The positive temperature dependence of yield stress (PTYS) has been observed (Kawabata et al., Acta Metall., 33, 1355 (1985)) in TiAl single crystals in all orientations operating ordinary dislocations with the Burgers vector 1/2<110] and superdislocations with the Burgers vectors <101] and 1/2<112]. A mechanism for the PTYS is proposed, which is a cross-slip and pinning mechanism of Shockley partials with the Burgers vector 1/6<112] or/and 1/6<121]. The results of strain rate dependence of yield stress, K value dependence of critical resolved shear stress (CRSS) and transmission electron microscopy support the theory. Here, K value is the Schmid factor ratio of a Shockley partial on {101) or {110) plane and a perfect ordinary dislocation or a perfect superdislocation on {111) plane.

The deformation structure of the alpha-2 and orthorhombic phase in two alloys of the Ti-Al-Nb system has been characterised by transmission electron microscopy after plastic deformation of about 2% in compression at room temperauture. The orthorhombic phase is shown to deform by all slip modes present in the alpha-2 phase. In addition, extensive “c” component slip is observed in the orthorhombic phase on slip planes that are not equivalent to those observed for “c”component slip in alpha-2. Dislocation dissociation on both basal and prismatic planes of dislocations without a c component is observed in both phases.

Single Crystals of L1o type Ti44Al54V2 alloy were deformed at high temperatures and various orientations. The dislocations thus produced by single slip were studied by TEM. 1/2<110] dislocations produced at 1073 K consist of dislocation loops and curled dislocations with spiral segments, which is in agreement with those shown in the polycrystalline Ti-Al-V, and -Nb alloys deformed under the same conditions. The normal dislocations produced at 873 K pile up in groups, each of which contains several straight dislocations with a screw character. On the other hand, the majority of superdislocations produced at 873 K were found in a dipole form, indicating that the dipole is a favorable configuration at this temperature.

Elevated temperature (815°C) compression tests were performed at strain rates between 10−2 & 10−5 sec−1 on a γ/α2 alloy, Ti-48Al-1V-0.2C (at%), for two different α2 volume fractions. A large change in the flow stress was observed to occur for strain rates between 1.10−5 & 1.10−3 sec−1. The dislocations observed in the primary γ grains deformed at the high strain rates were “regular” 1/2<110]; screw dislocations. The dislocations observed at slow strain rates were also regular 1/2<110] dislocations, however their orientation was changed so that they were aligned close to [001]. Dislocation structures produced during high cycle fatigue are identical to those observed during high strain rate compression tests, while dislocation structures produced during tensile (ė- 8.10−4 sec−1 ) tests are identical to those observed in the slow strain rate compression tests. Analysis of the orientation of deformed primary γ-TiAl grains shows that the CRSS for {111}<101] slip must be at least 1.4 times the CRSS of {111}1/2<110] slip.

Titanium trialuminide alloys suffer problems of intense segregation during solidification and extreme brittleness when subjected to tensile stresses. In this study, spray forming techniques have been used to obtain homogeneous materials of fine microstructure, containing various second phase particles, and the deformation behaviour of these materials examined over a range of temperatures.

The microstructure and deformation behaviour of an iron-modified titanium trialuminide with the L12 structure is reported here.Deformation mechanisms are interpreted by analysis of dislocation structures and further confirmation is obtained by activation volume measurements: the importance of Peierls effects at low temperatures and cross slip at high temperatures is shown. The material remains brittle in tension both at room temperature and at high temperatures: the second phase particles in the material are yet not present in sufficient volume fraction to greatly affect plastic behaviour.

In the L12 structure, the Kear-Wilsdorf lock on screw superdislocations is the best known example of a dislocation with a non-planar locked structure. Recent high resolution TEM work on Ni3Al and Ni3Ga has shown locked structures on the following dislocations: 30-degree <112>/3 super-Shockley partials, edge <110> {001} dislocations, and 45-degree <100>{001} dislocations. Dislocation reactions among intersecting <100>{001} and <110>{001} slip systems have also been found to form product dislocations with locked structures. This paper summarizes these locked structures and outlines their effects on the operation of related slip systems.

Weak beam electron microscopy has been used to study the substructure in octahedral planes in Ni3Ga deformed in the range of the yield stress anomaly. The substructure is formed by screw dislocations in the Kear Wilsdorf configuration, which contain numerous superkinks. A statistical analysis of the superkink height has been performed. The effect of a first high temperature deformation on a second low temperature one has been investigated. The behaviour of the dislocations has been deduced from these experimental results and from those of the literature.

The intermetallic compound Al3Sc is a trialuminide with the LI2 structure, which deforms easily in compression at room temperature, with yield stresses around 100 MPa. In tension Al3Sc fractures transgranularly in a brittle manner. The predominant cleavage plane is {011}. Regions, which are flat within experimental resolution are only occasionally observed. Numerous cleavage steps, which are aligned in 3 major crystallographic directions, are found. Some of these steps consist of {111} or {001} planes, but others are not distinctly crystallographic. Plastic deformation involving dislocation motion or twinning may have occurred at some of these steps. Reactions between different types of steps are also observed. One type of cleavage pattern found is strikingly similar to the typical appearance of fracture surfaces of fcc brass and Cu3Au after stress corrosion cracking. However, this particular pattern is only rarely observed in Al3Sc. Our observations indicate that an interpretation of cleavage fracture in Al3Sc in terms of surface energies alone is unlikely to be successful. A full understanding of the fracture morphology of Al3Sc will require detailed atomistic simulations taking dislocation motion and twinning into account.

Specimens of Nb-18 at. % Al were bend-tested at 20° C, 1200° C, and 1600° C, and the deformed microstructures were characterized using TEM to determine phase distributions and slip systems. Material deformed at 20° C showed brittle fracture and was characterized by large grains of a Nb-Al phase showing B2 ordering. Lamellar colonies of heavily dislocated massively transformed Nb3Al (A15) were present at grain boundaries. The slip system for dislocations in Nb3Al, was determined to be {100}/<001>, with defects typically extending in <011> directions. Fracture at 1200° C was ductile, and TEM observations revealed a bcc Nb-Al(ss) matrix with second phase growth of Nb3Al and α -Al2O3. The Nb3Al phase contained total dislocations and slip-induced planar defects on {100} planes terminating in partial dislocations. In tilting experiments, the planar defects exhibited stacking fault contrast consistent with a displacement vector of R=a/4<100>. Fracture at 1600° C was also ductile, and the deformed material consisted of a niobium matrix with equiaxed inclusions of alumina resulting from internal oxidation during deformation. The creep resistance reported previously was attributed to a combination of dispersion hardening of the refractory metal matrix and inherent creep resistance of Nb3Al.

Dissociation of the ao<110> screw dislocation in Ni3Al was studied using the embedded atom method of computer simulation. The dissociation occurred predominantly along the {111} plane, however, a {001}-plane step occurred in the APB at the center of the configuration. When a pair of ao/2<110> superpartials initially separated in the {111} plane was relaxed, the step formed once again but with a reduced height. When the pair was relaxed from larger distances the step was not formed. The results indicate that the elastic interaction “torque” due to elastic anisotropy is responsible for the formation of the {001} APB step. When a stress was applied to these dislocation configurations by simulation, results confirmed that the step in the APB and the octahedral cross-slipped-core dissociations can be significant barriers to glide of the screw dislocation.

Dislocation structure in a directionally solidified Ni3Al-based alloy deformed at temperatures below the peak in flow stress has been studied by the weak beam TEM technique. At room temperature, the screw superdislocations are partly transformed into Kear-Wilsdorf configurations. With increasing temperatures, the transformed Kear-Wilsdorf parts increase until the temperature reaches 450°C, at which the screw superdislocations are wholly transformed. Bending of the Kear-Wilsdorf configurations on {0101} planes is observed at 450°C, which is believed to be the cause of interaction of Kear-Wilsdorf configurations with superkinks. The Contribution of superkink motion to deformation is discussed. The results suggest that deformation in this temperature range occurs primarily by the slip of non-screw components on {lll} planes, which is consistent with the widely accepted cross slip pinning model.

Rapidly solidified Ni76 Al24 ribbons, with and without boron, have been the subject of in-situ TEM deformation studies, as well as X-ray and TEM characterization and TEM fractography. The aim has been to gain a better understanding of the influence of a reduced degree of order on grainboundary behavior and ductility. This, in turn, allows fresh insights to be gained, both into the manner in which boron additions can enhance ductility, and into the potential usefulness of sequential ordering in intermetallic alloys with a tendency to intergranular failure. Lower degrees of order are found to reduce stress concentrations at grain boundaries; this effect is due to easier generation of dislocations from boundary sources, and, to a lesser extent, the braking action of thermal APB's on dislocation motion. The beneficial effect of boron on ductility is seen to result, at least in part, from enhanced boundary cohesive strength.

Single crystal samples of a nickel base superalloy have been deformed by cyclic bending tests. At the higher temperatures of our experiments, an evolution of the apparent modulus as the advancement of disorientation of cubic planes has been detected.

The dislocation microstructures show that the development of dislocation networks is more advanced in the compressive zones than in the tensile ones.

The degree of long-range order in the trialuminides is determined by X-ray powder diffraction techniques. Long-range order exists to their melting points. For the binary trialuminides Al3Ti, Al73Ti27, and Al3Sc, the degree of long-range order is nearly perfect and is a measure of the lack of mixing of the aluminum atoms onto the sublattice occupied by either Ti or Sc. A calculation of the bond energy between neighboring pairs of atoms from the ordering (melting) temperature is made following the Bragg-Williams mean field theory approach. These bond energies compare favorably with more sophisticated calculations. Bond energies are found to be larger than the energy difference between the crystal structure forms DO22, Ll2, and DO23, and therefore, more relevant to understanding the mechanical and chemical behavior of the trialuminides. Ordering or melting temperatures of these intermetallics reflect the strong Al-metal near-neighbor pair potentials and may provide insights to their brittle properties.

X-ray scattering data obtained for multiple wavelengths with synchrotron radiation were analyzed by the Rietveld method to determine Ni and Pd distributions on the Cu(000) and Au(½½½) sites in the CuAuI tetragonal P4/mmm structure. Alloys of CuAuxM1-x containing 6 at. % Ni or 10 and 25 at. % Pd were processed to obtain maximum ordering. Nickel is predominantly found on the Cu site and most all the Pd is found on the Au site. The uncertainty in site occupation parameters is discussed for various contributions which affect powder intensity measurements. For highly absorbing materials, an observed powder roughness effect decreases the low angle (2θ) intensities relative to the high 2θ intensities. This effect reduces the reliability of the thermal parameters and obscures a proper description of the thermal motion of the two sublattices. Corrections to the X-ray intensity data for surface roughness/porosity effects reduce uncertainties to about ±1 at. % on the refined value of the site occupations. This use of variable wavelength X-rays with simultaneous refinement of the corresponding data is capable of distinguishing site occupations even between two elements of almost equal scattering factor as, for example, Cu and Ni atoms in this investigation. Chemical phase stability is related to the site occupation parameters.

The effects of oxygen on the deformation behavior of Ti-(48-52)Al alloys is reported. Two types of studies were conducted. In the first, high purity alloy buttons containing low oxygen (~250 ppm) were prepared, whereas in the second, alloys with additions of 1 at.% Er to scavenge the oxygen from the matrix were prepared. The alloys were heat treated to produce large grains and the microstructures characterized by analytical electron microscopy. Samples prepared from the heat treated alloys were electropolished and deformed in compression to a plastic strain of 1.0-1.5% at temperatures between 25 and 800°C and the yield stress measured. The morphology of deformation, that is, slip lines and the presence of twinning, was studied by optical microscopy and the dislocation structures were characterized by weak-beam imaging in the transmission electron microscope. The results of these various studies are presented and discussed in terms of recent developments regarding the factors that appear to control the dislocation structure and the mobility of dislocations.

In an effort to seek a way to toughen NiAL intermetallic compounds, deformation characteristics of copper containing NiAL martensite were studied by electron microscopy. Martensite plates with fine {111} <112> internal twins were well developed across the grains in as extruded alloys. Initial linear work hardening up to 7 % strain was observed and the migration of twin interface and martensite plate interfaces were suggested as major deformation modes at this stage. Increase in strain hardening rate after 7 % strain was associated with the development of dislocation structure and growth of new strain induced twins within the internal twin free martensite plates.

The relation between intergranular fracture and grain boundary character was studied in Ni3Al using the electron backscattering pattern (EBSP) technique. A new and improved set of experiments have been performed in which the problems of forced crack propagation and statistical validity of numerical results encountered in the earlier study were eliminated. It was found that low angle and σ3 boundaries do have a substantially lower propensity for cracking as compared to other boundaries, but there is no correlation between grain boundary σ value and the propensity for cracking for low σ boundaries. These results are in general agreement with recent theoretical predictions of grain boundary structure.

Titanium beryllide (TiBe12) has been processed by various techniques, including inert gas arc melting, sputter deposition and as second phase particles in atomized powder of alloys ranging in composition from 1 to 35 weight percent Ti. The TiBe12 that formed in the various alloys was characterized by transmission electron microscopy. Defect structures such as microtwinning, antiphase domain boundaries and dislocations have been observed within the beryllide phase. Descriptions of these defect structures will be presented in this paper, together with a discussion of their origin.

The majority of ordered intermetallics generally exhibit brittle fracture and poor ductility at ambient temperatures, and their brittleness is strongly associated with grain boundary properties. Recent efforts on alloying approaches and microstructural modifications gave promising results to improve the ductility of ordered intermetallics. This article summarizes recent studies on understanding the intergranular brittle fracture from the point of view of compositional and resultant structural (and electro-chemical) properties of grain boundaries. Significant improvement in ductility has been established by the control of chemical composition, that is, by the selection of constituent elements, the control of alloy stoichiometry, the addition of substitutional elements, the change of degree of ordering and the doping by the interstitial elements. Also, the gaseous atoms hydrogen and oxygen from environment or as residue are shown to strongly affect the grain boundary fracture behaviour. Emphasis will be placed on the L12-type ordered intermetallics including Ni3Al, Ni3Si and Co3Ti.