The stress-induced phase transformation between C36 and C15 structures in Fe2Zr is studied by electron microscopy. The nucleus of the transformation is believed to be some pre-existing C15 layers in C36 particles. Microstructural evidence for three mechanisms of growth of a new phase were found: Fault accumulation and rearrangement, moving of individual partial dislocations between two phases, and the migration of microscopic ledges composed of a series of Shockley partials between C36 and C15. Plastic deformation by slip on non-basal planes of C36 caused by indentation is studied.

Binary Fe-28 at.% Al alloys (Fe3Al-type) transform from B2→DO3 ordered phases at 550°C upon cooling. More complex Fe-28Al alloys can have B2↔DO3 transition temperatures that range from 530 to 670°C. The kinetics of the B2→DO3 ordered phase transition also appear to be affected by alloy composition and by matrix dislocation microstructure. The binary and Nb-containing ternary alloys have rapid B2→DO3 transition kinetics during cooling. However, the ternary or more complex alloys that contain Cr have relatively slower phase transition kinetics during similar cooling. Deformation of a complex Fe-28Al-5Cr alloy in the B2 phase regime creates a very high matrix dislocation concentration, which coincides with complete suppression of the B2→DO3 phase transition during cooling.

The microstructure and local chemistry in single crystal and polycrystalline stoichiometric NiAl, with and without boron additions, were investigated using transmission and analytical electron microscopy. Plate-like precipitates were present in the boron-doped NiAl. Since these precipitates were not observed in the boron-free material, they must be due to the formation of borides. After irradiation with 400kV electrons, fine scale changes in the image contrast and the appearance of diffuse scattering in the diffraction pattern indicate that a phase transformation has been induced by the electron beam.

The α→ γ transformation in a Ti-48 at% Al alloy was studied by a novel computer interfaced control-cum-data acquisition technique. In situ,real time, high speed measurements of temperature were made in the system engineered for this purpose. The samples were heated by controlled direct resistance heating and gas-jet quenched. Various cooling rates were achieved by control algorithms and controlling the pressure of the quenching medium. Cooling curves and thermal arrest data were used to determine the transformation temperatures and characteristics as a function of cooling rate. Subsequent microstructural analysis by optical microscopy was used to confirm the occurrence of the various transformation modes, namely, lamellar, Widmanstatten and massive. The transformation start and completion temperatures for the various reactions were determined for different cooling rates, and continuous cooling transformation diagrams were determined. The various results are presented and discussed in light of thermodynamic and phase diagram considerations.

The effect of cooling rate on transformations of the disordered, hcp α phase in TiAl alloys containing 43 to 50 at.% Al is reported. Cast and homogenized samples of the alloys were solutionized in the completely α region and cooled at various rates, and the microstructures examined by various microscopical techniques. The results indicate a very strong effect of cooling rate on transformations, and depending on it the γ forms from the α in three morphologies. At low cooling rates, a lamellar morphology is observed, at intermediate rates both Widmanstatten and feathery morphologies appear, and at very high rates a massive transformation dominates. The crystallography of the various forms of γ, together with orientation relationships, compositions, and nature of the parent/product interfaces have been examined. Controlled heat treatments were also performed to further elucidate the cooling rate dependence of the lamellar and Widmanstatten structures. These various results are presented and the temperature and composition dependence of transformations are discussed in light of phase diagram considerations.

Several alloying additions were made to NiAl and the effect on physical properties was evaluated. The values obtained were compared to available data on NiAl and NiAl alloys. Negligible effects were observed on thermal expansion and dynamic modulus with the additions studied. The high thermal conductivity of NiAl can be advantageously utilized in many applications, and it was found that additions of Hf lowered thermal conductivity only slightly, while larger decreases were found with other additions. However for the NiAl alloys currently under consideration for applications, significant advantages in physical properties are still maintained over nickel-base superalloys.

It has been found that additions of Ti to Nb3Al results in the formation of phases with the B2 crystal structure in as-cast samples. The microstructure and phase stability of two alloys, namely Nb-15Al-10Ti and Nb-15Al-40Ti (in at.%), have been studied. Heat-treatment of the Nb-15Al-10Ti alloy at 1100°C results in the precipitation of an A15 phase in the B2 matrix. A particular orientation relationship exists between the two phases. Heat-treatment above 1600°C results in the dissolution of the second phase. The ALCHEMI technique has been employed to assess qualitatively the distribution of atom types over the two sublattices of the B2 compound.

Formation energies of point defects in Ni3Al were calculated by EAM and static relaxations. The equilibrium equation of point defects in Ll2 type of intermetallic compounds was established. The concentration of point defects at 1000 K as a function of bulk composition and the effect of temperature on them was calculated for Ni3Al. The concentration of vacancies in Ni3Al with various Al and boron contents was also investigated by means of positron annihilation to verify the calculated results.

High resolution transmission electron microscopy (HRTEM) has been used to observe the core structures of a<100> and a<110> edge dislocations in bicrystals of stoichiometric NiAl. The images indicate that the core of a<100> edge dislocations are compact. Two different core structures have been observed for a<110> edge dislocations. In one configuration, the a<110> dislocation is decomposed into two a<100> dislocations, while in the other it is climb-dissociated into partial dislocations. Both configurations are non-planar on the scale of about 1 nm. EAM calculations have been performed to determine the relative energies and stabilities of these configurations. The implications of these core configurations are briefly discussed with respect to the macroscopic flow behavior of NiAl.

A transmission electron microscopy study of planar growth faults in the A15 intermetallic phase Nb3Al is presented. These faults are not observed in plasma melted material but a high density of these features is observed in arc-melted material which is cooled more slowly. The faults are found most frequently on planes {100}, often changing from one plane to another along their length. It has not been possible to obtain a full diffraction contrast analysis from the faults but HREM images indicate that they are intrinsic with a displacement vector R=l/4<120>. Since this vector does not lie in the plane, they cannot form by glide processes alone and therefore probably arise when vacancies coalesce into sheets on {100}. Since this value of R is consistent with the removal of an Al-rich plane {004}, it is argued that the formation of these faults may help to stabilise the Al5 phase to Nb-rich compositions.

TEM observations suggest the presence of edge-type strain fields around antiphase boundary tubes in deformed L12 compounds such as Ni3Al and Fe3Ge. The present paper aims at discussing the origin of these strain fields. Some preliminary results concerning the thermal instability of the tubes will also be discussed.

Properties of thermal vacancies in CoSi2 and defects in electron irradiated and proton irradiated CoSi2 have been studied by positron lifetime spectroscopy. It has been found that thermal vacancies in CoSi2 are easily quenched in. The effective formation enthalpy of vacancies has been estimated to be about 1.1 eV. The recovery of electron irradiated CoSi2 occurs in two stages; the first stage around 310 K attributed to the migration of vacancies forming secondary defects and the final stage between 670 and 700 K, where the secondary defects dissolve. Since the positron lifetime at the secondary defects is smaller than that at vacancies, the secondary defects must be collapsed vacancy clusters, such as faulted dislocation loops. In the recovery of proton irradiated CoSi2, microvoids, which are annealed out above 820 K, as well as collapsed vacancy clusters are also formed. The three dimensional vacancy clustering is probably due to the existence of implanted hydrogen atoms.

The Mössbauer measurements at room temperature have been carried out on the B2 intermetallic compound Fe-40Al and its Mn or Ti containing alloys. The results show that all the Mössbauer spectra of Fe-40Al and its Mn or Ti containing alloys are approximately one-peak singlets. The Fe atoms in these alloys show no significant magnetic moment. The spectra of Mn or Ti containing alloys have been fitted with two Lorentzian lines by the method of least squares. By studying the isomer shifts and other parameters of these fitted lines, it have been shown that Mn atoms occupy both Fe and A1 sublattices, Ti atoms occupy preferentially Fe sublattice, however, a few of Ti atoms occupy A1 sublattice only as Ti concentration is over certain amount (∼5-at%).

Recent in situ and post mortem observations in Ni3Al and Ni3Ga alloys have yielded consistent results concerning the structure and the movement of screw superdislocations in {111} planes. The different stresses involved in the observed cross-slip processes have been computed, and the outline of a new model of stress anomaly has been formulated on the basis of these results.

Ni3Al shows like many other L12 alloys an anomalous increase of the yield stress up to a peak temperature of about 800°C. This increase is generally explained by a thermal activated cross slip process of superlattice screw dislocations from (111) to (010). To study the influence of the external stress on this process two orientations were selected: compression axis [123] and [001]. They show a large difference of the Schmid factors for cube cross slip.

Single crystals of Ni3Al were deformed at RT and 400°C, and TEM foils cut parallel to (111) and (010) were studied. At RT which is below the anomalous temperature regime the dislocation structures are in both cases similar to those of fee structures without an order. At 400°C the TEM results_show a distinct difference: dislocations bowing out on (010) are a typical feature for the [123] axis; whereas in the case of the [001] axis dislocation reactions and long straight screws are predominant. They occur in the form of KW locks and contain frequently SISF converted KW locks. Therefore, despite of the different appearance, in both cases strong obstacles are contained in the observed dislocation structures. To overcome these strongly locked segments an Orowan type source model is proposed. When the stress is sufficiently high the dislocation can bow out between obstacles, move on and leave the locked segments behind, which are stored.

The interaction of 1/2<1 1 0> screw dislocations with symmetric [1 1 0] tilt boundaries was investigated by atomistic simulations using many-body potentials representing ordered compounds. The calculations were performed with and without an applied shear stress. The observations were: absorption into the grain boundary, attraction of a lattice Shockley partial dislocation towards the grain boundary and transmission through the grain boundary under the influence of a shear stress.

We studied the energetics and structure of simple point defects and defect clusters in the Ni-Al system. Atomistic computer simulation was used for this purpose with embedded atom interatomic potentials. In both, Ni3Al and NiAl the energetics are such that excess Ni is accommodated by anitisite defects more easily than excess Al. Structural vacancies are therefore more likely with stoichiometry deviations in the Al-rich side than in the Ni-rich side. The interaction of vacancies of different types to form divacancies was studied. Several configurations were found with a very similar energy to that of the two monovacancies far apart. Some configurations show attraction and others show repulsion. The vacancy interaction distance is limited to a few interatomic distances.

The relaxation around these defects was studied in detail and the results can be interpreted in terms of the larger size of the Al atom.

Atomistic simulations of dislocation motion in intermetallics have so far been limited to straight dislocations. In this work, this limitation is relaxed by developing a technique to construct and study dislocation kinks. Using the EAM method, the intermetallic compound, NiAl, is studied using OK simulations of single kinks on a mixed dislocation in the <001>{110} slip system. The threshold stress for the motion of the kink is calculated as 0.00125-0.00185μ, (∼160 Ħ230 MPa), which is fairly close to the CRSS measured at 77 K. These results, suggest that kink motion may be a contributory factor to the slip response of NiAl at low temperatures.

In the orthorhombic phase of Ti2AlNb-type, the occurrence of different dissociation modes of dislocations, glissile in the (001) basal plane is reported. [100] and 1/2[110] dislocations usually dissociate in order to form antiphase boundaries. Detailed weak-beam observations show that [100] dislocations, as well as [010] dislocations formed by interaction of 1/2<110> dislocations, can dissociate into three-fold configurations containing a pair of stacking faults.

In situ straining experiments are performed in Ti3Al, in order to study the glide of 1/3 <1120> superdislocations in prismatic planes. Two different dislocation behaviours are observed, corresponding to two different antiphase boundaries (APBs) in two parallel prismatic planes.