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

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.