Antiphase Boundary Fault Research Articles

Structural modifications during heating of bulk nanocrystalline FeAl produced by high-pressure torsion

The deformation-induced nanostructure developed during high-pressure torsion of B2 long-range ordered FeAl is shown to be unstable upon heating. The structural changes were analyzed using transmission electron microscopy, differential scanning calorimetry and microhardness measurements. Heating up to 220°C leads to the recurrence of the chemical long-range order that is destroyed during deformation. It is shown that the transition to the long-range-ordered phase evolves in the form of small ordered domains homogeneously distributed inside the nanosized grains. At temperatures between 220 and 370°C recovery of dislocations and antiphase boundary faults cause a reduction in the grain size from 77 to 35nm. Grain growth occurs at temperatures above 370°C. The evolution of the strength monitored by microhardness is discussed in the framework of grain-size hardening and hardening by defect recovery.

TEM investigation of the structure of deformation-induced antiphase boundary faults inNi3Al

Transmission electron microscopy (TEM) methods were used to analyze the deformation-induced antiphase boundary (APB) tubes in a ${\mathrm{Ni}}_{3}\mathrm{Al}$ single crystal. A high density of tubes is observed after room-temperature deformation showing strong contrasts in some fundamental reflections. This is not expected from a hard sphere model of the APB faults bounding the tubes since in this case the APB faults are just chemical faults (displacement vector ${R}_{C}\ensuremath{\rightarrow}=1/2〈110〉).$ The TEM analysis shows unambiguously that the APB faults contain a small additional structural displacement vector ${R}_{S}\ensuremath{\rightarrow}.$ ${R}_{S}\ensuremath{\rightarrow}$ lies in the plane of the fault and is perpendicular to ${R}_{C}\ensuremath{\rightarrow}.$

Three alternative experimental methods to determine the antiphase-boundary energies of the γ′ precipitates in superalloys

Abstract The {111} antiphase-boundary (APB) energies Γ of the L12 long-range-ordered γ′ phases that precipitation strengthen the commercial nickel-based superalloys Nimonic PE16 and Nimonic 105 were measured by three independent methods. Γ was derived, firstly, from the dependence of the critical resolved shear stress on the dispersion of the γ′ precipitates in these superalloys, secondly, from the minimum size of Orowan dislocation loops which can be sustained by γ′ precipitates and, thirdly, from the separations of the superpartial dislocations bounding APB faults in the single-phase γ′ alloys. The latter method is based on state-of-the-art transmission electron microscopy investigations and is considered as the most direct. The results determined by the three different methods show (with one exception) satisfactory agreement. This proves the validity of all three experimental approaches and thus supports the theoretical models on which the first two methods are based. Since Γ governs the strength of su...

On the origin of work softening of Ni 3Al deformed along [001] above the peak temperature

Single crystalline specimens of Ni 3Al were compressed along [001] at three different temperatures (600, 835 and 920 °C). Above the peak temperature ( T P=800 °C), the stress–strain curves show a yield drop caused by work softening. Transmission electron microscopy (TEM) methods were used to analyse the structures of the superlattice dislocations formed at different deformation temperatures showing that the Burgers vector and glide plane do not change (±a〈110〉{111}). Below T P, screws and screw dipoles consisting of Kear–Wilsdorf (KW) locks are dominating. Above T P, screw dipoles are not observed indicating their dynamic recovery. Above T P, dislocations of all characters contain antiphase boundary (APB) faults that lie inclined to their glide plane. It is proposed that their motion occurs by APB fault dragging. Calculations were carried out to model the work-softening behaviour observed above T P. It is concluded that both the yield stress and yield drop are governed by the value of the mobile dislocation density and its evolution with strain.

〈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.

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.

The effect of annealing on superdislocation structures and antiphase-boundary energies in Fe3Al alloys

Abstract The influence of annealing on 〈111〈 superdislocations introduced by prior deformation has been examined in several alloys based on Fe3Al, namely a binary Fe–28% Al alloy, a ternary Fe–28% Al–5% Cr alloy, and a more complex alloy based on Fe–28% Al–5% Cr (where the compositions are in atomic percentages). The 〈111〉 superdislocations show signs of the relaxation of the antiphase boundary (APB) between the partials, with the extent of relaxation varying considerably depending on the alloy considered. After longer anneals, particularly after high-temperature annealing, the 〈111〉 super-dislocations react to produce 〈100〉 superdislocations, with the APB between the new partials also providing evidence of APB relaxation at longer times and higher temperatures. It is not possible to provide a complete explanation of the results obtained, which clearly depend sensitively on the alloying additions in the material studied and the influence of these on such factors as order stability in the lattice and extent of solute segregation to the APB fault.

Dislocation dissociation in cubic Al3Ti alloys

Abstract Recently there have been conflicting reports concerning the dissociation scheme of superdislocations in Al3Ti alloyed to the cubic structure. This discrepancy can perhaps be explained by the difficulty of applying invisibility criteria for dislocations imaged under weak beam electron microscope conditions. The present study examines dissociated dislocations under a wide range of imaging conditions to deduce that dissociation by the creation of an antiphase boundary (APB) fault is the scheme operating, at room temperature as well as at high temperature. At the triple-point nodes of intersecting dislocations there are localized regions of superlattice intrinsic stacking faults (SISFs), which allow an estimation of SISF energy for comparison with the APB fault energy. This comparison confirms that dissociation by the creation of an APB fault should be the preferred scheme. After high-temperature deformation many of the superdislocations have their APB on cube planes and remain mobile with no evidenc...

The effect of annealing on dislocations in an al5ti2fe alloy

Abstract The influence of annealing at relatively low temperatures on the dislocations present in a deformed Al5Ti2Fe alloy has been examined by in situ transmission electron microscopy. It is shown that for this material the separation of the partial dislocations constituting the super-dislocations can increase rapidly in a non-recoverable manner during annealing. This variation corresponds to a significant decrease in the antiphase boundary fault energy from that of the deformed state. Such changes in fault energy may be responsible for a dynamic pinning of dislocations that can significantly affect mechanical properties of the material.

Lattice Defects in the Li2 Superlattice

At the AB3 composition, four families of phases exist that can be classified according to various stacking schemes of identical close-packed octahedral planes. These are listed in Table I. The noteworthy feature of all four stacking variants is that each A atom is surrounded by 12 nearest neighbor B atoms.1. Stacking FaultsStacking faults in the fee (Li2) superlattice are of three basic types - superlattice intrinsic (or extrinsic) faults, antiphase boundary faults, and complex faults.