The effects of the environment on the room temperature mechanical behavior of Fe-43Al single crystals have been studied. In both single slip and duplex slip crystals, fracture strains greater than 40% were obtained in specimens tested in oxygen, whereas elongations of ∼10% and ∼20% were obtained in air and vacuum, respectively. By comparison, similar elongations were obtained in boron-doped single-slip-oriented single crystals in both air and vacuum, but more ductility was obtained in air at slow strain rate. Fractography showed that testing in different environments produced marked differences in the fracture surfaces. Alternate loading of tensile specimens in air and under vacuum was performed at slow strain rates and showed changes in the flow stress between the two environments. The results are discussed in terms of the effects of moisture-produced hydrogen on the flow and fracture of FeAl.

The phase decomposition of the Al-rich γ TiAl intermetallic compound in the TiAl(L10) and Ti3Al5(P4/mbm) two phases region is investigated experimentally. On the phase decomposition of the Ti-56at%Al alloy, the single precipitate(Ti3Al5) shape is an oblate spheroid at the early stage of precipitation and each particle is aligned along certain direction of the orientation about 20 degrees from [100]. During coarsening, the precipitates encounter each other, then, the shape of the particle becomes the slanted or bended plate. In the case of phase decomposition of the Ti-58at%Al alloy, the tweed-like structure is observed at the beginning of the aging. The precipitates are connected each other during coarsening, finally the microstructure becomes the large layered structure with a zigzag-shaped interface. These microstructure changes are simulated based on the phase field model. The morphology and the time development of the simulated microstructure are in good agreement with the experimental results.

The Ni-33Al-33Cr-1Mo eutectic has been directionally solidified by a modified Bridgeman technique at growth rates ranging from 7.6 to 508 mm/h to produce grain/cellular microstructures containing alternating plates of NiAl and Cr alloyed with Mo. The grains had sharp boundaries for slower growth rates (≤ 12.7 mm/h), while faster growth rates (≥ 25.4 mm/h) lead to cells bounded by intercellular regions. Compressive testing at 1300 K indicated that alloys DS'ed at rates between 25.4 to 254 mm/h possessed the best strengths which exceed that for the as-cast alloy.

This study examines the influence of microstructural parameters (grain and dispersoid size, vacancy content) and some test parameters (strain rate, protective oxide coatings, air and water vapour excluding films, and surface geometrical quality) on the tensile behaviour (yield stress, work hardening rate, tensile stress, ductility) of a mechanically-alloyed, fine-grained Fe-40Al intermetallic. Major changes of strength and ductility are obtained by changing grain size (1% and 10% for grain sizes of 100μm and 1μm) and by avoiding premature stress/strain concentrators (ductility increased from 5% to 10% for imperfectly machined to prepolished samples). Ductility variations are interpreted using a slow-crack-propagation-to-instability model, where the roles of environment, surface state, deformation processes, and fracture mechanisms can be distinguished.

Single crystal Mo3Si specimens were grown and tested at room temperature using established nanoindentation techniques at various crystallographic orientations. The indentation modulus and hardness were obtained for loads that were large enough to determine bulk properties, yet small enough to avoid cracking in the specimens. From the indentation modulus results, anisotropic elastic constants were determined. As load was initially increased to approximately 1.5 mN, the hardness exhibited a sudden drop that corresponded to a jump in displacement. The resolved shear stress that was determined from initial yielding was 10–15% of the shear modulus, but 3 to 4 times the value obtained from the bulk hardness. Non-contact atomic force microscopy images in the vicinity of indents revealed features consistent with {100}(010) slip.

B2-type CoTi intermetallic compound that was hot-rolled and recrystallized was tensile-tested as functions of temperature and testing atmosphere. The tensile strength showed a peak at intermediate temperature (∼800K). The brittle-ductile transition (BDT) defined by tensile elongation took place at about 800K, above which large tensile elongation was observed. Corresponding to this transition, SEM fractography showed a change from cleavage-like pattern mixed with intergranular fracture pattern to large cross-sectional reduction, i.e. necking of the tensile specimen. Also, the observed mechanical properties were independent of heat-treatment procedures, indicating that retained vacancies did not affect the mechanical properties of CoTi intermetallic compound. However, the tensile elongation and UTS at room temperature were dependent on testing atmosphere, indicating that moisture-induced embrittlement occurred in CoTi intermetallic compound.

Thin foils of stoichiometric Ni3Al below 100 μm in thickness were successfully fabricated by cold rolling of the sheets which were sectioned from directionally solidified ingots. Maximum rolling reduction in thickness amounted to 96%, irrespective of the initial orientation or the existence of columnar grains in the starting sheets. The as-rolled foils were characterized in terms of microstructures, textures and dislocation structures. The deformation microstructures were of a dual banded structure composed of two different {110} textures in the case of <001> rolling direction, while a rather homogeneous structure with a single {110} texture resulted in the case of <112> rolling direction. TEM observation revealed homogenous dislocation structures in either case without cell formation, accompanied by very fine grained-regions at higher reduction.

The compression behavior in a multi-anvil apparatus of a foil of Ni3Al embedded in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction (EDX). At ambient temperature, the pressure and stresses, determined from line positions of NaCl, were constant throughout the sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. Ni3Al polycrystals, in contrast, undergo extensive (ductile) plastic deformation above 4 GPa due to the onset of high non-hydrostatic stresses and the introduction of stacking faults and dislocations. Plastic deformation due to stacking faults leads to a volume incompressibility followed by elastic compression of a fully plastically deformed state. The compression of a fully plastically deformed material is elastic and isotropic, independent of the presence and type of pressure medium. A discontinuity in the compressibility at the transition back from plastic to elastic compression is due to the yield strength of the plastically deformed material and corresponds to the Hugoniot elastic limit.

First-principles total-energy electronic structure calculations based on the full-potential linearized augmented plane wave (LAPW) method have been used to study the electronic and elastic properties of MoV, MoNb, and MoTa with the B2 (CsCl) estructure. From the calculated values for the bulk modulus we have determined the melting temperature using an empirical correlation. The chemical bond and the electronic structure around the Fermi level are analyzed. In particular, we found that MoTa which have the experimental determined highest melting point of the studied materials, present the largest bulk modulus and the highest degree of covalence bonding of these intermetallic compounds.

Microsample test specimens of single crystalline γ-Ti 55.5at%Al oriented near the [001], and [-110] crystal axes have been deformed in tension and compression at 973K. From these experiments, measurements of the 0.2% offset flow stress have been made as a function of temperature, crystal orientation and sense of the applied load. A measurable violation of Schmid's law was observed and a significant tension/compression asymmetry has been observed at this temperature for the above listed orientations. Single-cycle loading experiments designed to measure the tension/compression asymmetry of the yield strength on the same sample have been conducted along the ∼[-110] and [001] orientations. The flow strength measurements from the specimens, which underwent the fully reversed cyclic-loading experiment, fall near those of the monotonically loaded microsamples deformed at the same temperature, which suggests that the same deformation mode was active in both tension and compression.

This paper will describe the creep behavior of high-temperature Nb-silicide in-situ composites based on quaternary Nb-Hf-Ti-Si alloys. The effect of volume fraction of silicide on creep behavior, and the effects of Hf and Ti additions, will be described. The composites were tested in compression at temperatures up to 1200°C and stress levels in the range 70 to 280 MPa. At high (Nb) phase volume fractions the creep behavior is controlled by deformation of the (Nb) and, as the volume fraction of silicide is increased, the creep rate is reduced. However, at large silicide volume fractions (>0.7) damage in the silicide begins to degrade the creep performance. The creep rate has a minimum at a volume fraction of ∼0.6 silicide. The creep performance of the monolithic and silicide phases will also be discussed.

Effects of ternary additions on the deformation behavior of single crystals of MoSi2 with the hard [001] and soft [0 15 1] orientations have been investigated in compression and compression creep. The alloying elements studied include V, Cr, Nb and Al that form a C40 disilicide with Si and W and Re that form a C11b disilicide with Si. The addition of Al is found to decrease the yield strength of MoSi2 at all temperatures while the additions of V, Cr and Nb are found to decrease the yield strength at low temperatures and to increase the yield strength at high temperatures. In contrast, the additions of W and Re are found to increase the yield strength at all temperatures. The creep strain rate for the [001] orientation is significantly lower than that for the [0 15 1] orientation. The creep strain rate for both orientations is significantly improved by alloying with ternary elements such as Re and Nb.

In this work, the recent breakthroughs in the understanding of the fracture behavior and fracture toughness of L12-ordered titanium trialuminides are described and discussed. First, it is shown that, as opposed to many other intermetallics and specifically those with an L12 crystal structure, the fracture toughness of L12 titanium trialuminides is insensitive to testing in various environments such as air, water, argon, oxygen and vacuum (∼1.3×10–5 Pa). Second, it is reported here that by increasing the concentration of Ti combined with boron (B) doping, the room temperature fracture toughness of a Mn-stabilized titanium trialuminide can be improved by 100% from ∼4 MPam1/2 to ∼8 MPam1/2 and by 150–250% at 1000°C to ∼(10–12) MPam1/2 with a simultaneous suppression of intergranular fracture (IGF) to ∼(40–50%). Almost three fold increase in yield strength to ∼550 MPa is attained at room temperature for high Ti, boron-doped trialuminides. Both Vickers microhardness and strength increase linearly with increasing concentration of (Ti+B) indicating a classical solid solution strengthening response.

Intermetallic alloys are often doped with boron to suppress their intrinsic room-temperature intergranular brittleness. The commonly admitted mechanism of this effect, i.e. an intergranular segregation of boron, seems not to be the only important feature. In this work, boron interactions with numerous kinds of crystal defects (point defects, dislocations, grain boundaries) are studied in B-doped FeAl (B2) alloys containing 40 at. % Al. The intergranular segregation of boron is first characterized. Both an equilibrium and a non-equilibrium (due to a solute atom / thermal vacancy interaction) segregation mechanisms are identified. Strong tendency of boron to segregate to the dislocations lines is shown by direct measurements by atom probe field ion microscopy (AP FIM). This segregation is shown to induce a local depletion in Al in the vicinity of defects.

The effect of strain rate on tensile ductility of moisture-embrittled L12-type Co3Ti and Ni3(Si, Ti) ordered alloys was investigated at ambient temperatures (298K∼423K) by tensile test and SEM fractography. The anomalous increase of tensile elongation and ultimate tensile stress was observed in a low strain rate region and also at high temperatures, accompanied with an increased area fraction in ductile transgranular fracture pattern. The anomalous strain rate dependence of tensile ductility was shown to become more evident with decreasing grain size. As a process counteracting to the hydrogen decomposition from moisture in air, oxidation process on the alloy surface was suggested.

Iron aluminides are of considerable interest due to their low cost, relatively high melting point, relatively low density, and excellent resistance to oxidation, sulfidation and molten salts. However, poor ductility and fracture toughness at room temperature hinder their use as a structural material. Refining of the microstructure is known to be one method to increase the room temperature ductility. Mechanical alloying (MA) is an easy way to obtain fine microstructures. In addition, pulse discharge sintering (PDS) is a new technology which suppresses grain growth during sintering because the sparks generated during sintering break the surface oxide layer of the powder particles and thus speed up the sintering process. Therefore, the combination of MA and PDS processes results in final products with very fine microstructures. Fe-28at.%Al alloy and its composite reinforced with 5vol.% of TiB2 particles were fabricated by the MA-PDS process. The mechanical properties of these materials were improved significantly as compared to conventionally processed materials.

The structure of ternary and quaternary NiAl-based ordered intermetallic alloys is studied using the BFS method for alloys. A simple calculational procedure, based on the determination of the energetics of local environments surrounding defect atoms is introduced and applied to the study of the defect structure and phase formation in NiAl-based systems. The procedure is illustrated with two different examples: 1) the phase structure of Ni-Al-Fe alloys, focusing on the concentration dependence of the site preference behavior of Fe in NiAl, and 2) the precipitation of a β′ phase in NiAl(Ti,Hf) alloys, focusing on the role of Hf in lowering the solubility limit of Ti in NiAl, thus enhancing the precipitation of a Heusler Ni2Al(Ti,Hf) phase.

Molecular dynamics simulations of the diffusion process in ordered B2 NiAl at high temperature were performed using an embedded atom interatomic potential. Diffusion occurs through a variety of cyclic mechanisms that accomplish the motion of the vacancy through nearest neighbor jumps restoring order to the alloy at the end of the cycle. The traditionally postulated 6-jump cycle is only one of the various cycles observed and some of these are quite complex. A detailed sequential analysis of the observed 6-jump cycles was performed and the results are analyzed in terms of the activation energies for individual jumps calculated using molecular statics simulations.

Temperature dependence of thermoelectric power and resistivity was measured for the β-FeSi2 based cast alloys on which n-type Co doping was performed with or without Cu addition. The Cu-free alloy shows a homogeneous eutectic microstructure consisting of metallic phases α-FeSi2 and ε-FeSi, while the Cu-doped alloy has a quite heterogeneous microstructure dominated by coarse ε-FeSi dendrites in the α-FeSi2 matrix. Adequate heat treatment condition for the semiconductor β-FeSi2 phase formation has been evaluated as annealing at 923 K for less than 50 h, which is lower in temperature and shorter in duration than previously reported for alloys prepared by powder metallurgy. The resistivity measurement with the aid of microstructure observation has revealed that the β-FeSi2 formation takes place and almost finishes at very early stage of annealing process. Addition of Cu effectively promotes the β-FeSi2 formation rate of the present cast alloys.

Properties of ordinary dislocations and related debris are investigated in single crystals of γ-TiAl deformed in single slip at 20°C and 400°C. Typical of this temperature range are prismatic loops isolated or belonging to arrays, together with multipoles. The interaction of these with mobile ordinary dislocations generates various configurations including trailing-like hairpins. Certain features are consistent with an extrinsic origin for point pinning.