Fabrication of high strength carbon nanotube/7055Al composite by powder metallurgy combined with subsequent hot extrusion

Bimodal carbon nanotube reinforced 7055Al (CNT/7055Al) composites containing coarse grain bands and ultra-fine grain zones were fabricated by high energy ball milling, vacuum hot pressing followed by hot extrusion. The effect of extrusion temperature varied from 320 degrees C to 420 degrees C on the microstructure evolution and tensile properties were investigated. Microstructure observation indicates that the elongated coarse grain bands aligned along the extrusion direction after extrusion. The width of the coarse grain bands increased, and the length of the coarse grain bands increased firstly and then decreased with the increase of extrusion temperature. The grain size of the ultra-fine grain zones changed little after hot extrusion, but the ultra-fine grains coarsened after subsequent heat treatment, especially for the composite extruded at low temperature of 320 degrees C. By observing the CNT distribution, it was found that the higher temperature extrusion was beneficial to the CNT orientation along the extrusion direction. Furthermore, a precipitate free zone formed at the boundary between the coarse grain band and the ultra-fine grain zone as the composite extruded at high temperature of 420 degrees C. As the result of the comprehensive influence of the above microstructure, the tensile strength of the composite extruded at moderate temperature of 370 degrees C reached the highest of 826 MPa.