Enhanced grain refinement and mechanical properties of a high-strength Al-Zn-Mg-Cu-Zr alloy induced by TiC nano-particles

Grain refinement of Zr-containing Al alloys has attracted extensive attention for many years in both industry and academia, since "Zr-poisoning" phenomenon always happens when commercial refiner is applied in such alloys. This unrecoverable grain nucleating obstacle finally leads to poor grain refinement efficiency. In this work, a special Al-Ti-C master alloy rich in TiC nano-particles was applied to a high-strength Al-Zn-Mg-Cu-Zr alloy (i. e. 7050 alloy). The effects of TiC nano-particles on the microstructure and mechanical properties of 7050 alloy during solidification, hot extrusion and subsequent heat treatment were systematically investigated. The results show that prominent grain refinement occurs with TiC addition through both the growth restriction effect and heterogeneous nucleation. Meanwhile, TiC nano-particles are more inclined to distribute along grain boundaries than the interior of matrix grains after solidification. After subsequent homogenization and hot extrusion, TiC nano-particles tend to align along extrusion direction. In comparison with 7050 matrix alloy, the tensile properties especially ductility of 1% TiCp/7050 composite after heat treatment have been improved obviously. Furthermore, it is worth noting that precipitates in the composite have also been obviously refined. Synergetic effects of grain refinement, refined precipitates and increased dislocation density are responsible for the optimized mechanical properties of TiCp/7050 composite. This work provides new insights for controlling matrix grains of Zr-containing Al alloys and developing low-cost Al matrix composites.