Order-disorder changes in Fe3Al based alloys and the development of an iron-base alpha-alpha'' superalloy

Fe-Al alloys containing slightly less than 25% Al are a two phase mixture of disordered plus DO3-ordered phases at temperatures somewhat below 550 degrees C. The morphology and stability of microstructure has been examined in one such binary Fe-Al alloy when in the two-phase state, and compared with a ternary Fe-Al-Si alloy showing similar phase mixtures at similar temperatures. Phase changes in these alloys can be understood on the basis of the equilibrium phase diagram, including metastable phase boundary extensions and conditional spinodal decompositions to account for order changes occurring faster than chemical diffusional changes. Such two-phase microstructures are relatively stable against coarsening at these intermediate temperatures, particularly for the Fe-Al-Si alloy which is believed to be due to its lower diffusivity and smaller interface energy. Such two-phase alloys, particularly when the composition is carefully chosen for the required stability and ordered volume fraction at a given temperature, should represent an Fe-base b.c.c. equivalent to the Ni-base superalloys. Copyright (C) 1996 Acta Metallurgica Inc.