Modeling phase transformation kinetics during homogenization of aluminum alloy 7050

Abstract

Microstructural evolution and the changing kinetics of the phase transformations during homogenization of Al-Zn-Cu-Mg-Zr alloys are studied. The phase transformations kinetics during the process is modeled using a novel approach, coupled with Thermo-Calc™. A model for evolution of interdendritic phases which predicts transformation of the as-cast η to S phase and subsequent dissolution of the S phase is coupled with a model for precipitation of dispersoids of Al3Zr dispersoids across the grains. The η to S phase transformation is predicted to be interface reaction rate controlled while all the other phase transformations are diffusion controlled. Based on the numerical study, which is verified through SEM, EDS, XRD and DSC measurements, a three step homogenization process for aluminum alloy 7050 (Al-6.2Zn-2.3Cu-2.35Mg-0.13Zr) is proposed. Homogenization at 420°C for 10h, followed by steps at 470°C for 4h and 480°C for 15h, precipitates numerous nanosized Al3Zr dispersoids and minimizes the volume of the η and S phases. This microstructure is attractive because it is easy to extrude and does not readily recrystallize. The model provides a cost effective tool for optimizing the heat treatment schedules for 7050 and similar 7XXX alloys.