Disordering behaviour of alloys based on Fe3Al

Iron aluminides containing about 28 at.% Al have the D03 ordered structure in equilibrium below about 550°C and the B2 ordered structure above that, with the disordered state existing at temperatures above about 900°C. The transition from the D03 state to the B2 state has been examined for several alloys containing various alloying additions using X-ray diffraction, differential scanning calorimetry and in situ transmission electron microscopy. Some of the alloying additions investigated lead to changes in both the kinetics of ordering and disordering as well as modifying the temperature range of stability of the D03 state. While the alloys examined lose most of the D03 order at the critical transition temperature, there is clear evidence of remnant long range order for temperatures up to 100–200°C above this critical temperature, at least for some of the more complex alloys. The retention of this order may be caused solute remaining on specific lattice sites after disordering of the majority Fe and Al atoms.