Annealing out of quenched-in vacancies in an ordered B2 type Fe-Al single crystal

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Abstract
The annealing out of quenched-in vacancies has been studied, by transmission electron microscopy, in an ordered B2 type Fe-42 at.% Al single crystal. Slices of 〈111〉 and 〈001〉 orientation were quenched from 1000deg;C and annealed at 400deg;C for various times. In situ annealing has also been performed in a high-voltage electron microscope. The annealing process was found to be essentially heterogeneous, starting on grown-in dislocations. Vacancies promote the dissociation of dislocations of Burgers vector 〈100〉 into a series of partial loops of Burgers vector 1/2〈111〉. The loops are paired and separated by antiphase boundary. In order to minimize the energy the partials combine to give new dislocations of Burgers vector 〈100〉 and the process repeats giving rise to ‘bundles’ of dislocations which grow in the remaining part of the dislocation-free sample. The ‘bundles’ spread throughout the sample to produce, at the end of annealing, a high density of dislocations of Burgers vector 〈100〉. This peculiar mechanism, which is different from that encountered in pure metals, is thought to be due to the difficulty of nucleating a vacancy cluster to promote vacancy elimination in strongly ordered Fe-Al alloys.