Effects of the coherency of Al₃Zr on the microstructures and quench sensitivity of Al–Zn–Mg–Cu alloys

Effects of the coherency of Al₃Zr on the microstructures and quench sensitivity of Al-8.3Zn-2.3Mg-2.4Cu (AA7055) alloys were investigated. These alloys were subjected to two cold-rolling ratios (i.e., 5% and 20%) in order to vary the coherency of Al₃Zr particles. The main objectives of this study were to reduce the quench sensitivity, refine the grain size, and retain the coherency of Al₃Zr for high-strength Al–Zn–Mg–Cu production alloys. Results reveal that with the increase in the cold-rolling ratio, the quench sensitivity of the Zr-containing alloys increases. At a high cold-rolling ratio, the interface between Al₃Zr particles and the matrix became semi-coherent after solution treatment. This semi-coherent interface led to the heterogeneous nucleation of solute atoms during quenching, which was the leading cause for the high quench sensitivity. By contrast, at a low cold-rolling ratio, the alloys did not exhibit any noticeable quench sensitivity. The Al₃Zr particles retained coherence in the matrix and did not afford heterogeneous nucleation sites. On the other hand, regardless of the cold-rolling ratio, the alloys without Zr did not exhibit a high quench sensitivity. However, from the electron backscatter diffraction (EBSD) analysis, the grain size became coarser, and the recrystallization fraction was higher for alloys without Zr.