Multi-scale fatigue behaviour modelling of Al–Al2O3 short fibre composites

Using very heterogeneous materials in structural parts submitted to cyclic loadings, this paper presents an elasto-plastic micromechanical model. After recalling the homogenisation principle based on a mean field theory, non-linear kinematic and isotropic strain hardening is introduced into the matrix. Validation is made on an Al–3.5%Cu/SiC particle composite, and an Al–Si7Mg/Al₂O₃ fibre composite is treated as a first application. Damage is introduced into the model using a fibre failure criterion. It is based on the evolution of the volume fraction of broken fibres as a function of the maximum principal stress in the fibre family. The damage law is identified by means of in situ tensile tests performed inside the scanning electronic microscope. The number of broken fibres is determined as a function of the applied load and the number of cycles. The model predicts the fatigue behaviour, the loss of stiffness, the volume fraction of broken fibres for different volume fractions, aspect ratios, distributions of orientation and distributions of strength of the fibres. The effect of the mechanical fatigue properties of the matrix is also studied.     

Development of high performance Mg–Al2O3 composites containing Al2O3 in submicron length scale using microwave assisted rapid sintering

Abstract

In the present study, magnesium composites reinforced with different volume fraction of submicron size Al₂O₃ particulates were synthesised using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. The sintered materials were subsequently hot extruded for characterisation in terms of microstructural, physical and mechanical properties. Microstructural characterisation results revealed a reasonably uniform distribution of Al₂O₃ particulates, minimal porosity and good matrix reinforcement interfacial integrity. The average coefficient of thermal expansion (CTE) value for Mg–Al₂O₃ composites was found to decrease with increasing amount of submicron Al₂O₃ particulates. Mechanical characterisation of the composites revealed an increase in hardness, elastic modulus, 0·2% YS and ultimate tensile strength (UTS) with the increase in amount of alumina particulates. Ductility exhibited the reverse trend. An attempt is made in the present study to correlate the effect of the presence of submicron alumina and its increasing amount with the microstructural, physical and mechanical properties of magnesium.     

Fabrication of bioglass infiltrated Al2O3–(m-ZrO2) composites

Using 80 vol.% of poly methyl methacrylate (PMMA) as a pore-forming agent to obtain interconnected porous bodies, porous Al₂O₃–(m-ZrO₂) bodies were successfully fabricated. The pores were about 200 μm in diameter and were homogeneously dispersed in the Al₂O₃–25 vol.% (m-ZrO₂) matrix. To obtain Al₂O₃–(m-ZrO₂)/bioglass composites, the molten bioglass was infiltrated into porous Al₂O₃–(m-ZrO₂) bodies at 1400°C. The material properties of the Al₂O₃–(m-ZrO₂)/bioglass composites, such as relative density, hardness, compressive strength, fracture toughness and elastic modulus were investigated.     

Evaluation of subgrain formation in Al2O3–SiC nanocomposites

Both theoretical analysis and transmission electron microscopy (TEM) complementary studies have been conducted to evaluate the possible role of subgrain formation as a strengthening mechanism in a nanocomposite consisting of Al2O3 and 5 vol % 0.15 m SiC particles. The theoretical calculation predicted that the residual stresses due to thermal expansion mismatch between Al2O3 and SiC are insufficient to induce the extensive plastic deformation required for subgrain formation upon annealing. This prediction was consistent with TEM observations that the bulk of the material was completely free from subgrains, and that only a low density of dislocations was present in isolated areas. The results suggest, therefore, that microstructure refinement through subgrain formation cannot account for the superior mechanical behaviour of the nanocomposite reported in previous studies.TEM examination of the ground surfaces revealed significant plastic deformation in both single phase Al2O3 and the nanocomposite. Upon annealing at 1300°C for 2 h, dislocation-free subgrains were formed in Al2O3, whereas a high density of tangled dislocations were present in the nanocomposite. These observed differences are consistent with the fact that during annealing, residual stress relaxation is more difficult in the nanocomposite than in Al2O3.     

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.     

AC electrical and optical characterization of epoxy–Al2O3 composites

The AC electrical and optical characterizations of epoxy–alumina (Al₂O₃) composites have been investigated. Sheets filled with alumina were prepared with different alumina concentrations (0, 2, 5, 8, 10, and 15 wt%). The AC electrical properties were measured by using impedance spectroscopy as a function of applied frequency in range from 50 kHz to 1 MHz and filler concentration. The results obtained showed that the applied frequency and filler concentration was found to influence the AC electrical conductivity and dielectric behavior of the prepared composites. The UV-optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The absorption coefficient and the optical energy gap (E_opt) have been obtained from the direct allowed transitions in k-space at room temperature. The tail widths (ΔE) of the localized states in the band gap were evaluated using the Urbach-edges formula. It was found that both (E_opt) and (ΔE) vary with the alumina concentration dispersed in the epoxy matrix. The refractive index (n) for the composites was determined from the collected transmittance and reflectance spectra. The dispersion behavior of the refractive index is discussed in terms of the single oscillator model.     

Synthesis and characterization of barium borosilicate glass–Al2O3 composites

Barium borosilicate glass with composition 30BaO–60B₂O₃–10SiO₂ glass was prepared by melt-quenching technique. Different weight % of crystalline Al₂O₃ was mixed with the glass powder and sintered at optimum temperature. The changes in the structure and thermal properties of the glass with alumina content were investigated by X-ray powder diffraction, FT-IR spectroscopy and differential thermal analysis. The variations in the coefficient of thermal expansion and dielectric properties with composition were also studied and correlated with the structural changes.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.     

Dielectric properties of Lanxide™ Al2O3/Al composites

The dielectric properties of unreinforced Lanxide Al₂O₃/Al composites have been investigated over a wide range of temperatures and frequencies. These composites were formed by the directed oxidation of suitably doped aluminium-based alloy melts, with no filler or reinforcing material in the reaction path. As-grown composite materials were good electrical conductors in all directions owing to the presence of an interconnected metallic constituent. As the metallic phases were partially removed (in favour of porosity) by continuing the oxidation reaction to completion, the composites remained electrically conducting parallel to, and became insulating transverse to, the original growth direction of the composite. This anisotropy apparently was caused by different connectivity of the metal phase between the two directions. Thermal treatments at 1600°C in argon resulted in volatilization of the residual metal in the composite, thus further increasing the porosity. As the metal content was decreased, the composites changed from conducting to insulating along the growth direction. When the metallic phase was removed completely, the porous alumina ceramic maintained anisotropic dielectric properties, due to c-axis alignment of the alumina (corundum) phase along the growth direction. The dielectric constants were 8.0 and 6.4, respectively, parallel and perpendicular to the c-axis aligned directions of the porous alumina ceramic. A dielectric relaxation phenomenon was observed in some samples of both as-grown and thermally treated material, and was attributed to an unidentified impurity effect.     

Fabrication and properties of SiC fibre-reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

Ta₂O₅, Nb₂O₅ and TiO₂ were used separately as additives to a Li₂O·Al₂O₃·6SiO₂ glass-ceramic composition, to act as nucleating dopants and to aid the formation of an interfacial carbide layer (TaC and NbC) between the fibre and matrix in SiC fibre uniaxially reinforced glass-ceramic composites, The composites exhibited high modulus of rupture (>800 MPa) and fracture toughness (K _IC > 15 MPam^1/2). The interfacial amorphous carbon rich layer and carbide layer were responsible for lowered interfacial shear strength but permitted high composite fracture toughness. The composite with the TiO₂ additive in the matrix showed a lower flexural strength (<500MPa) and a smaller K _IC (-11 MPam^1/2) which resulted from the high interfacial shear strength between the SiC fibre and the matrix due to the formation of the interfacial TiC layer.     

Microstructures and properties of in-situ NiAl–Al2O3 functionally gradient composites

NiAl–Al₂O₃ functionally gradient composites (FGCs) have been fabricated by reactive hot compaction of Ni and Al powders in which one or both were partially preoxdized. The FGCs consisted of four or five layers with alumina content increasing up to about 35 vol.%. The gradient in the composition was obtained by stacking powder mixtures of desired compositions. It was found that the technique resulted in a gradual transition of the microstructure and microhardness from NiAl to NiAl–35 vol.%Al₂O₃. In contrast, the NiAl– (NiAl–35 vol.%Al₂O₃) bilayer compact showed a much steeper microhardness change with pronounced residual stress at the boundary. The FGCs were found to have higher fracture toughness values than the corresponding composites.     

Microstructure and mechanical properties of SiC Whisker reinforced ZrO2–Y2O3 composites

Abstract

The mechanical properties of 20 vol.-%SiC whisker reinforced ZrO₂?V₂O₃ composites containing 2 and 6 mol.-% Y₂O₃ were measured at room temperature and the fracture surface was examined. The results indicate that the mechanical behaviour of the composites is strongly influenced by the Y₂O₃ content. The magnitude of the enhancement of the toughness in composites containing 2 mol.-% Y₂O₃ compared with unreinforced ZrO₂?Y₂O₃ matrix is larger than that for the composites containing 6 mol.-% Y₂O₃. Crack propagation modes were characterised by crack deflection, whisker bridging, and whisker pullout. High resolution electron microscopic observations show that in composites containing 2 mol.-% Y₂O₃ the whiskers are directly bonded to the matrix. However, in composites containing 6 mol.-% Y₂O₃ there is always a thick amorphous layer at the interface, indicating that the high Y₂O₃ content has promoted the formation of interfacial amorphous layers. These interfacial amorphous layers strengthen the interfacial bonding, resulting in a composite with a low fracture toughness.

MST/2043     

Sintering behaviour of slip-cast Al2O3 – Y-TZP composites

The sintering behaviour of alumina–Y-TZP composites prepared by slip-casting technique were studied. Slip-cast samples containing varying amounts of Y-TZP ranging up to 90 vol% were prepared and evaluated. Sintering studies were carried out at 1450°C to 1600°C. Sintered samples were characterised where appropriate to determine phases present, grain sizes, bulk density and mechanical properties. Good correlation was obtained between the calculated prepared powder density and experimental results. The sintered bulk density of the composites was observed to increase with increasing Y-TZP content and sintering temperature up to 1550°C. Maximum hardness values (>14 GPa) were obtained for all samples containing <60 vol% Y-TZP and when sintered at 1550°C. It has been found that the additions of up to 50 vol% Y-TZP was effective in suppressing Al₂O₃ grain growth.     

Al3Ni2–Al composites with nanocrystalline intermetallic matrix produced by consolidation of milled powders

Mechanically alloyed nanocrystalline Al₆₃Ni₃₇ powder with a metastable structure of NiAl phase was mixed with 20, 30 and 40 vol.% of Al powder. The powder mixtures as well as pure powder of Al₆₃Ni₃₇ alloy were consolidated at 600 °C under the pressure of 7.7 GPa. The bulk materials were characterised by structural investigations (X-ray diffraction, light and scanning electron microscopy, energy dispersive spectroscopy), compression and hardness tests and measurements of density and open porosity. During the consolidation, the metastable NiAl phase transformed into the equilibrium Al₃Ni₂ intermetallic. The mean crystallite size of the Al₃Ni₂ intermetallic in the bulk materials is below 40 nm. The microstructure of the composite samples consists of Al₃Ni₂ intermetallic areas surrounded by lamellae-like Al regions. The hardness of the produced Al₃Ni₂–Al composites is in the range of 5–6.5 GPa (514–663 HV1), while that of the Al₃Ni₂ intermetallic is 9.18 GPa (936 HV1). The compressive strength of the composites increases with the decrease of Al content, ranging from 567 MPa to 876 MPa. The plastic elongation of the composites was increasing with the increase of Al content, while the Al₃Ni₂ intermetallic failed in the elastic region.     

Sintering, densification and crystallization of Ca–Al–B–Si–O glass/Al2O3 composites for LTCC application

Ca–Al–B–Si–O glass/Al₂O₃ composites were prepared based on the borosilicate glass powders (D₅₀ = 2.84) and Al₂O₃ ceramic powders (D₅₀ = 3.26), and the sintering, densification, crystallization of samples were investigated. The shrinkage of sample starts to have a sharp increase at 600 °C. The shrinkage of sample starts to have a further rapid increase after the glass softening temperature of about 713 °C. Glass/Al₂O₃ composites can be sintered at 875 °C/15 min and exhibit better properties of a relative density of 98.4 %, a λ value of 2.89 W/mK, a ε _r value of 7.82 and a tan δ value of 5.3 × 10^?4. The interface between glass and Al₂O₃ grains and the interface between anorthite and glass phase depicts a good compatibility according to transmission electron microcopy test. It is the low sintering temperature, high density and good compatibility with Ag electrodes that, guarantee borosilicate glass/Al₂O₃ composites suitable for low temperature co-fired ceramic materials.     

The effect of additives on Al2O3–SiO2–SiC–C ramming monolithic refractories

Abstract

The aim of this study is to describe the effect of containing additives on increasing cold crushing strength (CCS) and bulk density (BD) of Al₂O₃–SiO₂–SiC–C monolithic refractories. Two series of carbon containing monolithics were prepared from Iranian chamotte (Samples A) and Chinese bauxite (Samples B), as 65 wt.% in each case together with, 15wt.% SiC-containing material regenerates (crushed sagger), 10 wt.% fine coke (a total of 90% aggregate) and 10 wt.% resole (phenol formaldehyde resin) as a binder. Different types of additives (such as silicon and ferrosilicon metal) are added to a batch of 100 g of mixture and the physical and mechanical properties (such as BD, apparent porosity and CCS) are measured after tempering at 200°C for 2 h and firing at 1100°C and 1400°C for 2h. After low temperature tempering at 200°C, silicon and ferrosilicon contribute to the formation of stronger cross linking in the resulting structure and provides CCS values as high as 65 MPa. After high temperature sintering, at 1400°C, SiC whiskers of nano sized diameter are formed due to the presence of Si and FeSi₂ and increases the CCS values of the refractories as high as 3–4 times in sample containing 6wt.% ferrosilicon metal as additive, compared to the material without additive. The temperature of 1100°C is a transient temperature, used in high temperature sintering.     

Formation of Al3Ti and Al2O3 from an Al–TiO2 system for preparing in-situ aluminium matrix composites

In-situ processing offers significant advantages over conventional processing from both technical and economic standpoints. Al–TiO₂ is one of the interesting systems that has recently been receiving some attention. In this paper, the formation mechanism of Al₃Ti and Al₂O₃ from an Al–TiO₂ system is investigated by using thermal analysis, XRD and microstructural characterisation. It is found that the in-situ processing involves three intermediate steps. In addition, TiO and γ-Al₂O₃ are transitional phases, which form during the reactive process.     

Low temperature sintering and characteristics of K2O–B2O3–SiO2–Al2O3 glass/ceramic composites for LTCC applications

The K₂O–B₂O₃–SiO₂, K₂O–B₂O₃–SiO₂–2 %Al₂O₃, K₂O–B₂O₃–SiO₂–4 %Al₂O₃ glasses with different Al₂O₃ content were prepared. Different proportions (50, 55, 60, 65, 70 %) of the three glasses were respectively mixed with alumina ceramic-filler, then the mechanical and dielectric properties were investigated. The results showed K₂O–B₂O₃–SiO₂–2 %Al₂O₃ glass/alumina filler (glass:alumina = 60:40) had the excellent comprehensive properties, so further study was continued with part of alumina ceramic-filler replaced by the silica ceramic-filler on this composite. Then the X-ray diffraction analysis revealed that the alumina and silica fillers existed as the crystal phase, and the densification was seriously damaged when the silica content reached to three quarters of the fillers. With the increase of the silica-filler, the composites’ density and dielectric constant exhibited uniform decrease, but thermal expansion coefficient (TEC) uniformly increased. When the glass:alumina:silica was equal to 60:30:10, a best composite property was presented as a bulk density of 2.582 (g cm^?1), a dielectric constant of 6.1 and a dielectric loss of 2 × 10^?3 at 1 MHz, a flexural strength of 168 MPa, and a TEC of 8.62 × 10^?6 °C^?1.     

In situ fabrication of α-Al2O3 and Ni2Al3 reinforced aluminum matrix composites in an Al–Ni2O3 system

α-Al₂O₃ ceramic particles and Ni₂Al₃ intermetallic compound reinforced aluminum matrix composites were successfully fabricated via exothermic dispersion (XD) reaction in an Al–Ni₂O₃ system. Thermodynamic analysis indicated that the reaction between Al and Ni₂O₃ could occur spontaneously due to its negative Gibbs free energy. The reaction characteristic was discussed by using X-ray diffraction (XRD) method and differential scanning calorimetry (DSC) analysis. The results showed that the reactions of the Al–Ni₂O₃ system consisted of two steps as following: (1) the Al firstly reacted with Ni₂O₃ to form the stable α-Al₂O₃ particles and active Ni atoms; (2) the active Ni atoms further reacted with Al to form Ni₂Al₃. The values of activation energy of the two step reactions were around 457.3 and 282.4 kJ/mol, respectively. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) revealed that the Ni₂Al₃ blocks were uniformly distributed throughout the matrix, while the α-Al₂O₃ particles were slightly segregated in the matrix. The strength of the composite is controlled by the strength of Ni₂Al₃ phase, and the tensile strength and the elongation rate of the composite with 30 vol.% reinforcement volume fraction are 210 MPa and 8%, respectively.