Constant-stress compressive creep tests were carried out on an Al-rich a2 single-phase material, which had equiaxed-grains of 60μim in grain size, at 1050∼1250 K under 100∼500MPa. The type of the primary creep stage and the microstructures developed during creep depend greatly on the creep condition. The minimum creep-rate, however, can be represented by one set of parameters over the whole range of experimental condition. The stress exponent is 5.0±0.2 and the (modulus-compensated) activation energy is 360 ± 10kJ/mol. The Dorn-type plot of the minimum creep rate reveals that the normalized creep strength of fine-grained Ti-34mol%Al is not greatly different from that of disordered solid-solution hardened alloys.

The intermetallic compound NiAl is a promising high temperature material as it possesses a favorable combination of advantageous properties (high melting point, low density, high thermal conductivity, good oxidation resistance). However its creep resistance is far inferior to that of advanced superalloys. We have developed new powder-metallurgical NiAl alloys, where the creep strength is raised by incorporating fine yttria dispersoids. The materials are produced by mechanical alloying and subsequent coarse-grain recrystallization at temperatures close to the solidus temperature. The resulting alloys show exceptional creep strength up to 1400 °C, thereby exeeding the temperature capability of superalloys significantly. The creep properties are in agreement with our recently developed model for “detachment-controlled creep”, in which the release of dislocations from incoherent particles determines the creep rate. In addition, the brittleness problem at room temperature can be moderately alleviated by incorporating coarse ductile phases such as Mo. While the particles raise the critical stress intensity factor, they do not improve the room temperature tensile ductility.

The compressive and tensile strain rate sensitivities of polycrystalline and single crystal NiAl have beem evaluated at 877°C which is well above the static DBTT. Samples were prepared to specific grain sizes by hot press consolidation of appropriate powder sizes and minimizing post-consolidation thermal exposures. NiAl single crystals were grown in the near [100] orientation using a modified Bridgeman technique. The use of high oxygen content powders to fabricate polycrystalline NiAl with very fine grain sizes resulted in increased strengths but did not debit ductility. The yield and tensile strength of polycrystalline NiAl as a function of grain size generally follows a Hall-Petch type relationship. A tensile strain rate effect was found at strain rates of 1 to 10−4 sec−1. The strain rate sensitivity exponent, m, in the equation σ = Cεm is in the range 0.10 to 0.13. The highest strain rate (1 sec−1) resulted in significantly higher strengths and little or no observed tensile ductility. Cleavage-type transgranular fractures were found on samples tested at the highest strain rate (1 sec−1) and ductile fractures were found on samples tested at the lower strain rates (10−1 and 10−4 sec−1). Compression testing at strain rates of 10−5 to 10−3 sec−1 also indicated an effect of strain rate on strength.

Microcracking in polycrystalline near-stoichiometric NiAl produced by room temperature plastic deformation under uniaxial compression was investigated by means of optical microscopy, velocity of sound measurements, and acoustic emission monitoring. Results show that strains greater than 2 % are required to produce microcrack populations which can be evaluated by microscopical investigation or velocity of sound measurements. However, acoustic emission monitoring during compression testing indicates that microcracking starts at about 0.7 % compressive plastic strain which is identical with the typical tensile fracture strain for NiAl. Thus it is concluded that there is little or no stable microcracking prior to failure in tension. Acoustic emission results show also that the process of microcracking does not primarily occur during the applied compressive deformation. A considerable fraction of the microcracking takes place during the quasi-elastic unloading following deformation.

Single crystal NiAl and Ni-49.75Al-0.25Fe have been deformed along <110> at temperatures of 77, 298 and 773K and strain rates of 0.001/s, 0.1/s and 2000/s. The flow stress of <110> NiAl is rate and temperature sensitive. A significant decrease in the work hardening rate is observed after deformation at 77K and a strain rate of 2000/s. Coarse {110} slip traces are observed after deformation at a strain rate of 2000/s at 77K, while no slip traces were observed after deformation under all other conditions. TEM observations reveal distinct {110} slip bands after deformation at 77K and a strain rate of 2000/s.

The purpose of this work was to study the effect of ductile phases on the wear of brittle intermetallic compounds. The material system of interest was NiAl with an incorporated ductile Fe phase. Three different compositions were studied: Fe10Ni45Al45, Fe30Ni35Al35, and Fe44Ni28Al28, each after two different heat treatments. Their wear behavior in unlubricated sliding against zirconia was investigated using pin-on-disk tests at room temperature in air. Hardness and fracture toughness tests were run for the same materials to clarify the relationship between wear and deformation. Results for the ductile-phase toughened materials were compared with results of similar tests for unalloyed NiAl.

The amounts of intermetallic compounds and liquid phase are estimated as a function of temperature during reaction syntheses for Ni–Al binary alloys having arbitrary compositions under adiabatic conditions. It is assumed that the phases that would present during the process are only those appearing in the equilibrium binary phase diagram. Calculations are done by using available data on heat of formation of intermetallic compounds and those on enthalpy of liquid phase. The results obtained arc discussed in view of the estimation and control of the amount of liquid phase, which is significant for the densification and shape change of materials.

Fracture toughness of NiAl–based composite reinforced by TiC particulate was measured using single–edge notched three–point bend specimens. The composite was fabricated by HPES (hot pressing aided exothermic synthesis) technique. It is shown that the ambient toughness of stoichiometric NiAl was increased from 5.9 MPa m1/2 to 8.9 MPa m1/2 by incorporating 20 vol.% TiC particulates. From fracture surface analysis along with the high–resolution electron microscopy (HREM) observation, toughening mechanisms of the NiAl–TiC composite were also discussed.

The difference in developing of solubility lobe (s.l.) of B2 phase from the B2 NiAl point on each isothermal phase diagram of Ni-Al-Co, Ni-Al-Fe and Ni-Al-Mn is expected to be explained in terms of the pattern of free energy minimum (f.e.m.) line in the B2 phase region of these phase diagrams. To calculate such the f.e.m. line, a simple way has been proposed based on the tetrahedron approximation of the cluster variation method (CVM) making use of a relationship to correlate the conventional chemical potential of a species with some kinds of the CVM variables. The calculated results show that the f.e.m. line well reflects the solubility lobe developing of the B2 phase for both Ni-Al-Fe and Ni-Al-Co while some disagreement arises with Ni-Al-Mn. The results also show a theoretical possibility of transition from the high ordered state B2 to a low ordered one under a relatively low temperature regime so far as the present free energy model are concerned.

Cryomilling of prealloyed NiAl containing 53 at. % AJ was carried out to achieve high nitrogen levels. The consolidation of cryomilled powder by extrusion or hot pressing/hot isostatic pressing resulted in a fully dense NiAl-base composite containing 30 vol. % of inhomogeneously distributed, nanosized AlN particulate. The NiAl-30AlN composite exhibited the highest compression yield strengths at all temperatures between 300 and 1300 K as compared with other compositions of NiAl-AlN composite. The NiAl-30AlN specimens tested under compressive creep loading between 1300 and 1500 K also exhibited the highest creep resistance with very little surface oxidation indicating also their superior elevated temperature oxidation resistance. In the high stress exponent regime, the strength is proportional to the square root of the AlN content and in the low stress exponent regime, the influence of AlN content on strength appears to be less dramatic. The specific creep strength of this material at 1300 K is superior to a first generation Ni-base single crystal superalloy. The improvements in elevated temperature creep strength and oxidation resistance have been achieved without sacrificing the room temperature fracture toughness of the NiAl-base material. Based on its attractive combination of properties, the NiAl-30AlN composite is a potential candidate for advanced engine applications.

NiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmosphere. The mechanically alloyed (MA) powders were consolidated by vacuum hot pressing (VHP), resulting in a material which was fully dense. Analytical transmission electron microscopy has been used to understand the development of microstructure with processing. The dispersoid particles present are mostly A12O3 with small amount of A1N. It has been shown that morphology of the dispersoid particles are not uniform and the larger dispersoid particles are located mostly along the grain boundaries with the dispersoid-free zones near them. Coarsening behavior of the dispersoid particles is different depending on their locations, i.e., within the grain or along the grain boundaries. The accelerated coarsening of dispersoids along the grain boundaries occurs concurrently with that of the grains and the formation of dispersoid-free zones (DFZs) are observed along the grain boundaries. It is suggested that this abnormal dispersoid coarsening and the formation of DFZ are due to the coupled migration of dispersoids and grain boundaries.

The specific rotation relationships between adjacent γ grains have been measured in an alloy of composition Ti-47.5Al-2.5Cr consisting of lamellar and equiaxed distributions of γ grains in which the α2 phase was distributed as long lamellae or smaller globules respectively. Detailed analyses of active slip systems have been carried out by TEM observations of deformed samples and from these, the effect that specific rotations between γ grains have on the compatibility of deformation across adjacent grains has been discussed. Theoretical calculations of the geometric compatibility factor characterising the best slip transfer across adjacent grains have been used and compared to the experimental results. It has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.

The gamma lamellae in TiAI alloys are rotated relative to each other approximately by multiples of 60˚ so that close packed planes and directions nearly align. However, the deviations caused by the tetragonality of the lattice are important in determining the structure of the interfaces and the internal stresses in the lamellae. These, in turn, strongly affect the mechanical properties of the alloys. In this paper two new electron optical techniques are used to quantify the deviations from pure twist boundaries (1) selected area diffraction from two adjacent twin-related lamellae permits c/a to be measured with high precision, (2) convergent beam diffraction is used to measure tilt components of lamellar boundaries and also rotations of vectors within the lamellar boundaries. The results are discussed in connection with the weak beam observations of interface dislocation structures.

A consideration of {121}L10 boundaries in their unrelaxed form shows that there are at least three ィ2, six ィ3 and three ィ6 variants possible due to the tetragonal ordering of the L10 lattice. Experimental observations of コ-TiAl, obtained by HREM, show that two コ-grains with a true-twin orientation relationship exhibit a rigid body shift between the {111} planes perpendicular to and either side of a {121}L10 interface, and that the interfacial structure is distinct from those previously observed in f.c.c. metals.

Recent work has shown that antiphase domains are observed in alloys based around TiAl if they are quenched from the alpha field. The present work has shown that the anti phase domain boundaries (APDBs) are in fact not simple APDBs but are composed of very thin (about 2 nm) 90° domains. Although the contrast is dominated by the fact there is a 1/2<101> displacement between the domains either side of this thin 90° domain, there is somewhat more complex contrast associated with the APDBs than the hard sphere model would predict. Annealing studies of the complex APDBs in a Ti49Al alloy have shown that the originally irregular APDBs in the quenched material facet on {111} planes. The significance of these observations is discussed in terms of mechanisms by which the APDBs are formed in TiAl and in terms of the planar fault energies in the Llo structure.

The deformation structure of polycrystalline TiAl-based alloys after uniaxial compression at temperature range from 77K to 1073K has been examined using transmission electron microscopy. It was observed that a large number of faulted dipoles are commonly present in deformation structure of the alloys compressed at low temperature 77K and room temperature. The nature of the faulted dipoles has been determined to be intrinsic stacking fault lying on {111} plane, bounded by 1/6<112] partial dislocations. A possible mechanism for the formation of the faulted dipoles was suggested. The results of the statistic observation shows that faulted dipoles in deformed Ti-48A1 and Ti-(47–48) Al-X (X = V,Cr,Mn) alloys are less than those in single phase Ti-52A1 alloy, and the number of the faulted dipoles decreases with increasing deformation temperature. The effect of the faulted dipoles on plastic deformation of the alloy was discussed.

The processes controlling the dislocation mobility in two-phase γ-titanium aluminides have been investigated over a wide temperature range by determining the activation volumes and activation energies of thermally activated dislocation glide processes. The deformation tests are supplemented by electron microscope observations. Accordingly, at room temperature the mobility of ordinary dislocations is determined by a combination of localized pinning and lattice friction. Additional glide resistance arises from dislocation dipoles and debris defects, which are trailed and terminated at jogs in 1/2 <110] screw dislocations. Dislocation climb processes start above 900 K and seem to initiate the transition from brittle to ductile material behaviour.

Mechanical properties of a polycrystalline single phased γ Ti47Al51Mn2 alloy were studied by compression tests in a wide range of temperature (100 K - 1300 K). We report, in this paper, the temperature dependence of both the flow stress and its strain rate sensitivity. These dependencies show the existence of three temperature domains corresponding to different dislocation motion mechanisms. The temperature dependence of the flow stress strain rate sensitivity is compared with values measured in single crystals1.

Single crystal γ-TiAl with axial orientation [3 16 15] has been tested in compression between 4K and 1048K and the dislocation structures observed in TEM. The slip plane was found to be (111) over the entire temperature range tested. Three regimes exist in the variation of the yield stress with temperature, whereas the dislocation substructures are of two types, dominated by 30° 1/3[112] and 1/2 < 110] dislocations respectively. The anomalous yield stress is associated with 1/2 < 110] dislocations undergoing frequent cross-slip off the (111) plane.

The observations on quantity and configuration of dislocations by TEM conventional diffraction contrast method as well as the determinations of the electron charge density distributions by the quantitative electron crystallography method in Ti47. 5A12. 5V deformed at 400°C and room temperature (R. T.) have been carried out. The metallic bonding between Al—Al or Ti—Ti atom pair along <C110] and Ti — Ti along <C112] direction is strengthened; while the metallic bonding between Ti — Al atom pair both along <C101] and <12l] direction is weakened at 400°C. The quantities of a/2<110], a/2<112] and dissociated a <101] (a[101] →a/2[110] + a/3[112] + SISF+a/6[112] ) dislocations are increased at 400°C, compared with that at R. T.. The a/ 2 < 121] superdislocations have not been seen both at 400°C and R. T..