Thermomechanical synthesis of hybrid in-situ Al-(Al3Ti+Al2O3) composites through nanoscale Al-Al2TiO5 reactive system

Abstract

In this work, nanostructured aluminum titanate (Al2TiO5 or AT) was synthesized by the citrate sol gel method. Then, different volume fractions of this ceramic were blended with Al powder through different durations of the high-energy vibratory milling. The effect of mechanical milling on the thermal degradation of AT in exposure to Al and formation mechanism of in-situ Al2O3 and Al3Ti particles were explored in three conditions: (i) in the powder form; (ii) after annealing of green compact; and (iii) after hot extrusion. In the powder form, it was shown that the mechanical milling is able to significantly diminish the thermal stability of AT, so that the required temperature for the Al3Ti formation decreases from 600 to about 450 °C. Also, by extending the mechanical milling duration, the contact surface for interfacial reactions between Al and AT increases and the formation of Al3Ti and Al2O3 is facilitated. By selecting the optimum duration of mechanical milling, this study successfully fixed the microstructure problems of not-milled as-annealed composites i.e. unreacted AT particles and oxygen evolution-arisen cracks, and produced in-situ composite inside each Al particle. However, the induced work hardening of these milled particles hinders the perfect consolidation. To deal with this challenge, the hot extrusion at 535 °C was conducted. The obtained microstructure was composed of very fine (≤200 nm) semi-spherical Al3Ti and Al2O3 particles uniformly distributed within the Al matrix. To evaluate the mechanical response of the fabricated nanocomposites, the micro-hardness and nanoindentation testing were carried out. The results proved that the hardness and elastic modulus of the nanocomposites reach 143 HV and 90 GPa, respectively, indicating a significant improvement in the mechanical properties of these systems compared to not-milled extruded composites. Finally, the role of reinforcing particles in strengthening of composite was evaluated.