Synthesis of Al2O3 matrix composites by reactive infiltration

Reactive infiltration of a NiO-base blended powder with molten aluminium was attempted at 1673 K in order to obtain Al2O3 matrix composites containing a dispersion of Al3Ni, AlNi and/or AlNi3. The NiO powder was barely infiltrated by the molten aluminium after a 3600 s holding time at 1673 K. A continuous layer of Al2O3 was observed to exist at the infiltration front, which prevented any further infiltration. TiB2 particles were added to the NiO powder in order to absorb the heat of reaction between NiO and aluminium. When the TiB2 particle content in the [NiO+TiB2] powder blend was greater than 20 vol%, spontaneous infiltration occurred completely. Thus, it was shown that the addition of the TiB2 particles assisted in the spontaneous infiltration. The specimens produced by the in situ reaction consisted of Al2O3, TiB2 and Al3Ni. Al3Ni was mainly located between the TiB2 and Al2O3. The effect of the TiB2 addition on the infiltration kinetics was to decrease the maximum attainable temperature caused by the exothermic reaction. This in turn prevented the formation of a continuous Al2O3 film at the infiltration front. This resulted in the production of pathways for the infiltration of the molten aluminium and made possible the complete infiltration. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

Al2O3 reinforced Al/Ni intermetallic matrix composite by reactive sintering

An aluminium-nickel reinforced Al₂O₃ particulate composite was fabricated by a powder metallurgy route, where 35wt% aluminium and 30wt% nickel powders were mixed with 35wt% Al₂O₃ particles and compacted at 548 MPa. Sintering was carried out at 850 °C, where the synthesis reaction was sustained by the transient liquid phase resulting from the exothermic reaction associated with the formation of intermetallic compounds, i.e. reactive sintering. The resultant microstructure was studied using X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). It was found that the initial distribution of individual constituent powders affect the outcome of the reactive sintering and that the inward diffusion of aluminium in nickel was responsible for nickel aluminide formation.     

Microstructure and infiltration kinetics of Si3N4/Al–Mg composites fabricated by pressureless infiltration

Si₃N₄/ Al–Mg composites reinforced by ceramic interpenetrating network structure had been fabricated via pressureless infiltration technology. The matrix and the reinforcement phase, form an interconnected interpenetrating network structure. The Al–Mg/Si₃N₄ system exhibits an excellent wettability under moderate conditions. The increasing of Mg content (2–10 wt%) resulted in an increased amount of infiltration, once Mg content beyond 10 wt% has an adverse effect. Light chemical reaction occurs in the interface of Al–Mg/Si₃N₄ system and the reaction productions reduce the surface tension of melt and impulse the advance of infiltration. Infiltration temperature and infiltration time were the key parameters, which turn into the infiltration impetus. The appropriate infiltration temperature is 1050 °C and the corresponding infiltration time is 15 min, prolonging the infiltration time continuously has no significance.     

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.     

Nanostructured Ni3Al layers and Ni3Al/Ni multilayers obtained by magnetron sputtering

Intermetallic Ni₃Al thin layers and Ni₃Al/Ni multilayers were deposited on a Si wafer by means of magnetron sputtering. The structure and morphology of the layers have been characterized by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The polycrystalline films are textured in the (111) direction and have grain sizes below 20 nm. Superlattice reflections due to chemical order have been observed in the electron microscope. It is shown by x-ray diffraction that the multilayers grow coherently on the amorphous substrate.     

Microstructure and physical properties of Al/diamond composite fabricated by pressureless infiltration

Abstract

The Al/diamond composite was fabricated using a pressureless infiltration method. The microstructure and physical properties of the composite were investigated. The composite has a very low coefficient of thermal expansion (CTE) of 3·9 × 10^?6 K^?1. The thermal conductivity (TC) of the composite is 12% higher than that of the Al alloy matrix. The lower TC of the composite than the expected value was attributed to the existence of interfacial low conducting phases and the porocity of the composite.     

A micro initiator realized by reactive Ni/Al nanolaminates

This study described a micro initiator realized by reactive Ni/Al nanolaminates. A self-propagating reaction can be triggered in the Ni/Al film by applying a DC voltage of 1.5?V. This exothermic reaction can raise the temperature of the film (10?μm in thickness) surface to as high as 622?K. The measured ignition power to start the self-propagating reaction in the film was 3?mW with an ignition delay of around 0.63?s. The small ignition energy required and the large energy output make the Ni/Al film superior to the current resistive heater based initiators. Numerical simulation results demonstrated that different temperatures can be achieved by simply alternating the film thickness and the localization of high temperature exposure was realized to avoid unintentional fire of adjacent initiators. These findings were confirmed by the experiment using thermal indicators.     

Synthesis of TiC/Ti–Cu composites by pressureless reactive infiltration of TiCu alloy into carbon preforms fabricated by 3D-printing

The microstructure of dense TiC/Ti–Cu composites fabricated by pressureless infiltration of TiCu alloy into porous starch derived carbon preform produced by 3D-printing has been studied. The reactive melt infiltration was carried out at 1100 °C in a flowing Ar atmosphere and resulted in formation of a composite comprised predominantly of substoichiometric TiC_x (x=0.78), binary intermetallic Ti–Cu phases and residual carbon. SEM analyses revealed a microstructure consisting of a dispersed fine-grained TiC_0.78 (∼7 μm) in a Ti–Cu matrix.     

无压熔渗制备SiCp/Al复合材料的界面改性研究进展

研究表明,SiCp/Al间界面润湿性的好坏是采用无压熔渗法制备高体积分数SiCp/Al复合材料的最关键因素,也是影响复合材料性能的主要因素.本文从界面反应和界面润湿性角度出发,综述了近几年来国内外关于SiCp/Al复合材料的界面研究情况.     

Silicon-aluminium network composites fabricated by liquid metal infiltration

This paper provides a new method for fabricating interpenetrating silicon-aluminium network metal-matrix composites. This method involves the infiltration of an aluminium-silicon alloy (Al-12Si-1Mg or Al-30Si-1Mg) liquid into a silicon particle (50 vol %) preform. The silicon particles were partially dissolved by the liquid alloy and, together with silicon contributed by the original Al-Si-Mg matrix, resulted in an Si network after solidification. The network composites were metallurgically sound, with no porosity, and exhibited a thermal expansion coefficient down to 7.7×10^–6 °C^–1 at 50–100 °C, compressive strength up to 580 MPa, tensile strength up to 160 MPa and Vickers hardness up to 390.     

Titanium aluminide (Ti-48Al) powder synthesis,size refinement and sintering

Titanium aluminide based alloys have shown significant potential in high temperature applications, but the high production cost of TiAl considerably limits its utilisation. Although the use of powder metallurgy processes can reduce the cost by minimising post-machining, an economical powder production route is still required. Therefore, in the present study a pre-alloyed Ti-48Al powder is developed using an elemental Ti and Al powder blend prepared using a simple vacuum heat treatment. A formation model of the intermetallic phases (i.e. TiAl, Ti₃Al, TiAl₂, TiAl₃) during powder synthesis is proposed. In order to improve the sinterability, various milling methods (i.e. ball, attrition and shatterbox milling) are examined to reduce the particle size. The sintered microstructures, particularly the two-phased (α₂-Ti₃Al γ-TiAl) lamellar structures are also investigated. Improved densification is achieved at 1300 °C, held for 2 h, using the manufactured powder, compared to the elemental powder blend (～55%). With higher sintering temperatures or longer hold periods, increased density TiAl components are possible.