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-packed (11(1) over bar)Ni3Al//(010)Ni3Nb interfaces has been observed by high resolution electron microscopy. The interfaces separate Ni3Al (gamma') and Ni3Nb (delta) lamellae of a directionally solidified eutectic alloy. Three typical 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 or 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, whereas D and E are linked to a triple node junction involving gamma', delta, and an interfacial gamma' nanotwin. F limits an interfacial fault. Three types 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 accommodating an angular misfit. Only the 90 degrees Burgers vector content belongs to the gamma'-DSC lattice. It is inferred that the observed heterointerface 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 vector content of (1/2)(110), and accommodates partly an angular misfit. In region III three consecutive linear singularities have the same 90 degrees Burgers vector content, despite quite different environments. One of them interacts with a nearby 60 degrees gamma' superpartial so that the total dislocation content is (1/6)[<2(15)over bar>] or (1/6)[(1) over bar 2 (5) over bar]. It is concluded that the determination of the linear singularity structures at close-packed heterointerfaces is a more subtle and intricate question than apparently has been realized so far.