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

Abstract

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/cm1/2 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.