Patrick Veyssière

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 a...

The plastic deformation of silicon between 300°C and 600°C

Abstract Constant strain-rate compression of silicon single crystals has been performed under a hydrostatic pressure of 1500 MPa. The crystals were deformed at temperatures as low as 300°C and had resolved flow stress values of up to 1000 MPa. The variation of yield strength with temperature suggests a transition of the mechanism controlling the deformation near 600°C.

The geometrical configuration of kinks on screw superdislocations in Ni3Al deformed at room temperature

Abstract The role of kinks in moving locked screw superdislocations in L12 alloys is studied. In Ni3Al single crystals deformed at room temperature, two types of kinks are identified: single kinks that sit on one superpartial and paired ones. It is shown that both types glide on the primary octahedral plane and interconnect screw superdislocations which are themselves dissociated in the cube cross-slip plane. A geometrical glissile model for the kink is proposed.

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...

Plastic deformation of polycrystals of Co

The plastic behaviour of Co3(Al,W) polycrystals with the L12 structure has been investigated in compression from 77 to 1273 K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950 K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100 K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L12 compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co3(Al,W) and other L12 compounds, such as Ni3Al and Co3Ti, are discussed.

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...

Temperature effect on superdislocation dissociation on a cube plane in Ni3Al

Abstract Temperature dependence of the apparent antiphase boundary (APB) energy on cube planes is studied using weak-beam determination of dissociation distances of superdislocations originating from deformation at 20,650 and 850°C. It is difficult to assess how closely the observed dependence applies to the true cube ABP energy because of the uncertainty associated with lattice resistance to the motion of superpartials. It is estimated that the true energy of conservative APB on {001} is liable to decrease with temperature as much as 30% between room temperature and 850°C.

Slip systems in gadolinium gallium garnet single crystals

Gd3Ga5O12 single crystals with compression axes of different orientations have been deformed under creep conditions in air at 1450° C and 1550° C (0.86 TM and 0.92 TM, respectively, where TM is the melting temperature). After a few per cent of permanent strain the deformation substructure has been studied by optical and transmission electron microscopy. Etching and bi-refringence patterns indicate that slip on {110}, {112} and {123} can be activated depending on the orientation of the compression load. Dislocations with a/2 〈111〉 Burgers vectors have been observed to glide in {110 planes. They exhibit a segmented aspect suggesting their dissociation out of the glide plane.

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.

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.