In situ synthesis of Al2O3 particulate-reinforced Al matrix composite by low temperature sintering

The powder mixture of Al-10 wt.% SiO₂ was selected as a research system. Compared with an as-mixed powder, the phase structure and microstructure of an as-milled powder was investigated, and the temperature of the displacement reaction in the two kinds of powder was determined by thermal analysis. The preforms of the two kinds of powder were sintered based on the result of thermal analysis. The results indicate that the particle size of the Al-SiO₂ powder was refined greatly after 4 h of high energy ball milling, and diffusion couples were formed due to SiO₂ particles embedded in the Al matrix. The displacement reaction did not occur between Al and SiO₂ for the as-mixed powder, while it occurred in the range of 560–680°C for the as-milled powder. For the as-milled powder, an aluminum matrix composite reinforced with Al₂O₃ particles, which were homogeneously distributed in the Al matrix, can be fabricated by sintering at 640°C for 2 h.     

In situ synthesis and characterization of a hierarchically structured Al2O3/Al3Ti composite

A hierarchically structured α-Al₂O₃/Al₃Ti composite was fabricated by an in situ process called exothermic dispersive synthesis from a powder blend of Al and TiO₂. The microstructure of the composite was investigated by X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectroscopy. Three transitional phases, specifically TiO, Ti₂O₃, and γ-Al₂O₃, were found to form during the reactive process. Using differential scanning calorimetry, it was found that the reaction between the Al and TiO₂ occurred through three intermediate steps and their corresponding activation energies were 390, 205, and 197 kJ/mol, respectively. Moreover, the reaction rate of the third step was found to be much higher than that of the second step, and the time taken by each reaction step decreased with the increase of the heating rate. The findings are critical to understanding the microstructural development in the synthesis of strong and tough Al₂O₃/Al₃Ti composites.     

Reaction mechanism in SiO2-MnO2-Fe-Al system for producing ferrosilicomanganese powder

In this research, the ferrosilicomanganese powder was synthesized via mechanical alloying and SHS methods. Silicon oxide, manganese oxide, iron and aluminum powders were used as starting materials. SHS process was initiated by oxyacetylene flame. The activated and inactivated powder mixtures were used for producing ferrosilicomanganese grade 26%Si-53%Mn-21%Fe. The powder mixtures were characterized by X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. Thermodynamic analysis showed that aluminothermic reduction of MnO₂ is more thermodynamically favored as compared with aluminothermic reduction of SiO₂. Adiabatic temperature of MnO₂ and SiO₂ reduction by Al was calculated about 2946 K. It was found that no reaction took place during mechanical alloying. After annealing and SHS processes, first MnO₂, then SiO₂ oxides were reduced by Al and ferrosilicomanganese and Al₂O₃ phases were produced. Due to the high adiabatic temperature, all products were formed in liquid state, leading to produce sintered ferrosilicomanganese and isolated Al₂O₃ particles.     

Some physical properties of an Al2O3-SiC-TiC composite

Ceramic samples obtained by hot pressing from a mixture of Al₂O₃ with admixtures of 23 vol.% TiC powder and 30.9 vol.% SiC whisker crystals are investigated experimentally. The resistivity of the material is measured at temperatures of 4.2–300 K, the infrared reflection spectra are recorded in the region 400–4200 cm^–1, and the temperature dependence of the Young’s modulus is investigated at temperatures up to 1300 K. As a result it is it is shown that the conductivity and optical reflection of the high-strength composite have a semimetallic character, which is due to the titanium carbide particles contained in it. Pis’ma Zh. Tekh. Fiz. 23, 52–58 (December 12, 1997)     

Fatigue damage development in Al2O3/Al composite

Fatigue damage development in two aluminium matrix (Al7SiO.6Mg and Al5Si-3Cu1Mg) composites reinforced with discontinuous Al₂O₃ fibres has been monitored by means of acoustic emission (AE). The AE signals (RMS) recorded during the tests clearly exhibit three distinct stages which correspond to crack initiation, dominant crack formation and stable propagation. Generally speaking, the cracks initiated at a high load level form close together and a dominant crack forms easily. By contrast, at a low load, initiated cracks are widely separated and the formation of a dominant crack is difficult. If there are large defects in the composite, the first stage is absent, even at low load. In the first stage, little change in microstructure and modulus of the composite is observed; in the second, fibre fracture, interface debonding and matrix cracking occur and there are often sinusoidal cracks in the matrix; in the last stage, the principal characteristic is stable propagation of the dominant crack. The degradation of the elastic modulus of the composite in the last two stages is small.     

Superplasticity in an aluminum alloy 6061/Al(2)O(3)p composite

The superplasticity of an Al(2)O(3)p/6061Al composite, fabricated by powder metallurgy techniques, has been investigated. Instead of any special thermomechanical processing or hot rolling, simple hot extrusion has been employed to obtain a fine grained structure before superplastic testing. Superplastic tensile tests were performed at strain rates ranging from 10(-2) to 10(-4) s(-1) and at temperatures from 833 to 893 K. A maximum elongation of 200% was achieved at a temperature of 853 K and an initial strain rate of 1.67x10(-3) s(-1). The highest value obtained for the strain rate sensitivity index (m) was 0.32. Differential scanning calorimeter was used to ascertain the possibility of any partial melting in the vicinity of optimum superplastic temperature. These results suggested that no liquid phase existed where maximum elongation was achieved and deformation took place entirely in the solid state.     

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.     

Temperature-dependent Young's modulus of an SiCw/Al2O3 composite

Using a computer-controlled resonant-bar apparatus at frequencies near 5 kHz, we determined the temperature-dependent (86–732 K) Young's modulus of a ceramic-ceramic composite with a 0.30 volume fraction of SiC whiskers in an Al₂O₃ matrix. Using a megahertz-frequency pulse-echo method, it was verified that the composite shows little anisotropy (variation of the elastic properties with direction). Using a scattered-plane-wave ensemble-average method, we modelled the ambient-temperature elastic constants and found good model-observation agreement. To model the behaviour of the Young's modulus with temperature, Varshni's three-parameter relationship for Einstein-oscillator monocrystals was used. Again, good model-observation agreement was found. The mechanical-loss spectrum showed no remarkable features, indicating good whisker-matrix interface properties up to 732 K.     

Drilling studies of an Al2O3-Al metal matrix composite

Comparative drilling studies have been carried out for a 20% Al₂O₃ microsphere reinforced Al metal matrix composite (MMC) using a (a) high speed steel (HSS) drill, (b) tungsten carbide (WC) drill and (c) polycrystalline diamond (PCD) drill. In this part of the paper, the flank wear characteristics of the drills have been presented. It is found that for the HSS drill a flank wear of 1.00 mm is reached in drilling for as little as 12 s. In contrast, under similar conditions for the WC drill, a flank wear of 0.16 mm was observed after drilling for a period of 600 s and in the case of the PCD drill, after 2210 s of drilling a flank wear of only 0.12 mm was observed. The PCD drill, however, showed some signs of chipping in the early stage, but this seemed to stabilize later on. Scanning electron microscopic (SEM) observations of the worn drill tips revealed that in addition to flank wear the HSS exhibited margin wear, the WC drill exhibited both margin wear and chisel edge wear and the PCD drill displayed crater wear.