Alumina Grains Research Articles

Digital Image Correlation Characterization of Deformation Behavior and Cracking of Porous Segmented Alumina under Uniaxial Compression

In this study, the two-dimensional digital image correlation (DIC) technique has been applied to sequences of images taken from the surfaces of porous, segmented alumina samples during uniaxial compression tests. The sintered alumina was structurally composed of polycrystalline alumina grains with interior ~3–5-μm pores, a network of discontinuities that subdivided the sample into ~230 μm segments, and ~110 μm pores located at the discontinuity network nodes. Bimodal pore structure and the segment boundaries were the results of the evaporation and the outgassing of the paraffin and ultra-high-molecular-weight polyethylene admixed with alumina powder via slip casting. Only partial bonding bridges between the segments were formed during a low-temperature sintering at 1300 °C for 1 h. A special technological approach made it possible to change the strength of the partial bonding bridges between the segments, which significantly affected the deformation behavior ceramics during compression. The subpixel accuracy of the DIC results was achieved using an interpolation scheme for the identification functional. The vector fields obtained in the experiment made it possible to characterize the processes of deformation and destruction of a porous, segmented alumina using the strain localization in situ maps, cardinal plastic shear, and circulation of vector fields. The use of these characteristics made it possible to reveal new details in the mechanisms of deformation and destruction of segmented ceramics. The localizations of damage were identified and related to the characteristic structural heterogeneities of the tested porous segmented ceramics.

Ultrasonic melt processing upon direct-chill casting of aluminium alloys

Ultrasonic melt processing (USP) has been known for decades for beneficial effects in as-cast aluminium alloys, including degassing, grain and structure refinement. In the last 10 years the authors performed a series of research projects dedicated to understanding the nature of these effects, by dedicated advanced experiments and by developing numerical models that adequately reflect the complicated physics involved, aiming at further optimising the technology so that it becomes feasible for scale-up applications and attractive for industrial use. Based on the main USP mechanisms previously studied by in-situ observations coupled with acoustic pressure measurements, i.e. fragmentation, deagglomeration and dispersion of the solid phases and inclusions, technological approaches are suggested and tested for grain refinement upon direct-chill casting of Al alloys. Results showed that USP in the melt flow in the launder significantly improves the as-cast structure of a billet, opening the way for upscaling.

Effect of Al4B2O9 and Al18B4O33 whiskers synthesized in situ on the evolution of microstructure and mechanical properties of white fused alumina and Cubitron glass-ceramic composites

The present paper reports on the influence of in situ crystallization of aluminum borate whiskers on the evolution of the microstructure and mechanical properties of electrocorundum (white fused alumina) and microcrystalline alumina (Cubitron 321™) glass-ceramic porous composites. The investigated composites were obtained by sintering the white fused alumina (WFA) and Cubitron 321™ abrasive grains, glass-ceramic bond and various amounts of precursor for whisker growth. The results showed that in the presented technology, the crystallization of the composite reinforcing phases proceeds differently in both types of composites, which affects their microstructure and mechanical properties. An increase in the mechanical tensile strength of 36% was observed for both types of composites; however, depending on the type of abrasive grain, the composites with the highest tensile strength differed in the amount of whisker precursor added. A significant increase in the Young's modulus values was also observed for composites containing 40 wt% precursor: by 45% for WFA and by 52% for Cubitron 321™.

Effect of Thermochemical Boronizing of Alumina Surface on the Borate Crystals Growth and Interaction with Nickel and Nickel Alloy

Wettability at the metal-ceramic interface is highly important for the development of modern composite materials. Poor wettability by metal melts restricts the use of alumina in protective metal matrix composite (MMC) coatings. In the present experimental study, the possibility to modify wetting properties of alumina by thermochemical surface boronizing was investigated. The results of SEM, EDS, XRD and XPS characterisation of surfaces revealed the formation of oxygen containing Al–B compounds identified as aluminium borates (Al18B4O33/Al4B2O9); no signs of non-oxide Al–B compounds were observed. The shape of the single splats deposited on the boronized alumina surface by the thermal spray and re-melted in the furnace revealed that significant wetting improvement by self-fluxing nickel alloy did not occur. However, the improvement of adhesion between the nickel/nickel alloy and Al2O3 surface was obtained due to formation of an intermediate layer consisting of B, O, Al and Si between the metal and ceramic surfaces at the presence of some silicon at the modified surfaces. The presented study demonstrates that the thermochemical boronizing of alumina in amorphous boron medium is a simple method to obtain a thin aluminium borate layer consisting of oriented nano-rod-like crystals, whose growing direction is predetermined by the orientation of the alumina grains’ faces at surface.

Microstructure and Properties of Multilayer Niobium-Aluminum Composites Fabricated by Explosive Welding

In this study, a layered composite material consisting of alternating aluminum and niobium layers and cladded on both sides with titanium plates was obtained by explosive welding. Microstructure of the composite was thoroughly studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as by energy dispersive X-ray spectroscopy (EDX) and electron backscattered diffraction (EBSD). Microhardness measurements, tensile test, and impact strength test were carried out to evaluate the mechanical properties of the composite. Formation of mixing zones observed near all interfaces was explained by local melting and subsequent rapid solidification. Mixing zones at Nb/Al interfaces consisted of metastable amorphous and ultrafine crystalline phases, as well as NbAl3 and Nb2Al equilibrium phases. Niobium grains near the interface were significantly elongated, while aluminum grains were almost equiaxed. Crystalline grains inside the mixing zones did not have a distinct crystallographic texture. Microhardness of Al/Nb mixing zones was in the range 546–668 HV, which significantly exceeds the microhardness of initial materials. Tensile strength and impact strength of the composite were 535 MPa and 82 J/cm2, respectively. These results confirm the high bonding strength between the layers.

MXene-Based Ceramic Nanocomposites Enabled by Pressure-Assisted Sintering.

As MXenes become increasingly widespread, approaches to utilize this versatile class of 2D materials are sought. Recently, there has been growing interest in incorporating MXenes into metal or ceramic matrices to create advanced nanocomposites. This study presents a facile approach of mixing MXene with ceramic particles followed by pressure-assisted sintering to produce bulk MXene/ceramic nanocomposites. The effect of MXene addition on the densification behavior and properties of nanocomposites was explored through the Ti3C2Tz/alumina model system. We discovered that the presence of MXene altered the densification behavior and significantly enhanced the densification rate at low temperatures. In-depth microstructural characterization showed a homogeneous distribution of Ti3C2Tz MXene at the alumina grain boundaries. The Ti3C2Tz/alumina nanocomposites exhibited electrical conductivity but reduced hardness. We also demonstrated that using multilayered Ti3C2Tz as a precursor can produce composites with plate-like TiCx morphology. This work provides a conceptual approach for utilizing the diversity and versatility of MXenes in creating tunable advanced nanocomposites.

Evolution of microstructure and mechanical properties of graphene nanoplates and ZrB2 nanoparticles reinforced AA6111 composites during hot rolling deformation

Hot rolling deformation as a hot working process is commonly used to simultaneously improve the strength and toughness of aluminum matrix composites, owing to the excitation of the equiaxed recrystallized structure and the optimization of the distribution of the reinforcements. Therefore, the AA6111 composites reinforced by graphene nanoplates (GNPs) and in-situ ZrB 2 nanoparticles were prepared with the melt stirring casting technology and hot rolling. The evolution of the microstructure was characterized by electron backscatter diffraction (EBSD) technology. The results show that the clusters of GNPs and ZrB 2 particles were smaller and the grains were refined with the composites deformed from 75% to 92%. Besides, the textures of aluminum grains were transformed from simple to complex. The dynamic recrystallization (DRX) mechanisms of α-Al grains were explored through high-resolution transmission electron microscopes (HRTEM). It was found that the DRX was discovered to be caused by three dynamic recrystallization mechanisms: grain boundary bulging nucleation, mechanical fragmentation, and subgrain boundary coarsening. The role of ZrB 2 nanoparticles and GNPs for DRX revealed that GNPs and particle clusters have the combined effects of both promotion (larger than 1 µm) and hindering (smaller than 1 µm) on dynamic recrystallization. The strength of the composites was improved from 398 to 442 MPa by hot rolling deformation from 75% to 92%, and the elongation remained above 10%. The strengthening mechanism of the composites with increasing deformation was analyzed, providing a theoretical basis for the hot rolling deformation strengthening nanoparticles and GNPs reinforced aluminum matrix composites. • The Cu-GNPs and in situ ZrB2 nanoparticles synergistically reinforced composites. • The evolution of the size, orientation, and texture of α-Al grains was characterized. • Hot rolling deformation reduces the interlayer spacing and clusters of reinforcements. • Three dynamic recrystallization mechanisms of α-Al grains were analyzed. • The GNPs and particle clusters of different sizes can promote or hinder DRX.

The wear resistance of Ni alumina and NiAl2O4 alumina nanocomposites

The influence of metallic Ni or NiAl2O4 as a reinforcing particle on grain growth and wear resistance in alumina matrix composites was evaluated. Alumina composites with various Ni or NiAl2O4 concentrations were prepared by multiple-infiltrations of Ni-nitrate into bisque-fired (necked) alumina green bodies followed by heat treatment and sintering at 1600 °C for 2 h. Sintering in a reducing environment resulted in composites with metallic Ni nanoparticles, while NiAl2O4 alumina composites were formed when sintering in air. The addition of Ni or NiAl2O4 resulted in a reduction in alumina grain size after sintering. The material response to abrasive wear was estimated by measuring the time to section samples of a defined area using a diamond wafering saw and was compared to the wear resistance of undoped alumina. In both cases, reinforcing alumina with Ni or NiAl2O4 particles resulted in a significant increase in wear resistance, correlated to the reduced grain size.

Effect of TiO2 as sintering additive on microstructural, physical, and mechanical properties of CeO2 doped zirconia toughened alumina ceramic composite

Ceramic composites based on Zirconia Toughened Alumina (ZTA) are a good option to meet the requirements of outstanding mechanical properties in modern manufacturing and medical science. In this study, the influences of TiO2 on microstructural, physical, and mechanical properties of ZTA-CeO2 composites are examined by keeping CeO2 fixed at 5 wt% and varying TiO2 from 0 to 4 wt%. Each combination was pressed at a pressure of 300 MPa for 2 min in a hydraulic press. The pressed samples were sintered at 1450 °C for 4 h at atmospheric pressure. The prepared ceramic composites were characterized for their microstructure analysis, phase analysis, density, porosity, Vickers hardness, fracture toughness, and diametral tensile strength (DTS). TiO2 was found to have a substantial impact on the solidification of the composites by lowering the sintering temperature. SEM images showed that the lowest grain size of alumina was attained at 1.5 wt% TiO2, which resulted in the lowest porosity as well as the highest density, hardness, and fracture toughness of the composites. The XRD analysis revealed a secondary phase, Al2TiO5, after the 1.5 wt% TiO2 addition. This phase increased the size of the alumina grains and made the composites more porous and less dense, which resulted in lower hardness and fracture toughness than the maximum. Therefore, the composite (ZTA-CeO2) with 1.5 wt% TiO2 was found to have the optimum results with density (4.16 g/cm3), percentage of porosity (8%), Vickers hardness (1485 HV), fracture toughness (7.54 MPa√m), and DTS (126.4 MPa).

A Comparative Study of Self-Piercing Riveting and Friction Self-Piercing Riveting of Cast Aluminum Alloy Al–Si7Mg

Abstract Cast aluminum alloys are promising materials that can simplify the manufacturing process of automobile body structures. However, the low ductility of cast aluminum poses significant challenges to existing riveting technologies. In the present work, dissimilar AA6061-T6 aluminum alloy and Al–Si7Mg cast aluminum were joined by self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes to reveal the effect of friction heat on rivetability of low-ductility cast aluminum alloys. The joint macro-morphology, microstructure, peak tooling force, microhardness distribution, tensile-shear, and cross-tension performance of the two processes were comparatively studied. Results indicated that the in-situ softening effect of friction heat in the F-SPR process could effectively improve the ductility of cast aluminum, avoid cracking, and reduce the tooling force by 53%, compared to the SPR process. The severe plastic deformation and friction heat induced by rivet rotation results in refined equiaxed grains of aluminum near the rivets and solid-state bonding between aluminum sheets in the rivet cavity. The F-SPR joints are superior to SPR joints in both tensile-shear and cross-tension performance due to the avoidance of cracking, increase of mechanical interlocking, and solid-state bonding of interfaces. Significantly, when Al–Si7Mg is placed on the lower layer, the peak tensile-shear and cross-tension loads of the F-SPR joints are 7.2% and 45.5% higher than the corresponding SPR joints, respectively.

Graphene-coated alumina nano/microfibers as filler for composites

The present work is focused on the preparation and characterization of composite nano/microfibers based on alumina applicable as fillers in ceramic-matrix composites. The graphene-coated Al2O3 microfibers have been prepared in the form of continuous alumina/graphene fibers from electrospun precursors using needle-less electrospinning, calcination, and catalyst-free chemical vapor deposition of graphene layers on the surface of the microfibers. The phase composition and morphology of the composite fibers were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning, and transmission electron microscopy (SEM and TEM). The porosity of the final fibers was evaluated by low-temperature nitrogen adsorption. The surface microstructures of composite fibers were rough and their diameters were in the range from 300 nm to 2 μm. The phase transformation at 1100 °C led to the formation of a mixture of γ-, δ-, and α-Al2O3. TEM results confirmed that the alumina fibers were covered by the 3–4 layers of graphene-based structure. The specific surface area of pure alumina fibers after carbon deposition slightly increased from the initial 40 m2/g to 45 m2/g, due to a slightly wavy surface of graphene layers. Raman spectroscopy and XPS analysis results showed that the surface layer of fibers is formed as graphene oxide. Between the graphene oxide and alumina grains a chemical bonding C–O–Al, C–O–O–Al was discovered as the nucleation of the graphene oxide sheet. The growth mechanism of graphene oxide layers on the surface edges of Al2O3 grains was described.

Effect of the addition of ductile phase on mechanical properties of Alumina/nano SiC ceramic composite

ABSTRACT Silicon carbide reinforcement in Alumina possess better physical and mechanical properties at elevated temperature. However, addition of second phase ceramic to matrix enhances the strength and hardness but it moderately improves the fracture toughness. The reason is due to lack of plastic deformation. Addition of ductile metallic phase helps in bridging the advancing crack in the matrix. In the present study, Alumina/nano SiC ceramic composite is toughened by the addition of copper metal particles. The influence of these metallic particles on behavior and mechanical properties of Alumina/SiC ceramic composite have been studied. The composite is processed using microwave hybrid sintering technique (2800 Watt) for one hour at 1400℃ and 1500℃. After sintering, microstructure of sample, grain size, relative densification, hardness and toughness have been examined. Highest densification of 93% and hardness of 1590 HV is achieved at 1400℃ sintering temperature while highest fracture toughness of 3.9 MPa m1/2 is achieved at temperature of 1500℃ with coarsening of alumina grains. The ductile phase in ceramic has proven to be effective for better bonding. It creates ceramic-metal bonds, which act as a tiny metal matrix between ceramic particles and contribute to ductile phase toughening in developed (Alumina/5% nano SiC/5%Cu) ceramic composite.

Alumina grain growth and grain boundary segregation of FeCrAl-based oxide dispersion-strengthened alloys at high temperature

FeCrAl-based oxide dispersion-strengthened alloys are currently being considered for accident-tolerant fuel cladding applications. In this study, the microstructure of alumina scales that formed on the alloys after oxidation at 1100 °C and 1200 °C, in an atmosphere containing water vapor, was investigated. The alumina grain growth and grain boundary segregation were analyzed using multiscale techniques such as oxidation kinetics, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, and backscattered electron diffraction, thus evaluating the diffusion of the reactive elements and alumina grain structure at different scales. A mechanism supporting the development of the alumina grain structure was proposed.

Features of the Formation of Surface Structures under Dry Friction of Al-30Sn Composite against Steel

Features of the frictional interaction of a sintered Al-30Sn composite, which is used as a coating of bearing inserts, with steel counterbody in the absence of liquid lubricant were studied. The tribological tests were carried out according to the pin-on-disk scheme at room temperature. The friction coefficient μ of the friction pair is increased up to approximately 0.6 during the running-in process. Its growth stops when the stage of steady state friction begins, and then μ fluctuates around some relatively high and constant value. The study of the friction surfaces of the friction pair showed that the aforementioned increase in μ is due to the formation of a discrete transferred layer on the friction track surface and strain hardening of the subsurface layer of the Al-30Sn sample under the action of hard particles of the transferred layer. It was established that tin is transferred on the friction track surface mainly in a composition of wear particles, despite the high content of the solid lubricant in the investigated samples. The wear intensity of the samples subjected to processing by equal channel angular pressing with route A is much lower than that of the sintered (unprocessed) ones. The main wear mechanism of Al-30Sn composites under dry friction against steel is a delamination of the highly deformed subsurface aluminum grains along their interphase boundaries.

Design of newly effective grain refiner for aluminum based on medium-entropy metal diboride

In this paper, medium-entropy metal diboride (Ti1/3Cr1/3V1/3)B2 with a hexagonal TiB2-type crystal structure were synthesized via molten salt reaction, and its effect on refining pure aluminum grains was investigated. Compared with TiB2, the minimum lattice mismatch between Al and (Ti1/3Cr1/3V1/3)B2 is reduced from 5.6% to 3.7%, effectively reducing undercooling required for nucleation. By just adding 1.5 vol% (Ti1/3Cr1/3V1/3)B2, the average grain size of commercial pure aluminum is reduced to ∼32 μm. The synthesized particles are expected to be used in grain refinement during additive manufacturing and rapid solidification, and in the preparation of aluminum matrix composites. This study opens up a new direction in designing new compounds for grain refinement by tailoring lattice matching within multi-component composition space.

Zirconia-alumina multiphase ceramic fibers with exceptional thermal stability by melt-spinning from solid ceramic precursor

Zirconia-alumina multiphase ceramic fibers with 80 wt% (Z80A20 fiber) and 10 wt% (Z10A90 fiber) proportions of zirconia were prepared via melt-spinning and calcination from solid ceramic precursors synthesized by controllable hydrolysis of metallorganics. The zirconia-alumina multiphase fibers had a diameter of about 10 µm and were evenly distributed with alumina and zirconia grains. The Z80A20 and Z10A90 ceramic fibers had the highest filament tensile strength of 1.78 GPa and 1.87 GPa, respectively, with a peak value of 2.62 GPa and 2.71 GPa. The Z80A20 ceramic fiber has superior thermal stability compared to the Z10A90 ceramic fiber and a higher rate of filament strength retention due to the stability in grain size. After heat treatment at 1100 °C, 1200 °C, and 1300 °C for 1 h respectively, the filament tensile strength retention rate of Z80A20 ceramic fibers was 87 %, 80 %, and 40 %. While Z10A90 ceramic fiber was fragile after being heated at 1300 °C. The results showed that the high zirconia content facilitated the fiber's thermal stability.

Selective laser sintering of alumina-molybdenum nanocomposites

Alumina/molybdenum nanocomposites were obtained from alumina and molybdenum particles. Alumina with different molybdenum contents (0, 1, 2.5, 5, 10 and 20 wt %) was first uniaxially pressed at 100 MPa to obtain green compacts that were later sintered using a carbon dioxide (CO2) laser. Samples were characterized by X-ray diffraction and Scanning Electron Microscope to evaluate the morphological and microstructural characteristics of the composites. SEM results show that for mixtures with 1, 2.5, 5 and 10 wt % Mo, metallic Mo appears dispersed within the alumina grains (grain boundaries and triple points). Nevertheless, for the mixture with 20 wt % Mo, it also appears in the alumina matrix as Mo in triple points and grain boundaries, although MoO3 is also identified (as inclusion). The presence of these phases was confirmed by X-ray Diffraction technique. These metallic molybdenum particles distributed in the alumina matrix at triple points and grain boundaries promote the densification of the composite. Metallic molybdenum has also a pinning effect, which drastically affects the microstructural evolution during the sintering, mainly on the grain size of alumina. The best results were observed for the composite Al2O3-10 wt % Mo, with an average alumina grain size <10 μm and few pores.

Production of carbon nanomaterials and syngas from biogas reforming and decomposition on one-pot mesoporous nickel alumina catalysts

Biogas, a renewable energy source, is primarily composed of CH4 and CO2. It is a promising alternative to fossil fuels and can be used directly for electricity production as well as heat generation via combustion. Concerns about climate change and a greater emphasis on renewable energy sources have recently increased interest in biogas utilization. In this context, biogas reforming and decomposition (BRD) into synthesis gas and carbon nanofibers (CNFs) is viewed as a new and attractive way of efficiently valorising biogas. In this study, Ni-loaded (i.e., 20, 50 wt%) mesoporous alumina materials were prepared using one-pot evaporation-induced self-assembly method for BRD. Synthesized materials were characterized by various techniques: N2-physisorption, X-ray diffraction, temperature-programmed reduction, scanning electron microscopy, and thermal gravimetric analysis. Results showed that textural and structural properties of synthesised materials differed with Ni loading. High Ni-loaded catalyst displayed higher surface area, pore volume, pore size distribution, and average particle size which is the result of deposition of Ni species outside alumina grains creating thus, surface defects. BRD results were greatly influenced by Ni content with Ni50%Al2O3 reflecting catalytic behaviour similar to those expected for pure methane decomposition. Most importantly, this catalyst was also capable of generating, selectively, interesting carbon nanofibers.

Local deformation mechanisms in metal systems with a tailored grain size distribution

A large change in yield strength and ductility takes place in metals with a recrystallized average grain size in the near-micrometer grain size regime. This has been utilized to explore the influence of grain size heterogeneity on plastic deformation for recrystallized grains in aluminum prepared by spark plasma sintering, where tailoring of the grain size distribution and spatial arrangement can be achieved by selecting powder sizes and mixing conditions prior to sintering. The spatial variation of plastic deformation has been explored by digital image correlation (DIC), either at the sample scale or at the grain scale. For the latter colloidal SiO2 markers also allow repeated collection of electron backscatter diffraction data over the same area during in-situ deformation, proving correlated information on local displacement gradients and crystal lattice rotations. For samples prepared from well-mixed powders only small deviations from a rule-of-mixtures in the yield strength are observed. A mesoscale pattern of strain distribution is observed from DIC investigations where this is related to, but not fully consistent with, the spatial arrangement of fine and coarse grains. In the case of macroscopically layered arrangements of coarse and fine grains, deformation is dominated by development of local strain concentrations, which take place preferentially in the fine grain regions.

Microscopic quantum point contact formation as the electromigration mechanism in granular superconductor nanowires

Granular aluminium is a high kinetic inductance thin film superconductor which, when formed into nanowires can undergo an intrinsic electromigration process. We use a combination of experimental and computational approaches to investigate the role of grain morphology and distribution in granular aluminium thin films, when formed into nanowire constrictions. Treating the granular aluminium film as a network of randomly distributed resistors with parameters motivated by the film microstructure allows us to model the electrical characteristics of the nanowires. This model provides estimates of the dependence of sheet resistance on grain size and distribution, and the resulting device to device variation for superconducting nanowires. By fabricating a series of different length nanowires, we study the electromigration process as a function of applied current, and then compare directly to the results of our computational model. In doing so we show that the electromigration is driven by the formation of quantum point contacts between metallic aluminium grains.