Tensile and wear properties of rolled Al5Mg-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> or C particulate composites

Al5Mg alloy matrix composites reinforced with different percentages of Al₂O₃ (60 μm) or C (90 μm) particulates were prepared by the vortex method. The composites were then subjected to hot or cold rolling with different reduction ratios. The microstructures of the rolled composites revealed that the matrix grains moved around the particulate causing deformation. By continuing deformation, the particulates rearranged themselves in the matrix, leading to lensoid distortion. It was found that the addition of Al₂O₃ or C particulates increased the 0.2% proof stress and reduced both the tensile strength and ductility, compared with the monolithic alloy. Scanning electron microscopy (SEM) fractographic examinations showed that the composites reinforced with Al₂O₃ particulates failed through particulate fracture and matrix ligament rupture. However, the failure of the composites reinforced with C particulates was through particulate decohesion, followed by ductile failure of the matrix. Abrasive wear results showed that the wear rate of the Al5Mg alloy decreased with the addition of C particulates. However, increasing the volume fraction of C particulates did not have a prominent effect on the wear rate. The composites reinforced with Al₂O₃ particulates exhibited a higher wear rate than that of the unreinforced alloy. Furthermore, addition of both C and Al₂O₃ particulates into the Al5Mg matrix alloy did not significantly improve the wear resistance. For all composites studied in this work, hot or cold rolling had a marginal effect on the wear results.     

Fabrication of Al matrix composite reinforced with submicrometer-sized Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles formed by combustion reaction between HEMM Al and V<Subscript>2</Subscript>O<Subscript>5</Subscript> composite particles during sintering

To fabricate an Al-V matrix composite reinforced with submicron-sized Al₂O₃ and Al_xV_y (Al₃V, Al₁₀V) phases, high energy mechanical milling (HEMM) and sintering were employed. By increasing the milling time, the size of mechanically milled powder was significantly reduced. In this study, the average powder size of 59 μm for Al, and 178 μm for V₂O₅ decreased with the formation of a new product, Al-Al₂O₃-Al_xV_y, with a size range from 1.3 μm to 2.6 μm formed by the in-situ combustion reaction during sintering of HEM milled Al and V₂O₅ composite powders. The in-situ reaction between Al and V₂O₅ during the HEMM and sintering transformed the Al₂O₃ and Al_xV_y (Al₃V, Al₁₀V) phases. Most of the reduced V reacted with excess the Al to form Al_xV_y (Al₃V, Al₁₀V) with very little V dissolved into Al matrix. By increasing the milling time and weight percentage of V₂O₅, the hardness of the Al-Al₂O₃-Al_xV_y composite sintered at 1173 K increased. The composite fabricated with the HEMM Al-20wt.%V₂O₅ composite powder and sintering at 1173 K for 2 h had the highest hardness.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles on the microstructure and sliding wear of 7075 Al alloy manufactured by squeeze casting method

Aluminum (Al) alloy 7075 reinforced with Al₂O₃ particles was prepared using the stir casting method. The microstructure of the cast composites showed some degree of porosity and sites of Al₂O₃ particle clustering, especially at high-volume fractions of Al₂O₃ particles. Different squeeze pressures (25 and 50 MPa) were applied to the cast composite during solidification to reduce porosity and particle clusters. Microstructure examinations of the squeeze cast composites showed remarkable grain refining compared with that of the matrix alloy. As the volume fraction of particles and applied squeeze pressure increased, the hardness linearly increased. This increase was related to the modified structure and the decrease in the porosity. The effect of particle volume fraction and squeeze pressure on the dry-sliding wear of the composites was studied. Experiments were performed at 10, 30, and 50 N with a sliding speed of 1 m/s using a pin-on-ring apparatus. Increasing the particle volume fraction and squeeze pressure improved the wear resistance of the composite compared with that of the monolithic alloy, because the Al₂O₃ particles acted as load-bearing constituents. Also, these results can be attributed to the fact that the application of squeeze pressure during solidification led to a reduction in the porosity, and an increase in the solidification rate, leading to a finer structure. Moreover, the application of squeeze pressure improved the interface strength between the matrix and Al₂O₃ particles by elimination of the porosity at the interface, thereby providing better mechanical locking.     

Tribological Properties of Ni<Subscript>3</Subscript>Al Matrix Composite Sliding Against Si<Subscript>3</Subscript>N<Subscript>4</Subscript>, SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> at Elevated Temperatures

The Ni₃Al matrix self-lubricating composite was fabricated by powder metallurgy technique. The tribological behavior of the composite sliding against commercial Si₃N₄, SiC and Al₂O₃ ceramic balls was investigated from 20 to 1000 °C. It was found that the composite demonstrated excellent lubricating properties with different friction pairs at a wide temperature range, which can be attributed to the synergetic effect of Ag, fluorides, and molybdates formed by oxidations. The Ni₃Al matrix self-lubricating composite/Si₃N₄ couple possessed the stable friction coefficient and wear rate.     

Cold spray of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with soft metal incorporation: A technical contribution

Al + SiC, Al + Al₂O₃ composites as well as pure Al, SiC, and Al₂O₃ coatings were prepared on Si substrates by the cold gas dynamic spray process (CGDS or cold spray). The powder composition of metal (Al) and ceramic (SiC, Al₂O₃) was varied into 1:1 and 10:1 wt.%, respectively. The propellant gas was air heated up to 330 °C and the gas pressure was fixed at 0.7 MPa. SiC and Al₂O₃ have been successfully sprayed producing coatings with more than 50 μm in thickness with the incorporation of Al as a binder. Also, hard ceramic particles showed peening effects on the coating surfaces. In the case of pure Al metal coating, there was no crater formation on hard Si substrates. However, when Al mixed with SiC and Al₂O₃, craters were observed and their quantities and sizes depended on the composition, aggregation and size of raw materials.     

Oxidation resistant Mo-Mo<Subscript>2</Subscript>B-silica and Mo-Mo<Subscript>2</Subscript>B-silicate composites for high temperature applications

Development of Mo composites based on the Mo-Si-B system has been demonstrated as a possible new route to achieving a high temperature Mobased material. In this new system, the silicide phases are replaced directly with silica or other silicate materials. These composites avoid the high ductile to brittle transition temperature observed for Mo-Si-B alloys by removing the Si that exists in solid solution in Mo at equilibrium with its silicides. A variety of compositions is tested for room temperature ductility and oxidation resistance. A system based upon Mo, Mo₂B, and SrO·Al₂O₃·(SiO₂)₂ is shown to possess both ductility at 80 vol.% Mo and oxidation resistance at 60 vol.%. These composites can be produced using a powder processing approach and fired to greater than 95% theoretical density with a desirable microstructure of isolated boride and silicate phases within a ductile Mo matrix.     

Effect of Milling Time on Electrical Breakdown Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

In order to clarify the relationship between the microstructure and the arc erosion behavior of metal-matrix composite, Al₂O₃/Cu composites with different distributions of Al₂O₃ particles were prepared by high energy ball milling and powder metallurgy. The effect of milling time on microstructure, properties, and arc erosion behavior of Al₂O₃/Cu composite was investigated. The results show that the distribution of Al₂O₃ particles improves significantly with increase of milling time, but Al₂O₃ particles will be aggregated if milling time is too long. The optimal milling time is 24 h in the range of experiments. A uniform distribution of Al₂O₃ particles in copper matrix can improve the hardness, electrical conductivity, average breakdown strength, chopping level, and arc life. With improvement in the distribution of Al₂O₃ particles, the erosion area becomes larger, and the erosion pits become shallower and are dispersed more uniformly.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Content on Electrical Breakdown Properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

Al₂O₃/Cu composites were prepared by external addition of Al₂O₃, and the effect of Al₂O₃ content on microstructure, density, hardness, electrical conductivity and vacuum electrical breakdown properties was studied. The results show that with increasing Al₂O₃ addition, the density of Al₂O₃/Cu composite significantly decreases, the hardness sharply increases and then slowly decreases, but the electrical conductivity invariably decreases. The vacuum breakdown test shows that with increasing Al₂O₃ addition, the breakdown strength first sharply increases and then decreases when the Al₂O₃ content exceeds 1.2 wt.%; the chopping current always exhibits a decreasing trend and the arc life first increases and then decreases. According to the morphology of arc erosion and analysis, the arc erosion resistance increases and then decreases sharply. In the range of experiments, the optimal arc erosion resistance of Al₂O₃/Cu composite can be obtained with the addition of 1.2 wt.% Al₂O₃.     

Rapid consolidation of nanocrystalline Ti<Subscript>3</Subscript>Al-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites from mechanically synthesized powders by high frequency induction heated sintering

Nano-powders of Ti₃Al and ²Al₂O₃ were synthesized from 3TiO₂ and 5Al powders by high energy ball milling. Nanocrystalline Al₂O₃ reinforced composite was consolidated by high frequency induction heated sintering within 2 minutes from mechanochemically synthesized powders of 2Al₂O₃ and Ti₃Al. The relative density of the composite was 99.5%. The average hardness and fracture toughness were 1340 kg/mm² and 8 MPa·m^1/2, respectively.     

Microstructure Characterization of the FeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>-Based Nanostructured Composite Coating Synthesized by Plasma Spraying Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al Powders

The microstructure of the coating prepared by reactive plasma spraying Fe₂O₃/Al composite powders was characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicated that the coating exhibited nanostructured microstructure which consisted of FeAl₂O₄, Fe or Fe solid solution, Al₂O₃ and a little FeAl. In the composite coating, spherical Fe particles (tens of nanometers to hundreds of nanometers) were distributed uniformly within the equiaxed and columnar nanograins FeAl₂O₄ matrix. There were two kinds of Al₂O₃ phases present in the composite coating. One kind was nano-sized Al₂O₃ particles uniformly dispersed within the matrix, forming eutectic structure of (FeAl₂O₄ + γ-Al₂O₃); the other was 1-1.5 μm Al₂O₃ particles embedded individually within the matrix. The composite coating had higher toughness than the conventional microstructured Al₂O₃ coating.     

The Usability of Boric Acid as an Alternative Foaming Agent on the Fabrication of Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composite Foams

Pure Al and alumina (2, 5, 10 wt.% Al₂O₃)-added Al composite foams were fabricated through powder metallurgy technique, where boric acid (H₃BO₃) is employed as a new alternative foaming agent. It is aimed to determine the effects of boric acid on the foaming behavior and cellular structure and also purposed to develop the mechanical properties of Al foams by addition of Al₂O₃. Al and Al composite foams with porosity fraction in the range of 46-53% were achieved by sintering at 620 °C for 2 h. Cell morphology was characterized using a combination of stereomicroscope equipped with image analyzer and scanning electron microscopy. Microhardness values were measured via using Vickers indentation technique. Quasi-static compression tests were performed at strain rate of 10^?3 s^?1. Compressive strength and energy absorption of the composite foams enhanced not only by the increasing weight fraction of alumina, but also by the usage of boric acid which leads to formation of boron oxide (B₂O₃) acting as a binder in obtaining dense cell walls. The results revealed that the boric acid has outstanding potential as foaming agent in the fabrication of Al and Al composite foams by providing improved mechanical properties.     

Preparation of TiAl<Subscript>3</Subscript>-Al Composite Coating by Cold Spray and Its High Temperature Oxidation Behavior

A novel TiAl₃-Al coating was prepared by cold spray for high temperature protection of titanium aluminum-based alloy. The substrate alloy was orthorhombic-Ti-22Al-26Nb (at.%). The composite coating was mainly composed of TiAl₃ embedded in the matrix of residual aluminum. An interlayer about 10 μm was formed between the coating and the substrate. The oxidation test indicated that this composite coating was very effective in improving the high-temperature oxidation resistance of the substrate alloy at 950 °C in the tested 150 cycles without any sign of degradation. The microstructure analysis of the oxidized composite coating showed that an Al₂O₃ scale with a complex structure can be formed outside the interlayer during oxidation and no oxides beneath the interlayer were detected, which indicated that the complex continuous Al₂O₃ and the interlayer provide the protection of the substrate at high-temperature oxidation condition.     

The Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Powder Morphology on the Properties of Cu-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composites Designed for Functionally Graded Materials (FGM)

In order to meet the requirements of an increased efficiency applying to modern devices and in more general terms science and technology, it is necessary to develop new materials. Combining various types of materials (such as metals and ceramics) and developing composite materials seem to be suitable solutions. One of the most interesting materials includes Cu-Al₂O₃ composite and gradient materials (FGMs). Due to their potential properties, copper-alumina composites could be used in aerospace industry as rocket thrusters and components in aircraft engines. The main challenge posed by copper matrix composites reinforced by aluminum oxide particles is obtaining the uniform structure with no residual porosity (existing within the area of the ceramic phase). In the present paper, Cu-Al₂O₃ composites (also in a gradient form) with 1, 3, and 5 vol.% of aluminum oxide were fabricated by the hot pressing and spark plasma sintering methods. Two forms of aluminum oxide (αAl₂O₃ powder and electrocorundum) were used as a reinforcement. Microstructural investigations revealed that near fully dense materials with low porosity and a clear interface between the metal matrix and ceramics were obtained in the case of the SPS method. In this paper, the properties (mechanical, thermal, and tribological) of composite materials were also collected and compared. Technological tests were preceded by finite element method analyses of thermal stresses generated in the gradient structure, and additionally, the role of porosity in the formation process of composite properties was modeled. Based on the said modeling, technological conditions for obtaining FGMs were proposed.     

Rapid synthesis and consolidation of nanostuctured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgSiO<Subscript>3</Subscript> composites by high frequency induction heated sintering

Nanopowders of MgO, Al₂O₃ and SiO₂ were made by high energy ball milling. The rapid sintering of nanostuctured Al₂O₃-MgSiO₃ composites was investigated by the high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibits grain growth. Highly dense nanostructured Al₂O₃-MgSiO₃ composites were produced with the simultaneous application of 80 MPa pressure and the induced output current of total power capacity (15 kW) within 2 min. The sintering behavior, grain size and mechanical properties of Al₂O₃-MgSiO₃ composites were investigated.     

Synthesis and thermal expansion of 4J36/ZrW<Subscript>2</Subscript>O<Subscript>8</Subscript> composites

Gas atomized 4J36 alloy powder was milled for 72 h then mixed with ZrW₂O₈ powder and sintered at 600°C for 4 h under argon atmosphere. 4J36/ZrW₂O₈ composites containing 10 vol.%, 20 vol.%, 30 vol.%, and 40 vol.% ZrW₂O₈ were fabricated, the relative density of which ranged from 70% to 80%. Thermal expansion coefficients of the composites decreased as the amount of ZrW₂O₈ increased, in agreement with the rule of the mixture. The coefficient of thermal expansion of the 4J36/40 vol.%ZrW₂O₈ composite in 25–100°C is 0.55 × 10⁻⁶/°C.     

Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-40% TiO<Subscript>2</Subscript> Coatings

The formation and decomposition of aluminum titanate (Al₂TiO₅, tialite) in feedstock powders and coatings of the binary Al₂O₃-TiO₂ system are so far poorly understood. A commercial fused and crushed Al₂O₃-40%TiO₂ powder was selected as the feedstock for the experimental series presented in this paper, as the composition is close to that of Al₂TiO₅. Part of that powder was heat-treated in air at 1150 and 1500 °C in order to modify the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including EDS of metallographically prepared cross sections. Only a maximum content of about 45 wt.% Al₂TiO₅ was possible to obtain with the heat treatment at 1500 °C due to inhomogeneous distribution of Al and Ti in the original powder. Coatings were prepared by plasma spraying using a TriplexPro-210 (Oerlikon Metco) with Ar-H₂ and Ar-He plasma gas mixtures at plasma power levels of 41 and 48 kW. Coatings were studied by XRD, SEM including EDS linescans of metallographically prepared cross sections, and microhardness HV1. With the exception of the powder heat-treated at 1500 °C an Al₂TiO₅-Ti₃O₅ (tialite–anosovite) solid solution Al_2?xTi_1+xO₅ instead of Al₂TiO₅ existed in the initial powder and the coatings.     

Microstructure and hot deformation behavior of A356/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite fabricated by infiltration method

The hot deformation behavior of an A356/Al₂O₃ composite fabricated by the infiltration method was characterized in the temperature range of 300-500 °C and strain rate range of 0.001-1/s using compressive tests. The composite consists of an Al-Si based matrix and nano-sized Al₂O₃ particulates. A constitutive model was established based on the hyperbolic sine Arrhenius type equation and its hot workability was evaluated by means of processing maps based on Dynamic Material Modeling. The activation energy for hot deformation was calculated to be 223 kJ/mol, which is higher than the activation energy for self-diffusion of pure aluminum (142 kJ/mol). The optimum processing condition for the hot working of the composite was found to exist at 500 °C with a strain rate of 1/s, where a dynamic recrystallized microstructure was observed and the maximum efficiency was exhibited in the processing map. Voids were frequently detected at 500 °C with lower strain rates, deteriorating the workability of the composite.     

Mechano-thermal treatment of TiO<Subscript>2</Subscript>-Al powder mixture to prepare TiAl/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite

In this research, a composite comprising an intermetallic matrix and dispersed Al₂O₃ particles was processed. A mixture of TiO₂ and Al was mechanically activated in the presence of a process control agent and/or without it, in a high-energy planetary ball mill. As a subsequent process, the sample was sintered at various temperatures. The phase composition and morphology of the samples were evaluated by XRD and SEM techniques, respectively. The thermal behavior of the samples milled for 8 h with PVA and/or without it, were also assessed by the DTA technique and compared with one another. The DTA results revealed that addition of PVA shifted the aluminothermic reduction of TiO₂ to higher temperatures; therefore, final composite phases were developed at higher temperatures. The results also showed that addition of PVA during milling caused the final microstructure to coarsen. The XRD pattern of the sample sintered at 700 °C exhibits the existence of TiAl, Ti₃Al, and Al₂O₃ phases. In the sample sintered at 850 °C, the remaining Ti₃Al peak was attenuated and completely disappeared at 1000 °C.     

Microstructure properties and microhardness of rapidly solidified Al<Subscript>64</Subscript>Cu<Subscript>20</Subscript>Fe<Subscript>12</Subscript>Si<Subscript>4</Subscript> quasicrystal alloy

This paper presents differences in the microstructure and microhardness properties of conventional casting (ingot) and rapidly solidified Al₆₄Cu₂₀Fe₁₂Si₄ quasicrystal (QC) alloys. The phases present in the Al₆₄Cu₂₀Fe₁₂Si₄ ingot alloy were determined to be icosahedral quasicrystalline (IQC) Ψ-Al₆₅Cu₂₀Fe₁₅, cubic β-AlFe, tetragonal θ-Al₂Cu, and monoclinic λ-A1₃Fe₄ phases, whereas only IQC Ψ-Al₆₅Cu₂₀Fe₁₅ and cubic β-AlFe phases were identified in the rapidly solidified alloy. The microhardness value of the melt spun alloy was measured to be approximately 790 kg/mm². Microhardness increases with increasing solidification rates.     

Microstructure and Properties of Nanocrystalline Copper Strengthened by a Low Amount of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanoparticles

Dispersion-strengthened Cu-Al₂O₃ materials have been studied over recent years to find an optimum processing route to obtain a high strength, thermal-stable copper alloy designed for modern applications in electrical engineering. The study analyses the influence of 1 vol.% of alumina content on strengthening the copper matrix. Microstructure of the Cu-Al₂O₃ composite was studied by x-ray diffraction as well as scanning and transmission electron microscopy. The composite shows a homogeneous, thermal-stable nanostructure up to 900 °C due to dispersed alumina nanoparticles. The particles effectively strengthen crystallite/grain boundaries in processes of powder consolidation and annealing of the compact. In contrast to monolithic Cu, the Cu-1 vol.% Al₂O₃ exhibits more than double strength and hardness. The nanocrystalline matrix and the low amount of alumina particles result in a yield strength of 288 MPa and a ductility of 15% which is a good combination for practical utilization of the material.