Second Step Aging on Nanosized Precipitates and Properties of Al–Zn–Mg–Cu–Cr Spray-Deposited Alloys

The effects of second step aging (T76, T74, T73) on nano-sized precipitates and properties of Al–Zn–Mg–Cu–Cr spray-deposited alloys were explored through tensile tester, impact testing machine, metallographic microscope (OM), eddy-current device, scanning electron microscopy (SEM), twin-jet electro-polishing machine and transmission electron microscopy (TEM). Fine grain size (compared with as-deposited billet) and directional microstructures were obtained. T76 heat treatment of the alloy provided higher tensile strength, yield strength, impact toughness and hardness which were 767 MPa, 708 MPa, 39.41 J/cm 1/2 99.1 HRB, respectively in comparison with T74 and T73 samples. However, they provided lower elongation and electrical conductivity which were 7.6% and 31.1% IACS, respectively in comparison with T74 and T73 samples. This resulted from the larger quantity and volume of tiny η ′ precipitates that distribute homogeneously in matrix. However, coarse precipitates with increasing second step aging time (T74, T73) made wider grain boundary width and discontinuous η precipitates boosted conductivity of the Al–Zn–Mg–Cu–Cr alloy. Furthermore, proportion of white precipitated phase in the matrix decreased slightly and volume became larger with increasing second step aging time.