Titanium aluminides containing 20-30 atom percent Al can suffer from severe subsurface embrittlement (alpha-case) when exposed to air at elevated temperatures for extended periods. For example.the alloys Ti-24Al-12.5Nb-l.5Mo and Ti-24Al-8Nb-2Mo-2Ta (based on the Ti3Al alpha-2 intermetallic compound) embrittle to a depth of about 80 microns during 1000 hours exposure to air at 815°C (1500°F), resulting in major reductions in tensile properties. Plasma-sprayed coatings of the MCrAlY and MCr types deposited over a thin diffusion barrier of chromium or tungsten, have been found effective in protecting these alloys against oxidation and embrittlement at this temperatures.

Oxidation behavior of Nb3Al has been studied in the temperature range from room temperature to 1500°C. It was revealed in this study that the oxide scale that formed at elevated temperatures was not protective against the propagation of the oxidation front. The products of oxidized Nb3Al alloys melted below 1500 °C, even though the melting point of Nb3Al is 1960°C.

We expected that the addition of third elements would improve oxidation resistance because atomic sizes smaller than Nb and Al might reduce the solubility and diffusivity of oxygen in the alloy. The elements V, Mn, Fe, Mo, Ta, W and Re were added as the third element for the examination of oxidation resistance. The oxidation resistance tests were carried out by measuring weight gain during temperature elevation and during isothermal heating in air. It was concluded that the addition of Re or Mo improved the oxidation resistance for Nb3Al alloys.

The phenomenon of “MoSi2 pest” has been studied in detail by low temperature oxidation of various polycrystalline MoSi2 monoliths and composites. Single crystalline samples were studied to address the nucleation and growth of pest reaction. Pest of MoSi2 was observed to occur at temperatures between 375 and 500°C. Samples after pest reactions yielded to powdery reaction products of MoO3 whiskers, SiO2 clusters, and residual MoSi2 particles. Although all the samples showed complete disintegration after sufficient oxidation durations were rendered, the kinetics of pest disintegration varied among different samples. By performing interrupted oxidation tests, pest reaction was found to consist of nucleation and growth stages, with the former stage acung as the rate-limiting step. Furthermore, the nucleation of pest reaction was found to be closely related to the formation of MoO3, which was preferentially formed at such defect sites as intergranular boundaries and cracks. Mechanisms leading to the pest disintegration of MoSi2 are discussed in light of the nucleation and growth of pest reaction.

A systematic study of iron-aluminum alloys has shown that Fe-16 at. % Al alloys are not very sensitive to environmental embrittlement. The Fe-22 and -28 at. % Al alloys are sensitive to environmental embrittlement, and the effect can be reduced by the addition of chromium and through the control of grain size by additions of zirconium and carbon. The Fe-16 at. % Al binary, and alloys based on it, yielded over 20% room-temperature (RT) elongation even after high-temperature annealing treatments at 1100°C. The best values for the Fe-22 and -28 at. % Al-base alloys after similar annealing treatments were 5 and 10%, respectively. A multi-component alloy, FAP, based on Fe-16 at. % Al was designed, which gave an RT ductility of over 25%.

We present some results regarding the influence of grinding atmosphere on the phenomena occurring during mechanical milling of a Ni3Al-base alloy. We have disordered this permanently ordered alloy by ball-milling in a vibratory mill, using alternatively argon and hydrogen atmospheres. We observed a slower reduction of the long-range order parameter during experiments carried out under a hydrogen atmosphere, regardless of the used grinding media. We discuss our Findings in terms of the effects, such as in grain boundary cohesive strength or localize ductility enhancement, that hydrogen may induce in such alloy. The microstructural parameters of the powders at different stages of disordering have been investigated using calorimetric tests, x-ray diffraction analysis and microscopic observations.

Iron aluminides based on Fe3Al are of interest as structural materials because of their excellent corrosion resistance in many environments. However, studies have shown that one of the major causes of low room temperature tensile ductility, which so far has limited the use of these alloys, is an environmental reaction involving aluminum in the presence of water vapor. During this reaction, atomic hydrogen is released, moves into the sample ahead of the crack tip during stressing, and causes failure before the true ultimate tensile strength of the material is reached. This reaction is reduced significantly by testing in oxygen or vacuum. In the present study, an Fe3Al-based alloy was tensile tested as a function of the level of water vapor in the test environment, from a vacuum of 10−4 Pa to a water vapor partial pressure of 1330 Pa. The results show that a water vapor level of as low as 133 Pa (1 torr) can result in significant embrittlement. The fracture mode remains transgranular cleavage, but the scale of the cleavage facets changes with the water vapor level.

Hydrogen embrittlement has been studied in continuous cast sheet of a Ni 3 Alalloy (NI 77.83 AL 21.73 ZR 0.34 B 0.1), known as IC-50, after introducing various amounts of hydrogen cathodically. The elongation and UTS decreased with the increasing content of hydrogen. When tensile-tested at a strain rate of 5.8 × 10−5 s−1, the elongation decreased from 32.7% for no charging to 1.9% for 330 min of charging with 50 mA cm−2 current. The yield stress, however, did not change. When tested at a higher strain rate of 5.8 × 10−3 s−1, the embrittlement was less, but the yield stress increased with the hydrogen content. With increasing hydrogen content the fracture mode changed from dimpled to intergranular and cleavage modes.

Ti-25Al, Ti-25Al-10Nb-2V, and Ti-25Al-10Nb-3V-1Mo (at. %) α2-based alloys, and Ti-48Al and Ti-52Al γ-based alloys were exposed to gaseous hydrogen at elevated temperatures. A novel ternary hydride was observed in Ti-25Al and Ti-25Al-10Nb-3V-1Mo, identified as Ti3AlH. A highly faulted ternary hydride was seen in two phase α2 + γ Ti-48Al which did not have the crystal structure or chemistry of any known Ti- or Ti-Al-hydride. Very fine, oriented, needle-shaped hydrides were observed in single-phase γ Ti-52Al.

Environmental effects on the room temperature ductility of polysynthetically twinned (PST) crystals of binary and some ternary TiAl compounds have been investigated through tensile tests conducted in four different atmospheres. The tensile ductility of TiAl PST crystals is sensitive to test environment. It is higher when tested in vacuum or in dry air than in air or in hydrogen gas. The environmental loss in ductility of PST crystals decreases with increasing strain rate. The environmental embrittlement of binary TiAl PST crystals can be interpreted in terms of hydrogen embrittlement. The ternary TiAl PST crystals containing Cr, Mo or Mn exhibit higher ductility than the binary TiAl PST crystals when tested in air, while the ternary ones show lower ductility than the binary ones in vacuum. Additions of alloying elements such as Cr, Mo and Mn seem to be effective in reducing the environmental loss in ductility of TiAl PST crystals.

Oxidation tests were performed isothermally at 1000° C on chill block melt spun metallic ribbons with alloy compositions consisting of Ni(46-50 at%), Al(31-33 at%), Co( 17-20 at%) and Hf(0-1 at%). The oxidation rates were calculated and the microstructure was examined. Results indicated the difference in microstructural and oxidation characteristics could largely be attributed to the differences in the superheat of the melt. There was no indication that hafnium retarded grain growth. The oxidation behavior of two of the ribbons exhibited a logarithmic rate law.

During oxidation at 1000°C in dry, flowing O2, β-NiAl first forms an oxide scale of primarily θ-Al2O3. After 1-4 hr of continued exposure this metastable oxide transforms into the stable α-Al2O3 structure. An implant of 2 × 1016Y/ cm2 at 70kV was found to stabilize the first- forming θ- Al203 scale up to 100hr at 1000°C. β-NiAl was also implanted with Al and Cr to study the effect of the implantation damage and of another more noble element on the oxidation behavior. At 1200°C, only a short term effect of the Y-implant was observed.

The composition and bonding of incipient and atmospheric oxides grown on a Ti - 47.0 A1 - 2.2 Nb alloy were examined using Auger, x-ray, and low-energy ion scattering spectroscopies. The depth distribution of various components was determined with secondary-ion mass spectroscopy (SIMS). An oxide grown on the material in air at room temperature showed advanced A1 and Ti oxide bonding states, which had very different stabilities during heating in vacuum up to 600°C. The Ti-oxygen bonding state was markedly reduced while the Al oxide was somewhat enhanced during this process. Further controlled studies at 600°C, involving oxygen adsorption at pressures below ∼. 10-6 Torr, showed an initial A1 cation surface enrichment until the detectable A1 2p peak was 90% oxidized. Further oxygen exposure gradually produced a fully oxidized Ti valence state with a pronounced enrichment of the Ti cation at the vacuum-solid interface. The results are discussed in terms of the kinetic and thermodynamic factors of the rate-limiting steps in the oxidation process.

Oxidation behavior of binary Ti-48 Al and ternary alloys with 1.5 at.% Cr, 1.4 at.%Mn, 0.2 at.% W, 2.2 at% V, 20 vol.% TiNb particles were examined in an air atmosphere at 704°C, 815°C and 982°C. Results indicate that the addition of Cr is detrimental at all temperatures. The Cr and V containing alloys exhibited a linear oxidation behavior at 815°C while the Mn containing alloy showed a linear behavior at 982°C. At 982 °C the alloys with W and V exhibit the best oxidation behavior. The mechanism of oxidation has been explained with the help of a simple vacancy model for oxygen diffusion.

Coating of silicon carbide on the surface of structural materials is being used in industry to improve the oxidation resistance and wear resistance properties. In this study, silicon carbide coatings were applied on titanium aluminide by both chemical vapor deposition and sputter deposition techniques. Chemical vapor deposition of SiC was accomplished by reacting methyl trichlorosilane (MTS) and hydrogen in the presence of argon on the substrate surface at 1100 C under 1 atm pressure. Sputter deposition of SiC was completed in a Cooke sputter deposition system using a SiC target at 10 mTorr argon total pressure and 100 watt power. Under the experimental conditions of this study, the deposition rate of CVD SiC is about 1 μm per hour whereas that of sputtered SiC is about 1 urn per hour. To minimize the thermal residual stress between the aluminide substrate and SiC coating as a result of the coefficient of thermal expansion mismatch and temperature variation, a thin layer of TiC was coated prior to the SiC coating. For sputter deposited SiC, it was found that when the coating was done at room temperature, the adhesion of coating to the aluminide substrate was poor. The coating layer spalled off when the coated specimen was thermal cycled to 900 C. By coating the carbides at an elevated temperature, e.g. 560 C. strong adhesion between carbide coating and the substrate was obtained. The coating survived many thermal cyclings. The degree of surface roughness ranging from #80 to #800 grit surface polish did not affect the adhesion of coating.

The mechanical properties and oxidation resistance of HIPed Ti5Si3 have been measured. HIPing submicron size powder compacts produces a crack-free, fine-grained microstructure with significantly higher hardness and toughness than a coarse-grained microstructure which contains microcracks within larger grains. Oxidation resistance is influenced by the grain size. Coarse-grained material has much lower mass gain than finegrained material in an oxidizing atmosphere and exhibits parabolic oxidation kinetics. The oxidation resistance of fine-grained material was measured between 700°C and 1000°C in air. Mass gain at 120 hours was measured to be 0.07 mg/cm2 at 700°C. At 900°C cracking of the scale leads to linear oxidation kinetics and significantly higher mass gain.

The effect of hydrogen on the tensile and fatigue behavior of an Fe3Al, Cr type intermetallic alloy is examined. Hydrogen due to moisture in the air is found to be a major cause of embrittlement. The rates and mechanisms of the observed embritdement appear to be temperature dependant. In addition, the alloy was found to have no notch sensitivity.

The microstructure of MoSi2 composite materials reinforced with 30vol% SiC particulate were investigated by TEM. These materials were initially produced by the XD™ process followed by either HIPing or forging.

The microstructural study revealed no significant differences between the HIPed and forged materials. In both materials, the size of the SiC particulate was found to vary from submicron to 30jtm and twins and stacking faults were observed in the SiC. No orientation relationships between the SiC and MoSi2 were found and dislocations emitted from the SiC/MoSi2 interfaces were noticed.

The distribution of dislocations in the MoSi2 matrix was nonuniform. The morphology of the dislocation structures may be categorized into three groups: 1) individual; 2) array; and 3) networks. [100]/[010] prismatic loops and (110) twins also were observed in some grains.

Silicon carbide fibers in a Ti24Al+llNb matrix were debonded under varying combinations of shear and compression in off axis loading in a pushout test. Evidence of debond crack branching was recorded for etched specimens using scanning electron microscopy and the interface microstructure was examined by transmission electron microscopy. The regions of high shear stress and multiple weak interfaces promoting crack branching were analyzed in terms of an energy balance for brittle fracture.

A two-phase Ti-47Al-2Ta composite reinforced with a titanium diboride (TiB2) and two alumina fibers was processed by hot isotatic pressing and heat treated to simulate service conditions. Diffusion couples were heat treated between 593°C and 982°C for up to 500 h and analyzed using a scanning electron microscope and energy dispersive x-ray spectroscopy.

The fibers used were 3M® alumina, Sapphikon® sapphire, and Textron Specialty Materials TiB2. Observations reveal reaction layers, cracking fibers due to thermal stress, and chemical interactions between ternary addition of Ta with the fibers. Alumina is thermodynamically stable in TiAl, and its coefficient of thermal expansion closely matches TiAl. Observations revealing alumina as a good potential reinforcement in TiAl are discussed. Thermodynamic information supports TiB2 stability in two-phase TiAl. A three-fold increase of Ta at the fiber/ matrix interface of TiB2 is evidence of a lack of homogeneity due to Ta mobility. This indicates a possible loss of oxidation resistance provided by the Ta addition.

High temperature compression workability tests have been performed on a Ti-44a/oA1-3a/oV- 7.5v/oTiB2 XD™ composite over a range of temperatures (1273 to 1473 K), strain rates (10−3 to 10 s−1), and interrupted strains. Three types of specimen configurations were used in the tests. Fracture caused by secondary tensile stress at the surface of the specimens was investigated in terms of crack initiation and propagation. The fractured specimens were analyzed using both optical and SEM microscopy. Cracks started at the equator of the specimens and propagated into the specimens through their longitudinal axis. The fracture mode varied with temperature and strain rate. At the lowest temperature (1273 K) and the lower strain rate (0.1 s−1), the fracture was transgranular with a crack arrester type. At the lowest and intermediate temperatures (1273 and 1373 K) and the highest strain rate (10 s−1), intergranular cracks were observed. At the highest temperature (1473 K) and the highest strain rate (10 s−1), transgranular failures with delamination and crack divider types were observed.