Non-isothermal retrogression kinetics for grain boundary precipitate of 7A55 aluminum alloy

The retrogression kinetics for grain boundary precipitate (GBP) of 7A55 aluminum alloy was investigated by transmission electron microscopy (TEM) observation. The results reveal that the coarsening behavior of GBP obeys “LSW” theory, namely, the cube of GBP average size has a linear dependence relation to retrogression time, and the coarsening rate accelerates at the elevated retrogression temperature. The GBP coarsening activation energy Qd of (115.2±1.3) kJ/mol is obtained subsequently. Taking the retrogression treatment schedule of 190 °C, 45 min derived from AA7055 thin plate as reference, the non-isothermal retrogression model for GBP coarsening behavior is established based on “LSW” theory and “iso-kinetics” solution, which includes an Arrhenius form equation. After that, the average size of GBP r(t) is predicted successfully at any non-isothermal process T(t) when the initial size of GBP r0 is given. Finally, the universal characterization method for the microstructure homogeneity along the thickness direction of 7A55 aluminum alloy thick plate is also set up.