Effect of Minor Cr, Mn, Zr or Ti on Recrystallization, Secondary Phases and Fracture Behaviour of Al-Zn-Mg-Cu-Yb Alloys

The effects of sole Cr, Mn, Zr, Ti addition on the precipitation of secondary phases (especially AlCuYb), matrix recrystallization behaviour and tensile intergranular fracture of a AlZnMgCuYb based alloy were investigated by tensile test, together with microstructural characterization including TEM, SEM, and EBSD technique. The results indicate that the precipitation of coarse micro-scaled AlCuYb phase is inevitable in the Al alloy containing Yb and Cu. It is interesting that the least coarse phases are formed in the AlZnMgCuYb-Mn alloy due to the precipitation of submicro-scaled Al20Cu2Mn3 phase, resulting in an effective decrease of coarse AlCuYb phases. The addition of Zr to AlZnMgCuYb alloy can effectively inhibit the recrystallization of a (Al) matrix by the formation of nano-scale coherent Al-3(Yb, Zr) dispersoid. However, the formation of AlCuYb phases consumes Yb, which will reduce the precipitation of secondary coherent Al-3(Yb, Zr) and the particle-stimulated partial recrystallization nucleation, resulting in a decline in the strength of AlZnMgCuYb-Zr alloy. T6-tempered AlZnMgCuYb-Zr and AlZnMgCuYb-Mn alloys still maintain an unrecrystallized fiber-like structure, and the fraction of low-angle grain boundaries (LAGBs) increases to 50% and the average grain size decreases to 2 similar to 7 mu m. However, more homogeneous recrystallization grains are observed in AlZnMgCuYb alloy containing Cr or Ti, and the fraction of high-angle grain boundaries (HAGBs) and the average grain size achieve 80% and 40 similar to 96 mu m, respectively. The primary 1 similar to 3 mu m Al2CuMg particles are preferentially cracked, rather than coarse AlCuYb, and cracks propagate along high-angle recrystallized grain boundaries or original grain boundaries with continuous or coarse grain boundary precipitates and broadening precipitate-free zones (PFZs) at its periphery.