Two-phase alloys based on B2 type intermetallic compound (Co,Ni)Al with either Co primary solid solution or Li2 type Ni3Al, γ, phase are investigated for their variations in microstructure and mechanical properties. The aim of the present work is to improve ductility and to enhance fracture resistance in extremely brittle B2 CoAl or NiAl by introducing second phase. To start with, microstructure-mechanical property relationship is investigated in two-phase alloys consisting of B2 CoAl and Co primary solid solution in the Co-Al binary system. Subsequently, the similar strategy is extended into the Co-Ni-Al ternary system. Mechanical properties are measured by compression test conducted over the wide temperature range from 77 K to 1273 K at strain rates ranging from 1.4xl0-3 to 1.4-4 s-1. Micro-structure observations are carried out using both optical and transmission electron microscope.

The role of the microstructure on the room-temperature tensile properties of the alloy Ti-25Al-10Nb-3V-lMo (at.%) was investigated. A wide spectrum of microstructures was obtained by varying the cooling rate after a solutionizing treatment in the β phase field. The strength was found to increase monotonically with increasing cooling rate. It is proposed that strength is controlled by (α2+βR) boundary strengthening in β-solutionized microstructures. The ductility was observed to go through a maximum at a cooling rate of 1.5°C/s. Different deformation and failure mechanisms were identified, depending on the cooling rate regime, and correlated to the ductility trends. The β2 nature, α2 lath size, α2 lath arrangement and prior β grain boundaries appeared to be the principal features governing die deformation and failure mechanisms.

Two types of aluminum-titanium-vanadium ternary intermetallic compounds have been prepared by arc melting under argon atmosphere. Their compositions were nominally AI65Ti25V10 and Al57 Ti 22V21; the numbers represent the molar fractions in mol%. These two alloys stand within the gamma-phase field where the L10 face-centered tetragonal structure is produced. Metallographic structure and mechanical properties have been investigated. It is suggested that vanadium addition to titanium aluminides and titanium trialuminides can enhance their strength.

Near γ-TiAl alloys which incorporated Cr, and Mo or Re additions [Ti-48Al-2Cr-1.5Mo-0.1Y, Ti-48Al-2Cr-1.5Re-0.1Y, and Ti-46Al-2Cr-1.5Re-0.1Y(at.%)], were prepared by arc casting and hot isostatic pressing; then heat treated at 1200 and 900°C to produce a duplex microstructure. All three alloys contained a bimodal size distribution of B2 phase, which was enriched in Cr+Mo or Cr+Re. Very little α2 was present in the 48A1 alloys; whereas the 46A1, 1.5Re alloy contained areas of transformed lamellar α2/γ, within which short lengths of B2 periodically interrupted the α2 plates. The composition of both the equiaxed and lamellar γ in each alloy was 49±0.5A1,1.6±0.4Cr and 蝶 1.2Mo or <lRe(at%). Similarly, the composition of the B2 in the three alloys was experimentally equivalent: 33±1A1, 8.5±2Cr and 6.5±1.5Mo or Re(at%). In the Re containing alloys, the α2 composition was 蝶33A1, 1.7±0.3Cr, and 0.5±0.3Re. Room temperature tensile ductilities were low, ranging from 0.5 to 1%; while ultimate strengths approached 500MPa [7x10-4s-1 strain rate]. Testing at 700°C produced little change in alloy strength, while ductility increased to as much as 2.5%.

Single crystal AI66Ti25Mng has been produced and tested in compression as a function of temperature and orientation. Yield strengths for orientations near [001] and [111] continuously decrease with increasing temperature in a manner similar to other single crystal L12 trialuminides, and also to polycrystals of these materials. Slip was determined to occur on the {111} octahedral planes using two-surface analysis. Critical resolved shear stress (CRSS) variation with temperature, calculated on {111} planes, overlap closely for both orientations. Dislocation analysis confirmed the Burgers vectors to be of the type a<110> at both 298K and 1073K. A limited number of uniaxial tension tests were conducted near [001] at 1073K; the HIPed specimens contained a small amount of residual porosity and second phases which resulted in lastic failure even at this high temperature.

Transformations, microstructures and mechanical properties of Nb(10-16) at.% Al alloys are reported. Cast buttons of the alloys were solutionized in the high temperature single phase Nb-Al region and cooled to retain supersaturated solid solutions (SS). These SSs are observed to show B2 ordering, with both the degree of ordering and hardness increasing with increase in Al. The precipitation of ND3AI from the supersaturated SS, which was studied as a function of temperature (1000-1600°C) and time (to 100h) by hardness measurements and microscopy, is sluggish, proceeds by initial heterogeneous nucleation at grain boundaries followed by growth into the grains in a discontinuous fashion, follows C-curve kinetics, and leads to significant hardening. The size, spacing and amount of the Nb3Al precipitates depend strongly on Al content, temperature and time. The compressive yield strength of the SS increases with %Al and high values of 900 and 500 MPa at 25 and 900°C, respectively, are observed. The presence of Nb3Al leads to a very significant increase in yield strength, with values being as high as 1550 and 1100 MPa at 25 and 900°C, respectively. Also, a strong dependence of strength on the size and spacing of the Nb3Al precipitates is observed. These results are presented and discussed.

Binary, coarse-grained polycrystalline Ti-48, 50 and 52 Al (in at.%) alloys, containing low (∼250 wt.ppm) levels of interstitials (O+N) have been deformed at various temperatures in compression and four-point bending. The 0.2% proof strength-temperature profiles are observed to comprise of three distinct regimes. Differences in the material response between bending and compression modes of deformation have been observed. Importantly, in both cases a flow stress anomaly is observed in the 50 and 52 Al alloys. Dislocation fine structures of samples deformed in compression, have been observed in the TEM. Analyses suggests the possible influence of Al contents on the line directions of dislocations, superdislocation dissociation modes, SISF dissociation distances and hence planar fault energies. Distinct differences in the deformation structures at RT and at the flow stress peak temperature (800°C) have been observed. These results are presented and discussed in relation with the observed mechanical properties.

The low cycle fatigue response of polycrystalline NiAl above the brittle-to-ductile transition temperature (BDTT) was investigated. Samples of nominally stoichiometric NiAl were prepared from extruded ingots and from hot isostatically pressed (HIP'ed) prealloyed powders. The fatigue samples were cycled in a fully reversible fashion at plastic strain ranges between 0.06 and 1.0% in air. The HIP'ed NiAl material was also tested in vacuum at 1000 K. Both processing route and environment were found to have an effect on fatigue life. The lives of the powder samples were about a factor of three less than the cast and extruded material, which was attributed to the lower flow stress of the wrought NiAl. An environmental effect was noted for the HIP'ed material with a factor of 2-3 increase in fatigue life when samples were tested in vacuum compared to samples tested in air. In general, fatigue behavior for NiAl at high plastic strain ranges was typical of most metals, exhibiting a Coffin-Manson strain life behavior with a slope of -0.7. However, fatigue life of the HIP'ed powder material at low plastic strain ranges was controlled by intergranular cavitation and creep processes leading to a change in the slope of the fatigue life curve. Both materials exhibited cyclic softening over the majority of their fatigue lives with fatigue crack propagation occurring predominantly by intergranular mechanisms and final fracture by tensile overload occurring in a transgranular manner. Overall, the 1000 K fatigue life of NiAl was superior to conventional superalloys on a plastic strain range basis partly because of its high ductility and low flow stress but NiAl was not competitive on a stress range basis. The results indicate that binary NiAl has excellent plastic strain cycling capabilities and would be an acceptable material for the matrix phase of intermetallic matrix composites.

The deformation transients associated with changes in stress and strain rate have been studied as a means of determining the controlling deformation mechanisms in NiAl. Stress change experiments in tension, and strain rate change tests in compression have been performed on single crystals of NiAl at temperatures between 850 and 1200°C. The orientation dependence of these transients was studied by testing in the hard [001] orientation and a soft [223] orientation. Strain rate change experiments suggest increased contribution from structure-controlled mechanisms in hard oriented crystals. Stress change experiments in samples tested along the hard orientation produce transients that are characteristic of the evolution of a stable dislocation substructure. In soft oriented crystals, however, stress change transients suggest that deformation is not significantly affected by the formation of a dislocation substructure. These results are consistent with observations of dislocation substructure formation and strain hardening in hard oriented crystals.

The effects of boron addition on defect hardening at room temperature and on high temperature creep properties are investigated In the B2 NiAl intermetallic compound. It is found that boron addition is effective in increasing the room temperature hardness on the Ni-rich side but has no effect on the Al-rich side of stoichiometry. These observations are attributed to interstitial dissolution of boron in Ni-rich NiAl and due to a lack of solubility, and consequently an enrichment at grain boundaries on the Al-rich side. The similar effect is found for high temperature creep resistance of NiAl by boron addition, where it is increased at Ni-rich side but is unaffected at Al-rich side of offstoiciometry.

This work concerns the temperature and stress dependencies of the steady-state creep rate of a Ti-24Al-llNb alloy in the temperature range between 593 and 760°C (1,100 and 1,400°F) and stress levels 137.9 to 275.8 MPa (20 to 40 ksi). Tests were conducted on both as-extruded and heat-treated specimens. A transition occurs in the creep curves at 704°C (1,300°F) and 206.9 MPa (30 ksi), indicating a creep mechanism change. Generally speaking, the fracture surface of as-extruded specimens shows a ductile transgranular fracture mechanism; a more ductile fracture mode is observed for heat-treated specimens.

Notch stress rupture behavior of a titanium aluminide Ti3Al alloy, Ti-24A1-11Nb, was investigated. Three axisymetric test bar geometries were used: smooth bar (kt =1), circular notch (U-notch, kt = 1.6) and British standard notch (V-notch, kt = 4.2). Tests were performed at 650 °C, with a real-time DC potential drop (DCPD) automated data acquisition system continuously monitoring local creep deformation and damage accumulation in the notched specimens. Two microstructures were investigated, an equiaxed grain structure produced by an α2+β heat treatment and a transformed β microstructure produced by a β heat treatment. It was found that the effects of notches on the stress rupture lives are dependent on microstructure, notch geometry, and applied stress level. For the α2+β treated Ti-24A1-11Nb alloy, a U-notch has a notch strengthening effect in the high stress or short life region and a notch weakening effect in the low stress, long life region; a V-notch always has a notch weakening effect. The β treated Ti-24A1-1 INb alloy shows notch weakening effects both for U-notch and V-notch, with V-notch and high stress levels being the most deleterious. The theoretical DCPD response corresponding to a FEM-calculated creep deformation at a given time t was estimated and compared with the real time recorded DCPD changes. This combined DCPD-FEM analysis method provided quantitative information about the relative contributions of creep deformation and damage mechanisms (cavitation, micro- and macro-cracking) to the experimental DCPD curve in a notch stress rupture test.

As a part of an ongoing study of creep deformation mechanisms, mechanical twinning behavior was investigated in the lamellar region of a creep specimen. A multi-stress jump creep test was performed and the microstructures before and after deformation were investigated using optical and transmission electron microscopy. The identification of mechanical twins in a lamellar microstructure is discussed. Extensive mechanical twinning was observed in lamellar regions, in addition to slip and subgrain formation. The occurrence of mechanical twinning depended on the lamellar orientation with respect to the tensile axis. The mechanical twins are analyzed and discussed in terms of possible crystallographic twinning systems. In this case, a maximum resolved shear stress criterion for mechanical twinning is proposed to account for the observed orientations.

Several recent reports on the mechanical behavior of polycrystalline L12 trialuminides, particularly those based on the ternary Al-Ti-Cr and Al-Ti-Mn systems, have indicated limited tensile ductility at low temperatures (≤623K) as measured in uniaxial tension or in three- and four-point bending. While plastic elongation in bending and in uniaxial tension was not observed for forged Al66Ti25Cr9 at and below 473K, some room temperature ductility was reported in bending for this compound in the cast and HIPed as well as in the extruded and annealed conditions. This study attempts to understand this discrepancy by subjecting the forged compound to a variety of heat treatments that influence the residual dislocation content and the grain size, and then evaluating bend specimens at 473K. After identifying the heat treatment that provided the maximum ductility, a second set of bend specimens were heat treated and tested at 473K as a function of strain rate (crosshead speed). All bend tests were conducted in air. Bend specimens that were optimally heat treated were also tested at 473K at a slow strain rate in diffusion pump oil to isolate the possible role of environment on ductility. Notched bend specimens were independently tested in the as-forged condition as a function of temperature to obtain a measure of fracture toughness.

In this study, new combinations of heat treatment and extrusion conditions was employed to produce fine-grained, nearly lamellar microstructures in a binary Ti-48 at.% Al alloy. The effect of subsequent heat treatment on microstructure was studied and room temperature tensile properties were determined for some representative microstructures with relatively high volume fractions of lamellar grains. High tensile strengths and ductilities (upto 2.3%) were obtained and microstructural factors thought to be responsible for these are discussed. Constant load tensile creep properties for the same microstructures were evaluated in the temperature range of 700 to 815°C for stresses from 103 to 241 MPa. This data was used to determine minimum creep rates, activation energies and stress exponents and their dependence on microstructure. A significant effect of microstructure, particularly volume fraction of lamellar grains on the creep rates was observed, and creep rates comparable to those reported in quarternary Ti-48Al alloys with similar microstructures were obtained. The various results suggest that it is possible to obtain an attractive and balanced combination of room temperature ductility and high temperature creep resistance in these materials by suitably varying thermomechanical processing and heat treatment conditions.

Alloy design of a new Co-base heat resisting alloy is being pursued. Similar to Ni3Al in the Ni-base superalloys, the introduction of E21 type intermetallic compound Co3AlC as a strengthening phase in a Co solid solution matrix is being attempted. Three experimental alloys are prepared which consist of either two or three phases. Two phase alloys consist of the Co terminal solid solution and the Co3AlC, while a three phase alloy contains additional B2 phase based on CoAl. Temperature dependence of strength is measured for all the three alloys to reveal that the alloys could exhibit comparable intermediate to high temperature strength with Ni-base superalloys and that, most astonishingly, are found to be quite ductile over all the test temperature including 77 K.

Recent research on the low- and high-temperature properties of two beryllium-niobium intermetallic compounds, Be12Nb and Be17Nb2, is reviewed and discussed. Strength (bend and compression), hardness and fracture toughness has been mapped as a function of test temperature up to 1200°C. Results for hot-isostatically-pressed Be12Nb and Be17Nb2 are highlighted illustrating the potential for reasonable strength at both low and high temperatures. Chemical compatibility between Be12Nb and several high-temperature materials such as SiC, Al2O3, MoSi 2and various refractory metals is evaluated. Limitations for the structural use of the beryllides are identified and discussed including low-temperature toughness, intermediate-temperature embrittlement, high-temperature creep strength and composite compatibility.

Near Nb3AI alloys, having Nb3Al with the ordered A15 crystalographic structure within regions of Nb solid solution, can be considered as in-situ reinforced composites. Nb-18 at.%AI ingots were arc melted and heat treated to provide several microstructures. Small compact tension specimens were cracked in compression- compression cyclic loading and subsequently grown in tension-tension cyclic loading. Fatigue crack growth rates were found to be similar to crack growth through a TiAI-based alloy with a lamellar microstructure. Threshold stress intensity factor was estimated as 5.3 MPaȡm, and fracture toughness as about 10 MPaȡm.

The effects of microalloying elements, Y(0.02at%, 0.04at% and 0.06at%) and Nb(0.2at%, 0.5at% and 0.8at%) on the wettability of intermetallic Fe-40A1 with polycrystalline α-Al2O3 were investigated experimentally by means of sessile drop method. The addition of 0.02at%Y and 0.04at%Y showed no significant effects on the wettability. However, the addition of 0.06at%Y improved the wettability in temperature range from 1673K to 1823K. The addition of different amount of Nb were not beneficial to the FeAl-Al2O3 wettability under conditions investigated. The SEM study of the shear fractured surfaces showed that the additions of Y and Nb coursed Al2O3 particles more adherent to FeAl matrix, compared with the case of pure Fe-40A1. The degree of adherence varied with the amount of Y and Nb. These phenomena are discussed in terms of interfacial reactions and interfacial bonding.

TiAl alloys with Cr, V, and Nb additions show promise as high temperature materials due to their high temperature strength and modulus. Dynamic recrystallization has been shown to be important for the processing and superplastic forming of these materials. Earlier studies have indicated that dynamic recrystallization may also occur during tensile straining at or near the proposed use temperature of these alloys. A systematic study has been conducted to determine the effects of various parameters - temperature, strain, strain rate, and microstructure, on the occurrence of dynamic recrystallization near expected use conditions. The conditions near the ductile to brittle transition temperature which bound the onset of dynamic recrystallization in tension and compression were investigated. The findings will be presented and affrelations will be drawn between the contributions of dynamic recrystallization and high temperature to the observed ductility.