Kinetic study of an AA7075 alloy under RRA heat treatment

The kinetics of phase precipitation of an AA7075 alloy subjected to retrogression and reaging (RRA) temper were theoretically characterized in each of the stages of that treatment. Calorimetric measurements were used to determine the exothermic reactions promoting the precipitation process of phases coexisting in the temperature range from 200 to 450 A degrees C. Using deconvolution methods, the reactions were separated by means of Weibull transfer functions, and each deconvolved reaction was analyzed by combining isoconversion and linear regression methods. A kinetic analysis was performed through the Sestak-Berggren kinetic function, thus determining each kinetic parameter. The RRA temper in the temperature range studied required at least two reactions to have the minimum experimental-theoretical correlation condition set at R (2) ae<yen> 0.99, and three reactions to attain a value of R (2) ae<yen> 0.998. This study concerns deconvolution with three reactions, two, respectively, associated with the precipitation of phases eta' and eta; and a third phase containing Cu (S phase). The results permit to predict that the physical mechanism controlling the precipitation process during the stages of homogenization and retrogression is the migration of alloying elements that exploit quenching vacancy oversaturation, whereas the dominant mechanism during aging and reaging is the diffusion of Zn, Mg, and Cu through the matrix.