Microstructural evolution and mechanical behavior of ultrasonically synthesized Al6061-nano alumina composites

Al6061 alloy matrix composites reinforced with 1, 2 and 3 wt% nano-alumina particles are fabricated by ultrasound solidification technique. Application of high intensity ultrasonic vibrations to the melt resulted in uniform dispersion of alumina particles up to 2 wt% additions of reinforcement. Tensile strength of the composites increased while the ductility is retained as compared to the base alloy. Addition of nano-particles above this threshold value of 2 wt% led to poor distribution and clusters of alumina particles, along with thermal stress induced micro-cracks. Theoretical calculations and TEM studies revealed that thermal mismatch between the matrix and reinforcement contributed dominantly to the yield strength up to 2 wt% reinforcement additions, beyond which the thermal mismatch stresses led to formation of micro cracks. From among the various strengthening models, theoretical yield strength values calculated using the quadratic summation method showed good agreement with the experimental yield strength values for 1 and 2 wt% reinforced nano-composites.