Characterization of η′ Precipitates and Mn Dispersoids in Al–Zn–Mg–(Mn) Alloys Using Small-Angle Neutron Scattering and Electron Microscopy

Three Al–Zn–Mg–(Mn) alloys with 0, 0.6, and 1.0 wt pct Mn were thermomechanically processed from cast billets to wrought aluminum sheet using hot extrusion and cold rolling. Small-angle neutron scattering (SANS) experiments, performed in conjunction with electron microscopy characterization, allowed detailed microstructural characterization on sample sizes orders of magnitude larger than possible with microscopy alone. Accordingly, robust assessment of microstructural changes after the different stages of thermomechanical processing were possible for two distinct populations of precipitates and dispersoids differing in size by a factor of ~ 20. Size distributions and volume distributions of sub-micron Al₆Mn and nano-sized Mg–Zn η′ precipitate phases were obtained from SANS data and compared with those estimated from Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The SANS derived η′ size distributions and volume fractions were found to be similar for the alloys, suggesting that the prior precipitated Al₆Mn dispersoids did not interfere with the precipitation of the η′ phase. The TEM derived size and volume distributions for the η′ phase were found to be in good agreement with those derived from SANS. The agreement between SANS and SEM derived size and volume distributions for the Al₆Mn dispersoids was less robust. The anisotropy in the SANS data allowed confirmation of a preferred orientation of Al₆Mn dispersoids with extrusion/rolling direction, noted in microscopy observations, on the much larger spatial extent typical of SANS.