Microstructure and mechanical properties of a directionally solidified Al2O3/Y3Al5O12/ZrO2 hypoeutectic in situ composite

Directionally solidified ternary Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ hypoeutectic rod composites were successfully fabricated by the laser zone remelting technique. The microstructure and mechanical properties of the composite were investigated. The microstructure presented a complex three-dimensional network structure consisting of fine Al₂O₃ (41 vol.%) and YAG (49 vol.%) phases, with smaller ZrO₂ (10 vol.%) phases partially distributed at the Al₂O₃/YAG interfaces. The irregular growth behavior in the hypoeutectic was revealed. The hardness and fracture toughness at ambient temperature were measured to be 17.3 GPa and 5.2 MPa m^1/2, respectively. The toughness enhancement in comparison with previous binary Al₂O₃/YAG composites was mainly attributed to the refined microstructure, and crack deflection, branching and bridging. Moreover, the residual stresses, generated by different thermal expansion coefficients of the component phases, also importantly contributed to the improved toughness. Correlations between the addition of the third component ZrO₂ and the microstructure and properties were discussed as well.     

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.     

Compaction and sintering behaviour of glass–alumina composites

The effects of Al₂O₃ additions on the compaction and sintering behaviour of a leadborosilicate glass (LG) have been investigated. LG powder was prepared by melting, fritting and milling a glass of the composition: 77PbO, 10B₂O₃, 10SiO₂, 2Al₂O₃ and 1P₂O₅ (wt.%). The mean particle sizes of the powders were: LG, 6.5 μm and Al₂O₃, 3.3 μm. The compaction behaviour of LG–Al₂O₃ powder mixtures can be represented by a new compaction equation: [(D_g−D₀)/(1−D₀)]=(P/P_f)ⁿ, where D_g is the relative green density, D₀ the relative tap density and n and P_f are material constants. The exponent n decreases from 0.192 to 0.065 as the Al₂O₃ content is increased from 0 to 100 vol.%. The Frenkel equation for isothermal shrinkage has been found to be valid. It is shown that in the glass matrix composites the minimum sintering temperature can be determined by measuring the dilatometric deformation temperature. The presence of Al₂O₃ in excess of 15 vol.% has been found to strongly retard the sintering kinetics. An addition of 45 vol.% Al₂O₃ increases the activation energy for sintering from 67 to 112 kcal mol⁻¹. The presence of Al₂O₃ particles also induced a partial crystallisation in LG matrix.     

Characterization of hot-pressed short ZrO2 fiber toughened ZrB2-based ultra-high temperature ceramics

Two different ZrB₂-based ultra-high temperature ceramics were produced by hot pressing: ZrB₂ + 20 vol.% SiC particle + 15 vol.% ZrO₂ fiber and ZrB₂ + 20 vol.% SiC whisker + 15 vol.% ZrO₂ fiber. The microstructures were analyzed by using transmission electron microscopy and high-resolution transmission electron microscopy. It was shown that a clean interface without any impurities was identified in ZrB₂-based hybrid ceramics with SiC whiskers and ZrO₂ fibers, which would significantly improve the toughening mechanism. The results of high-resolution transmission electron microscopy showed that stacking faults in SiC whiskers resulted from an insertion of a (111) layer, which would be one of the main reasons for material anisotropy. However, the interface between the SiC particle and ZrO₂ fiber was found to be ambiguous in ZrB₂-based hybrid ceramics with SiC particles and ZrO₂ fibers due to the slight reaction. The orientation relationship between t-ZrO₂ and m-ZrO₂ phases obeyed the classical correspondence: (100)_m//{100}_t and [001]_m//〈001〉_t, which further verified the feasibility of phase transformation toughening mechanism.     

Isothermal oxidation behavior of the TZAV-20 alloy

Oxidation behavior is important for secure and long-life service of metals and alloys. The isothermal oxidation behavior of the Ti–20Zr–6.5Al–4V alloy at 470 to 670 °C was investigated. The kinetics analysis shows that the oxidation of TZAV-20 alloy below 570 °C accords with the parabolic law. While the alloy oxidized at 670 °C, obeys the linear law. As oxidation temperature increases from 470 to 670 °C, the oxidation products change as: TiO₂ → TiO₂ + ZrO₂ → TiO₂ + ZrO₂ + Al₂O₃. Relation between weight gain and thickness of oxidation film shows that the weight will increase 0.171 mg/cm² for every 1 μm increasing in thickness. The surface hardness increases from approximately 380 HV for base material to 689 HV for 670 °C oxidized specimen. In short, the TZAV-20 alloy has favorable inoxidizability below 570 °C. The findings will not only promote practical applications of the new TiZrAlV series alloys but also supplement the oxidation theory.     

Superplasticity and machinability in a four-phase ceramic

Fine grained four-phase ceramic materials were fabricated to have a combination of high temperature superplasticity and room temperature machinability. The composite ceramics were made of 25 vol.% 3Y-TZP/8YSZ–25 vol.% Al₂O₃–25 vol.% MgAl₂O₄–25 vol.% LaPO₄, using LaPO₄ powders fabricated in-house. X-ray diffraction and scanning electron microscopy revealed that the grain size for the best mixed samples was in the range of 600 nm, tetragonal ZrO₂ transformed into monoclinic, and reactions in the sintered samples produced a new phase, magnetoplumbite (LaMgAl₁₁O₁₉) with lath-like grains. The formation of magnetoplumbite was facilitated by the presence of yttria and by a liquid phase generated at elevated temperatures. These four-phase ceramics had a maximum hardness of 12 GPa and fracture toughness was no more than 3 MPa√m. Deformation rates at 1400 °C under 40 MPa stress were in the superplastic range of 10^?3 s^?1 for most compositions. These four-phase ceramics were machinable as demonstrated using conventional tungsten drill bits.     

Fracture behaviour of α-Al2O3 ceramics reinforced with a mixture of single-wall and multi-wall carbon nanotubes

The extraordinary mechanical properties of single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) have generated interest in incorporating them as toughening agents in ceramics. This work describes the fracture behaviour of an alumina (Al₂O₃) ceramic reinforced with a mixture of 0.05 wt% MWCNTs + 0.05 wt% SWCNTs. The CNT/Al₂O₃ nanocomposite was pressureless sintered in air using graphite powder as bed powder at 1520 °C for 1 h. The hardnesses and fracture toughnesses were lower than for pure Al₂O₃ and Al₂O₃ + 0.1 wt% SWCNTs and Al₂O₃ + 0.1 wt% MWCNTs. A predominantly transgranular fracture mode with a decrease in crack deflection and no pull-out was observed in the SWCNT + MWCNT–Al₂O₃ nanocomposite. MWCNTs had to the best reinforcing effect in Al₂O₃ nanocomposite.     

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.     

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.     

A study on the mechanochemical behavior of TiO2–Al–Si system to produce Ti5Si3–Al2O3 nanocomposite

In the present study Ti₅Si₃–Al₂O₃ nanocomposite was synthesized by a displacement reaction between Al and TiO₂ in ball milling of TiO₂, Al and Si powders. The effect of milling time and heat treatment temperatures were also investigated. The structural changes of powder particles during mechanical alloying were investigated by X-ray diffraction (XRD). Morphology and microstructure of powders were characterized by scanning electron microscopy (SEM). It was found that after 10 h of MA, the reaction between Al and TiO₂ initiated in a gradual mode and after about 45 h of milling, the reaction was successfully completed. The final product consisted of Ti₅Si₃ intermetallic compound with a crystallite size of 13 nm and amorphous Al₂O₃. Heat treatment of this structure at 1050 °C led to the crystallization of Al₂O₃ and ordering of Ti₅Si₃. The crystallite size of Ti₅Si₃ and Al₂O₃ after annealing at 1050 °C for 1 h remained in nanometer scale. So the final product appeared to be stable upon annealing.     

Effect of stirring on the dissolution of coal fly ash and synthesis of pure-form Na-A and -X zeolites by two-step process

Using the coal fly ash (FA), pure-form Na-A and -X zeolites were synthesized by two-step process. The FA was pretreated in aqueous NaOH solution under stirring condition at 85 °C for 18 h. The amorphous aluminosilicate of FA was dissolved during pretreatment. Increasing the stirring speed accelerated the dissolution of FA and increased Si⁴⁺ and Al³⁺ concentrations in the solution. This fact indicated that the stirring during pretreatment significantly affected on the dissolution of FA. After pretreatment, remaining FA was removed and aqueous NaAlO₂ solution was added to the residual solution to control the molar ratio SiO₂/Al₂O₃ of 0.5–4.5. After aging the resultant at 85 °C for 24 h, white precipitates were generated over the whole SiO₂/Al₂O₃ range. Increment of Si⁴⁺ concentration by stirring during pretreatment increases the yield of the product. At SiO₂/Al₂O₃ = 0.5, the material was identified as Na-A zeolite with a trace amount of hydroxysodalite. A single phase Na-A zeolite was obtained at SiO₂/Al₂O₃ = 1.0. The Na-X zeolite was emerged at SiO₂/Al₂O₃ ? 2.0. At SiO₂/Al₂O₃ = 4.5, a single phase Na-X zeolite was formed. The cation exchange capacity of synthetic single phase Na-A and -X zeolites was respectively 4.78 and 3.88 meq./g.     

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.     

Effect of cup and ball types on alumina–tungsten carbide nanocomposite powder synthesized by mechanical alloying

Alumina-based nanocomposite powders with tungsten carbides particulates were synthesized by ball milling WO₃, Al and graphite powders. X-ray Diffraction (XRD) was used to characterize the milled and annealed powders. Microstructures of milled powders were studied by Transmission Electron Microscopy (TEM). Results showed that Al₂O₃–W₂C composite formed after 5 h of milling with major amount of un-reacted W in stainless steel cup. The remained W was decreased to minor amount by increasing carbon content up to 10 wt.%. When milled with ZrO₂ cup and balls, Al₂O₃–W₂C composite was completely synthesized after 20 h of milling with the major impurity of ZrO₂. In the case of stainless steel cup and balls with 10 wt.% carbon, Fe impurity after 5 h of milling (maximum 0.09 wt.%) was removed from the powder by leaching in 3HCl·HNO₃ solution. The mean grain size of the powder milled for 5 h was less than 60 nm. The powder preserved its nanocrystalline nature after annealing at 800 °C.     

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.     

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.%.     

Influence of nano-alumina and sub-micron copper on mechanical properties of magnesium alloy AZ31

Magnesium composites of AZ31–Al₂O₃–Cu formulations were produced using the disintegrated melt deposition technique following by hot extrusion. Microstructural characterization showed reasonable distribution of secondary phases up to 1 vol.% of copper. A tendency to form clustered agglomeration and longer shape of secondary phases was observed when the amount of copper was increased to 1.5 vol.%. Mechanical tests indicated remarkable improvements in 0.2%YS, UTS and microhardness when nano-alumina and sub-micron copper were added into AZ31. The ductility was increased up to 9.3% in the case of AZ31–1.5Al₂O₃–1.0Cu sample and significantly reduced (5.5%) when the amount of copper was increased to 1.5 vol.%. Heat treated sample of AZ31–1.5Al₂O₃–1.0Cu showed overall improvement in both tensile strength and ductility. The results suggest that the judicious selection of composition and heat treatment has the capability to enhance overall tensile response of Mg–Al₂O₃–Cu nanocomposites.     

The effects of various reinforcements on dry sliding wear behaviour of AA 6061 nanocomposites

The present work aims to investigate the dry sliding wear behaviour of AA 6061 nanocomposites reinforced with various nanolevel reinforcements, such as titanium carbide (TiC), gamma phase alumina (γ-Al₂O₃) and hybrid (TiC + Al₂O₃) nanoparticles with two weight percentages (wt.%) prepared by 30 h of mechanical alloying (MA). The tests were performed using a pin-on-disk wear tester by sliding these pin specimens at sliding speeds of 0.6, 0.9 and 1.2 m/s against an oil-hardened non-shrinking (OHNS) steel disk at room temperature. Wear tests were conducted for normal loads of 5, 7 and 10 N at different sliding speeds at room temperature. The variations of the friction coefficient and the wear rate with the sliding distances (500 m, 1000 m and 1600 m) for different normal loads and sliding velocities were plotted and investigated. To observe the wear characteristics and to investigate the wear mechanism, the morphologies of the worn surfaces were analysed using a scanning electron microscope (SEM). The formation of an oxide layer on the worn surface was examined by energy dispersive spectroscopy (EDS). The wear rate was found to increase with the load and sliding velocity for all prepared nanocomposites. Hybrid (TiC + Al₂O₃) reinforced AA 6061 nanocomposites had lower wear rates and friction coefficients compared with TiC and Al₂O₃ reinforced AA 6061 nanocomposites.     

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.     

Effect of deflocculants on hardness and densification of YSZ–Al2O3 (whiskers & particulates) composites

In the present study effect of deflocculants like P-Aminobenzoic Acid (PABA) and Cetyltrimethyl ammonium bromide (CTAB) on densification and hardness of 3 mol.% Yttria-stabilized ZrO₂ (abridged as YSZ) + Al₂O₃ (whiskers or particulates) composite have been studied. Maximum hardness & density were achieved at 1 wt% of CTAB or PABA, while further addition (5, 10 and 15 wt%) had no significant affect on the aforementioned properties. It was also observed that alumina addition in form of particulates only improved the density while its addition in form of whiskers significantly increased the hardness of YSZ + alumina composite. The maximum hardness achieved was more than 14 GPa in case of sample containing alumina in form of whiskers.     

Controlled crystallization of glass–ceramics with two nucleating agents

The optimum nucleating agent had been investigated in Li₂O–Al₂O₃–SiO₂ glass–ceramics system with 2% ZrO₂ and different amounts of TiO₂ as nucleating agents. The activation energy (E) of crystallization and the Avrami parameter (n) for the LAS glasses obtained from the DTA and results show that the most effective addition of TiO₂ was about 2.36 wt.%. With the optimum nucleating agents (2% ZrO₂ + 2.36% TiO₂) addition, LAS glass–ceramics with fine grain, high transparency and good mechanical properties were obtained, due to the β-quartz solid solution formed after the crystallization process.