The aim of this research is to characterize the unique microstructural features of Al-matrix nanocomposites reinforced by graphene nano-platelets (GNPs), fabricated by multi-pass friction-stir ...processing (FSP). During this process, secondary phase GNPs were dispersed within the stir zone (SZ) of an AA5052 alloy matrix, with a homogenous distribution achieved after five cumulative passes. The microstructural characteristics and crystallographic textures of different regions in the FSPed nanocomposite, i.e., base metal (BM), heat affected zone (HAZ), thermo-mechanical affected zone (TMAZ), and SZ, were evaluated using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The annealed BM consisted of a nearly random crystal orientation distribution with an average grain size of 10.7μm. The SZ exhibited equiaxed recrystallized grains with a mean size of 2μm and a high fraction of high-angle grain boundaries (HAGBs) caused by a discontinuous dynamic recrystallization (DDRX) enhanced by pinning of grain boundaries by GNPs. The sub-grains and grain structure modification within the HAZ and TMAZ regions are governed by dislocation annihilation and reorganization in the grain interiors/within grains which convert low-angle to high-angle grain boundaries via dynamic recovery (DRV). The FSP process and incorporation of GNPs produced a pre-dominantly {100} cube texture component in the SZ induced by the stirring action of the rotating tool and hindering effect of nano-platelets. Although, a very strong {112} simple shear texture was found in the HAZ and TMAZ regions governed by additional heating and deformation imposed by the tool shoulder. These grain structure and texture features lead to a hardness and tensile strength increases of about 55% and 220%, respectively.
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•A new Al-matrix nanocomposite was prepared by friction stir processing.•Improved hardness and strength were attained by incorporation of graphene nano-platelets.•Microstructural changes, restoration mechanisms and textural developments were studied.•The correlation between the microstructural features and textural components was established.
The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The ...properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.
Summary
Multipass friction‐stir processing was employed to uniformly disperse multiwalled carbon nanotubes (MW‐CNTs) within an Al–Mg alloy metal matrix. Decomposition of MW‐CNTs occurs in situ as a ...result of solid‐state chemical reactions, forming fullerene (C60) and aluminium carbide (Al4C3) phases during reactive high temperature severe plastic processing. The effects of this decomposition on the microstructural features, dynamic restoration mechanisms and crystallographic microtextural developments are studied for the first time by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analysis. The formation of an equiaxed grain structure with an average size of ∼1.5 μm occurs within the stirred zone (SZ) under the influence of inclusions which hinder grain boundary migration via Zener‐Smith pinning mechanisms during the discontinuous dynamic recrystallisation (DDRX). Formation of two strong Cubic and Brass microtextural components in the heat affected zone (HAZ) and thermomechanical affected zone (TMAZ) was noted as compared to the completely random and Cube components found in the base and SZ regions, respectively. The microstructural modification led to hardening and tensile strength improvement for the processed nanocomposite by ∼55% and 110%, respectively with respect to the annealed Al–Mg base alloy.
Plates of AA5052 (Al–Mg) alloy in both annealed (solution-treated) and wrought (rolled) temper conditions were subjected to friction stir processing (FSP) at various w/v pitch ratios from 4 to ...28rev.min/mm. The role of stored strain energy on the evolution of restoration mechanisms and crystallographic texture components were assessed in terms of microstructural features evaluated using electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis. The results revealed that FSP significantly refined the grain structure and changed the crystallographic micro-texture components. The grain size of the annealed and wrought alloy was reduced from 49.4 and 9.7μm initial values to 3.3 and 3.6μm, respectively when w/v=4. Also, the formation of a {112}⟨110⟩ crystallographic shear texture with a strong B/B¯ component intensity were observed. The microstructural changes in the annealed alloy were related to the occurrence of discontinuous dynamic recrystallization (DDRX) mechanism, while operation of a static recrystallization (SRX) prior to a continuous (CDRX) mode in the wrought one, revealing the effect of stored strain energy. Evaluations of the mechanical properties also determined enhanced ultimate tensile strength, elongation, and indentation Vickers hardness while preserving the yield stress.
Aluminum matrix nanocomposites were fabricated by friction stir processing of Al–Mg alloy sheets with pre-placed TiO2 nanoparticles at a concentration of 2 to 6vol%. Microstructural studies showed ...that solid state reactions between the metal matrix and TiO2 particles caused in situ formation of MgO and Al3Ti nanophases with an average size ~50nm. These nanophases were homogenously distributed in an ultra-fine grain structure (0.2–2µm) of the base metal. The results of pole figures evaluation obtained by electron back scattered diffraction studies revealed that the random orientation of initial annealed sheet was changed to components near to shear and silver texture in the friction stir processed alloy without and with pre-placed powder, respectively. The concentration of TiO2 particles affected the preferred texture orientation as the ceramic inclusion restricted the severe plastic deformation and dynamic recrystalization of the metal matrix. Hardness and tensile yield strength of the Al–Mg alloy sheet were also significantly improved by employing friction stir processing in the presence of TiO2 nanoparticles (up to ~3.1vol%). Fractographic studies showed a mixture of ductile–brittle fracture modes with an increase in the content of catastrophic manner at higher TiO2 fractions.
An Nb-Ti low carbon HSLA steel with the application of Coiled Tubing steel in petroleum industry, was subjected to hot rolling followed by the subsequent cooling procedures including cooling and ...coiling. The effect of cooling rate and coiling temperature on final microstructure and texture was investigated via optical microscopy, SEM, and EBSD analyses. It was found that rolling at 800 °C results in the appearance of elongated ferrites due to the activation of the deformation-induced ferrite transformation mechanism. Cooling from the finish rolling with different rates of 2.4 to 15 °C/s leads to the formation of polygonal ferrite and retained austenite. Higher cooling rate increases the retained austenite, decreases the ferrite grain size and improves the grain size homogeneity. Coiling at 600 °C results in coarser ferrite with the texture of γ fiber which is due to the recrystallization of ferrite. On the other hand, coiling at 500 °C intensifies the {332}
113
and {113}
110
components indicating deformed austenite transformed to ferrite.
The study purpose was to determine the possibility of using pattern recognition methods to study the impact of physical exercises modes on teaching primary school children throwing a small ball at a ...vertical target.
Materials and methods. The study participants were boys aged 7 years (n=48). The paper relied on analysis and generalization of data of scientific and methodological literature, general scientific methods of theoretical level, such as analogy, analysis, synthesis, abstraction, induction, as well as general scientific methods of empirical level: observation, testing, experiment.
Results. The effectiveness of teaching boys aged 7 “throwing a ball at a vertical target” is positively influenced by 6-12 sets, 3 repetitions per set, rest interval of 60-180 seconds. The focus in choosing a teaching mode is on the number of repetitions per set.
Conclusions. A full factorial experiment method makes it possible to mathematically describe the process in some local area of the factorial space and to verify the regression model. Regression equations provide an opportunity to select the modes of performing for each exercise being studied. Discriminant analysis has made it possible to determine the modes of physical exercises in the process of motor skills development; to answer the question as to how significantly the modes of training differ by the effectiveness of motor skills development; what motor tasks most substantially influence the differentiation of classes; what class the object belongs to based on the values of discriminant variables. To select the most rational mode of performing exercises in the process of motor skills development in boys aged 7, the first and second discriminant functions can be used, with a focus on the most informative variables.
Solid-state butt-joining of powder metallurgy (PM) fabricated Al-Al2O3 nanocomposites was assessed using friction-stir welding (FSW), in which the PM materials were prepared from aluminum powder with ...different particle size distributions of <9 μm, <63 μm, and <250 μm. After establishing a suitable working window in terms of rotational speed (w) and traverse velocity (v), the effects of initial powder particle size on weldability, microstructure and mechanical properties joints were studied. The joint quality was assessed by macro cross-sectional examinations, where deterioration occurs due to defects at the weld root with increasing traversal speed. A defect-free friction stir weld with a more homogenous cross-section is produced for all powder sizes using FSW parameters of w = 1200 rpm and v = 300 mm/min. The native amorphous aluminium oxide (am-Al2O3) layer on the powder is redistributed and partially transformed to crystalline γ-Al2O3 nanoparticles depending on FSW parameters which is revealed by transmission electron microscopy (TEM). Detailed electron back-scattered diffraction (EBSD) analysis indicates the formation of a fine equiaxed grain structure in the range of 1.5–3.7 μm as a function of initial aluminum particle sizes and FSW parameters with a mixture of ideal random and shear components as the dominant texture. Transverse tensile and indentation hardness testing revealed no significant changes in mechanical properties for the composite weld, with ductile fracture occurring in the base metal with hardening only occurring in the stir zone for the powder particle size.
Accumulative fold-forging (AFF) as a newly developed severe plastic deformation (SPD) process based on the repetitive fold-forging steps is implemented for the production of the layered UFG (~200 nm) ...AA8006 alloy and AA8006-B4C nanocomposite (~35 nm, 10 vol%) materials from the initial AA8006 alloy foil. The remarkably refined grains and nanoparticles can control metallic materials' mechanical properties, including the strength, strain rate dependency, and thermal stability behavior. In this context, nano-grains' local mechanical response during nanoindentation can vary considerably depending on the testing temperature, and this has yet to be discussed. In this research, after materials characterization of produced nanostructured materials according to the AFF route, the relating depth-sensing thermal stability of them assessed by conducting the nanoindentation testing at different temperatures in the range of 300–523 K. Depth sensing softening behavior is elaborated to identify the low-temperature thermal stability of processed materials. The results enunciated the occurrence of thermal softening by refining the grain structure. However, introducing the reinforcing nanoparticles lead to a pinning action that stabilized the grain boundaries.
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•The UFG alloy and NS nanocomposite were produced by the new AFF route.•Formation of nano-grains and nanoparticles dispersion studied.•Depth-sensing thermal property interrogated the more stability of nanocomposite.•The importance of nanoparticles is retarding the coalescence of nano-grains.
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