Based on simple thermodynamic models free energies of chemisorption of water vapor in transition metal aluminides and enthalpies and entropies of formation of solid solutions of hydrogen in the aluminides are derived. Susceptibilities of the aluminides to moisture-induced embrittlement are evaluated with these thermodynamic properties and compared with the susceptibilities to hydrogen-induced embrittlement.

TiAl binary intermetallics were aluminized in a vacuum system in order to improve their oxidation resistance. From the observations of optical microscopy and EPMA line scan, several different reaction layers were observed to be formed on respective substrates: pure Ti, Ti-32at.%Al, Ti-50at.%Al. From the results of the X-ray diffraction, the primary reaction layers were identified as TiAl3 structure in these three substrates This reacted layer grows with preferred orientation having [002] axis of TiAl3 perpendicular to the substrate surface from XRD experiments. High temperature oxidation tests revealed that vacuum aluminized layer was responsible for remarkable improvement on the oxidation resistance.

Effect of surface treatment on the formation of oxygen enriched brittle layer in a Ti3Al-Nb alloy was investigated. The results show that a mixed Ti2N and TiN layer was formed at the surface by the titanium nitride vapor deposit treatment, and this titanium nitride layer makes it possible to suppress the formation of the oxygen enriched brittle layer during thermal exposure. An Al2O3 film was formed at the surface of the alloy, when the surface treatment was conducted under a low partial pressure oxygen atmosphere. However, such surface treatment is not able to depress the formation of the oxygen enriched brittle layer during thermal exposure, due to the higher affinity of α2 phase for oxygen as well as the higher diffusivity of titanium in Al2O3.

Details of the structure and previous work on the deformation of C15 Laves phases are reviewed. The phase diagram of the Hf-V-Nb system, some metallurgical and physical properties, mechanical behavior, and the deformation mechanisms of HfV2+Nb (C15 HfV2+Nb and V-rich bcc solution) are presented based on our previous work. Theoretical approaches to understanding the results of these studies are discussed.

Neutron powder diffraction measurements have been carried out on cubic Al3TiX compounds (X = Cu, Cr, Mn, Fe and Ni) as a function of temperature from room temperature to 1200°C. The data were analysed by the Rietveld method in order to determine the ordering and lattice site location of the constituent atoms. All these intermetallic compounds were found to be highly ordered at room temperature, and a significant increase in disorder at higher temperatures was observed only in alloys containing Cr and Cu.

Alloys based on Al3Ti with the ordered L12 structure show slight ductility when the Ti content is near 25% and about 8% of Mn or Cr is present as ternary addition. Such materials are relatively soft due to the easy movement of APB-dissociated superdislocations but remain almost completely brittle. While the precise reasons for such brittleness are not clear, it seems reasonable to consider that alloying to lower fault energies may soften the material and enhance ductility. In the present study, new alloys are selected on the basis of electronic structure calculations using the discrete variational cluster method, and the ordered state, mechanical behaviour, and dislocation and fault characteristics examined.

The alloys examined were based on the Al-26%Ti-8%Mn composition, with lower Ti and Mn contents, in an attempt to maintain a single-phase matrix and weaker, less-directional bonding and lower fault energies. In addition, for some alloys, Al was partially substituted by Mg.

The single-phase region of the L12 Al-Ti-Mn phase is very small and second phases such as DO22 and complex AlxMn can appear which lead to hardening and strain ageing, much as Al2Ti does in more Ti-rich alloys. Mg substitution is limited to a few percent before a Mg-rich phase appears and the alloys examined are complex mixtures of this, the L12 phase, and γTiAl. The dislocation structures observed after deformation are examined to determine fault energies, and it is shown that these values can be rationalised in terms of the structural instabilities of the matrix phases and the secondary phases produced.

TEM investigation associated with computer image simulations were conducted to determine the dislocation core structure in the Ll2 Al67MnsTi25 alloy deformed at room temperature. It is identified that the a<110> superdislocations dissociated into pairs of a/3<112> superpartials with SISF between them. The theoretical and experimental micrographs are in good agreement under various diffraction conditions. The unusual temperature dependence of flow stress at low temperature can be related to the SISF-type dissociation.

Single phase LI2 (Al,Cr)3Ti alloys show no anomalous strengthening at intermediate temperatures, but the strength rises rapidly at low temperatures. It is likely that a transition from SISF to APB superdislocation dissociation occurs with increasing temperature based on the more rapid decrease in APB energy compared to SISF energy because of entropy considerations. High strain hardening rates are suggested to arise in large part from the formation of large numbers of dipoles. Calculations of interaction forces illustrate the complexity of superdislocation interactions and provide a reasonable mechanism for initiating cross slip.

Two types of aluminum–titanium–iron–vanadium ( Al–Ti–Fe–V ) quarternary intermetallic compounds have been prepared by arc melting under argon atmosphere. Their compositions were nominally Al66Ti25Fe6V3 and Al66Ti25Fe3V6. These alloys are based on the iron–modified titanium trialuminide with L12 cubic structure. Vanadium addition up to about 6 mol% did not destroy the cubic symmetry, and L12 solid solution compounds were produced in these two Al–Ti–Fe–V quarternary alloys. Microstructure and mechanical properties have been investigated. It has been demonstrated that vanadium addition to iron–modified L12 titanium trialuminides can enhance their strength.

An attempt was made to change the crystal structure of the intermetallic compound Al3Ti from DO22 to Ll2, by replacing Ti with several alloying elements. The alloying elements chosen in this study were Sc, Er and Lu with which Al forms intermetallic compounds having LI, structure. Ingots weighing about 50g of Al3(Ti1-xMx) — M represents alloying element and the x value is 0, 0.25 or 0.5 — were melted in an argon plasma arc furnace. X-ray diffraction, energy dispersive spectroscopy and scanning electron and optical microscopy were carried out to examine the microstructurc in relation to the kind and amount of alloying elements. Microstructural observation revealed the occurrence of an Ll2 phase in all M bearing alloys. Compression tests were also carried out to assess the ductility of these alloys. The results showed that the Sc alloying is effective to increase the room temperature ductility of Al3Ti.

An alloy with composition 27Nb-33Ti-40Al (at. %) was heat treated to produce a fine two-phase microstructure of σ (Nb2Al) and γ (TiAl) phases. This microstructure exhibited a peak in the flow curves obtained in the strain rate range 5 × 10−4 to 5 × 10−6s−1 at 1000°C, with a stress exponent of 3. The deformation occurred by dislocation activity and twinning accompanied by dynamic recrystallization of the γ-phase and extensive grain boundary sliding of the σ-phase, which resulted in the separation and fragmentation of the latter phase.

A hot workability study was conducted on an orthorhombic Ti-21AI-23Nb alloy using isothermal, constant velocity compression tests, over the temperature range 900–1155°C and nominal strain rate range 0.001 to 1.0 s−1. Three different microstructures corresponding to (a) hot rolled and mill annealed, (b) β0 -heat treated, and (c) β0 solution treated and direct aged conditions were examined. These microstructures varied from a relatively coarse distribution of O-phase precipitates in a matrix of transformed βO to the metastable retained βO. Flow stress displayed a sharp dependence on temperature, strain rate and on the initial structure. While uniform plastic flow was obtained at high temperatures and for structures containing primary O, βO-heat treated structures led to pronounced flow softening. Strong yield point phenomena and serrated flow were observed in selected test conditions and for microstructures containing high volume fractions of metastable βO. These flow instabilities resulting from Luder band propagation and dynamic strain aging are interpreted in terms of dislocation interactions in supersaturated βO.

The microstructure-property relations were evaluated for a nominally Ti-25Al-17Nb orthorhombic alloy, in terms of the deformation and failure mechanisms of the constituent ordered phases (orthorhombic, alpha-2, and B2). The mechanisms were characterized through observation of slip traces, crack initiation sites, and TEM. Properties of interest were the room temperature elongation, fatigue crack growth (FCG) resistance, and creep resistance.

The discovery of ductile cubic phases in the Nb-Ti-Al system has led to increased study of these high-temperature intermetallics. I have performed first-principles calculations for ordered crystal structures in this system, paying particular attention to the Nb7Ti7Al2 structure. Somewhat surprisingly, the electronic density of states, lattice constant, and bulk modulus are nearly independent of the ordering of these materials, even though the changes in the total energy are significant.

Oxidation resistant intermetallic compounds based on NiAl, TiAl, and MoSi2 are of interest for high temperature applications. Each system exhibits different life-limiting degradation modes due to oxidation. β-NiAl forms protective α-Al2O3 scales. Breakdown follows well-established diffusion controlled processes resulting in survival for thousands of hours. The effect of thermal cycling and spalling is well established. Ti3Al and TiAl compounds form less protective mixed TiO2 and A12O3 scales. However at realistic use temperatures (600°–800 °C), scale growth rates are acceptably low. The critical factor is embrittlement due to interstitial oxygen diffusion over a matter of hours. Solutions based on alloy development and coatings have not been satisfactory. MoSi2 materials exhibit very low oxidation rates at very high temperatures. However, low temperature (500°C) pest oxidation can be a catastrophic transient effect. Material integrity is a key factor. Fracture occurs because of accelerated growth of non-protective mixed MoO2–SiO2 scales in pores and microcracks.

MoSi2, (γ-NiAl and TiAl with Cr additions are of interest for high temperature applications in oxidizing environments, where an oxide layer such as SiO2 or Al2O3 should form to protect the base material. At elevated temperatures (600-1700°C), a protective SiO2 layer forms on MoSi2, while near 500°C pesting and/or accelerated oxidation could disintegrate the material to powder as Mo and Si oxidize to form a complex, thick, non-protective oxide layer. Use of γ -TiAl is limited by poor oxidation resistance, whereby layered mixed oxides of TiO2 and Al2O3 form. With the addition of Cr from 4 to 34 at%, results are varied: protective Al2O3 formation, mixed oxide formation as with TiAl or more rapid oxidation than TiAl. NiAl is currently used as a diffusion coating on Ni-based superalloys and is being considered for use as a structural material itself because of its excellent oxidation resistance, i.e. forming α-alumina above 1000°C. Recent work indicates that pure NiAl oxidized under low oxygen partial pressures in a contained atmosphere develops nonprotective oxide scales similar to accelerated oxidation of MoSi2. This study explores the parameters defining protective behavior of these intermetallics and attempts to describe and explain anomalies at low temperatures and pressures.

The effect of Y and Zr on the oxidation behaviour of NiAl has been investigated using high resolution SIMS and analytical TEM. 18O tracer experiments in conjunction with SIMS showed that Y and Zr reduce the outward transport of cations during the growth of the AI2O3 scale. Complementary STEM-EDS measurements revealed segregation of Y and Zr at the metal/oxide interface as well as the grain boundaries in the AI2O3 scale. The amounts of segregation at both the interface and grain boundary were calculated to be less than one monolayer.

Small punch (SP) tests on single grained titanium aluminide (Ti-48 at.%Al) specimens with 12° and 80° lamellar orientations with respect to the tensile stress axis were conducted at 1123 K in air. Brittle cracks readily extended through the thickness in the 80° lamellar structure. In a SP specimen with the 12° lamellar structure load-interrupted at the strain of 0.43%, surface cracks with the depth of 15–25 μm were formed along lamellar boundaries. Local oxidation behavior on partly sputtered surfaces in the load-interrupted 12° lamellar specimen was examined using scanning Auger microprobe (SAM). Oxygen enriched regions were observed near cracks and some lamellar layers. The mechanisms of high temperature oxygen-induced cracking are discussed in terms of the local oxidation near cracks and lamellar boundaries.

Al2Ti is a promising material for use in elevated temperature structural applications. It has a lower density than Ti3Al and TiAl. We have previously shown that the compressive yield strength of Al2Ti is higher than that of similarly processed TiAl and Al3Ti between room temperature and 900°C. In this study, two additional processing methods were utilized to produce the material: uniaxial hot pressing of rapidly solidified powder and hot forging of cast ingots. Effects of hot forging or annealing of as-cast Al2Ti on the microhardness and load for crack initiation were studied. The microhardness and crack initiation load were also used to compare powder processed material consolidated either by hot pressing or hot isostatic pressing. The oxidation behavior of as-cast, cast & hot isostatically pressed (cast & hip’ed), and powder processed & hip’ed (PP) Al2Ti was also investigated. The oxidation tests were performed in air at 815°C and 982°C for 100 hr. PP Al2Ti exhibited the lowest weight gain and as-cast exhibited the highest. Al2Ti shows superior oxidation resistance compared to Ti3Al- and TiAl- based alloys tested under similar conditions and shows comparable oxidation resistance to Al3Ti at 800°C. SEM/EDS was used to study the morphological development of the oxide scale and to identify the chemical constituents present in the various layers of the scale.

The relationships between alloy microstructure and air oxidation kinetics and alloy microstructure and microhardness in the Al-Ti-Cr system for exposures at 800°C and 1000°C were investigated. The relevant phases were identified as τ (Ll2), γ (Ll0), r-Al2Ti, TiCrAl (laves), and Cr2Al. Protective alumina formation was associated with τ, Al-rich TiCrAl, and γ/TiCrAl mixtures. Brittleness was associated with the TiCrAl phase and τ decomposition to Al2Ti + Cr2Al. It was concluded that two-phase γ + TiCrAl alloys offer the greatest potential for oxidation resistance and room temperature ductility in the Al-Ti-Cr system.