On the presence of super lattice intrinsic stacking faults in plastically deformed Ni3Al

Abstract

Abstract Superlattice intrinsic stacking faults (SISFs) have been observed in Ni3Al polycrystals deformed in compression between room temperature and 800°C. A detailed weak-beam analysis indicates that the SISFs originate at a screw partial with a Burgers vector of 1/2〈110〉. Starting from the stable dissociation into two partials with collinear Burgers vectors and with an antiphase boundary stabilized on {001}, one of the two 1/2〈110〉 partials may split under the effect of a high local stress into an edge Shockley and a partial with a Burgers vector of 1/2〈112〉. The latter produces a SISF as it escapes from its initial position in a {111} plane. Calculations of the total energy of the threefold dissociation shows the presence of a secondary minimum when the 1/3〈112〉 partial lies between 25 and 50 nm from the Shockley partial, in good agreement with the weak-beam observations. However, the SISFs exhibit large deviations from this equilibrium distance when they result from a deformation at low temperature and, in any case, they show a strong tendency towards serration, with segmentation of the 1/3〈112〉 partial along 〈110〉 directions. These features attest to high Peierls forces acting on the 1/3〈112〉 partials. The equilibrium shape of a SISF is described in terms of a thermally aided process: the higher the temperature, the larger the probability of recombination; deformation or annealing at an intermediate temperature should favour the presence of elongated SISFs with almost homogeneous width. Steps on SISFs have been revealed in weak-beam experiments; they result from the intersection of the SISFs by superdisloc-ations. It is shown that such events affect the equilibrium shape of the outer 1/3〈112〉 partial. Surface energies have been measured for several split configurations: the energies of the antiphase boundaries on {100} and {111} are 140 ±; 14mJm−2 and 180 ± 30 mJ m − 2 respectively, the SISF energy should lie between 5 and 15 mJ m −2.