Quantitative Investigation on Nano-Scale Precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr Alloy under Various Aging Tempers

Abstract

In the present study, a systematic study of nanoscale precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy have been carried out for various typical aging tempers, including T6, T76, T74, T73 and RRA treatments, by combining synchrotron-based small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Based on the TEM observations, quantitative and statistical structural information, including precipitate size, volume fraction, number density and inter-precipitate distance have been extracted from SAXS data through model fitting. The results show that the T6 peak-aged alloy with the smallest precipitate size has the highest number density and lowest volume fraction of precipitates. Under two-step T7X over-aged tempers, with the deepen of aging degree, the precipitate size, volume fraction and inter-precipitate distance increases, but the number density decreases. The size distribution of precipitates for the RRA-treated alloy is in between that of T6 and T76. The results also show that as the degree of over-aging deepens, the precipitate size distribution interval becomes broader.

Quantitative study of nanoscale precipitates in Al–Zn–Mg–Cu alloys with different chemical compositions

“…This can be attributed to the lower Cu content of 7B85 alloy compared with the other alloys. The lower Cu content is designed for a low sensitivity to quench rate [23,24]. This may affect the mechanical strength of the alloy in two ways.…”