Effects of Zinc on the RRA behavior of high purity Al-Zn-Mg-Cu alloy with different Zn content were investigated by measuring hardness, electrical conductivity changes and Stress corrosion crack growth rates.
Stress corrosion crack growth rates were measured as function of stress intensity factor by means of constant loading machine. The variation of microstructural, both in the matrix and at grain boundary, during Retrogression and Reaging (RRA) treatment was investigated using Transmission electron microscopy.
The GP zone solvus temperature was found to be increased from 165℃ in 5. 5wt%Zn alloy to 172℃ and 175℃ in 6.8wt%Zn, and 7.8wt%Zn alloy respectively. At the same time, the addition of Zn decreased the incubation time for the GP zone formation.
The RRA treatment on the T6 tempered alloys improves the SCC resistance without sacrificing their original maximum strength of all the alloys regardless of Zn content.
The addition of Zn content up to a certain amount substantially improves the effect of the RRA heat treatment, Particularly on retrogression below 220℃: the maximum retrogression time (within which the alloy can its recover maximum strength) increases on increasing Zn content from 5.5wt% to 6.8wt%. However a further increase in the Zn content rather decreased the maximum retrogression time.
The result of microstructural study suggests that the RRA heat treatment results in the formation of microstructure containing fine particle of high density.
The major role of Zn is believed to be in the increasing solvus temperature of GP zone and their nucleation rate. This effect thus influences the microstructural stability during the retrogression and the reading treatment.
The variation in the dispersion of grain boundary precipates, particularly in the volume fraction of grain boundary precipates, appeared to be closely to the Stress corrosion crack growth rate. The SCC Crack growth rate decreased with the increased in the volume fraction of grain boundary precipates. However the variation of Zinc content appeared not to significantly influence the volume fraction of grain boundary precipates.
The effect of the variation of Zn content is believed to be on the effect on the microchemistry near grain boundary.
