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.     

Effect of Al2O3 layer on improving high-temperature oxidation resistance of siliconized TiAl-based alloy

TiAl-based specimens were siliconized with two different kinds of cementation respectively, one is 23 vol.% Si + 77 vol.% Al₂O₃, and the other is 23 vol.% Si + 77 vol.% ZrO₂. SEM observation showed that a Ti₅Si₃-based layer, in which some Al₂O₃ particles dispersed, formed on the surface after siliconization. Further observation showed that an extra outer Al₂O₃ layer existed on the surface of specimens siliconized with 23 vol.% Si + 77 vol.% Al₂O₃, while no such Al₂O₃ layer was found in specimens siliconized with 23 vol.% Si + 77 vol.% ZrO₂. The cyclic oxidation test performed at 900 °C shows that the oxidation resistance was significantly improved by siliconizing. By comparison, the specimens that siliconized with 23 vol.% Si + 77 vol.% Al₂O₃ exhibits a better oxidation resistance than that with 23 vol.% Si + 77 vol.% ZrO₂. It was deduced that the extra outer Al₂O₃ layer is beneficial to the oxidation resistance of siliconized TiAl-based alloy.     

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.     

Mechanical properties of Al2O3/Ni laminated composites

Al₂O₃/Ni laminated composites were prepared by aqueous tape casting and hot pressing with intent to study mechanical properties including the fracture strength and toughness. The residual stress was evaluated and proved. The relations of mechanical properties with the thermal residual stress, the ductility of metal layers and the layer thickness ratio were studied, respectively. It was found that the toughness and work of fracture of Al₂O₃/Ni laminar reached to 12.56 MPa m^1/2 and 12 450 J m^− 2, which are 3.6 and 478.8 times that of pure Al₂O₃.     

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.     

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.     

Al2O3–FeCrAl composites and functionally graded materials fabricated by reactive hot pressing

Al₂O₃–FeCrAl composites were fabricated by mixing Fe₂O₃, Al and Cr powders and then reactive hot pressing. The high temperature alloy FeCrAl was formed by the reaction of extra Al, Cr and the Fe reduced from Fe₂O₃. The Al₂O₃–FeCrAl composites with various Al₂O₃ fractions were successfully fabricated by the proper addition of extra Fe, Cr, Al or Al₂O₃ powders. A five-layer functionally graded material of YSZ–FeCrAl was fabricated using the Al₂O₃–FeCrAl composites with compositions of 25, 53.2 and 75 vol.% Al₂O₃ as interlayer. The results from XRD analysis, optical microscope observation and thermal cycling test show that the composites fabricated by this method consist of α-Al₂O₃ phase and (Fe, Cr, Al) solid solution. The α-Al₂O₃ grain formed by this in-situ reaction between Fe₂O₃ and Fe is ultrafine and uniform distribution. The three-point bending strength is 305.0 MPa for the composite with 53.2 vol.% Al₂O₃ prepared by the reactive hot pressing, about 20% higher than that of the composite with same composition prepared by ex situ hot pressing method (252.0 MPa). No cracking was found in the functionally graded materials after 10 thermal cycles up to 1000 °C due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM.     

Y2O3–Nd2O3 double stabilized ZrO2–TiCN nanocomposites

Yttria-neodymia double stabilized ZrO₂-based nanocomposites with 40 vol% electrical conductive TiCN were fully densified by means of pulsed electric current sintering (PECS) in the 1400–1500 °C range. The Y₂O₃ stabilizer content was fixed at 1 mol% whereas the Nd₂O₃ co-stabilizer content was varied between 0.75 and 2 mol% in order to optimise the mechanical properties. The mechanical (Vickers hardness, fracture toughness and bending strength), electrical (electrical resistivity) and microstructural properties were investigated and the hydrothermal stability in steam at 200 °C was assessed.The nanocomposites with 1–1.75 mol% Nd₂O₃, PECS at 1400 or 1450 °C, have an excellent fracture toughness of 8 MPa m^1/2, although the grain size of both ZrO₂ and TiCN phases after densification is in the 100 ± 30 nm range. Moreover, the composites combine a hardness of about 13 GPa, a bending strength of 1.1–1.3 GPa with a low electrical resistivity (1.6–2.2 × 10^?5 Ω m) allowing electrical discharge machining. The hydrothermal stability of the double stabilizer nanocomposites was higher than for yttria-stabilized ZrO₂-based composites with the same overall stabilizer content.     

Mechanical and magnetic properties of γ-Ni–xFe/Al2O3 composites

Ultra-fine grained γ-Ni–xFe (x = 20, 50, and 64 (nominal)) dispersed Al₂O₃-matrix composites were fabricated by a mechano-chemical process plus hot-pressing, and their mechanical and magnetic properties were explored. The results indicated that all composites incorporated with different γ-Ni–xFe alloys possessed high densities (relative density D ⩾ 98%) and sub-micrometer-sized matrix dispersed with γ-Ni–xFe particles of sizes below ∼500 nm. As compared to other two composite systems, γ-Ni–20Fe/Al₂O₃ had finer microstructures and displayed superior fracture toughness and strength. In high iron-contained γ-Ni–64Fe/Al₂O₃ composite undesired FeAl₂O₄ phase formed on the matrix grain boundaries, which is mainly responsible for its inferior mechanical properties. Although Young’s modulus and hardness of Ni–20Fe/Al₂O₃ composite system decreased, its fracture toughness increased monotonously with increasing the alloy content in the composition range investigated. Moreover, incorporation of ferromagnetic γ-Ni–xFe particles led all the composite systems to display ferromagnetism with their saturation magnetization increasing almost linearly with increasing alloy content. In addition, experiments showed that their ferromagnetism had high thermal stability (T_c = ∼580 °C), no obvious magnetism degradation and magnetic interactions of the alloys with the matrix being observed. The combination of good mechanical properties with excellent magnetic performance would make this material be very valuable in industry.     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

In situ synthesis and properties of Zr2Al3C4/ZrB2 composites

The Zr₂Al₃C₄/ZrB₂ composites are in situ synthesized by spark plasma sintering using Zr, Al, graphite, and B₄C powders as the initial materials. The introduction of ZrB₂ can not only evidently hinder the coarsening of Zr₂Al₃C₄ grains, but also benefit the densification and improve the hardness and Young’s modulus of the Zr₂Al₃C₄/ZrB₂ composites. When the ZrB₂ content is 20 vol.%, the composite shows an optimum fracture toughness value of 4.37 MPa m^1/2, about 20% higher than that of the monolithic Zr₂Al₃C₄. The unique mechanical properties can be mainly ascribed to the contribution of ZrB₂ as the reinforcing phase hindering the crack propagating. Compared with Zr₂Al₃C₄, the Zr₂Al₃C₄/20 vol.%ZrB₂ composite also exhibits a relatively higher thermal conductivity and better oxidation resistance.     

Synthesis of an Al2O3/Al co-continuous composite by reactive melt infiltration

The route for the fabrication of an Al₂O₃/Al co-continuous composite by reactive melt infiltration was investigated using scanning electron microscopy, energy dispersive X-ray microanalysis and X-ray diffraction analysis. It was found that in the process of molten aluminium infiltration into the SiO₂ preform, the chemical reaction of 3SiO₂ + 4Al → 2Al₂O₃ + 3Si occurred at the infiltration front, and generated a transition zone containing a new type of continuous porosity about 100 μm in width. The reaction continued with further infiltration of molten aluminium alloy into this porosity which reacted with the residual SiO₂ until all the SiO₂ was transformed into Al₂O₃. A comparison was made between this route and that by direct infiltration of molten aluminium alloy into the open porosity of an Al₂O₃ preform. As a result of the increased wetting ability of the molten aluminium alloy by the chemical reaction, reactive melt infiltration took place at a higher rate for the SiO₂ preform than that for the direct infiltration of the Al₂O₃ preform. A fracture surface examination demonstrated a toughening effect provided by the continuous aluminium alloy in the composite.     

Doping of iron phosphate glasses with Al2O3, SiO2 or B2O3 for improved thermal stability

The effects of doping 60 P₂O₅–40 Fe₂O₃ (mol%) glasses with 5–10 mol% SiO₂, Al₂O₃ or B₂O₃ on their thermal stability, iron environments and redox were investigated. Thermal stability improved markedly with 5% dopant addition in the order Al₂O₃ > SiO₂ > B₂O₃ ≫ base glass. Solubility of pro rata additions when melted at 1150 °C was 5–10% SiO₂, <5% Al₂O₃, and >10% B₂O₃. It was possible to dissolve 5% Al₂O₃ by replacing Fe₂O₃. These additions generally had little effect on dilatometric measurements and iron environments, however the Fe²⁺/ΣFe redox ratio increased in the order base glass < Al₂O₃ < SiO₂ < B₂O₃. This behaviour was broadly consistent with the effects of glass basicity. The increased thermal stability of these glasses may improve their suitability for applications such as waste immobilisation or sealing.     

Synthesis of Ti3AlC2/Al2O3 nanopowders by mechano-chemical reaction

Ti₃AlC₂/Al₂O₃ nanopowders were synthesized by the combination of mechanically-induce self-propagating reaction (MSR) of Ti, C, Al and TiO₂ powder mixtures and subsequently heat treatment. Effects of high energy milling and heat treatment temperatures on the phase transformation were investigated in detail. X-ray diffraction (XRD) was used to characterize the powders of milled and annealed, respectively. The morphology and microstructure of as fabricated products were also studied by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Results show that TiC, Ti_xAl_y and Al₂O₃ transitional phases were formed when the initial powder mixtures were milled for 24 h. The desired Ti₃AlC₂/Al₂O₃ nanopowders with high purity were obtained when annealed the as-milled powders at 1100 °C. SEM image confirmed that the as fabricated Ti₃AlC₂/Al₂O₃ particles has nanocrystalline layered structural matrix of Ti₃AlC₂, and the second phase of nanosized Al₂O₃ disperses uniformly in the Ti₃AlC₂ matrix.     

Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites

New Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ ternary Melt Growth Composites (MGCs) with a novel microstructure have been fabricated by unidirectional solidification. These MGCs displayed superior high-temperature strength characteristics. The flexural strength increases progressively in the range 650–800 MPa with a rise in temperature from room temperature up to 1873 K. These excellent high-temperature characteristics are closely linked to such factors as: a microstructure consisting of three-dimensionally continuous and complexly entangled single-crystal Al₂O₃ with a hexagonal structure, single-crystal YAG with a garnet structure and fine ZrO₂ with a cubic structure; characteristic dimensions of the microstructure of Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics of around 2–3 m for YAG, around 2–3 m for Al₂O₃ and around 0.4–0.8 m for ZrO₂; and the fact that no amorphous phase is formed at interfaces between any of the phases.     

Fabrication and mechanical properties of Al2O3/Ti(C0.7N0.3) nanocomposites

Two kinds of Al₂O₃/Ti(C_0.7N_0.3) nanocomposites were fabricated with traditional hot pressed sintering and repetitious-hot-pressing technology, one is added with nano-scale SiC, and the other is without SiC. The results showed that the mechanical properties of the former are higher than that of the latter, especially the fracture toughness can reach up to 8.3 MPa m^1/2. Although the fracture toughness remains high, repetitious-hot-pressing results in the reduction of flexural strength. The improvement of the mechanical properties is interpreted from the different microstructure and fracture mode. The microstructure shows that the addition of nano-scale Ti(C_0.7N_0.3) and nano-scale SiC lead to the refinement of matrix grain, and the inter/intragranular microstructure can be formed instead of the intergranular microstructure in monolithic alumina. The higher fracture toughness resulted mainly from the transgranular fracture mode.     

In-situ fabrication of Al3V/Al2O3 nanocomposite through mechanochemical synthesis and evaluation of its mechanism

In this research, in situ fabrication of Al₃V based nanocomposite and its formation mechanism have been investigated. In order to synthesize Al₃V/Al₂O₃ nanocomposite, a mixture of Al and V₂O₅ powders was subjected to high-energy ball milling and the nanocomposite was produced through a mechanochemical reaction. The produced structure was isothermally heat-treated at 500–600 °C for 0.5–2 h under argon atmosphere. In order to evaluate the structural changes during milling and annealing, the synthesized powders were characterized by X-ray diffraction (XRD). Moreover, the powder morphological changes were studied by scanning electron microscopy (SEM). It was observed that the reaction between Al and V₂O₅ occurred after about 30 min and, the Al₃V and Al₂O₃ were formed in nanocrystalline structure with the continuing mechanical milling. Calculation of adiabatic temperature confirmed that reaction took place in combustion mode. In final stage of milling up to 40 h; it was observed that the Al₃V decomposed to Al and V so that the optimum time of milling to achieve fabrication of nanocomposite was determined to be about 20 h. Calculations based on Miedema’s model verified partial disordering of Al₃V during further milling and annealing of as-milled powder at 600 °C led to the ordering of Al₃V. The crystallite size of Al₃V and Al₂O₃ after annealing at 600 °C for 2 h remained in nanometer scale. So the final product appeared to be stable even after annealing.     

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.     

Monocrystalline silicon surface passivation by Al2O3/porous silicon combined treatment

In this paper, we report on the effect of Al₂O₃/porous silicon combined treatment on the surface passivation of monocrystalline silicon (c-Si). Al₂O₃ films with a thickness of 5, 20 and 80 nm are deposited by pulsed laser deposition (PLD). It was demonstrated that Al₂O₃ coating is a very interesting low temperature solution for surface passivation. The level of surface passivation is determined by techniques based on photoconductance and FTIR. As a result, the effective minority carrier lifetime increase from 2 μs to 7 μs at a minority carrier density (Δn) of 1 × 10¹⁵ cm^?3 and the reflectivity reduce from 28% to about 7% after Al₂O₃/PS coating.     

Mechanochemical synthesis of Al2O3/Co nanocomposite by aluminothermic reaction

In this work, Al₂O₃/Co nanocomposite was successfully prepared by mechanochemical reaction between Co₃O₄ and Al powders in a planetary high energy ball mill. The mechanism of the reaction was dealt using X-ray diffraction (XRD), differential thermal analysis (DTA), and thermodynamics calculations. It was found that Co₃O₄ reacts with Al through a self-sustaining combustion reaction after an incubation period of 50 min and the reaction between Co₃O₄ and Al involves two steps. First, Co₃O₄ reacts with Al to form CoO and Al₂O₃ at the temperature around melting point of Al, and at higher temperature, CoO reacts with remaining Al to form Co and Al₂O₃. Mechanical activation process decreases the reaction temperature from 1041 °C for as-received Co₃O₄ and Al powder mixture to 869 °C for 45 min milled powders. After annealing of powder milled for 12 h, no phase transformation has been detected. The crystallite sizes of both α-Al₂O₃ and Co remained in nanometeric scale after annealing at 1000 °C for 1 h.