KINETIC STUDY OF PHASE PRECIPITATION OF AN AA7075 ALLOY UNDER T6 AND T7 TEMPER

Abstract

T6 and T7 tempers are used to reinforce the mechanical strength and resistance to stress cracking of the aluminum alloys of the 7000 series .This is achieved because such treatments regulate the dissolution and precipitation of various precipitates which varies the volume fraction inside and the grain boundaries. Hitherto have been used DSC measurements to characterize the different exo and endothermic transformations, which are associated With precipitation and dissolution of phases that Occur during that process, relapsing on the reaction peak values the principal weight of the characteristic thermal analysis. While this is true on simple reactions, on typical complex reactions of multicomponent alloys these peaks are the result of numerous processes. So far in the literature best experimentally defined peaks correspond to phase dissolution reactions, being frequently reported their kinetic parameters, but not for the parameters of the precipitation reaction. Hence it aims to study the kinetics of precipitation of phases η',η and S on an AA7075 alloy subjected to the T6 and T7 tempers through deconvolution of the peaks obtained by DSC. Asymmetric transfer functions of Weibull and the Sestak-Berggren kinetic model combined with the isoconversion method to determine the kinetic parameters of each reaction are used. Through these parameters it is established that the pattern of precipitation of the η phase is similar for both temper, but not for the h and S phases, being the physical mechanism responsible of precipitation of these phases predominantly the migration of alloying elements favored by the initial pre-precipitation of Guinier Preston zones, serving as precursor of subsequent processes.