Al TiO2 System Research Articles

Особенности фазообразования в смеси порошков Al-TiO2 в условиях изменяющейся температуры

The paper analyzes the reaction of phase formation occurring during the synthesis in the Al-TiO2 system. The problem of the phase composition change in the vicinity of the particles, where the change of the regions occupied by the phases is related to the moving boundaries, is formulated. An approximate analytical solution is constructed. Kinetic regularities of the strengthening particle formation and the matrix composition evolution in its vicinity have been studied under the assumption that the reactions start at a temperature higher than the melting temperature of aluminum, which surrounds the titanium oxide solid particles. The temperature is a function of the time and follows from the solution of the macro problem. The accompanying stresses and strains both in the vicinity of the interface and averaged over the cell volume are evaluated. During cooling both diffusion and reactions are inhibited, which leads to slowing down the growth of stresses, however, their values remain rather high.

Synthesis and characterization of in-situ (Al–Al3Ti–Al2O3)/Al dual matrix composite

Abstract Aluminum matrix composites (AMCs) with high volume fraction of reinforcement exhibit superior mechanical properties at the expense of ductility. To overcome this shortcoming, a dual matrix composite microstructural design has proven to be beneficial in increasing the ductility while maintaining the optimal strength. In the present investigation, an in-situ dual matrix composite from Al-TiO2 system was successfully fabricated by a mechanical and thermal synthesis process via powder metallurgy route. The in-situ dual matrix composite microstructure comprises of Al-Al2O3-Al3Ti composite regions separated by ductile unmilled outer Al matrix. The composite region provides strength to the dual matrix composite while the outer Al matrix provides plasticity during deformation. The differential scanning calorimetry (DSC) technique was used to analyze the chemical reactions occurring during the synthesis. A comparative study on the microstructure evolution of in-situ single and dual matrix composites was done using XRD, SEM, EDS and EBSD. The in-situ dual matrix composite shows lower microhardness and compressive strength with significant increase in ductility and wear resistance compared to the in-situ single matrix composite with the same amount of reinforcement.

Production of a high strength Al/(TiAl3+Al2O3) composite from an Al-TiO2 system by accumulative roll-bonding and spark plasma sintering

A bulk in-situ Al/(TiAl3+Al2O3) composite was fabricated by accumulative roll-bonding (ARB) and spark plasma sintering (SPS) from pure Al sheets and TiO2 nanoparticles. The microstructure and mechanical properties of the composite were investigated. A uniform dispersion of TiO2 nanoparticles in Al matrix was first achieved by ARB processing in the primary Al/TiO2 composites. Subsequent SPS resulted in the formation of hierarchical TiAl3 (∼60 nm) and Al2O3 (∼750 nm) particles that were distributed uniformly in the ultrafine-grained Al matrix. Microhardness and strength (at ambient and high temperatures) of the final composite were substantially enhanced compared with those of pure Al and the primary Al/TiO2 composite, reaching ∼198.6 HV, ∼628.8 MPa at room temperature and ∼384.2 MPa at 300 °C, respectively. The high strength of the Al/(TiAl3+Al2O3) composite was attributed to a superimposition of different strengthening mechanisms.

Design, microstructure and high temperature properties of in-situ Al3Ti and nano-Al2O3 reinforced 2024Al matrix composites from Al-TiO2 system

This study was conducted to obtain a type of aluminum matrix composites exhibiting a good strength and certain ductility at high temperature. The 25 vol% TiO2-75 vol%2024 Al systems were selected to fabricate the (Al3Ti+Al2O3)/2024 Al composites with residual ∼32 vol% Al matrix through powder metallurgy. The (Al3Ti+Al2O3)/2024 Al exhibits a good strength and certain ductility at high temperature as in the design. The microstructure of (Al3Ti+Al2O3)/2024 Al composites was investigated. It was discovered that the in-situ Al3Ti reinforcement was in coarse block-shaped particles of approximately 6.9 μm in size and the Al-Al3Ti interface was clean. The Al2O3 particles were in the nano-scale and distributed in the Al matrix in a cluster form. The high temperature compression testing of the composites was conducted at the temperatures of 573 K, 623 K, 673 K, 723 K and 773 K with the strain rate of 10−3 ∼ 0.42 s−1. The results demonstrated that the composites exhibited higher strength at the same high temperature than the other Al matrix composites with a similar volume fraction. The massive Al3Ti and Al2O3 phases played a load bearing role at high temperatures. The residual ∼32 vol% Al matrix led the composites to acquire certain ductility.

Effect of Fe2O3 as an accelerator on the reaction mechanism of Al–TiO2 nanothermite system

Thermite reactions between aluminum and metal oxides could lead to the formation of intermetallic matrix composites used in high-temperature industrial applications. Thermite reaction in Al–TiO2 system needs a considerable amount of energy to take place by mechanochemical or by the combustion synthesis (CS) method due to the low amount of reaction enthalpy in Al–TiO2 system. In this study, Fe2O3 was chosen as a accelerator for this system, to generate a high amount of heat which could be released between Fe2O3 and Al, leading to a more convenient reaction between Al and TiO2 in the CS process. The results of XRD, SEM, and DSC analyses indicated that both the mechanical activation of Al–TiO2 system in a high-energy ball mill and the Fe2O3 addition led to considerable effects of reduction in the reaction temperature and increase in the reaction intensity in Al–TiO2 nanothermite system. Finally, it was shown that Fe3Al intermetallic compounds as well as γ-AlTi and alumina phases in the final products were formed after the CS of the milled powders.

Effects of Processing Parameters on the Microstructures and Mechanical Properties of In Situ (Al3Ti + Al2O3)/Al Composites Fabricated by Hot Pressing and Subsequent Friction-Stir Processing

In situ (Al3Ti + Al2O3)/Al composites were fabricated in an Al-TiO2 system by the combination of hot pressing (HP) and friction-stir processing (FSP). The effects of HP and FSP parameters on the microstructures and mechanical properties of the in situ composites were studied. The Al-TiO2 reaction extent increased with increasing the temperature and holding time of HP. Subsequent FSP not only induced Al-TiO2 reaction, resulting in the formation of nanosized Al3Ti and Al2O3, but also rounded the polygonal Al3Ti particles formed during HP. The tensile strength of the FSP sample in which the Al-TiO2 reaction took place completely during HP was lower than that of the FSP samples in which the Al-TiO2 reaction took place hardly or partly during HP. For the FSP samples in which the Al-TiO2 reaction took place hardly during HP, the volume fraction of reinforcing particles increased with decreasing the traverse speed, resulting in the increase in tensile strength of the FSP samples. At a traverse speed of 25 mm/min, increasing the rotation rate from 1000 to 2000 rpm produced little influence on the microstructures and mechanical properties of the in situ composites. Additional 2-pass FSP in water refined the matrix grains, resulting in the significant increase in the tensile strength.

Reaction pathways, activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures

In-situ aluminum matrix composites were fabricated from Al–TiO2–graphitic C powder mixtures using exothermic dispersion method. The effects of C/TiO2 molar ratio on the reaction processes, activation energies and mechanical properties of the resulting materials were investigated. When the C/TiO2 molar ratio is 0, Al reacts with TiO2 to produce fine α-Al2O3 particles and Ti, which then reacts with Al to form large rod-like Al3Ti phase. By adding graphite C into the Al–TiO2 system, the activation energy of the first reactive step increases; in addition, the resultant Ti preferentially reacts with C to form hard TiC particles. When the C/TiO2 molar ratio increases to 1.0, the Al3Ti phase disappears and the reinforcements consist of nano-sized α-Al2O3 and TiC phases. The tensile strength of the composites increases from 239.2 MPa to 351.8 MPa and the elongation increases from 4.1% to 5.6%, suggesting a marked increase in damage tolerance (i.e., toughness).

Reactive mechanism and mechanical properties of in situ composites fabricated from an Al–TiO2 system by friction stir processing

In situ (Al3Ti+Al2O3)/Al composites were fabricated from powder mixtures of Al and TiO2 using hot pressing, forging and subsequent multiple-pass friction stir processing (FSP). The reactive mechanism and mechanical properties of the FSPed composites were investigated. Four-pass FSP with 100% overlapping induced the Al–TiO2 reaction, as a result of the enhanced solid diffusion and mechanical activation effect caused by the severe deformation of FSP. Decreasing the size of TiO2 from 450 to 150nm resulted in the formation of more Al3Ti and Al2O3 particles. The formation mechanisms of Al2O3 and Al3Ti during FSP are understood to be a deformation-assisted interfacial reaction and deformation-assisted solution-precipitation, respectively, based on detailed microstructural observations. The microhardness, Young’s modulus and tensile strength of the FSPed composites were substantially enhanced compared with those of FSPed pure Al with the same processing history, and increased as the TiO2 size decreased from 450 to 150nm. The strengthening mechanisms of the FSPed composites included load transferring, grain refinement and Orowan strengthening, among which Orowan strengthening contributed the most to the yield strength of the composites.

Reaction Mechanism of an Al-TiO<sub>2</sub> System

The reaction mechanism of an Al-TiO2 system is discussed. Thermodynamic analysis indicates that the reaction between Al and TiO2 can occur spontaneously due to the negative Gibbs free energy of the Al-TiO2 reaction system. When the reinforcement volume fraction is 30%, there is an endothermic peak and an exothermic peak in the DSC curve. But when the reinforcement volume fraction increases to 100%, there are two independent exothermic peaks and the height of them increases obviously. With increasing the heating rate, the ignite temperature becomes higher and all the peaks move to the higher temperature direction. The reactions between Al and TiO2 consist of two steps: first, Al reacts with TiO2 to form the stable α-Al2O3 particles and the active Ti atoms; second, the active Ti atoms react with Al to form Al3Ti.

Investigation of a discus-milling process using a powder mixture of Al and TiO2

In this study, a powder mixture of Al and TiO2 was employed to investigate the milling process in a discus mill. In this first report on this novel mechanical mill, several variables, including the milling time and powder charge and their effects on the microstructural evolution of powder particles, are monitored and studied. The study reveals that the dominant parameters of the milling process are the milling time and the starting powder charge, similar to the other high-energy ball-milling processes. The longer the milling time and the smaller the starting powder charge, the more homogeneous the mixing and the finer the microstructure of the powder particles. The reaction between Al and TiO2 was not observed with a milling period as long as 6 hours, for the present materials. However, the reaction between Al and TiO2, during the subsequent heat treatment, is influenced by the milling condition. The powders with the longer milling times and finer mixing microstructures also form a finer microstructure, after the reaction between Al and TiO2 during heat treatment. The methods for achieving an optimal milling efficiency for the Al-TiO2 system are discussed.

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

Abstract In-situ processing offers significant advantages over conventional processing from both technical and economic standpoints. Al–TiO 2 is one of the interesting systems that has recently been receiving some attention. In this paper, the formation mechanism of Al 3 Ti and Al 2 O 3 from an Al–TiO 2 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-Al 2 O 3 are transitional phases, which form during the reactive process.