Theory and direct observation of dislocations in the Fe 3Al superlattices

Abstract

It has been found that in order for dislocations to travel through superlattices of the type D0 3, L2 1 and B2 without creating any net disorder across the slip plane, they must move as superlattice dislocations. A theoretical analysis of the superlattice dislocations in the D0 3 and L2 1 type structures shows that they should consist of four 1 2 a 0 〈111〉 type ordinary dislocations bound together by two different type antiphase boundaries. On the other hand, superlattice dislocations in the B2 structure consist of pairs of coupled dislocations. Calculations have been made of the separation between the superlattice dislocations for the specific case of the Fe 3Al alloy exhibiting both D0 3 and B2 type ordered lattices. It has been found that for this specific alloy, the antiphase boundary energy associated with the superlattice dislocations is relatively low, resulting in a large extension of the individual dislocations constituting the superlattice dislocation. Because of this low energy, the possibility is suggested that dislocations might move through the Fe 3Al lattice as ordinary 1 2 a 0 〈111〉 types. In order to determine experimentally whether or not dislocations travel through the lattice as superlattice dislocations or as ordinary dislocations, for the particular Fe 3Al alloy possessing both D0 3 and B2 type structures, thin foils of the alloy were examined using transmission electron microscopy. It was indeed found that the dislocations in this alloy travel as ordinary dislocations, leaving behind on their slip plane ribbons of antiphase boundaries. In addition, a preponderance of screw dislocations were observed to be present, which in turn gives rise to wavy or noncrystallographic type slip behavior.