Abstract
The morhpology of the ordered and disordered domains of the α+B2 phase field in Fe3Al alloys has been investigated by means of electron microscopy, and the process is discussed with respect to the phase separation into two phases in combination with the results of our previous electron microscopy. The alloys of the composition within the α+B2 phase field are decomposed by isothermal annealing in this region after quenching from the disordered α or the ordered B2 single phase field. In the alloys in the vicinity of the phase boundaries leading to the transition α→α+B2 or B2→α+B2, phase separation is observed to take place by the nucleation and growth of the B2 domains in the α matrix or the α in B2. In the latter transition, the α-free zone is formed along the B2 type APB’s. In the central region within the α+B2 phase field we observed a microstructure which resembles the morphology of a spinodal decomposition in an isotropic system. This structure may be interpreted as the growth of fluctuations in the local composition and degree of order in the transition B2→α+B2, or as the diminution in the reverse case. In the case of nucleation and growth, the characteristic strain field contrast of coherent precipitates is observed due to the difference in the lattice parameter between α and B2 phases.
