G. Saada

The dissociation of a screw superdislocation in the L12 structure

Abstract The transition by cross-slip from an infinite superdislocation fully dissociated in the primary octahedral slip plane to the Kear-Wilsdorf (KW) configuration, which results from dissociation in the cube cross-slip plane, is examined analytically in the approximation of linear anisotropic elasticity. In the absence of lattice friction, a twofold (or a manifold) configuration that straddles the cube and the octahedral planes is always unstable. Lattice friction may temporarily freeze twofold configurations, but there is no mechanical driving force that tends to favour a nonplanar configuration, irrespective of the application of an external stress. The preference for either of the two planar stable configurations, which is controlled by the antiphaseboundary (APB) energy ratio on these planes, z = γo/γc, and by the deviation from isotropic elasticity, can be modified by an appropriate choice of the shear stresses resolved on the two planes under consideration. The role of the subdissociation of eac...

Twinning nucleation in Cu-8 at.% Al single crystals

Abstract Transmission electron microscopy observations of the dislocation substructure in Cu-8at.% Al single crystals deformed in tension have been carried out to elucidate the origin of the twinning dislocation which produces conjugate twinning in this alloy. It is argued that the ‘correct’ Heidenreich-Shockley dislocation is obtained in the reaction of the primary dislocation with the faulted (Frank) dipole. These interactions are expected to occur frequently during the deformation. Different configurations of the twinning sources are analysed with respect to the nucleation and the growth of the twin. Geometrical issues related to the pinning of the component dislocations at the nodes formed, the motion of the single twinning dislocation and the non-coherent twinning front and the passing barriers encountered during twin growth are analysed. It is shown that there are no geometrical and energetic obstacles to the development of a macroscopic twin by the pole mechanism from the source proposed here.

A simulation of dislocation dynamics and of the flow stress anomaly in L12 alloys

Abstract This work examines the flow stress anomaly of Ll2 alloys by means of a mesoscopic two-dimensional simulation of dislocation dynamics. The basic properties modelled are slip in the octahedral plane, the conditions at which screw dislocation segments are locked by formation of Kear-Wilsdorf locks and subsequently unlocked, and the mobility of jogs in the cube plane. The range of temperatures investigated varies between 200 and 600 K, scaling with the domain of anomaly of Ni3Al-based alloys. The simulations indicate that strain is mostly provided by the sliding of kinks. Two conditions are simultaneously required in order to reproduce the flow stress anomaly: firstly kink mobility should be hindered via the dragging of jogs and secondly, irrespective of the probability of locking, locks should not be destroyed easily. The simulations suggest, in addition, that two different flow stress regimes take place in the temperature domain of the stress anomaly. At the onset of the anomaly, the flow stress is...

Stacking fault energy in short-range ordered γ-phases of Ni-based superalloys

Several aspects on the stacking fault energy (SFE) in short-range ordered -phase model alloys of new nickel based superalloys are discussed. Using transmission electron microscope weak beam observations and computer simulated images of dissociation widths, the SFE was determined as a function of temperature. The values for both alloys are close (31 4m J m −2 for the Re-containing alloy and 28 6m J m − 2 for the Ru-containing alloy). In both cases, the dissociation widths remain quite constant up to 350 °C and a slight decrease is observed at higher temperature. The deformation micromechanisms and the constancy of the SFE up to 750 °C are analysed in connection with short-range order. The deformation at low temperature is characterised by dislocation pile ups, the influence on the dissociation widths of the internal stress due to such configuration is analysed. The calculation reveals a strong effect on the leading dislocation of the pile-up whose dissociation distance can be reduced at most by 50%.

Simulation of the plastic flow in Ni3Al: Work hardening and strain-rate sensitivity

Abstract The origin of the anomalous temperature dependence of the giant work-hardening rate and of the modest strain-rate sensitivity observed in many L12 alloys is investigated by means of an ‘end-on’ simulation of dislocation dynamics in Ni3Al. The present model reproduces most of the atypical mechanical properties observed experimentally. The study of the evolution of the distribution of the Kear-Wilsdorf locks during plastic deformation indicates that the order of magnitude of the work-hardening rate stems from a preferred exhaustion of the weakest locks. In addition, the low strain-rate sensitivity measured is found to rely on the absence of a correlation between the dislocation properties which provide the plastic strain and those which determine the flow stress level.

The formation of antiphase-boundary tubes in Ni3Al

Abstract Based on the observation of antiphase-boundary (APB) tubes connected to mixed superdislocations in Ni3Al, the hypothesis that APB tubes are trailed behind moving kinks is explored. Evidence is given to show that APB tubes originate in particular at the junction between a kink and a Kear-Wilsdorf lock. The mechanism proposed here for the formation of APB tubes involves the local relaxation by short-range athermal climb motion of the connecting jog; once this is achieved, a straight APB tube is trailed conservatively by the moving kink.

Role of internal stresses in co-deformed two-phase materials

The mechanical response of two-phase materials during plastic co-deformation is examined with a particular emphasis on the role of the internal stresses developed during deformation. A framework is developed to describe the accumulation of these internal stresses during plastic co-deformation in terms of a small number of physical parameters. This simplified framework is used to rationalize the observed behaviour of copper–chromium and copper–niobium two-phase materials in terms of their monotonic strain hardening rates, the response to changes in path (tension/compression) and their dimensional stability on heating.

In situ straining investigation of slip transfer across α2 lamellae at room temperature in a lamellar TiAl alloy

Processes by which deformation spreads throughout a lamellar TiAl alloy have been investigated by in situ tensile experiments performed at room temperature in a transmission electron microscope. Several situations are found and analysed in which dislocations cross the γ/α2 interfaces and the α2 lamellae – the hard phase of the structure. Conditions by which strain transfer can be elastically mediated across sufficiently thin α2 lamellae are discussed.

The annihilation of superdislocations in Ni3Al during deformation at room temperature

Abstract Transmission electron microscopy evidence is given of mechanisms. of annihilation that take place in the course of the room-temperature plastic deformation of Ni3Al. Under a load orientation that favours the operation of a single-slip system, the short dipoles that are present in foils sectioned parallel to the slip plane are identified as elongated closed superdislocation loops instead of dipoles terminating at free surfaces. The fact that dipoles are often aligned in rows such that one extremity of a given dipole corresponds with the closest extremity of the next dipole in the screw orientation is explained in terms of the cross-slip properties of Ni3Al-based alloys. The presence of two distinct categories of dipole rows is interpreted in terms of two different approach mechanisms. The formation of antiphase-boundary tubes-by direct annihilation of screw segments is questioned.

Motion under stress of a screw superdislocation in the L12 structure

Abstract The transformation of a dissociated screw superdislocation under an external stress is analysed. Analytical expressions of the forces exerted on to superpartials are derived which enable one to check the role of every parameter directly. A graphical method to predict the evolution of a given configuration is presented that can be used to simulate the transformation paths under stress. The evolution of a twofold (or incomplete Kear-Wilsdorf (KW)) configuration is controlled by a stress-dependent parameter ζ, just as in the unstressed case presented by Saada and Veyssiere in 1992, and three distinct domains can be defined: ζ, 31/2, evolution is towards the planar configuration in the octahedral plane; α < ζ < 3½, the situation is bistable and further evolution towards a planar configuration located either in the octahedral plane or in the cube plane, is controlled by the antiphase-boundary (APB) ext...