The measurement of grain boundary particle distributions in aged Al-Zn-Mg alloys

Abstract

Three alloys based on 7005 have been studied in the peak-aged condition and after a retrogression and reaging (RRA) treatment in an attempt to change the distribution of the grain boundary precipitates. Transmission electron microscopy of thin foils of known thickness was employed to characterize the grain boundary particle distribution. Average sizes (d) and deviations (sigma) were calculated and found to vary widely from boundary to boundary, sigma being smallest where d was smallest. The number of particles per unit area of boundary (N(A)) also varied widely, its value decreasing as d increased, with a discontinuity at d almost-equal-to 65 nm. A reason for this discontinuity is suggested. When d > 70 nm, particle coarsening is not accompanied by any significant change in the average surface-to-surface spacing, suggesting that any RRA treatments aiming to enhance fracture toughness via this mechanism will not be successful in this system.