Single crystals of γ′-Ni3Al((001)-oriented) were oxidized at 1223 K under an oxygen partial pressure of ∼4 ×10−19 atm for times ranging from 0.1 to 50 hours. Microstructural development of the oxide scale and subscale metal was studied by electron microscopy. A special technique permitted the reproducible and efficient preparation of TEM cross section specimens. Initially, a fine-grained γ-Al2O3 scale formed with a preferred orientation. Depletion of Al from the γ′-Ni3Al resultedin a Ni-Al solid solution zone between the oxide scale and the intermetallic. After 20 hours oxidation, a discontinuous α-A12O3 layer between the γ-A12O3 and the metal was observed. The α-A12O3 layer exhibited a much larger grain size than that of the γ-A12O3 and was continuous after 50 hours oxidation. Formation of the α-A12O3 layer correlated with a decreasing parabolic oxidation rate constant kp, as measured by thermogravimetric analysis (TGA).

XAS was used to investigate the oxidation behavior of Ni3Al aluminides with and without the addition of Cr. At 450 C the Cr formed a Cr2O3 like oxide layer which inhibited the oxidation of the Ni at the surface. Ni oxidation did occur in the sample without Cr at 450 C. At higher temperatures the Ni at the surface oxidized to NiO, but in the bulk no oxidation of the Ni was observed whether Cr was present or not. When no Cr was present a minimum was observed at 700 C in the fraction of oxidized Ni. The Cr (8at.%) did not cluster and substituted for Ni or Al in the Ni3Al aluminide.

Nickel aluminides based on Ni3Al were continuously cast into 12.4 mm diameter wire in an industrial production machine. Cyclic corrosion tests were carried out on this material in oxidizing, carburizing, sulfidizing and chloridizing atmospheres at temperatures between 750°C and 1100 °C in order to study the time and temperature dependence of high temperature corrosion. The results show, that Ni3Al has a very good oxidation resistance up to 1000 °C. The sulfidation resistance is poor, while resistance against carburization and chlorination is significantly better than that of the commercial alloy 800H andalloy 617.

The oxidation behavior of a number of ordered high melting point intermetallic materials has been studied in flowing air in the temperature range 1000 to 1500°C. The alloys were selected for acceptable toughness and ductility at temperatures above 1000°C. Included were ruthenium aluminides, chromium silicide and compounds of iridium and ruthenium with elements such as titanium, tantalum, nickel, iron, cobalt, silicon, vanadium and rhenium. In general, alloys containing the metals iron, cobalt and nickel, and also titanium and tantalum, oxidized rapidly at 1000°C and above. Oxidation-resistant materials were those capable of forming scales of chromia/silica or alumina. Intermetallic compounds of chromium and ruthenium formed volatile oxides above 1200°C, but binary compounds of these elements with silicon and aluminum were highly oxidation resistant at lower temperatures.

The oxidation resistance of intermetallic compound TiAl was greatly improved in cyclic heating at 900°C by means of a new type of surface treatment, which is the heat treatment under a low partial pressure oxygen atmosphere. The treatment becomes more effective with increasing aluminum content in the specimen. It was presumed that the excellent oxidation resistance resulting from this method is due to preferential formation of a thin and tighit Al2 O3 surface layer, which is formed during the surface treatment at a temperature vicinity of 1000°C and air pressure of about 10−3 Pa. The oxidation resistance of Al2O3 film formed, however, was found to be insufficient at 950°C. A combined treatment consisting of the heat treatment under a low partial pressure oxygen atmosphere followed by a diffusion coating with aluminum showed an excellent resistance to cyclic oxidation even at 950°C.

NiAl was ion implanted with yttrium (2×1016 cm−2) in order to study its effect on the very high temperature (1000–1500°C) oxidation properties. At 1000°C, implanted Y stabilizes the faster growing θ-Al2O3 phase, thus slightly increasing the oxidation rate. At 1200°C, where predominantly α-Al2 O3 is formed with and without Y, there is a factor of 4 reduction in the oxidation rate. However at higher than 1200°C, there is little effect by Y on the oxidation rate. Cyclic testing showed that the Y implant had an imperfect and short-lived improvement on adherence relative to other Y-containing alloys. Variations in aluminum content from 23.5 to 36.0wt%(40–55at%) had little effect on the oxidation properties. Initial experiments at 1500°C with a novel Rh marker indicate that alumina grows at least partially by outward cation diffusion both with and without Y.

This paper is concerned with the processing and mechanical properties of intermetallic-matrix composites. The effects of processing variables on fabrication of compounds including Ni3Al, NiAl, TiTaAl2, MoSi2 and their composites is described. A key concern is with processing effects on microstructure, selection of compatible ceramic reinforcing phases, and whisker alignment through injection molding.

Brittle silicide intermetallics such as Nb5Si3 have been significantly toughened by Nb particles incorporated during in-situ processing techniques. Ten-fold increases in toughness as well as resistance-curve behavior have been realized at conventional testing rates with these ductile Nb particles. Under more rapid loading rates the efficiency of the body-centered cubic (bcc) Nb particles in enhancing the toughness may be affected due to the strain-rate sensitivity of the Nb and/or the possible strain-rate sensitivity of interfacial debonding. Toughness tests conducted at loading rates of 0.0042 to 8.5 mm/sec have indicated a significant sensitivity to testing rate, with a drop in fracture toughness from 28 to 9 MPa√m over this range accompanied by a change in fracture mode from primarily ductile to cleavage failure in the Nb.

Previous work has shown that ternary Ti-Al-Si alloys with hypoeutectic and eutectic microstructures containing the intermetallic phases Ti3 (Al,Si) and Ti5 (Si,Al)3 have promising high temperature mechanical properties [1]. In the present investigation alloying additions of Niobium have been made to selected Ti-Al-Si alloys based on hypoeutectic compositions and the effects of Niobium on the microstructure and mechanical properties have been studied. The high temperature creep and oxidation behaviour of such alloys appears to be superior to that of currently available Ti3Al-based alloys.

The possibility of producing NiAl reinforced with the G-phase (Ni16X6Si7), where X is Zr or Hf, has been investigated. The microstructures of these NiAl alloys have been characterized in the as-cast and annealed conditions. The G-phases are present as fine cuboidal precipitates (10 to 40 nm) and have lattice parameters almost four times that of NiAl. They are coherent with the matrix and fairly resistant to coarsening during annealing heat treatments. Segregation and non-uniform precipitate distribution observed in as-cast materials were eliminated by homogenization at temperatures near 1600 K. Slow cooling from these temperatures resulted in large plate shaped precipitates, denuded zones, and a loss of coherency in some of the large particles. Faster cooling produced a homogeneous fine distribution of cuboidal G-phase particles (≤10 nm) in the matrix. Preliminary mechanical properties for the Zr-doped alloy are presented and compared to binary single crystal NiAl. The presence of these precipitates appears to have an important strengthening effect at temperatures≥1000 K compared to binary NiAl single crystals.

In this study, the creep properties of two intermetallics; NiAl and TiAl have been evaluated. The former was selected based on its relatively high melting point, and the latter was based on its specific strength and high melting point. Tensile creep deformation rates in range of 760 to 930°C were determined as a function of stress for both monolithic NiAl, NiAl reinforced with HfC dispersoids and TiAl reinforced with TiB2 dispersoid. Microstructure and dislocation structures were examined both beore and after creep in both alloys to identify the creep mechanisms. In both systems, deformation occurs by power-law creep with diffusion controlling creep rate. Power-law breakdown regimes have been identified for both systems. In addition, TiAl undergoes dynamic recrystallization during creep resulting in smaller grain sizes than the initial value.

An oxidation study of pure, dense MoSi 2 at 400-600°C and MoSi2/20 vol. % (TiB2, Al2O3, Nb) mixtures at 400–1200°C has been performed. Dense MoSi2 does not fail catastrophically (“pest”) in dry air, wet air, or oxygen, and oxidizes heavily only near 500°C. Heavy oxidation is related to the slow growth of a protective, amorphous SiO2 layer; a low rate of Mo removal as vapor species; and cracking of an initially protective Mo9O26-SiO2 layer. Pre-oxidation of MoSi2 at 1200°C prevents subsequent heavy oxidation at 500°C. MoSi2/TiB2 and MoSi2/A12O3 mixtures oxidize more slowly than MoSi2 for T < 600°C and more rapidly for T > 600°C. Protective oxides are formed on MoSi2/reinforcement mixtures except at 500°C, for which high-temperature pre-oxidation is again effective. MoSi2/Nb mixtures suffer severe mass losses upon oxidation at both 500 and 1200°C owing to rapid oxidation of the Nb and spalling of the product Nb2O5 Only the latter case poses problems for the application of MoSi2-matrix composites as high-temperature materials.

Two β-NiAl plus α -Re alloys with near eutectic composition were produced by rapid solidification and then consolidated through a powder extrusion process. Microstructures of the rapidly solidified, as-consolidated, and deformed alloys were characterized by transmission electron microscopy. Both compression and tensile properties of these alloys were determined over the temperature range of 300–1300 K. The Re additions were found to greatly enhance low and intermediate temperature yield strengths when compared to other β-NiA1 alloys. This enhancement was attributed to a combination of solid solution strengthening, precipitate hardening, and grain size refinement. Above 1200K the strength of these Re-modified alloys was comparable to the binary β-NiAl compound.

Compression studies were conducted on monolithic and TiB2 particulatereinforced composites of AI22 Fe3 Ti8, both with and without minor quaternary alloying additions (2 at.% Nb and 2 at.% Mn) as a function of temperature and as a function of strain rate at high temperature. The volume fraction of reinforcement was varied between 0 and 20 percent. The particulate reinforcements were found to be effective in increasing ambient- and warm-temperature strength; at high temperatures, the monolithic material is stronger than the composites, although the composites are superior at slow strain rates. The microstructures of the monolithic and composite specimens were examined before and after deformation to explain these observations.

Solid state reactions of Ni3A1 with SiC have been studied at 1000°C. Three layered reaction products consisting of NiAl Ni5,4Al1Si2, and Ni(5.4−x)A11Si2+C were formed in the reaction zone. The Ni5.4AllSi2 layer showed carbon precipitation free, while modulated carbon bands were formed in the Ni(5.4−x)A11Si2+C layer. Carbon precipitates were found to exist in either a disordered or partially ordered (graphitic) state, depending upon their locations from the SiC reaction interface. The NiAl layer showed dramatic contrast difference compared to the Ni3Al and Ni5.4Al1Si2 layers, and was bounded by the Ni3Al/NiAl and NiAl/Ni5.4A1lSi2 phase boundaries. The kinetics of the NiAl formation was limited by diffusion, and the growth rate constant was measured to be 2 × 10−10cm2/s. The thickness of the reaction zone on the Ni3Al side was always greater than that on the SiC side, suggesting that the decomposition of the SiC may be a rate limiting step for the SiC/Ni3Al reactions. An Al-rejection model, based on a lower solid solubility of Al in the Ni-Al-Si ternary phase than in the Ni3A1 intermetallic compound, is proposed to explain the formation of NiAl phase. The rate limiting step for the Ni3Al/SiC reactions is discussed in light of discontinuous decomposition of SiC, which leads to the formation of alternating C and Ni(5 4−x)Al1Si2 layers in the reaction zone. Preliminary results on C precipitation behavior in NiAl/SiC system are reported.

Hot-extrusion was used to produce Ni3Al-based alloys to which 10 vol% TiN, NbC, HfO2, or HfN was added for reinforcement. The TiN, NbC, and HfO2 particulates produced Ni3Al-matrix composites in which no reaction was noted at the particle-matrix interface. However, the addition of HfN resulted in extensive reaction in which the hafnium appeared to diffuse into the matrix. Microstructures of this alloy showed a complex array of phases and voids where the HfN particles are presumed to have been originally. Hot hardness, compression, and compression creep tests were performed on specimens cut from the extruded bar of each alloy. No significant strengthening was observed for the alloys containing TiN, NbC, or HfO2. However the HfN-containing alloy did show significant strengthening in simple compression and compression creep. This presentation will include microstructures and the results of the mechanical properties tests.

Results of an experimental study on unidirectional wave combustion synthesized Nb-Al-B composite alloys are described. The effect of combustion initiation temperature and composition on the microstructural morphology and phases is studied. Correlations with the initial parameters and reaction velocities have been sought to understand the kinetics of the rate controlling step for sustained combustion and for the explanation of the synthesized microstructures. Strategies for controlling the distribution of the reinforcement phases (both ductile and hard) in the composite are discussed.

Dislocation structures in 0, 10, and 20 V% TiB2/NiAl composites have been thoroughly investigated with a 1 MeV HVEM after compression testing at 760–1000° C. Samples with 0 and 10 V% TiBl2/NiAl additions have almost identical dislocation structures which can be described as a<100> screw dislocations with extensive jogs and superjoqs. Prismatically punched dislocations were observed in all of the deformed composites and deformed samples of 20 V% TiB2/NiAl had extensive dislocation generation around the particles. Dislocation density, grain size, and the tendency for dislocation reactions or networks forming during deformation decrease as the volume fraction of TiB2 increases. Also, since a predominance of screw dislocation was observed, the rate controlling process is not likely to be dislocation annihilation or climb, but dislocation generation. The grain size refinement could play an important role in the strengthening of the composites.