Oxide-dispersed Fe-28at.%Al-2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. Yttria and ceria were added to the base alloy to assess the effect of these dopants on the oxidation behavior. The amount of dopant was varied from 0.05–0.5 at.% Y in a series of Y2O2-dispersed alloys. Isothermal and cyclic oxidation testing was conducted at temperatures from 800˚ to 1300˚C. A Ce02 addition was detrimental to the oxidation behavior. The Y2O3 improved the (α-Al2O3 scale adhesion relative to an undoped alloy, but was not as effective as similar additions to an oxide-dispersed FeCrAl alloy.

The microstructures of arc melted ingots and melt spun ribbons of NbSi2 and NbSi2-(8,20 at%)Cr alloys are discussed. Rapid quenching from the melt refined the grain size of the alloys by two orders of magnitude and suppressed the formation of Nb5Si3 in Zone A of the binary alloy ribbons. In the ternary alloy ingots and ribbons formation of the Nb5Si3 was suppressed, and the C40 NbSi2 and CrSi2 and orthorhombic Nb2Si6Cr3 phases were formed. Zone A was not formed in the ribbons of the ternary alloys. The microhardness of these phases and the oxidation behaviour of the alloys up to 1350°C in air and an Ar atmosphere are discussed. NbSi2 did not form a protective silica layer on oxidation. Instead a mixed oxide scale of silica and α-Nb2O5 formed which cracked during growth of the oxide and hindered protection of the alloy. It is shown that the addition of Cr modifies the oxide to a duplex oxide of SiO2 and Cr2O3 and that the underlying matrix becomes depleted in Cr. Pest oxidation at 750°C was suppressed in Nb-60Si-20Cr.

The high-temperature oxidation resistance of Nb-40Ti-15Al-(0 or 5)Cr alloys has been studied. The kinetics and activation energy for the isothermal oxidation of Nb-40Ti-15Al are similar to pure titanium. Detailed TEM analysis of the oxidized Nb-40Ti-15Al alloy reveals that oxygen dissolution has produced a case layer by the decomposition of the matrix B2 phase into oxygen saturated alpha-Ti and an A15 phase. Internal oxidation of the A15 phase to form AlNbO4 occurred in the case layer near the scale/metal interface. The phase separated case layer evolved into a segregated oxide layer containing a co-continuous TiO2 phase. These morphologies of the oxide scale and case layer correlate well with the kinetic data, and suggest that oxygen dissolution in α-Ti and diffusion through TiO2 control the high-temperature oxidation rates. The Cr additions reduced the thickness of the case layer and slowed the oxidation kinetics, but did not change the mechanism. In addition, excellent cyclic oxidation resistant was provided by the silicide diffusion coatings.

A series of two-phase chromium silicide base intermetallics have been investigated with respect to creep behavior and oxidation resistance. The first results of these investigations are reported. Good oxidation resistance has been demonstrated in one composition, which was due to the formation of the protective vitrious SiO2 layer, while the chromium oxide layer is very helpful at the intermediate temperature range.

Oxidation resistance (1000-1400°C) was characterized for intermetallics M2Al, M3Al, M2Cr, and M5Si3 (M=Nb, Nb+Ti), and metal-toughened intermetallic composite systems. Aluminides were poorest in oxidation, and Laves phases were best. Substitution of Ti for Nb was beneficial for each intermetallic. Addition of metal to produce two-phase castings had differing effects on oxidation, depending on the metal and intermetallic. Modulus and expansion were also measured.

The results of a recent study of the fatigue crack growth in a new class of niobium aluminide (Nb3Al) intermetallics alloyed with various levels of Ti are presented. The alloys studied have the following nominal compositions (note that all compositions are quoted in atomic &): Nb-15Al-10Ti(10Ti alloy), Nb-15Al-25Ti(25Ti alloy), Nb-15Al-40Ti(40Ti alloy) and Nb-12.5A1–41Ti-1.5Mo(41Ti alloy). Fatigue crack growth behavior was studied at room-temperature in all the alloys. Fracture mechanisms were determined by inspection of SEM images of fracture surfaces. Faceted cleavage fracture modes were apparent on the lOTi and 25Ti alloys. The 40Ti and 41Ti alloys exhibited clear evidence of ductile dimpled fracture. The ductile 40Ti and 41Ti alloys are shown to have comparable or better fatigue crack growth resistance than mill annealed Ti-6A1–4V. The effects of crack orientation (relative to elongated grains) on the fatigue crack growth behavior of the 41Ti alloy are also discussed.

The microstructure of arc melted ingots and melt-spun ribbons of Nb-18A1, Nb-18Al-(20–40) Mo (at%) were studied. Rapid quenching from the melt refined the grain size by at least two orders of magnitude. In the binary alloy ingots and ribbons the Nb3Al (A15) and B2 phases were present. Molybdenum exhibited complete solid solubility in the bcc Nb-Al solid solution as well as in the Nb3Al (A15 phase). The former was the only phase present in the ribbons of the ternary alloys, while Nb3Al was detected only in the ingots. B2 phase was formed in the alloy with 20 at% Mo and the A2 phase was present in the alloy with 40 at% Mo. Furthermore, Mo addition increased the room temperature microhardness of the Nb-Al solid solution by ≈7 Kgmm−2/ at%Mo.

An alloy with nominal composition of Nb-15Al-25Ti (in at.%) has been developed for high temperature structural applications. This alloy exhibits a B2 ordered structure in the as-cast form [1]. In addition to dislocation slip, pseudo-twinning occurs in this B2 alloy when deformed in compression at room temperature at a strain rate of ∼ 10−4 sec−1 to 3% strain. The pseudo-twinning plane has been deduced to be the {112} planes, which is similar to that in twinning of bcc metals. The mechanism of deformation pseudo-twinning in this B2 alloy is found to be analogous to the normal twinning mechanism in bcc metals, involving glide of partial dislocations with Burgers vector (b) given by b=l/6<111= on {112} planes. This pseudo-twinning mechanism causes a phase transformation from the B2 to an ordered orthorhombic structure.

Plasma-Melt-Gas-Atomizing (PMGA) technique has been developed to produce rapidly solidified powders of intermetallic alloys for ultra-high temperature structural applications. Using this technique, rapidly solidified Nb3Al powders have been produced. These powders consist mainly of supersaturated solid solution of Al in Nb (A2), while the conventional cast structure consists of ordered A 15, and may be a good source for most powder metallurgical applications. Furthermore, a unique technique to control the powder size distribution is currently under development. In-situ measurement method of the atomized powder size distribution has been examined by using phase Doppler particle analyzer to control final powder size distribution by the feedback from the in-situ measurement data to the control system of the Plasma-Melt-Gas-Atomizer.

The Co-Al-C ternary phase diagram has been experimentally examined for the Co-corner with a particular interest in phase relations among B2 type intermetallic compound, the E21 type Co3AlC, and cobalt primary solid solution, denoted as (Co). Reaction scheme, liquidus surface, isothermal sections and isoplethals at constant concentration were determined. Mechanical properties of the B2/E21/(Co) three-phase alloys were investigated by compression tests carried out at a temperature range from 77 to 1273 K, and by tensile tests at room temperature. It has been revealed that both excellent ambient temperature ductility and sufficient high temperature strength can be achieved by proper choice of alloy compositions.

An attempt is made to develop a two-phase alloy consisting of the Ll2 and L21(Heusler) phases in the Co–Al–Ni–Ti quaternary sytem exhibiting a high elevated temperature strength as well as some room temperature ductility as a new class of heat resisting structural materials. The idea behind this approach is expectations for the L21 phase to provide high elevated temperature strength, whereas the Ll2 phase provides some room temperature ductility. Compositional optimization in the room temperature ductility of the Ll2 (Co,Ni)3(Al,Ti) is first carried out and then, based on the result, several L12/L21 two phase alloys are designed. It is found that a few to several per cent room temperature bend ductility is obtained in such two-phase alloys.

Three alloys, single-phase Cr2Hf, a two-phase alloy consisting of Cr solid solution and Cr2Hf, and a two-phase alloy consisting of Hf solid solution and Cr2Hf were cast and heat treated. The C14-to- C15 transformation of the Laves phase, Cr2Hf was studied as a function of heat treatment. According to the existing phase diagram, the Cr2Hf phase exhibits a C14 structure at elevated temperature but transforms to the C15 structure at lower temperatures. Such transformations are known to be extremely sluggish. In the present study, the Cr2Hf phase was found to retain the C14 structure at room temperature in all three compositions in the cast or cast and forged conditions; upon subsequent heat-treatment at various temperatures and time-at-temperatures, however, the C14 structure decomposes to a variety of higher order structures including the 16H, 10H, and 4H structures. These superstructures can be viewed as containing various percentages of the cubic and hexagonal stacking. The C15 structure was not observed for any of the conditions considered.

The phase stability of C15 HfV2 was studied by specific heat measurements. The elastic constants of C15 HfV2 were measured by the resonant ultrasound spectroscopy. Total energy and electronic structure of CI5 intermetallic compounds MV2 (M=Zr, Hf and Ta) were calculated using the linear muffin tin orbital (LMTO) method. The band structures at the X-point near the Fermi level were used to understand the anomalous shear moduli of HfV2 and ZrV2. It was found that the double degeneracy with a linear dispersion relation of electronic levels at the X-point near the Fermi surface is mainly responsible for the C15 anomalous elasticity at high temperatures. The density of states at the Fermi level and the geometry of the Fermi surface were used to explain the low temperature phase instability of C15 HfV2 and ZrV2 and the stability of C15 TaV2. The relationship between the anomalous elasticity and structural instability of C15 HfV2 and ZrV2 were also studied.

The Cr2Nb intermetallic compounds with C15 Laves structure, which are considered to be attractive candidate as extremely high temperature structural materials, are deformed by compression test. The Cr2Nb intermetallic compounds are deformable at temperatures above 1473 K. The Burgers vector of dislocations activated at temperatures above BDTT are determined to be 1/2<011= which is split into a pair of 1/6<112= (i.e. the Shockley partials) connected by the stacking fault (SF). Also, the morphological feature of the activated dislocations and the SF energy are evaluated.

Ti-Cr alloys near the TiCr2 composition have been studied to determine the single-phase Laves field and the associated defects accompanying off-stoichiometry. A combination of metallography, x-ray diffraction, lattice parameter measurements, density measurements and electron microprobe analysis have been used to establish a narrow single-phase region extending towards Ti-rich compositions. All three Laves crystal structures (C14, C36 and C15) were found to exist at different temperatures. Hardness and fracture toughness values determined by Vickers microindentation were studied as a function of alloy composition. Effects of adding Fe, Nb, Mo, and V to TiCr2 on lattice parameter, crystal structure, hardness and fracture toughness are reported.