Dislocation structures in disordered and ordered Ni 3Fe

Abstract

Transmission electron microscopy (TEM) methods have been used to examine the dislocation structures in thin foils of Ni 3Fe in four different states, corresponding to disordered and deformed; fully ordered and deformed; deformed when disordered and afterwards fully ordered; and deformed when disordered, afterwards fully ordered and additionally deformed. The study has been carried out on single crystals deformed at room temperature. In the disordered alloy slip is coarse and group motion of dislocations is prevailing, as is confirmed by the abundance of planar dislocation arrays. The dislocation structure of such a disordered deformed crystal remains unchanged after additional ordering by annealing 1000 h at 460°C. No rearrangement of the unit dislocations into superlattice dislocations is observed. The dislocations are preserved and since they are unit dislocations they are sessile. The additional deformation of this disordered deformed and afterwards fully ordered crystal proceeds by the glide of superlattice dislocations. They originate in the high internal stresses of the preserved unit dislocations. Cross slip from (111) onto (11̄1) planes is very frequent, whereas cross slip from (111) onto (010) planes is rather rare. The structure of the superlattice dislocations in fully ordered and deformed specimens consists of dipoles and bundles of dipoles of near edge orientation. Superlattice dislocations of near screw orientation are rarely observed, since they cross slip from (111) onto (11̄1) planes and annihilate in most cases. The experimental results on ordered Ni 3Fe samples differ characteristically from those reported in the literature on other alloys having the L1 2 long range ordered structure (e.g. Ni 3Al, Cu 3Au, Ni 3Ga).