Effect of extrusion dies angle on the microstructure and properties of (TiB+TiC)/Ti6Al4V in situ titanium matrix composite

Forged 5 vol% (TiB+TiC)/Ti6Al4V titanium matrix composite billets with a heterogeneous reinforced structure was successfully hot indirect extruded with different dies angle (θ=45°, 60° and 75°) at 1303 K. The dies angle was found to work as one key factor to change the microstructures and mechanical properties. Detail investigation on the microstructural evolution revealed that the significant grain refinement was obtained after extrusion due to dynamic recrystallization, the fully dynamic recrystallization takes place during indirect extrusion at dies angle 75°. In situ TiB and TiC reinforcements were also significantly refined by the indirect extrusion process, while highly developed preferred orientation of TiB short fibers along extrusion direction. Meanwhile, the mechanical properties showed that the ultimate tensile strength, yield strength and elongation exhibited a simultaneous increase after the indirect extrusion, especially on the ductility of the materials. The strength was slightly improved but not too much by hot extrusion. However, both tensile strength and elongation to fracture were reduced as the extrusion dies angle enlarged from 45° to 75°. Higher strength and elongation were obtained with optimal dies angle 45°, confirmed by matrix refinement, dynamical recrystallization and the optimal aspect ratio of TiB short fibers. Therefore, the optimal hot extrusion with the extrusion dies angle should be controlled less than 60°.