The mechanical behavior and microstructures of TiAl alloys after tensile and bend tests at room temperature and elevated temperatures were studied. The results for two-phase Ti52Al48 alloys are compared with those of single phase Ti48Al52, and the effect of adding 3 at. pct. vanadium as a substitute for Ti in these two alloys is considered. It is shown that Ti52Al48 has greater strength and ductility than Ti48Al52 at room temperature and elevated temperatures up to 871°C (1600°F). Adding vanadium increases the ductility of both binary alloys. The microstructure of the Ti52Al48 alloy deformed at room temperature contains primarily twins and 1/2<110> easy slip dislocations, whereas the similar Ti48Al52 sample exhibits superdislocations and associated pinned faulted dipoles. If these samples are deformed at 540°C (1000°F) or above, the Ti52Al48 exhibits extensive twinning, and the pinned faulted dipoles in the Ti48Al52 sample disappear. The vanadium additions do not noticeably change the deformation microstructure at room temperature. It is suggested that the strength and ductility of these alloys may be controlled by tetragonality, bonding, interstitial element, and grain size effects, which in turn are affected by the presence of second phases and by the alloy composition.

Constant stress compression creep behavior at elevated temperature was investigated on Ti-53.4mol%Al polycrystalline intermetallics of gamma single phase in different grain morphologies: coarse columnar grains, fine recrystallized grains and equiaxed grains. The shape of creep curves depended on the applied stress: normal primary transient followed by steady creep was found at higher stresses, but at lower stresses steep acceleration was observed after taking minimum creep rates. This behavior was common to all the grain morphology. The creep rate itself was not greatly different among the three types of structure, and the stress exponent of about 4 and the apparent activation energy of about 340kJ/mol at 0.3 true strain were common to all three types. The shape and size of grains do not affect the essential creep features of this material.

The dislocation structures in rapidly solidified Al3Ti with the DO22 structure and the ternary Al-25Ti-8Ni (at.%) alloy with the L12 structure deformed in compression in the temperature range of 25 to 800°C have been studied by transmission electron microscopy. The room temperature deformation microstructure of the Al3Ti compound is characterized by the presence of stacking faults/order twins on {111} planes bounded by partial dislocations with Burgers vector b=1/6<112], as reported by others. At intermediate temperatures, besides the stacking faults, slip is also observed as bands on the {001] plane delineated by dislocations with b=1/2<110] which bound APB's. At 600°C, the reported increase in ductility is associated here with additional slip on the {001)<110], {001)[100] and {001)[010] systems. Dislocations with b=<110] exist as pairs of partial dislocations with b=1/2<110] connected by APB's. The mean separation between the partials was measured to be 30 nm, corresponding to an APB energy of ≍32 mJ.m-2 on the (001) plane. Observations also indicate that the APB energy is anisotropic, i.e., is considerably higher on the {111} planes compared to the {001) plane. The deformation microstructure of the Al-25Ti-8Ni L12 alloy is characterized by slip of dislocations with b=<110> gliding on {111} planes, a major fraction of which exist as dipoles. Following deformation at 300°C, there is essentially no evidence of dissociation of these dislocations, although some dissociated dislocations on (001) having b=l/2<110> are also observed. With an increase in temperature, there is a considerable increase in dislocation activity and strong evidence for 1/2<110> dissociated dislocations is present.

The effect of hydrogen on the phase stability of γ-TiAl and α2-Ti3Al phases was studied, with emphasis on the investigation of decomposition of a metastable α2 phase to a stable γ phase in gamma alloy powders. In this work, progressive decomposition of α2 → γ was observed to occur during aging with increasing temperatures or time under vacuum and under H2. The microstructural analysis indicated that hydrogen may facilitate such decomposition to occur at lower temperatures and shorter aging time.

A new approach to powder processing is employed in forming titanium aluminide composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of a homopolar generator. The high-energy high-rate “1MJ in 1s” pulse permits rapid heating of an electrically conducting powder mixture in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible with standard powder processing approaches. This paper describes the consolidation results of titanium aluminide-based powder composite materials. The focus of this study was the definition of microstructure/processing relationships for each of the composite constituents, first as monoliths and then in composite forms. Non-equilibrium phases present in rapidly solidified TiAl powders are transformed to metastable intermediates en route to the equilibrium gamma phase. The initial single phase beta in Nb-stabilized Ti3Al is transformed to alpha two with an intermediate beta two phase. In composite blends of TiAl powders mixed with Nb-stabilized Ti3Al powders a 10 μm thick interfacial layer is formed on the dispersed TiAl. Limited control of post-pulse heat extraction prevents full retention of the rapidly solidified powder microstructures.

The exothermic formation of intermetallic compounds from the constituent powders through reactive sintering at relatively low temperatures is a promising new process being explored for the preparation of nickel aluminides. The process is useful for synthesis of other intermetallics as well.

We describe here the preparation of Ti2CuAl5 using the technique of reactive sintering. This intermetallic compound has a L12 structure which is also the structure of the very interesting Ni3Al intermetallic. Characterization of the Ti2CuAl5 phase is done by x-ray diffraction, microhardness, and microstructure.

In order to correlate the flow stress anomaly of Ni3Si with dislocation properties, a weakbeam study ofpolycrystalline samples deformed between ambient and the peak temperature was carried out. Samples with two extreme Ni/Si ratios were tested.The most frequently activated slip system changes progressively from octahedral to cubic with increasing temperature. The transformation of superdislocations into Kear-Wilsdorf configurations gives rise to screw dislocations that are rectilinear only after deformation at room temperature. The effect of temperature is to gradually promote bending of Kear-Wisdorf configurations in the cube plane, from a few nanometers at 230°C to several tenths of micrometers at intermediate temperature. Cube slip begins to be massively activated a little below the peak temperature. It is suggested that the flow stress anomaly is controlled by progressive exhaustion of octahedral slip by thermally-activated expansion of superdislocations on the cube cross-slip plane.

β-brass single crystals oriented along <001> were deformed between room temperature and 300°C. The deformation microstructure and dissociation properties were studied by transmission electron microscopy under weak-beam imaging conditions.

Whatever the deformation temperature, superdislocations with <111> Burgers vector and strong edge component dominate within the microstructure. In addition, below the temperature of the flow stress peak (≈ 250°C), the density of screw relative to mixed superdislocations decreases as straining temperature increases. Dissociation does not always occur on the slip plane neither does it proceed exclusively by glide, even in samples deformed at 100°C.

It is demonstrated that the hardness of NiBe is dependant upon its stoichiometry; a minimum hardness is observed at the equiatomic composition. This behavior is similar to intermetallics that have the same crystallographic structure, e.g., NiAI and CoAl. The hardness increase for the off-stoichiometric compositions is a result of defect and anti-site defect structures, but may also, in part, be caused by interstitial oxygen. Nickel beryllides appear to have some intrinsic room temperature ductility, as evidenced by the absence of cracking near hardness indentations.

The domain structure of the alloy Ni-20 at. % Mo depends upon the heat treatment below the critical temperature (868°C). Upon aging around 750–850°C, initially a very fine domain structure forms inside the a grains, which then coarsens, and then migration of these boundaries occurs with a coarser domains structure behind them. In the ordered condition, the alloy is very brittle. Fractographs are presented which correlate the fracture morphology to the microstructures of the ordered alloy, showing that fracture occurs along the migrated, former α grain boundaries.

Rods of five FeAl alloys whose compositions span the B2 phase field (34–51 at. % Al) were produced by multiple hot extrusions of ingots. Samples of each alloy were annealed in air at 1173K and 1473K in order to study the effect of alloy composition on both the grain growth kinetics and on the rate of oxidation. Little oxidation occurred at 1173K but at 1473K the oxidation rate was found to decrease with increasing aluminum concentration. The predominant oxide was found to be α-A12O3. The grain size was found to increase rapidly during the first hour of annealing at both temperatures after which it increased more slowly. The grain growth rate was found to decrease with increasing aluminum content.

Hardness measurements were made on both as-extruded and annealed samples of FeAl. Hardness increased with increasing aluminum content. The hardness was higher in annealed and air-cooled samples due to vacancy retention. Compression tests of air-cooled, annealed samples also showed higher yield strengths than as-extruded samples. Yield strength and strain hardening rate were found to increase and the ductility was found to decrease with increasing aluminum content.

Alloying additions of silicon to the intermetallic titanium-aluminide Ti4Al lead to two-phase microstructures consisting of α2-Ti3(Al, Si) and Ti5(Si, Al)3.

Although details of the ternary Ti-Al-Si phase diagram are not available, theoretical analysis and experimental screening have indicated the existence of a eutectic line in the Ti-rich region of the Ti3Al + Ti5Si3 two-phase region. The eutectic alloys display dual phase structures with Ti3(Al, Si) being the matrix and Ti5(Si, Al)3 the second phase.

In this paper the microstructures of the eutectic as well as hypoeutectic and hyper-eutectic alloys, which contain additionally primary Ti3(Al, Si) and Ti5(Si, Al)3 needles, respectively, are described. The influence of the particular microstructure on various temperature-dependent mechanical properties such as strength, ductility, creep and fracture behaviour has been investigated. Alloys with hypo-eutectic microstructures show promising high temperature mechanical properties.