.0% to 59.3% with the increase of sputtering pressure p, which can improve the grain size uniformity. The surface roughness increases due to the insufficient atom diffusion, which is comparable to its decrease due to the etching effect at p < 0.8 Pa, while it increases drastically at p > 0.8 Pa, and this increase is dominated by the atom diffusion. The electrical resistivity values of Be films range from 1.7 μΩ·m to 2.7 μΩ·m in the range 0.4 Pa–1.2 Pa, which is 50 times larger than the bulk resistivity.

Influences of in situ annealing on microstructure, residual stress and electrical resistivity for sputter-deposited Be coating

The Be coating grown on K9 substrate are fabricated at different annealing temperatures by dc magnetron sputtering, and they are composed of α-Be phase with hcp structure from XRD analysis. As the in situ annealing temperatures increase from 60°C to 410°C, the coating surfaces show the transition from fibrous grains dominated by surface diffusion to diamond-like grains, and then to recrystallized grains dominated by bulk diffusion. For the coating cross-section morphologies, they are always characterized by the coarsening columnar grains with distinct boundaries. Both surface roughness and grain size increase with annealing temperatures, and obey the modified Anelli model. Moreover, the constitutive relationship of Be coating was explored. The residual stress is related closely to the microstructures and annealing temperatures, not only displays the tensile stress but also compressive stress. The compression–tension transition temperature occurs near 250°C (0.34Tm), which is just recrystallization transition temperature. The film electrical resistivity values decrease gradually with the increase of grain size due to the reduction of grain boundary.

Microstructural evolution and grain refinement mechanism of pure tungsten under explosive loading condition

The microstructural evolution and grain refinement mechanism of pure tungsten prepared by chemical vapor deposition under explosive loading condition were systematically investigated. Results show that dynamic recrystallization is the main deformation mechanism of pure tungsten under explosive loading condition, and new refined tungsten grains were observed in the deformed tungsten. However, the grain refinement mechanism of tungsten is obviously different from that of subgrain rotational dynamic recrystallization (RDR) mechanism and progressive subgrain misorientation recrystallization (PriSM) mechanism. Under the explosive loading condition, the tungsten grains are severely elongated into fibrous grains. Because the deformation time is ultra-short, dislocation slips are conducted through single slip system within the elongated tungsten grain, resulting in the formation of high density dislocation walls consisting of parallel dislocation lines. With continuous deformation, the high density dislocation walls are transformed into subgrain boundaries, which further evolve into refined boundaries. Then the fibrous tungsten grains are fragmented into equiaxed grains. No deformation twins were observed in the deformed pure tungsten.