The domain structure and the evolution of antiphase boundaries (APBs) have been investigated in Fe-Al by means of “in-situ” transmission electron microscopy (TEM) heating experiments. Single crystals with composition Fe22.1at%Al and Fe25.6at%Al have been used. The grown-in structure of the Fe22.1at%al single crystal is composed of DO3 ordered particles embedded in the disorderd ±-matrix. A bimodal distribution of the particles was found. Small ordered particles are in between the large precipitates which are surrounded by particle-free zones. Numerous of this large ordered precipitates contain APBs. Crossing the transition temperature to the disordered phase, the small particles dissolve into the ±-matrix and the large particles start to shrink by dissolving. The single crystal with composition Fe25.6at%Al was found to be completely DO3 ordered. The grown-in domains are separated by APBs of type a′0/2〈100〉. At temperatures far below the transition temperature to the B2 phase no significant change in the APB and domain structure has been detected. In contrast, a remarkable evolution in the APB structure has been observed approaching the transition temperature. Coarsening of the domains has been found. Furthermore, APBs of B2-type (a′0/4〈lll〉 shear) are dragged out by dislocation motion. B2- and DC3-type APBs react and junctions are formed. With increasing annealing time, the density of B2-type boundaries increases. The TEM image is dominated by B2-type boundaries linked by the D03-type boundaries. The DO3 superlattice spots are clearly excited approaching the transition temperature to B2. Above the transition temperature, the DO3 spots disappear completely and the diffraction pattern reveals B2 long range order.

Abstract Single crystals of disordered Ni3Fe oriented for single slip were deformed in tension at room temperature. Ni3Fe represents a f.c.c. alloy in the solid solution range having both a high value of stacking fault energy γ and friction stress τf. Electron microscopy methods are used to study undissociated glide dislocations (b = a/2[110]) bowing out on (001) planes. The local radius of curvature of these half loops is rather low for dislocation characters near 35° and 90° and therefore their shape deviates from the elliptical one normally observed for glide dislocations on {111} planes in f.c.c. metals. The experimentally observed loop shapes agree very well with shapes calculated using a line tension model. From this agreement it can be concluded that τf balancing the stress of the loop does not depend on the character of the dislocation. This result is confirmed by computations showing that τf mainly arises from modulus and short-range-order (SRO) effects. Finally the values of the locally effectiv...

Alloys of Cu–15at.% Al, Ag–21at.% Zn and Au–14at.% Fe are compared with respect to short-range ordering in the recrystallized state and after high deformation by cold-rolling, respectively. Additional information on the defect annealing behaviour is gained from an investigation of microhardness and by microscopy.

It is well investigated that concentrated {alpha}-CuAl shows effects of short-range order (SRO) which for thermodynamical reasons are a function of temperature. In most cases, however, it was tried rather to carefully avoid complications by a defect structure than to study SRO-effects in the presence of a heavily defected microstructure after plastic deformation. Depending on the temperature ranges of recovery and recrystallization, defect annihilation of a cold-worked alloy may occur simultaneously with changes in the degree of SRO during an annealing treatment. In previous works on {alpha}-AgZn it has been shown that these processes of defect recovery and SRO could be separated. For the technical application of a material, but also as a basis for the separation of SRO-effects from effects of defect annealing it is of great interest to investigate the microstructural changes during post-deformation annealing. In a preliminary study on the interaction between SRO and post-deformation annealing certain stages of defect annealing could be detected. In the present work the authors concentrate on the interpretation of the microstructure of these stages as studied by TEM.

The annealing processes of deformation-induced defects after cold-rolling to about 30 and 60% thickness reduction were investigated for Ag21, 23, 28 at% Zn by means of residual electrical resistivity, microhardness, light microscopy and transmission electron microscopy (TEM). Three annealing stages have been detected and consistently interpreted as annealing-out of mainly point defects and their agglomerates (stage I), formation of dislocation cells and recrystallized strain-free grains (stage II) and grain growth (stage III). Further, it is tried to determine the evolution of defect production (point defects and dislocations) during rolling deformation from measured changes of electrical resistivity and microhardness.

Dislocation mechanisms and their influence on mechanical properties (e.g. flow stress and work-hardening) can be studied in detail by critical prestraining experiments. In this study, the influence of prestrain on the flow stress of an Ni3(Al, Ti) alloy with L12 long-range order structure was investigated. The yield strength has a positive temperature dependence (it increases with increasing temperature) from room temperature (RT) up to 450 °C. Prestraining of Ni3(Al, Ti) single crystals at 800 °C activated the primary ± a[110](001) cube slip system. Climb dissociated segments interconnect superlattice dislocations of Lomer-Cottrell (LC) type. A subsequent additional deformation at RT introduced superlattice dislocations of the primary ± a[101](111) glide system. This subsequent deformation results in almost the same flow stress as the flow stress found in single crystals subjected to RT deformation only. Prestraining was found to have a huge influence on work-hardening. This is consistent with dislocation interactions of cube and octahedral glide systems. It is concluded that yielding is an intrinsic property of dislocation motion due to extended core configurations.

In-situ heating experiments have been performed on thin foils of a single crystal with composition Fe-25.6 at % Al. The isothermal evolution of antiphase boundaries (APBs) has been investigated by means of transmission electron microscopy. APBs play an important role in phase changes of long-range ordered alloys. The grown-in domains were found to have the DO3 long-range order structure with randomly shaped APBs (swirl-shaped domains). Up to 400 degrees C, no significant change in the domain and APE structure was observed. At 500 degrees C, which is below-the transition temperature. (T-c approximate to 550 degrees C) from DO3 to B2 coarsening of the domains has been analysed and dislocation motion was detected. The dislocations were analysed to have a Burgers vector of (a(0)'/4) (111) (a(0)' is the lattice parameter of the DO3 lattice). The dislocations drag behind them APBs of B2 type. Reactions of the B2-type APBs with the DO3-type grown-in boundaries have been analysed. With increasing annealing time the density of the B2-type boundaries increases and the size of the domains becomes smaller. The associated thickening of the B2 domain walls has been imaged.

Cross-slip of screw superlattice dislocations in long range ordered alloys is investigated by using the weak-beam darkfield imaging method. A Ni3(Al,Ti) alloy and Ni3Fe were selected. The former alloy belongs to the category whose representatives are ordered up to the melting temperature; the latter alloy has an order/disorder transition at 503°C. Ni3(Al,Ti) exhibits an anomalous increase of the yield stress with increasing temperature. Cross slip of screw superlattice dislocations onto cube planes is a characteristic feature which was found to occur with high frequency at the temperature of the onset of the anomaly. Screws dissociated on cube planes are predominantly observed. In the temperature regime below where the flow stress slightly decreases with increasing temperature octahedral cross-slip was found. In Ni3Fe there is almost no temperature dependence of the flow stress. Octahedral cross-slip is frequently observed but cube cross-slip is rarely found. The edge superlattice dislocations are fourfold dissociated.

Ni3(Al,Ti) belongs to the large category whose representatives illustrate anomalous flow stress behaviour. Up to 450°C the flow stress increases with increasing temperature. It is therefore a challenge to study the structure of superlattice dislocations and their intrinsic properties in compressed L12 long-range-ordered Ni3(Al,Ti) by means of transmission electron microscopy (TEM). A detailed analysis of the changes in the dislocation structure in the wide temperature range between −196°C and 800°C is presented. The fine structure and dissociation modes of dislocations were identified by weak-beam microscopy. Slip behaviour and transition of slip systems are discussed with respect to comprehending yielding properties. The onset of the flow stress anomaly is strongly correlated with the observation of screw superlattice b = ± a[101] dislocations are predominant and octahedral (111) cross-slip occurs. Approaching the temperature temperatures edge dislocations are predominant and octahedral (111) cross-slip occurs. Approaching the temperature where the flow stress reaches its maximum, octahedral ±a[101](111) slip is gradually replaced by cube ±a[110](001) slip. Above the peak temperature single cube slip controls deformation. Thermally assisted exhaustion of primary octahedral slip by cube cross-slip is regarded as responsible for the anomalous yielding behaviour.

Electron microscope images of superlattice dislocations taken with g.b > 2 can give rise to multiple image peaks whose position and separation are not necessarily directly related to dislocation dissociation schemes. This was proved by image simulations and is consistent with the results of the kinematical theory of image formation.A study of the separation widths of dissociated superlattice dislocations in Ni3(Al, Ti) was made using g.b = 4 and 8 and large absolute value of s. Dissociation widths were determined to be in the range 3 nm < DELTA(APB) < 5 nm and 0 nm < DELTA(CSF) < 1.5 nm, from which antiphase-boundary and complex stacking-fault energies were determined to be in the ranges 250 mJ m-2 < gamma(APB) < 420 mJ m-2 and gamma(CSF) > 370 mJ m-2.

A single crystal of L1(2) long-range-ordered Ni3(Al, Ti) has been deformed in compression at -196-degrees-C. The dislocation structure in the (111) planes is predominated by edge superlattice dislocations which are arranged in dipoles and bundles of dipoles. The density of screw superlattice dislocations is low. They are mainly dissociated on octahedral (111) and (111) planes. This provides strong evidence for octahedral (111BAR) cross-slip. Cube (010) cross-slip was only observed locally presumably assisted by the help of localized internal stresses. It is assumed that, at very low temperatures, cross-slip of screw dislocation from octahedral (111) to cube (010) planes cannot be thermally activated.Altogether the results are consistent with the fact that the increase in the flow stress is strongly correlated to cross-slip of screw superlattice dislocations from (111) to (010) planes.

Although Ni3Fe belongs to the group of L1(2) long-range ordered intermetallics the critical resolved shear stress (CRSS) does not depend strongly on the deformation temperature. Dipoles and bundles of dipoles near edge orientations are characteristic features in the dislocation structure. Few screw superlattice dislocations are observed. There are striking similarities to the dislocation structure of pure f.c.c. metals or dilute solid solutions after low and medium plastic deformation.Cross-slip of screw superlattice dislocations onto cube (010) planes plays a minor role and is not regarded to be the rate-controlling factor for dislocation motion. The reason for the absence of cube cross-slip is the high activation enthalpy necessary to constrict widely dissociated Shockley partials to form unit screw superlattice partials.

The dislocation structure in Ni3(Al, Ti) deformed at 1073 K, a temperature above that at which the flow stress peaks, illustrates features of planar localized single slip. Deformation proceeds by movement and multiplication of dislocations of the ±a[110](001) slip system. Edge dislocations transform into locked configurations. APB dragging is observed to operate locally but is not regarded to be a rate-controlling process for dislocation motion. Delocalization of the APB on several atomic planes parallel to the fault plane could be the reason for glide plane ‘softening’ and the formation of a planar dislocation structure.

Abstract The weak-beam method of transmission electron microscopy has been applied to measure the dissociation width of superlattice dislocations in Ni3(Al,Ti) as a function of temperature. Dislocations in thin foils parallel to (111) and (010) planes were investigated. No significant variation of the distance between the unit dislocations which constitute the superlattice dislocation was found over the temperature range from 298 to around 950 K. The experimental observations are discussed in the framework of the model proposed by Brown. It is based on the transformation of geometrical antiphase boundaries (APBs) into APBs in their thermodynamic equilibrium configuration. The role of this APB-based model is examined with respect to the anomalous increase of the flow stress with increasing temperature.