Effects of lubrication and aspect ratio on the consolidation of metal matrix composites under cyclic pressure

The effects of powder compaction under cyclic load were compared to traditional single-cycle compaction. The effect of interparticle friction as modified by lubrication and compact aspect ratio received particular attention. Mixtures of Al and Al₂O₃ were consolidated at room temperature in contained uniaxial consolidation experiments. The experiments showed that in static compaction, lubrication aids densification at low pressures but can inhibit consolidation at high pressures. Enhanced densification was observed following pressure cycling. These improvements were more pronounced in compacts having smaller aspect ratios. The efficiency of pressure cycling was reduced by the lubricant. Lubrication also decreased the effects of aspect ratio in both the static and cyclic compaction cases. Although lubrication did increase density uniformity, the resulting compact green strength was much lower. Both single and double action compaction were studied and the best green strength and density distribution were obtained with double-action compaction under cyclic pressure without lubricant.     

Prediction of effect of volume fraction, compact pressure and milling time on properties of Al-Al2O3 MMCs using neural networks

An artificial neural network (ANN) model was developed to predict the effect of volume fraction, compact pressure and milling time on green density, sintered density and hardness of Al-Al₂O₃ metal matrix composites (MMCs). Al-Al₂O₃ powder mixtures with various reinforcement volume fractions of 5, 10, 15% Al₂O₃ and milling times (0 h to 7 h) were prepared by mechanical milling process and composite powders were compacted at various pressure (300, 500 and 700 MPa). The three input parameters in the proposed ANN were the volume fraction, compact pressure and duration of the milling process. Green density, sintered density and hardness of the composites were the outputs obtained from the proposed ANN. As a result of this study the ANN was found to be successful for predicting the green density, sintered density and hardness of Al-Al₂O₃ MMCs. The mean absolute percentage error for the predicted values didn’t exceed 5.53%. This model can be used for predicting Al-Al₂O₃ MMCs properties produced with different reinforcement volume fractions, compact pressures and milling times.     

Improvement of cyclic oxidation resistance of a NiAl-based alloy modified by Dy

The effect of the rare-earth-element dysprosium on the cyclic oxidation behavior of NiAl-31Cr-3Mo alloy at 1300 and 1400 K in static air atmosphere was investigated in this paper. The results revealed that doping 0.1.% atom fraction of Dy greatly improved the cyclic oxidation resistance at 1300 and 1400 K. The microstructure and composition of the oxidation layer were studied by means of X-ray diffractometer (XRD), scanning electron microscopy (SEM) with EDAX unit. The oxidation scale, which is located on the surface of NiAl-31Cr-3Mo alloy, mainly consisted of α-Al₂O₃ and a little Cr₂O₃. While the continuous and sole α-Al₂O₃ scale formed on the surface of the alloys doped with the dysprosium. The adherency of scale improved due to a Cr-rich phase decreased the internal stress by thermal cycling and transformation of Al₂O₃ or transverse growing induced for the testing alloy modified by Dy. The oxidation mechanism was discussed from phase constitution.     

Processing and microstructure of alumina-based composites

A study of powder structure and its effect on the sintering tendency of certain alumina-based ceramic systems, that is, Al₂O₃-SiO₂ and Al₂O₃-ZrO₂, was carried out to improve their mechanical strength and fracture toughness. The compacting behavior and the sintering characteristics were optimized through control of various parameters such as composition, compaction pressure, sintering temperature, and time. Best densification was obtained for mixtures prepared using very fine and deagglomerated alumina powders.     

Microstructure and strength of a squeeze cast aluminium piston alloy composite reinforced with alumina short fibre using Al2O3 binder

Abstract

Alumina short fibre preforms were fabricated using an Al₂O₃ binder and infiltrated with aluminium piston alloy melt by squeeze casting. Al₂O₃ binder is thermodynamically more stable than the conventional SiO₂ binder and reduces the fibre/matrix interfacial reaction. The effects of fibre volume fraction, temperature and heat treatment on the yield strength and tensile strength of the composite were investigated. The Al₂O₃ binder provided a satisfactory interfacial bond between the fibre and the matrix without any interfacial reaction or fibre damage. Aging behaviour was not changed by reinforcement. At every temperature, the composites showed the highest strength with a fibre volume fraction of 18%. The strength of the composite was improved by T6 heat treatment. Examination of the fracture surfaces and calculation of the tensile strength using the rule of mixtures indicated that the 18% fibre reinforced composite had a strong interfacial bond even at high temperatures.     

Effect of deformation temperature on the mechanical behavior and deformation mechanisms of Al-Al2O3 metal matrix composites

Aluminum-alumina (Al-Al₂O₃) metal matrix composite (MMC) materials were fabricated using the powder metallurgy (PM) techniques of hot pressing followed by hot extrusion. Different reinforcement weight fractions were used, that is, 0, 2.5, 5, and 10 wt% Al₂O₃. The effect of deformation temperature was investigated through hot tensile deformation conducted at different temperatures. The microstructures of the tested specimens were also investigated to characterize the operative softening mechanisms. The yield and tensile strength of the Al-Al₂O₃ were found to improve as a function of reinforcement weight fraction. With the exception of Al-10wt%Al₂O₃, the MMC showed better strength and behavior at high temperatures than the unreinforced matrix. The uniform deformation range was found to decrease for the same reinforcement weight fraction, as a function of temperature. For the same deformation temperature, it increases as a function of reinforcement weight fraction. Both dynamic recovery and dynamic recrystallization were found to be operative in Al-Al₂O₃ MMC as a function of deformation temperature. Dynamic recovery is dominant in the lower temperature range, while dynamic recrystallization is more dominant at the higher range. The increase in reinforcement weight fraction was found to lead to early nucleation of recrystallization. No direct relationship was established as far as the number of grains nucleated due to each reinforcement particle.     

The influence of ceramic particles on bond strength of cold spray composite coatings on AZ91 alloy substrate

Al-Al₂O₃ composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al₂O₃ particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al₂O₃ particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al₂O₃ content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al-Al₂O₃ bonds compared with stronger Al-Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted.     

Reactive synthesis of alumina-boron nitride composites

The reactions of aluminum borates (9Al₂O₃ · 2B₂O₃ and 2Al₂O₃ · B₂O₃) with aluminum nitride (AlN) have been used as a new chemical route to synthesize alumina-boron nitride (Al₂O₃–BN) composites. Reaction mechanisms were investigated by TG-DTA and static reaction process. The reactions started at around 1200 °C and completed at around 1500 °C. Soaking at temperatures higher than 1800 °C resulted in the reverse reaction that caused great weight loss. Hot pressing promoted the reactions due to the improved diffusion process. The in situ formed BN phase was in agglomerate shape located at the pockets of Al₂O₃ matrix particles and this distribution was suggested to be beneficial to the strength of materials with weak phase dispersoids. The fracture surface analysis demonstrated that the main fracture mode was transgranular, indicating the existence of a strong Al₂O₃ network in the in situ synthesized composites. The prepared composites exhibited high strength, low Young’s modulus and high strain tolerance.     

Al2O3晶须与石墨烯纳米片共增强铜基复合材料的显微结构及界面行为(英文)

研究Al₂O₃晶须和石墨烯纳米片共增强铜基复合材料的力学性能和显微结构。采用机械合金化、真空热压烧结和热等静压工艺制备不同石墨烯含量的铜基复合材料。含0.5%石墨烯(质量分数)的铜基复合材料(GNP-0.5)具有良好的Cu/C和Cu/Al₂O₃界面结合性能;复合材料的硬度和抗压强度随石墨烯含量的增加呈现先增加到一个临界值后减小的趋势。研究结果表明,石墨烯和Al₂O₃晶须在铜基复合材料中最主要的强化机制是能量耗散和载荷传递以及石墨烯导致的晶粒细化。石墨烯与Al₂O₃晶须的双相混杂增强效应在于：当Al₂O₃/Cu界面存在微裂纹并沿着界面扩展时,嵌于铜基复合材料中的石墨烯会阻碍裂纹扩展路径,从而强化Al₂O₃晶须在铜基复合材料中的增强作用。     

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.     

Mechanical Properties and Thermal Shock Resistance of Al2O3-TiC-Co Composites

Ductile cobalt was introduced into Al₂O₃-TiC (AT) composites by using a chemical deposition method to improve toughness and resistance to thermal shock. The mixture of Co-coated Al₂O₃ and TiC powders was hot-pressed into an Al₂O₃-TiC-Co (ATC) composite. The flexure strength and fracture toughness of the ATC composites have been improved considerably, compared with AT and Al₂O₃. The fracture surface of ATC shows a large proportion of transgranular cracks with some intergranular type, unlike the intergranular fracture modes of AT and Al₂O₃. The thermal shock properties of the composites were evaluated by water quenching technique and compared with the traditional AT and Al₂O₃. The composites containing only 3.96 vol.% cobalt exhibited higher critical temperature difference and retained flexure strength. The SEM examination of the fracture surfaces of the ATC composites after single thermal cycle showed that voids increased in number and size, and most isolated voids coalesced with increasing temperature difference, which caused the density and strength to decrease. The ATC composite is less sensitive to repeated thermal shock than the AT composite.     

The Microstructures and Mechanical Properties of V2O5-Doped Al2O3/TiAl In-Situ Composites by Reactive Hot Pressing Process

Al₂O₃/TiAl composites are successfully fabricated by the in-situ hot pressing method from the elemental powders of Ti, Al, TiO₂, and V₂O₅. The effect of V₂O₅ addition on the microstructure and mechanical properties of the Al₂O₃/TiAl in-situ composites is investigated in detail. It is found that the as-synthesized composites mainly consist of V-dissolved γ-TiAl, α₂-Ti₃Al, and Al₂O₃ particles along with a small amount of V₃Al phase, and the in-situ-formed fine Al₂O₃ particles tend to disperse on the grain boundaries of TiAl matrix. With increasing V₂O₅ content, the density and Vickers hardness of the resulting composites gradually increase, whereas the fracture toughness and flexural strength first increase and then decrease with the increase of V₂O₅ content. The composite with 3.5 wt.% V₂O₅ has the maximum value of 9.35 MPa m^1/2 and 713.36 MPa for the fracture toughness and flexural strength, respectively. The toughening mechanism is also discussed in detail.     

Pressure consolidation of amorphous ZrO2–Al2O3 by plastic deformation of powder particles

Amorphous ZrO₂–Al₂O₃ powders undergo densification at low temperatures (<650°C) and moderate uniaxial pressures (~750 MPa). It is established that large pressure dependent densification and little time dependent densification occur. Viscous sintering is not the dominant densification mechanism. Study of the particle size effect in densification of amorphous ZrO₂–40% Al₂O₃, and comparison with hot pressing of borosilicate glass powder at 500 and 550°C and cold compaction of silver powder, clearly indicate the possibility of compaction of amorphous ZrO₂–Al₂O₃ by plastic deformation. Good agreement was seen between a model for the compaction of ductile metal powders and the observed hot pressing behaviour.     

A facile method for fabrication of nano-structured Ni-Al2O3 graded coatings: Structural characterization

Powder charges of micron-size Ni and Al₂O₃ were utilized to deposit nano-structured Ni-Al₂O₃ composite coatings on an aluminum plate fixed at the top end of a milling vial using a planetary ball mill. Composite coatings were fabricated using powder mixtures with a wide range of Ni/Al₂O₃ mass ratio varying from 1:1 to plain Ni. XRD, SEM and TEM techniques were employed to study the structural characteristics of the coatings. It was found that the composition of the starting mixture strongly affects the Al₂O₃ content and the microstructure of the final coating. Mixtures containing higher contents of Al₂O₃ yield higher volume fractions of the Al₂O₃ particles in the coating. Though Ni-Al₂O₃ composite coatings with about 50% of Al₂O₃ particles were successfully deposited, well-compacted and free of cracks and/or voids coatings included less than 20% (volume fraction) of Al₂O₃ particles which were deposited from powder mixtures with Ni/Al₂O₃ mass ratios of 4:1 or higher. Moreover, mechanical and metallurgical bondings are the main mechanisms of the adhesion of the coating to the Al substrate. Finally, functionally graded composite coatings with noticeable compaction and integrity were produced by deposition of two separate layers under identical coating conditions.     

Reactive mechanism and mechanical properties of in situ composites fabricated from an Al–TiO2 system by friction stir processing

In situ (Al₃Ti + Al₂O₃)/Al composites were fabricated from powder mixtures of Al and TiO₂ using hot pressing, forging and subsequent multiple-pass friction stir processing (FSP). The reactive mechanism and mechanical properties of the FSPed composites were investigated. Four-pass FSP with 100% overlapping induced the Al–TiO₂ reaction, as a result of the enhanced solid diffusion and mechanical activation effect caused by the severe deformation of FSP. Decreasing the size of TiO₂ from 450 to 150 nm resulted in the formation of more Al₃Ti and Al₂O₃ particles. The formation mechanisms of Al₂O₃ and Al₃Ti during FSP are understood to be a deformation-assisted interfacial reaction and deformation-assisted solution-precipitation, respectively, based on detailed microstructural observations. The microhardness, Young’s modulus and tensile strength of the FSPed composites were substantially enhanced compared with those of FSPed pure Al with the same processing history, and increased as the TiO₂ size decreased from 450 to 150 nm. The strengthening mechanisms of the FSPed composites included load transferring, grain refinement and Orowan strengthening, among which Orowan strengthening contributed the most to the yield strength of the composites.     

Effect of hot isostatic pressing (HIP) on Al composite parts made from laser consolidated Al/Fe2O3 powder mixtures

Selective laser melting (SLM) was used to manufacture Al composite parts from Al/5–15 wt%Fe₂O₃ powder mixtures and followed by hot isostatic pressing (HIP) post-treatment, in order to assess the influence of HIP on density, hardness and microstructure. It was found that the HIP post-treatment slightly increased the density, but it failed to efficiently densify the material due to formation of thick oxide bands between solids within SLM process, expanding with increasing the iron oxide. The hardness also decreased after HIP, attributed to high temperature annealing impact of HIP post-treatment on microstructure (such as coarsening, coalescence and transformation of phases). The microstructural phases before HIP were a combination of equilibrium and stable phases (i.e. Al₁₃Fe₄ and α-Al₂O₃) plus phases such as Al₂Fe, AlFe, Fe₃Al, and intermediate-Al₂O₃ (formed in high Fe₂O₃ contents), but only equilibrium and stable phases remained after HIP post-treatment.     

The effect of particle size,sintering temperature and sintering time on the properties of Al–Al2O3 composites,made by powder metallurgy

Al₂O₃ is a major reinforcement in aluminum-based composites, which have been developing rapidly in recent years. The aim of this paper is to investigate the effect of alumina particle size, sintering temperature and sintering time on the properties of Al–Al₂O₃ composite. The average particle size of alumina were 3, 12 and 48 μm. Sintering temperature and time were in the range of 500–600 °C for 30–90 min. A correlation is established between the microstructure and mechanical properties. The investigated properties include density, hardness, microstructure, yield strength, compressive strength and elongation to fracture. It has been concluded that as the particle size of alumina is reduced, the density is increased followed by a fall in density. In addition, at low particle size, the hardness and yield strength and compressive strength and elongation to fracture were higher, compared to coarse particles size of alumina. The variations in properties of Al–Al₂O₃ composite are dependent on both sintering temperature and time. Prolonged sintering times had an adverse effect on the strength of the composite.     

Influence of Water Vapor on the Cyclic-Oxidation Behavior of a Low-Pressure Plasma-Sprayed NiCrAlY Coating

The oxidation of a low-pressure plasma-sprayed (LPPS) NiCrAlY coating on a nickel-base superalloy was studied at 1050 °C in flows of O₂, and mixture of O₂ and 5% H₂O under atmospheric pressure. Water vapor has an obvious effect on the cyclic oxidation of the NiCrAlY coating. There is more decrease in weight gain when exposure to O₂ is replaced by exposure to O₂ + 5% H₂O. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in pure oxygen is composed mainly of Cr₂O₃, and a thin Al₂O₃-rich layer is formed at the interface between the Cr₂O₃-rich layer and the coating. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in a mixture of O₂ + H₂O is composed of NiCr₂O₄, NiO and Cr₂O₃. The effect of water vapor on the oxidation of the NiCrAlY coating may be attributed to an increase in Ni and Cr cation transport, stress-corrosion cracking of Al₂O₃ and moisture-enhanced volatility of the Cr₂O₃ scale.     

The influence of powder characteristics on mechanical properties of Al2O3–TiC–Co ceramic materials prepared by Co‐coated Al2O3/TiC powders

Ultrafine Al₂O₃–TiC–Co (ATC) ceramic is prepared in order to improve the bending strength and fracture toughness of ceramic materials. The ultrafine Co‐coated Al₂O₃ and TiC powders have been synthesized by electroless plating at room temperature, and the composite powders were sintered by hot‐pressing to compact ATC samples. The average bending strength, average hardness and average fracture toughness values of ATC ceramic with different particle sizes and Co contents were investigated. The toughening mechanism of the ultrafine ATC ceramic was studied by transmission electron microscopy (TEM) and ceramic performance testing methods. The results show that the relative density, bending strength and fracture toughness values increase remarkably with the increase of Co content. The ultrafine grain of original powders is beneficial to improve the relative density, strength and toughness values of ATC ceramic. The Co phase hinders the growth of ATC ceramic grains during sintering. The Co phase forms a three‐dimensional mesh skeleton structure during sintering, improving the fracture toughness and strength of the composite ceramic.     

Sintering of Al2O3–TiB2 nano-composite derived from milling assisted sol–gel method

Synthesis and sintering of an alumina /titanium diboride nano-composite have been studied as an alternative for pure titanium diboride for ceramic armor applications. Addition of TiB₂ particles to an Al₂O₃ matrix can improve its fracture toughness, hardness and flexural strength and offer advantages with respect to wear and fracture behavior. This contribution, for the first time, reports the sintering, microstructure, and properties of Al₂O₃–TiB₂ nano-composite densified with no sintering aids. Nano-composite powder was produced by combination of sol–gel and mechano-chemical methods. The densification experiments were carried out using both hot pressing and pressureless sintering routes. In the pressureless sintering route, a maximum of 92.3% of the theoretical density was achieved after sintering at 1850 °C for 2 h under vacuum. However, hot pressing at 1500 °C for 2 h under the same condition led to achieving a 99% of the theoretical density. The hot pressed Al₂O₃–TiB₂ nano-composites exhibit high Vickers hardness (16.1 GPa) and a modest indentation toughness (~ 4.2 MPa.m^1/2).