Atomic scale observation of the displacement fields generated by the intrinsic defects of {111}Ni3Al/(010) Ni3Nb close-packed interfaces

The deformation generated by the intrinsic structure of facetted close-pack ed (11(1) over bar)Ni3Al//(010)Ni3Nb interfaces has been observed by high r esolution electron microscopy. The interfaces separate Ni3Al (gamma') and N i3Nb (delta) lamellae of a directionally solidified eutectic alloy. Three t ypical interfacial regions, I, II and III, have been examined. In region I a series of ten consecutive linear singularities has been analysed. Six of them, denoted A-F, have different core structures. They accommodate fully o r partly the 2.1% length misfit according to the sequence denoted A-A-A-B-C -A-D-E-A-F. Of these singularities, A, B and C are dislocation ledges, wher eas D and E are linked to a triple node junction involving gamma', delta, a nd an interfacial gamma' nanotwin. F limits an interfacial fault. Three typ es of Burgers vector content have been found (relative to the yl lattice): (i) 90 degrees, (1/6)[(112) over bar], for A, D E, F; (ii) 30 degrees, (1/6 )[<(2)over bar 1(1)over bar>], for B; and (iii) 60 degrees, (1/6)[<(41)over bar>1] or (1/6)[<(14)over bar>1], for C. This latter contributes to accomm odating an angular misfit. Only the 90 degrees Burgers vector content belon gs to the gamma'-DSC lattice. It is inferred that the observed heterointerf ace consists of at least four different interfacial regions with different energies. In regions II and III the heterointerfaces exhibit other complex atomic structures involving gamma' interfacial Sigma 3 nanotwins. In region II a linear singularity linked to a triple node junction has a Burgers vec tor content of (1/2)(110), and accommodates partly an angular misfit. In re gion III three consecutive linear singularities have the same 90 degrees Bu rgers vector content, despite quite different environments. One of them int eracts with a nearby 60 degrees gamma' superpartial so that the total dislo cation content is (1/6)[<2(15)over bar>] or (1/6)[(1) over bar 2 (5) over b ar]. It is concluded that the determination of the linear singularity struc tures at close-packed heterointerfaces is a more subtle and intricate quest ion than apparently has been realized so far.