Microstructures and mechanical properties of AlCoCrFeNi2.1/6061-T6 aluminum-matrix composites prepared by friction stir processing

Abstract

A novel 6061-T6 aluminum matrix composites reinforced by AlCoCrFeNi2.1 high-entropy alloy (HEA) were obtained using friction stir processing (FSP), and the resultant composites were further strengthened by T6 heat treatment. The effects of reinforcement phase content on the microstructure and mechanical properties of composites were investigated. The results indicate that AlCoCrFeNi2.1 HEA is suitable as a reinforcement material to strengthen the aluminum matrix, which effectively suppressed the formation and growth of intermetallic compounds unlike selecting pure metal particles as reinforcements. As the content of HEA reinforcement increased, the recrystallization degree of composites increased, while the grain size decreased. The continuous and dense interface zone shows sound metallurgical bonding between the aluminum alloy matrix and HEA particles. Owing to the low processing temperature of FSP, the drastic chemical reaction can be effectively avoided, and there were no intermetallic compounds formed at the interface. The hardness and tensile strength of the composites improved with the reinforcement content. Compared with friction stir processed-6061, the hardness and tensile strength of the composites with 15% HEA increased by 28.6% and 25.6%, which reached 72 HV and 222.9 MPa respectively. After T6 aging heat treatment, the hardness and the tensile strength reached 125 HV and 332.1 MPa, further improved by 73.6% and 49.0%, respectively. It shows that Hall-Petch and thermal mismatch strengthening played the significant positive influences to strengthening, and Orowan strengthening made the least contribution. The findings can promote the wide application of aluminum-matrix composites in the automotive, marine and aerospace industries.