Atomistic studies of dislocation cores in a model TiAl compound (L10) carried out using Finnis-Sinclair type many body potentials are presented. Two different core configurations, one planar and the other non-planar, were found for both 1/2〈110] ordinary dislocations and 〈101] superdislocations while a core containing a microtwin was found in the case of the l/2[112] dislocation. The impact of these core structures upon the plastic behavior or PST and single-phase TiAl crystals is discussed.

Interatomic potentials of the Embedded Atom and Embedded Defect types were used to study the effect of the angular dependent term in the Embedded Defect potential on the properties of defects in TiAl. The defect properties were computed with interatomic potentials developed with and without angular dependent terms. It was found that the inclusion of the angular dependent terms tends to increase the energies of the APB’s and lower the energies of stacking faults. The effects of the angular term on the relaxation around vacancies and antisites in TiAl was also studied, as well as the core structure of several dislocations in this compound.

The stability of bcc-based phases in the Ti-Al-Nb alloy system has been studied from first-principles using a combination of ab-initio total energy and cluster variation method (CVM) calculations. Total energies have been computed for 18 binary and ternary bcc superstructures in order to determine low temperature ordering tendencies. From the results of these calculations a set of effective cluster interaction parameters have been derived. These interaction parameters are required input for CVM computations of alloy thermodynamic properties. The CVM has been used to study the effect of composition on finite-temperature ordering tendencies and site preferences for bcc-based phases. Strong ordering tendencies are observed for binary Nb-Al and Ti-Al bcc phases as well as for ternary alloys with compositions near Ti2AlNb. For selected superstructures we have also analyzed structural stabilities with respect to tetragonal distortions which transform the bcc into an fcc lattice. Instabilities with respect to such distortions are found to exist for binary but not ternary bcc compounds.

The yield anomaly of TiAl is modelled in terms of a mechanism of mobile dislocation exhaustion. Several consequences on mechanical properties are derived, that are shown to be applicable to a wide range of anomalies, irrespective of the specific dislocation mechanisms.

Deformation microstructures in γ-TiAl + α2Ti3Al based fully lamellar (FL) and nearly lamellar (NL) microstructures have been simulated using micro-mechanical methods. The deformation is extremely inhomogenous resulting in a large accumulation of hydrostatic stresses at the grain boundaries, thereby promoting intergranular fracture initiation. In particular, the increase in ductility with increasing equiaxed γ-grain volume fraction (with compliant deformation characteristics) in nearly lamellar alloys is explained by the reduction of the hydrostatic stress buildup at the boundaries, consequently mitigating fracture.

In recent years, the determination of x-ray structure factors and hence the electron charge density distribution in TiAl has received considerable attention. Experimental assessment of electron charge densities requires accurate values of the Debye-Waller (D-W) factors. This work attempts to determine the Debye-Waller factors from measurements of an Al rich off-stoichiometric single crystal of TiAl using the four circle x-ray diffraction method. The results suggest an ordered substitution of the excess Al atoms on the Ti-sublattice. This result is consistent with previous site occupancy studies.

Reaction sequence of the massive transformation from the high-temperature α-Ti phase to the γ-TiAl phase (αm) in a Ti-48at.% Al alloy has been examined in terms of optical and transmission electron microscopes. Both transformed and untransformed regions were macroscopically observed in the sample quenched from the high-temperature α phase field, when the sample was held there for a extended period of time prior to quenching. The transformed region consists of randomly oriented fine α single phase grains, in which many thermal anti-phase domains (TAPDs), together with a number of stacking faults were observed. In contrast, the untransformed region comprises extremely fine lamellae of the α and α2-Ti3Al phases, and the α plates were found to run through the TAPDs caused by α ə α2 ordering. Subsequent aging at 1273K causes the microstructure change in the untransformed region from α2/γ lamellae to γ/γ lamellae spontaneously and expands the γm region. These observations suggest that the α ə γm transformation proceeds through formation of fine γ plates.

The microstructures in arc melted ingots and melt spun ribbons have been investigated by electron microscopy and thermodynamic modelling has been used to study the phase transformations. In the ingot, solidification starts with the bcc β phase and at room temperature the structure consists of B2, ωordered, γ and α2 phases. The calculated equilibrium phase transformation sequence during cooling is L → L+ β→β→β + α→β2+α → α+β2+γ → α2+γ + B2. The phase transformation sequence is dramatically changed by rapid quenching from the melt. Athermal ordered w phase is formed in metastable B2 and the α→α2 ordering process is completely suppressed in the melt spun ribbons. The volume fraction of the α precipitates is also dependent on cooling rates.

Fatigue crack growth in intermetallics and their composites published in the literature have been examined in the light of the new concepts developed by the authors. It is shown that the two parametric approach can adequately account for the observed behavior without resorting to an extraneous factor such as crack closure. Implication of the two parametric approach in design is discussed.

Thermal conductivity data at room temperature, which are measured by laser-flash method, are presented in B2 aluminides and titanides, and in nickel based L12 compounds. The thermal conductivity λ is changed in the following order of the compounds. λ(NiAl) >λ(CoAl)>λ(FeAl), λ(NiTi)<λ(CoTi)<λ(FeTi), λ(Ni3Al)>λ(Ni3Si) and λ(Ni3Ga)>λ(Ni3Ge). According to Norbury rule originally proposed for the concentration dependence of electrical resistivity, the increasing rate is greater in the solid solution, where the position of solute elements is more remote in horizontal distance from a host metal in the periodic table. It is found that this rule holds for the thermal conductivity measured for the intermetallic compounds with the combination of a series of guest constituents and a fixed host constituent both in the B2 and Ll2 intermetallic compounds.

Fracture toughness data of stoichiometric FeAl and NiAl single crystals were measured in four-point-bending tests and the crystallographic orientations of the resultant fracture surfaces were determined.

For FeAl single crystals unexpectedly high fracture toughness values were measured even at low temperatures (17 MPa [001] at 77K). However, these crystals failed in a brittle manner for temperatures up to 300K. The crystallographic orientation of the fracture surfaces were strongly dependent on the environment due to hydrogen embrittlement.

The NiAl specimens were precracked and the fracture toughness was measured in the weakest crystal orientation. The fracture surfaces were similar to those obtained in previous investigations. Although the room temperature (RT) KIC-value was the highest for pure weak-oriented NiAl (7.5 MPa [001]) the BDT temperature was found in the same region where it occurs for the hard orientation (673K to 693K). The onset of the BDT will be discussed.

The ordered structure of Fe–40A1 B2 powders was completely disordered by high energy ball-milling. The defect structures of the powders disordered to different extents were characterized using X-Ray diffraction. A significant lattice expansion was observed, which we tried to relate to the high concentration of point defects introduced into the material as a consequence of milling.

A complete reordering was accomplished by heating up the specimens under an inert Ar atmosphere. We followed the kinetics of the disorder to order transformation by Differential Scanning Calorimetry and Thermogravimetric Magnetic Measurements. The two techniques allowed us to identify the temperature range and the apparent activation energy of the process. As expected, a decrease in the magnetic moment of the originally disordered powders as a consequence of the reordering treatments was also observed.

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.

The phenomenon of superplasticity in an ordered FeAl based alloy is reported in this paper for the first time. The behavior of superplastic deformation for the FeAl based alloy (Fe-36.5at.%-2at.%Ti) with large grains of above 300 μm has been examined at 1000°C in a strain rate range from 1.39×10− 4 /s to 2.78×l0−2 /s. The maximum elongation of 208% for the FeAl based alloy with large grains has been obtained and the index of strain rate sensitivity, the m value, has a range of values from 0.25 to 0.42. The reason for the large elongation is ascribed to the dynamic recovery and recrystallization during deformation in this large grained alloy at high temperatures.

The mechanical properties of B2 structural FeAl alloys, prepared by hot rolling, at elevated temperatures have been measured by tensile tests. The alloys of Fe-36.5at.%A1, Fe-36.5at.%A1-5at.%Cr and Fe-36.5at.%Al-2at.%Ti were taken for tensile tests at a temperature range from room temperature to 1000°C. The fracture surfaces of these alloys were observed by SEM. The results showed that elongations of these alloys increased with increasing temperature when the testing temperatures were above 600°C. All the maximum elongations of these alloys appeared at 1000°C and those of Fe-36.5A1, Fe-36.5Al-5Cr, and Fe-36.5Al-2Ti alloys were 120%, 183% and 208% respectively. Fracture surfaces showed that failure of these alloys was by a combination of intergranular fracture and transgranular cleavage below 700°C. but showed a ductile fracture above 700°C. The ductility and strength of ternary alloys were higher than that of binary FeAl alloy at elevated temperatures, especially at high temperature. The <111> dislocations and helices have been observed in Fe-36.5A1 alloy by TEM. The large elongation of FeAl alloy at high temperature resulted from <111> dislocations slipping and <111> helices climbing.

Capacitor Discharge Welding (CDW) is a rapid solidification joining process capable of cooling rates greater than 106 K/s. The Bureau of Mines is investigating the CDW process as a method of joining TiAl, Fe3A1 and MoSi2. Experimental results show that the fusion zone of the CDW welds is less than 0.1 mm wide, is uniform in composition, and has a cellular solidification microstructure. This paper compares the CDW microstructure of several intermetallics to the microstructures obtained from the gas tungsten arc welding.

The U.S. Bureau of Mines is investigating the wear behavior of a variety of advanced materials. Among the many materials under evaluation are intermetallic alloys based on the compounds: Fe3Al, Ti3Al, TiAl, Al3Ti, NiAl and MoSi2. The high hardness, high modulus, low density, and superior environmental stability of these compounds make them attractive for wear materials. This paper reports on the abrasive wear of alloys and composites based on the above compounds. The abrasive wear behavior of these alloys and composites are compared to other engineering materials used in wear applications.

The effects of heat treatment at 1150°C on the creep-rupture properties of a Fe3Al-based alloy were studied. Tests were conducted in air using various test temperatures and stresses. By plotting the data to a power-law equation, the activation energy for creep was determined to be approximately 150 kcal/mole and the creep exponent to be about 8. Optical metallography and scanning electron microscopy of ruptured specimens indicated that failure occurred in a ductile manner by microvoid coalescence at grain boundaries and triple points. Analytical electron microscopy was used to study characteristics of the microstructure and to identify Zr-rich precipitates that formed during heat treatment or creep testing. These precipitates were found to provide strengthening by pinning dislocations and grain boundaries.

By TEM and metallographic examination, we found that dynamic recovery and following continuous recrystallization, which connects with the elevated temperature ductility, takes place in a Fe3Al based alloy during elevated temperature deformation. Dynamic restoration consists of the following process: (1) by climbing or cross-slipping, glide dislocations change into dislocation arrays, (2) the dislocation array attracts lattice dislocations, (3) with increasing dislocation density, a non-equilibrium subboundary forms, which is easy to migrate, slide or rotate under external force, and (4) when the misorientation angle of the subboundary increases to a critical value, the subboundary changes into a grain boundary with continuous misfit. By relaxation, the boundary then changes into a grain boundary consisting of periodical structure units.

Slip in <111> directions is a deformation mode found in many intermetallic compounds ordered into the B2 (CsCl) structure, but in NiAl it is active only under the limiting conditions of uniaxial loading along <100> and low testing temperatures. This paper investigates the deformation characteristics of <100> oriented NiAl tested between 4.2K and 300K. Compression tests have shown that below about 100K, deformation takes place by slip in <111> directions without kinking, whereas between 100K and 300K kinking in the form of shearing on {100} becomes the dominant mode of plastic deformation, occurring usually near one or both ends of the compression samples. The yield stress has been found to exhibit a normal decrease with temperature, but at a rate that is smaller than most bcc metals. The sensitivity of the flow stress to strain-rate changes is also characteristically small, giving activation volumes around b3, b being the Burgers vector of a<111> superdislocation. Both slip line observations and transmission electron microscopy have shown slip mainly on {110} near 80K, but with increasing activity on {112} and cross-slip between {110} and {112} with the lowering of temperature to 4.2K.