Microanalysis of Precipitate Free Zones (PFZ) In Al-Zn-Mg and Cu-Ni-Nb Alloys

Abstract

Due to their possible detrimental effect on the stress corrosion and mechanical properties, Precipitate Free Zones (PFZ) in precipitation hardening alloy systems have been the subject of several investigations. Earlier studies by Nicholson and other workers convincingly demonstrated that PFZ can form as a result of vacancy concentration profile near the grain boundaries (1,2). Based on these and other investigations, PFZ in alloys are generally attributed as due to either solute depletion or vacancy depletion. Recently, several investigations have applied microanalytical techniques (plasmon loss, electron energy loss and X-ray microanalysis) to understand the solute profile in the PFZ particularly in the Al-Mg and Al-Zn-Mg systems (3,4). The present investigation was conducted to provide a better understanding of the formation of PFZ in alloys by analyzing the solute profiles in the PFZ by X-ray analysis in two alloys (5). One was an Al-2.2% Zn-4.7% Mg* alloy which has been demonstrated to form PFZ due to the vacancy concentration profile (1,2). The other alloy is a Cu-30% Ni-0.9% Nb alloy in which the PFZ was suggested to form due to depletion of Nb (6). Samples of these alloy samples were examined after selected heat treatments.