Precipitation Kinetics of Aluminium Nitride in Austenite in Microalloyed HSLA Steels

Abstract

In this work, the thermodynamic information on aluminium nitride formation and experimental precipitation kinetics data are reviewed. A revised expression for the Gibbs energy of AlN is developed with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation kinetics are performed and compared to several independent experimental results from literature. To mimic the geometrical arrangement of AlN precipitates along austenite grain boundaries, a new model for precipitation at grain boundaries is used, which takes into account fast short-circuit diffusion along grain boundaries as well as the slower bulk diffusion of atoms from inside the grain to the grain boundaries. This is essential for the calculation of AlN precipitation in austenite where nucleation occurs predominantly on grain boundaries. By studying the AlN precipitation at grain boundaries numerically, and by comparison with experimental data, it is demonstrated that the precipitation kinetics of AlN differs significantly from the simulated precipitation kinetics of randomly distributed precipitates assuming spherical diffusion fields.