Stable Stacking Fault Research Articles

331 slip on {013} planes in molybdenum disilicide

Abstract Branches of the generalized stacking-fault energy surfaces for {013} and {110} planes in molybdenum disilicide are calculated by first-principles density functional methods. The possible antiphase-boundary faults are found to be unstable, but stable stacking faults which permit (331) dislocations to lower their energy by dissociation are found on both planes. Based on the properties of the predicted dissociated dislocations, a qualitative model is advanced to account for the observed features of {013}(331) slip in molybdenum disilicide, including the ductile-to-brittle transition.

Prediction of a {1122} hcp stacking fault using a modified generalized stacking-fault calculation

Abstract We present a new approach for calculating generalized stacking-fault energies, which allows for in-plane relaxation of atoms. When applied to the {112} slip plane in hcp materials, our approach predicts that a stable stacking fault occurs, associated with a slip of 1/6 (11 ) [tbnd] ½ (c + a). This is consistent with the edge dislocation with b = (c + a) breaking into two partial dislocations, each with b = 1/2 (c + a), observed in previous simulations of the dislocation cores. The resulting generalized stacking-fault energy profile may be useful in understanding the competition between {112} (11 ) twinning (observed in Zr and Ti) and the related slip mode, observed in Mg, Co and Zn.

Dissociation of a dislocation into partial dislocations bounding an unstable stacking fault

Though a stable stacking fault is usually assumed to exist for a dislocation to dissociate into partial dislocations, we show in this paper that a dissociation can occur even when the stacking fault is unstable. In the case where both the undissociated and dissociated states are stable and the former state has the lower energy than the latter, a yield stress anomaly is expected to occur because the metastable state can have much higher mobility than the stable state.

Stacking Fault Energies in TiCx

ABSTRACTWe have used a Tight Binding model to calculate the surface energies of various (111) stacking faults in TiC1.0 and TiC0.5. Based upon our preliminary results we have identified the stable stacking faults and have examined possible dislocation dissociation reactions.

Nature and stability of tem observed defects in 14-MeV neutron-irradiated Au and Cu

Cryo-transfer transmission electron microscopy and isochronal annealing experiments have been made on low temperature 14-MeV neutron-irradiated Au, Cu, Cu-0.1 at% Ge and Cu-0.2 at% Ni specimens. A two and a half dimensional stereo analysis has revealed the effects of specimen thickness and neutron dose on the formation of cascade defects. Interstitial-type defects disappear and some of the submicroscopic vacancy-type defects grow and become visible at the stage-III temperature because of migrating vacancies. Both disappearance and appearance of interstitial-type defects are observed below stage-III, which may be due to beam effects in the electron microscope. Above stage-III, residual interstitial-type defects disappear by absorbing vacancies at the temperature at which vacancy-type defects thermally emit these vacancies and disappear. Stable vacancy-type stacking fault tetrahedra seem to be transformed from vacancy-type clusters during isochronal annealing.

Point-defect and stacking-fault properties in body-centred-cubic metals withn-body interatomic potentials

Abstract The properties of vacancies, self-interstitial atoms and translation stacking faults have been studied by computer simulation using the n-body empirical potentials recently developed for the body-centred-cubic transition metals by Finnis and Sinclair. Vacancy formation energy, formation volume and migration energy have been computed and the results are compared with other theoretical and known experimental values. Formation energy and volume have been calculated for six possible interstitial configurations and, although the expected 〈110〉 dumbbell is most stable in Mo and α-Fe, the 〈111〉 crowdion is favoured in Ta, Cr and W. The energetics of interstitial migration have been studied in detail for Mo, and the migration steps which lead to two-dimensional diffusion do not have the lowest migration energy. The n-body potentials predict the absence of stable translation stacking faults on the {110} and {112} planes.

Multi-layer faults on (112) planes and interatomic potentials in b.c.c. metals

Abstract The possibility of stable multi-layer stacking faults on (112) planes in b.c.c. metals has been studied by computer simulation. The relaxation method and the lithium potential used are described in the accompanying paper (Beauchamp 1978). γ-surface-type calculations have been performed for two-and three-layer faults. The possible extension of the results to all simple metals, and to transition metals described by polynomial model potentials, is discussed. For simple metals, no stable n-layer faults exist for n3, but a stable four-layer fault is found. The successive fault vectors are-b/6,-b/3,-b/3 and-b/6, where b = a½[1 11]. This can be regarded as a twin lamella limited by two twin boundaries in configuration 2. The total fault vector is-b, and thus a perfect dislocation could create this fault, n-layer faults with n  5 consist of a twin lamella with twin boundaries in configuration 1 or configuration 2. For most polynomial potentials, no stable n-layer faults are found for n  2, but faults ...

Cross-slip on {110} planes in aluminum single crystals compressed along 〈100〉 axis

In order to investigate the {110} 〈11̄0〉 glide system in f.c.c. metals, single crystals of aluminum have been deformed in compression parallel to [001] at temperatures between 225 and 365°C and strain rates between 9 × 10 −6 and 9 × 10 −4/ sec. The stress strain curve exhibits three stages: Stage I: Slip occurs on all 4{111} 〈11̄0〉 systems and a cellular dislocation structure builds up. Stage II: For a stress σ 110, heterogeneous {110} 〈11̄0〉 slip appears. Coarse bundles of slip lines fill the whole sample. The average consolidation is low. Stage III: The consolidation increases again. All slip systems probably participate in the deformation. The stress σ 110 is thermally activated. The activation energy for ⋗ at constant σ 110 is close to 28 k The results are interpreted in terms of cross-slip of a/2〈11̄0〉 screw dislocations from {111} on to {110} planes. The glide may be stabilized in {110} planes by the existence of a stable stacking fault in this plane.

Effect of prolonged loading on the structure and properties of alloy KhN77TYu

When alloy KhN77TYu is subjected to preliminary creep dispersed particles of γ′ phase appear in the structure, and consequently plastic deformation occurs by cutting of the particles by dislocations, with formation of stable stacking faults, which are retained during subsequent aging and strengthen the structure. After long-term strength tests the structure contains dislocation loops evenly distributed through the grains, with no coplanar distribution of dislocations in grain boundaries. All these changes in the fine structure lead to an increase of the service life of alloy KhN77TYu.

Intrinsic Stacking Faults on {112} Planes in the B.C.C. Lattice

AbstractThe possibility of intrinsic stacking faults in the b.c.c. lattice is discussed for a central force model. The existence of stable stacking faults depends on the cut‐off radius of the interatomic potential. For rather small cut‐off radii the model predicts stacking faults. However, we obtain stacking fault vectors different from those predicted by the hard‐sphere model. Further, the asymmetry of the stacking fault energy surface with respect to displacements in ± 〈111〉 directions depends on the cut‐off radius.

Intrinsic stacking faults in body-centred cubic crystals

Abstract A study of the possibility of the existence of stacking faults in b.c.c. crystals on {110} and {112} planes has been performed, representing the lattice by a central force interaction between atoms. The same study was also carried out for {111} planes in f.c.c. crystals. The results for the f.c.c. lattice are in full agreement with the predictions based on a hard-sphere model. However, the results for the b.c.c. lattice are very different and suggest that no stable instrinsic stacking faults of the same type as in f.c.c. crystals can exist either on {110} or on {112} planes in b.c.c. crystals.

Stacking faults in the refractory metals and alloys — A review

A potential mechanism of strengthening in refractory alloys is the formation of stable stacking faults. Development of alloys based on the group Va (V, Nb, Ta) and VIa (Cr, Mo, W) metals should include consideration of this lattice imperfection. The available literature indicates some confusion with regard to the formation of stacking faults in such body-centered-cubic (b.c.c.) structures. Therefore, it is first shown what energetically favorable dislocation dissociation reactions may take place in the b.c.c. structure. The various methods of measuring the stacking fault energy are critically reviewed. Reasonably accurate values can be obtained from observation of thin films by transmission electron microscopy. Perhaps the most important obstacle to utilizing this lattice defect is an imcomplete understanding of the effect of solute atoms (alloying additions and impurities) on the stacking fault energy. It is shown that at thermodynamic equilibrium the solute segregates at the fault and, thereby, reduces the fault energy and increases its width. The refractory metals have high intrinsic fault energies and solute segregation would appear to be necessary if faults are to be formed. Stacking faults have been observed in impure columbium and tungsten, and a molybdenum-35 at.% rhenium alloy. Such dislocation processes as cross slip and climb have been shown to be a function of the fault energy; that is, the activation energies for such processes increase with decreasing fault energy. In addition, the motion of complete slip dislocations may be impeded by faults; on the other hand, the Peierls-Nabarro friction stress may be lower for partial dislocations which form the boundaries of stacking faults. It is concluded that with suitable alloying and thermal treatment reasonably large faults can be formed. By studying the mechanical behavior of refractory alloys and giving consideration to the presence of stacking faults, the effect of this lattice defect on the high temperature strength and low temperature ductility of these materials may be determined.