EBSD Analysis On Texture And Microstructure Evolution Of MgAl₂O_4w/reinforced 6061 Al Composites In Hot Extrusion And Recrystallization

Discontinuously reinforced aluminum matrix composites?AMCs?are widely used in structural parts especially in aerospace and transportation industries.The reinforcement of AMCs,in-situ fabricated though a powder metallurgy method,was distributed uniformly in the matrix and had a good interfacial bonding with the matrix,guaranteeing the effect of reinforcement.Moreover,MgAl₂O₄ spinel whisker was an ideal reinforcement because of its high strength,high modules and high thermo stability.Meanwhile,MgAl₂O₄ spinel whisker had a very low mismatch with the aluminum matrix,which is beneficial to the interfacial bonding with the matrix.The reinforcements in AMCs have crucial influence on the microstructure evolution in thermomechanical processes and the mechanical properties at elevated temperatures.Herein,6061 Al powder,Mg powder and H₃BO₃ powder were used as raw materials,and in-situ synthesized MgAl₂O₄ spinel whisker reinforced 6061 Al composites were fabricated by hot-press sintering followed by hot-extrusion.The influence of ball milling process on the morphology of whiskers was investigated,and mechanical properties were tested.EBSD examine was carried out to analyze the microstructure and texture evolution of the composite samples during hot-extrusion and annealing.The results showed that using two-step ball milling process can improve sintering ability of the composites.In-site synthesized MgAl₂O₄ spinel whiskers had an average diameter of 30-100 nm,and the aspect ratio ranged from 10 to 30.The MgAl₂O₄ spinel whiskers were uniformly distributed in the 6061 Al matrix,and had a good interfacial bonding with the matrix.The UTS of 5 wt.%-whisker composite reached 285 MPa,and its elongation was 9.4%.A partially recrystallized grain structure was identified after hot-extrusion.The extruded samples were annealed at530°C,580°C,600°C and 630°C for 1 h and the microstructure of the annealed samples showed little difference at 530°C,580°C and 600°C.But the grains of the sample annealed at 630°C grew coarse sharply.Compared with the 6061 Al alloys,the MgAl₂O₄ spinel whisker reinforced 6061 Al composites showed great recrystallization resistance,which was caused by Zener's drag of MgAl₂O₄ spinel whiskers.The texture component hardly evolved with the annealing temperature,but the volume fraction of texture first decreased and then increased with increasing the annealing temperature.Compared with as-extruded samples,the UTS of all annealed samples declined,but the UTS dropped slightly when samples were annealed at530°C,580°C,600°C,indicating the obtained composites possessed good resistance to elevated temperatures.