Microstructural evolution and mechanical behavior of in situ synthesized MgAl₂O₄ whiskers reinforced 6061 Al alloy composite after hot extrusion and annealing

In situ synthesized MgAl₂O₄ whiskers reinforced 6061 Al alloy composites were fabricated by a powder metallurgy method. Nano-sized MgAl₂O₄ spinel whiskers distributed uniformly in 6061 Al alloy matrix without any interfacial product and laid along the extrusion direction after hot extrusion. As-extruded composite samples were annealed at 530, 580, 600 and 630 °C for 1 h and examined by electron backscattered diffraction to investigate the recrystallization behavior of obtained composites. Microstructure and texture evolution was investigated, and it is found that the recrystallization process is inhibited by whiskers because of Zener pinning. Slender fibrous grains remain until 630 °C, and deformation textures still exist even in specimen annealed at 630 °C. Hardness and tensile tests show that the completely recrystallized sample has an ultimate tensile strength of 218 MPa and a hardness of HV 52 compared with 283 MPa and HV 69 of the unannealed sample. Mechanical properties of the annealed samples decline slightly with annealing temperature increasing up to 600 °C.