κ‐Phase Formation in Fe–Mn–Al–C Austenitic Steels

Abstract

In the present work, a combined approach of ab initio theoretical calculations and in situ synchrotron X‐ray diffraction is used to investigate the κ‐phase formation in Fe–Mn–Al–C austenitic steels. In order to understand the driving forces for short‐ and long‐range ordering in the Fe–Mn–Al–C steels, a fully systematic approach is used for DFT calculations. A weak Al–Al as well as a strong Al–C repulsion in austenite matrix is established which both gradually decrease with increasing interatomic distance. An exception is the Al–C–Al configuration that also exists identically in κ‐phases. It is found that, in an austenite matrix with Al contents exceeding 25%, the ordered κ‐phase has a higher thermodynamic stability than the disordered κ‐phase. The synchrotron X‐ray diffraction results further reveal that the long‐range ordered κ‐phase already starts precipitating from the austenitic matrix within the first 15 min during aging at 600 °C in Fe‐30Mn‐8Al‐1.2C austenitic steel. The lattice misfit between the ordered κ‐phase and the austenite matrix expands upon aging time. Up to 9 h aging, however, the lattice misfit still maintains being very small (less than 2%) which leads to an effective coherent precipitation hardening in the material.