Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

Improvement of yield strength of LM24 alloy

In this study, Al₂O₃ particles were employed to improve the microstructure of LM24 and therefore, to increase the yield strength and tensile strength of this kind of alloy. In situ Al₂O₃ particles were obtained by direct reaction between oxygen and Al melt at 750–800 °C. Microstructure examination shows that the size of in situ formed Al₂O₃ particles was about 1–2 μm, and interestingly, with addition of in situ Al₂O₃ particles, the coarse primary Si phase was disappeared completely. More important, the yield strength and the tensile strength of Al₂O₃/LM24 are increased by 52 MPa, 16 MPa than that of LM24 alloy with 0.1% Sb addition. The value of 181 MPa and 315 MPa is for yield strength and tensile strength of Al₂O₃/LM24 respectively. Besides, the yield strength and tensile strength are 180 MPa and 314 MPa respectively for Al₂O₃/LM24 alloy after remelting and casting. This verifies that the improvement of mechanical properties of such kind of material possesses stability and reliability.     

Sinter-HIP of polymer-derived Al2O3–SiC composites with high SiC contents

Submicrometer Al₂O₃ composites with more than 20 vol.% of SiC particles were produced using a multiple infiltration of porous bodies with a liquid polymer SiC precursor. The fully dense composites were successfully densified using a sinter-HIP process. Parameters of sintering and HIP steps are discussed with respect to both densification and microstructure evolution of the composites. The initial pressure during the sintering step plays an important role for the preparation of fully dense composites with a submicrometer alumina matrix at 1750 °C. Optimized densification schedule of sinter-HIP represents a novel approach of densification at relatively mild conditions compared to previously reported or common densification methods of Al₂O₃–SiC composites with high SiC content, such as pressureless sintering, hot pressing and post-HIPing. The method expands the possibilities for preparation of alumina based composites with SiC volume fraction > 20 vol.%, filling the gap in available literature data.     

SiC nanoparticle-reinforced Al2O3–Nb composite as a potential femoral head material in total hip arthroplasty

A ceramic–metal composite consisting of SiC nanoparticle-reinforced Al₂O₃ and Nb (referred to as SiC/Al₂O₃–Nb), was prepared and evaluated in vitro for potential application as a femoral head material in total hip arthroplasty. Dense bi-layer laminates of SiC nanoparticle-reinforced Al₂O₃ and Nb were fabricated by hot pressing of powders (1425 °C; 35 MPa), and evaluated using scanning electron microscopy, microchemical analysis, and mechanical testing. The flexural strength of the SiC/Al₂O₃–Nb laminate (960 ± 20 MPa) was higher than the value (720 ± 40 MPa) for an Al₂O₃–Nb laminate, and far higher than the value (620 ± 50 MPa) for SiC nanoparticle-reinforced Al₂O₃ (SiC/Al₂O₃). The Vickers hardness of SiC/Al₂O₃ was 17 ± 2 GPa, compared to 12 ± 1 GPa for Al₂O₃. A high interfacial shear strength of the SiC/Al₂O₃–Nb laminate (310 ± 100 MPa), coupled with SEM observation of the interfacial region, showed strong bonding between the SiC/Al₂O₃ and Nb layers. Composite femoral heads consisting of a SiC/Al₂O₃ surface layer and a Nb core could potentially lead to a reduction in the tendency for brittle failure as well as to lower wear, when compared to Al₂O₃ femoral heads.     

Evaluation of microstructure and mechanical properties of Al/Al2O3/SiC hybrid composite fabricated by accumulative roll bonding process

In this investigation, a new kind of metal matrix composites with a matrix of pure aluminum and hybrid reinforcement of Al₂O₃ and SiC particles was fabricated for the first time by anodizing followed by eight cycles accumulative roll bonding (ARB). The resulting microstructures and the corresponding mechanical properties of composites within different stages of ARB process were studied. It was found that with increasing the ARB cycles, alumina layers were fractured, resulting in homogenous distribution of Al₂O₃ particles in the aluminum matrix. Also, the distribution of SiC particles was improved and the porosity between particles and the matrix was decreased. It was observed that the tensile strength of composites improved by increasing the ARB passes, i.e. the tensile strength of the Al/1.6 vol.% Al₂O₃/1 vol.% SiC composite was measured to be about 3.1 times higher than as-received material. In addition, tensile strength of composites decreased by increasing volume fraction of SiC particles to more than 1 vol.%. Scanning electron microscopy (SEM) observation of fractured surfaces showed that the failure mechanism of broken hybrid composite was shear ductile rupture.     

Structural and mechanical characterization of Al-based composite reinforced with heat treated Al2O3 particles

In this study, the addition of 1.00 wt.% Al₂O₃ crystals to the metal matrix of the liquid aluminum was studied. In order to investigate the influence of heat treatment on activation of Al₂O₃ powders and mechanical properties of Al–Al₂O₃ composites, the Al₂O₃ particles were heated at 1000 °C. X-ray Diffraction (XRD) analysis used to characterize the crystal lattice of Al₂O₃ and its variation during heat treatment. The size and morphology of the Al₂O₃ grains was evaluated by Scanning Electron Microscopy (SEM). The results showed a considerable change in morphology of Al₂O₃ grains during the heat treatment. Mechanical evaluation such as hardness, compression and wear tests showed enhancement in the properties of Al–1.00 wt.% heat treated Al₂O₃ vs. Al–1.00 wt.% Al₂O₃ composite.     

Effect of 10Ce-TZP/Al2O3 nanocomposite particle amount and sintering temperature on the microstructure and mechanical properties of Al/(10Ce-TZP/Al2O3) nanocomposites

A zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al₂O₃ nanocomposite) can be a good substitute as reinforcement in metal matrix composites. In the present study, the effect of the amount of 10Ce-TZP/Al₂O₃ particles on the microstructure and properties of Al/(10Ce-TZP/Al₂O₃) nanocomposites was investigated. For this purpose, aluminum powders with average size of 30 μm were ball-milled with 10Ce-TZP/Al₂O₃ nanocomposite powders (synthesized by aqueous combustion) in varying amounts of 1, 3, 5, 7, and 10 wt.%. Cylindrical-shape samples were prepared by pressing the powders at 600 MPa for 60 min while heating at 400–450 °C. The specimens were then characterized by scanning and transmission electron microscopy (SEM and TEM) in addition to different physical and mechanical testing methods in order to establish the optimal processing conditions. The highest compression strength was obtained in the composite with 7 wt.% (10Ce-TZP/Al₂O₃) sintered at 450 °C.     

Microstructure and mechanical properties of in situ TiC and Nd2O3 particles reinforced Ti–4.5 wt.%Si alloy composites

(TiC + Nd₂O₃)/Ti–4.5 wt.%Si composites were in situ synthesized by a non-consumable arc-melting technology. The phases in the composites were identified by X-ray diffraction. Microstructures of the composites were observed by optical microscope and scanning electron microscope. The composite contains four phases: TiC, Nd₂O₃, Ti₅Si₃ and Ti. The TiC and Nd₂O₃ particles with dendritic and near-equiaxed shapes are well distributed in Ti–4.5 wt.%Si alloy matrix, and the fine Nd₂O₃ particles exist in the network Ti + Ti₅Si₃ eutectic cells and Ti matrix of the composites. The hardness and compressive strength of the composites are markedly higher than that of Ti–4.5 wt.%Si alloy. When the TiC content is fixed as 10 wt.% in the composites, the hardness is enhanced as the Nd₂O₃ content increases from 8 wt.% to 13 wt.%, but the compressive strength peaks at the Nd₂O₃ content of 8 wt.%.     

High strength Al–Al2O3p composites: Optimization of extrusion parameters

Composite aluminium alloys reinforced with Al₂O_3p particles have been produced by squeeze casting followed by hot extrusion and a precipitation hardening treatment. Good mechanical properties can be achieved, and in this paper we describe an optimization of the key processing parameters. The parameters investigated are the extrusion temperature, the extrusion rate and the extrusion ratio. The materials chosen are AA 2024 and AA 6061, each reinforced with 30 vol.% Al₂O₃ particles of diameter typically in the range from 0.15 to 0.3 μm. The extruded composites have been evaluated based on an investigation of their mechanical properties and microstructure, as well as on the surface quality of the extruded samples. The evaluation shows that material with good strength, though with limited ductility, can be reliably obtained using a production route of squeeze casting, followed by hot extrusion and a precipitation hardening treatment. For the extrusion step optimized processing parameters have been determined as: (i) extrusion temperature = 500 °C–560 °C; (ii) extrusion rate = 5 mm/s; (iii) extrusion ratio = 10:1.     

Effect of aluminum on microstructure,mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels

Effect of aluminum on microstructure, mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels has been investigated. Aluminum can significantly increase the ratio of equiaxed crystal grains, but the promotion effect has great relation with aluminum content. Aluminum can stabilize ferrite phase and significantly reduce recrystallization temperature. Increased aluminum content can also lead to the precipitate of AlN and Al₂O₃ at higher temperature. The increased amount of AlN may partly contribute to the reduced nitrogen element to form austenite at high temperature, hence the high temperature phase transformation of α + γ → α occurs. The fine and large number of Al₂O₃ particles can refine grain size and then promote recrystallization. The highest intensity of γ-fiber texture {1 1 1}〈1 1 2〉 is observed in the steel with 0.19 wt.% aluminum, which can improve the formability of steels. With the increase of aluminum content, the tensile strength increases linearly but the elongation and plastic strain ratio first increase then decrease, the working hardening index varies slightly among the steels. Appearance of Al₂O₃ inclusions with small size and decreased content of MnS benefit pitting corrosion resistance. However, the large dimension Al₂O₃ inclusions have significantly negative influence on pitting corrosion resistance.     

Improving the mechanical properties of Al2O3/Ni laminated composites by adding Ni particles in Al2O3 layers

The (Al₂O₃ + Ni) composite, (Al₂O₃ + Ni)/Ni and Al₂O₃/(Al₂O₃ + Ni)/Ni laminated materials were prepared by aqueous tape casting and hot pressing. Results indicated that the (Al₂O₃ + Ni) composite had higher strength and fracture toughness than those of pure Al₂O₃. The fracture toughness of (Al₂O₃ + Ni)/Ni and Al₂O₃/(Al₂O₃ + Ni)/Ni laminated materials was higher than not only those of pure Al₂O₃, but also those of Al₂O₃/Ni laminar with the same layer numbers and thickness ratio. It was found that the toughness of the Al₂O₃/(Al₂O₃ + Ni)/Ni laminated material with five layers and layer thickness ratio = 2 could reach 16.10 MPa m^1/2, which were about 4.6 times of pure Al₂O₃. The strength and toughness of the (Al₂O₃ + Ni)/Ni laminated material with three layers and layer thickness ratio = 2 could reach 417.41 MPa and 12.42 MPa m^1/2. It indicated the material had better mechanical property.     

Fabrication of in situ Al2O3 reinforced nanostructure 304 stainless steel matrix composite by self-propagating high temperature synthesis process

304 austenitic stainless steel reinforced by Al₂O₃ particles was prepared by microwave assisted self-propagating high temperature synthesis process using the Fe₂O₃Cr₂O₃NiOAlFe reaction system. Furthermore, effects of mechanical activation of the reactants and the addition of 21.2 wt.% extra Al to the chemical composition of the reactants on the chemical composition of the produced stainless steel was investigated. Atomic absorption spectroscopy analysis results indicated that by the addition of extra Al to the reactant mixture and using 30 minute mechanical activation, stainless steel containing 17.27 wt.% Cr and 7.73 wt.% Ni could be produced with its chemical composition very close to the chemical composition of 304 stainless steel. X-ray diffraction analysis showed that the stainless steel contains nanostructured austenite and ferrite phases. Also microstructural characterizations indicated that there is a uniform distribution of black particles in the steel matrix. Energy dispersive spectroscopy analysis showed that these particles are composed of Al and O elements while the matrix contains Fe, Cr and Ni elements. The presence of Al₂O₃ particles and nanostructure matrix improved the hardness and therefore the wear properties of the composite in comparison with the wrought 304 stainless steel plate.     

The bimodal effect of La on the microstructures and mechanical properties of in-situ A356–TiB2 composites

In this work, in-situ A356–2.5 wt.% TiB₂ composites were fabricated via a re-melting and diluting (RD) approach. The effects of lanthanum (La) on the microstructures and mechanical properties of the composites were investigated. The results demonstrated that the addition of La can, simultaneously, refine the second dendrite arm spacing (SDAS) of the α-Al grains and modify the coarse lamellar eutectic Si into fine fibrous particles. The optimum addition levels for refinement and modification lay at 0.1 wt.% and 0.8 wt.%, respectively, due to the distinct mechanisms between the two. Since the resultant mechanical properties are a combined function of refinement and modification, the tensile properties of the composites show a bimodal distribution with the first peak value at 0.1 wt.% and the latter one at 0.8 wt.% La, in accordance with the optimum refinement and modification, respectively.     

Characterization of centrifugal cast functionally graded aluminum-silicon carbide metal matrix composites

The present investigation is on characterization of functionally graded composites based on 356 cast and 2124 wrought aluminum alloys reinforced with SiC particles of 23 μm average particle size processed by liquid metal stir casting followed by horizontal centrifugal casting. A maximum of 45 and 40% SiC particles are obtained at the outer periphery of the Al(356)-SiC and Al(2124)-SiC FGMMC casting respectively. The maximum hardness obtained at the outer periphery after heat treatment for Al(356)-SiC and Al(2124)-SiC FGMMC are 155 BHN and 145 BHN respectively. The freezing range of the matrix alloy has been found to dictate the nature of transition from particle enriched to depleted zone. These composites are suitable for making engineering components, which require very high surface hardness and wear resistances with high specific strength.     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Wear behaviors of Fe-based amorphous composite coatings reinforced by Al2O3 particles in air and in NaCl solution

In this paper, the influence of the addition of Al₂O₃ particles on the microstructure and wear properties of Fe-based amorphous coatings prepared by high velocity oxygen fuel (HVOF) has been studied. The wear behaviors of the composite coatings were evaluated against Si₃N₄ in a pin-on-disk mode in air and in 3.5 wt.% NaCl solution. It was found that the Al₂O₃ particles were homogenously distributed in the amorphous matrix and the composite coatings exhibited improved wear resistance and reduced coefficient of friction (COF) in both air and wet conditions as compared to the monolithic amorphous coating. The composite coating reinforced with 20 wt.% Al₂O₃ particles exhibit the best wear performance, which, for example, has extremely low COF (< 0.2) and high wear resistance (2–3 times higher than monolithic amorphous coating). Detailed analysis on the worn surface indicated that the wear mechanism for the amorphous and composite coatings is similar and is dominated by oxidative delamination in air and by corrosion wear in 3.5% NaCl solution. The enhanced wear resistance is mainly attributed to the addition of Al₂O₃ particles which exhibit high hardness, good corrosion resistance and excellent chemical and thermal stability.     

Consolidation of Al2O3–GdAlO3 eutectic powder prepared from induction-melted solid and strength at high temperatures

A eutectic powder of Al₂O₃–GdAlO₃ was melted using a Mo crucible by induction heating. The melt was slowly solidified, resulting in a eutectic solid with coarse Al₂O₃ and GdAlO₃ phases. The eutectic solid was ground and sieved into 3–44 μm and 64–124 μm particles. The powders were consolidated to produce a eutectic composite by spark plasma sintering. Mechanical properties of the consolidated eutectic composite were measured at room temperature. High temperature strength was obtained at temperatures up to 1673 K. Superplastic deformation of the eutectic composite was not observed on stress–strain curves at 1673 K, but did occur in the case of a conventional composite at 1573 K.     

Effect of re-melting on particle distribution and interface formation in SiC reinforced 2124Al matrix composite

The interface between metal matrix and ceramic reinforcement particles plays an important role in improving properties of the metal matrix composites. Hence, it is important to find out the interface structure of composite after re-melting. In the present investigation, the 2124Al matrix with 10 wt.% SiC particle reinforced composite was re-melted at 800 °C and 900 °C for 10 min followed by pouring into a permanent mould. The microstructures reveal that the SiC particles are distributed throughout the Al-matrix. The volume fraction of SiC particles varies from top to bottom of the composite plate and the difference increases with the decrease of re-melting temperature. The interfacial structure of re-melted 2124Al–10 wt.%SiC composite was investigated using scanning electron microscopy, an electron probe micro-analyzer, a scanning transmission electron detector fitted with scanning electron microscopy and an X-ray energy dispersive spectrometer. It is found that a thick layer of reaction product is formed at the interface of composite after re-melting. The experimental results show that the reaction products at the interface are associated with high concentration of Cu, Mg, Si and C. At re-melting temperature, liquid Al reacts with SiC to form Al₄C₃ and Al–Si eutectic phase or elemental Si at the interface. High concentration of Si at the interface indicates that SiC is dissociated during re-melting. The X-ray energy dispersive spectrometer analyses confirm that Mg- and Cu-enrich phases are formed at the interface region. The Mg is segregated at the interface region and formed MgAl₂O₄ in the presence of oxygen. The several elements identified at the interface region indicate that different types of interfaces are formed in between Al matrix and SiC particles. The Al–Si eutectic phase is formed around SiC particles during re-melting which restricts the SiC dissolution.     

The measurement of the adhesion force between ceramic particles and metal matrix in ceramic reinforced-metal matrix composites

This paper presents the method for measurement of the adhesion force and fracture strength of the interface between ceramic particles and metal matrix in ceramic reinforced-metal matrix composites. Three samples with the following Cu to Al₂O₃ ratio (in vol.%) were prepared: 98.0Cu/2.0Al₂O₃, 95.0Cu/5.0Al₂O₃ and 90Cu/10Al₂O₃. Furthermore, microwires which contain a few ceramic particles were produced by means of electro etching. The microwires with clearly exposed interface were tested with use of the microtensile tester. The microwires usually break exactly at the interface between the metal matrix and ceramic particle. The force and the interface area were carefully measured and then the fracture strength of the interface was determined. The strength of the interface between ceramic particle and metal matrix was equal to 59 ± 8 MPa and 59 ± 11 MPa in the case of 2% and 5% Al₂O₃ to Cu ratio, respectively. On the other hand, it was significantly lower (38 ± 5 MPa) for the wires made of composite with 10% Al₂O₃.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.