Phase equilibria and stability of ordered BCC phases in the Fe-rich portion of the Fe–Al system

Phase stability of the BCC phases with A2, B2 and D03 structures and phase equilibria among these phases in the Fe-rich portion of the Fe–Al binary system were examined using diffusion couples, transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS) and Vickers hardness (VH) measurement. TEM observations of the diffusion couples annealed at the temperatures between 300 and 700 °C were performed to identify the crystal structure of the ordered phases and to measure the average size of the B2 and D03 ordered domains. Phase boundaries between A2 and B2 or D03 phases were then evaluated by comparison between the concentration profile and the profiles of the ordered phase domain size or Vickers hardness. It was clarified that the present results in the high temperature range over 450 °C are in agreement with the previous data, while the A2+D03 two-phase field extends beyond the A2/D03 phase boundary in the currently available phase diagram into a lower Al range at temperatures below 450 °C. The A2→B2, B2→D03 and para→ferro continuous ordering temperatures were also measured by differential scanning calorimetry (DSC).