Fe-Al-X alloys (X = Cr, Mo) in the composition range 50 to 80 at. % Fe, 0 to 20 at. % X were rapidly solidified by chill-block melt-spinning. As-spun and heat treated ribbons were characterised by optical and electron microscopy and X-ray diffraction. Strengthening mechanisms were identified using microhardness measurements. Tensile fracture surfaces of ribbons reveal ductile to cleavage to intergranular fracture transitions with increasing Al-content. An increasing preponderance of cleavage fracture with increasing ternary substitution for Fe was observed.

Based on studies of several binary trialuminides, it has been concluded that those with the L12 structure offer the best prospects for developing useful mechanical properties. Further, evaluation of a series of cubic ternary alloys produced by alloying Al3Ti with fourth period transition elements from Cr to Cu has shown that the mechanical properties and lattice parameters of the cubic alloys vary in a consistent manner with the location of the ternary alloying element in the periodic table. These structure/property relationships have been interpreted in terms of the number of d-shell electrons available for bonding. The Cr- and Mn-modified alloys have been shown to be the most attractive in terms of hightemperature structural applications. Finally, the work has resulted in a suggested mechanism for the D022 to Ll2 transformation on alloying.

Titanium aluminide alloys with compositions near Ti-25A1-25Nb at.% were prepared by both rapid solidification and ingot techniques. Their tensile and creep properties were studied after heat treatment to produce various microstructures containing ordered orthorhombic (O) [1], ordered beta (βo), and α2 phases. It was found that these alloys had higher specific strength from room temperature to 760°C than conventional α2 alloys. Ductility and tensile strength of O+βo alloys were strongly dependent upon heat treatment, with the highest strength observed as-heat-treated, and the highest ductility after long term aging. The creep resistance of single phase O and two phase O+βo alloys was strongly dependent upon heat treatment.

Development of microcracking in a ternary titanium trialuminide modified with copper (Al5CuTi2) has been investigated. It has been observed that indenting the material by diamond pyramid indenter (Vickers) causes two types of cracking: (a) on the faces of the indentation and (b) on the surface around the indentation. Compression tests carried up to various levels of permanent deformation have shown a different response of low porosity (2.6±0.5%) and high porosity (5.3±1.5%) material to the formation of microcracks.

The mechanical behavior of intermetallic alloys is related to crystal bonding and the influence of bonding on the core structure of dislocations formed in these compounds. These combined effects influence the deformation behavior in an, as yet, undefined manner. However, in a way that gives rise to unusual behavior, such as the anomalous temperature dependence of flow stress observed in TiAl. Recent studies have suggested a particular relationship between the directional bonding in TiAl and dislocation mobility. To better understand the flow behavior of intermetallics, and as a beginning toward bridging the gap between electronic structure and flow behavior, we have calculated the electronic structure of various planar faults in TiAl. The self consistent electronic structure has been determined using the layered Korringa Kohn Rostoker (LKKR) method which embeds the region containing the defect between two semi-infinite perfect crystals. Calculated defect energies agree reasonably well with other theoretical estimates, though overestimating experimental values. The changes in bonding taking place in the vicinity of the planar defect will be discussed and illustrated through the density of states and charge density plots.

Tensile creep properties have been studied in Ti-50mol%Al intermetallics prepared by ingot metallurgy. Four types of structure were obtained in as-cast material (C), homogenized material (CA), isothermally forged material (F) and forged-and-recrystallized material (FA). Structural effects was investigated by comparing creep properties such as minimum creep rates, fracture strains and creep life among C, CA, F, and FA materials. Creep characteristics were strongly dependent upon the structure.

The hot-workability of a two-phase (γ+α2) alloy, Ti-48A1-2Cr-2Nb, has been studied by conducting isothermal compression tests to 0.8 true strain over the temperature range of 975–1200°C at strain rates between 1×l0−1 and 3×10−3s−1. A deformation map showing temperature, strain rate, soundness of deformation, and isostress contours was constructed. Good workability is found from the low temperature/low strain rate regime to combinations of high temperature and either high or low strain rate. The upper-limit flow stress for good workability is between 450 and 500 MPa. Deformation induced softening occurs at all conditions. SEM and TEM examinations of the deformed specimens reveal that non-uniform deformation takes place at all strain rates, but cracking occurs mostly at high strain rates (e.g. 1×10−1s−1), especially combined with low temperatures. The cracking appears to progress primarily along γ/α2interfaces. It is thought that non-uniform deformation develops channels of shear bands, which in turn promote localized recrystallization, thus accommodating higher strains.

On the basis of creep strength, ultimate tensile strength and oxidation resistance, seven intermetallic compounds with melting temperatures above 1600°C have been selected as possible candidate materials for high temperature structural applications in advanced aero-turbines. These compounds, Nb3Al, Cr3Si, Co2Nb, Cr2Nb, MoSi2, Mo5Si3 and Nb2Al, have been evaluated and their properties reported herein. All seven of the compounds displayed excellent creep resistance at 1200°C with Mo5Si3 and Nb2Al being the strongest. Nb3Al, with the precipitation of the niobium solid solution displayed the greatest low temperature toughness. The greatest ultimate tensile strengths were observed for Co2Nb and MoSi2, while MoSi2 was by far the most oxidation resistant.

A basic characterization of the deformation behavior of Cr2Nb by microindention at ambient and elevated temperatures (up to 1400 °C) was undertaken. The microhardness of this system was seen to decrease with increasing temperature, from 1040 MPa at 25°C to 322 MPa at 1400 °C. Further, the microindention creep behavior of this system was studied by varying time on load at T = 1000 and 1200°C. Analysis of the data showed that m = 24 and Qapp = 477.61 kJ/mole. These unusually high values are indicative of the existence of an effective resisting stress against creep. When the data was fit against a microindention creep deformation law which was modified to incorporate an effective resisting stress term, it was determined that m = 4.5, Qcreep = 357 kJ/mole and the resisting stress term σr = 300 MPa.

High temperature compression tests were performed on Cr 3Si single crystalline and poly crystalline samples. Slip systems were determined to be of the {001}<010> type based on an analysis of slip traces and Laue spots. Single crystals show significant compressive ductility at temperatures above 0.7Tm. The implication of cube slip on the ductility of A15-type intermetallic compounds is discussed.

Preliminary work was conducted to investigate whether 0.1 at% boron affects the ductility of Ni-rich Ni3Ge (Ni-23.5 at%Ge). Cracks were observed in the unmodified alloy around hardness indentations and on fresh surfaces upon abrasive cutting. No cracks appeared in the modified alloy. Also, SEM fractographs showed that with the boron addition the morphology changed from pure intergranular fracture to a mixture of intergranular and transgranular fracture, albeit predominantly intergranular. Tensile tests indicated that the boron addition slightly improves the ductility of Ni-rich Ni3 Ge.

The results of miniaturized disk-bend tests on samples of Ni 3Al of different stoichiometry and boron content are presented. The yield strengths and ductilities of alloys containing 24, 25 or 26 %Al. either boron-free or doped with 0.3 or 0.35 %B, were measured. Specimens 3 mm in diameter and approximately 200 μm thick were tested, some of these having been cut from the grip sections of previously tested tensile bars. The yield strengths were in excellent agreement with the results obtained from the uniaxial tensile tests. The load-displacement curves for the brittle alloys (all but the boron-doped 24 %Al alloy) exhibited a maximum load corresponding to crack initiation. The shapes of the deformed specimens confirmed the assumption that they deform as if they were clamped even though they are not. The fracture surfaces of the brittle alloys are consistent with intergranular failure. Nevertheless, the ductility of the alloys increases with decreasing Al content and decreasing grain size, even for the boron-free alloys which are all brittle. The fracture stress of the boron-doped 26 %Al alloy is about 30% greater than that of the boron-free alloy. It is argued that this is most likely a consequence of the depletion of aluminum at grain boundaries, coupled with boron segregation. Independent evidence suggests that this should increase the cohesive strength of grain boundaries in the boron-doped 26 %Al alloy.

The mechanical behavior of Zr3Al irradiated with 3.8 MeV Zr3+, ions has been investigated using a miniaturized disk bend test (MDBT). The samples tested were 3 mm in diameter and either nominally 100 or 250 μ m thick. They were irradiated at 250 °C to peak doses varying from 0.0041 to 2 displacements per atom (dpa). The yield stresses, σy, of the thin specimens were independent of dose, within experimental error, and comparable to that of unirradiated Zr3 Al. However, for the thick specimens σy was significantly larger for the irradiated material, possibly increasing to a maximum at a peak dose between 0.02 and 0.06 dpa. The relative insensitivity of σy of the thin specimens to irradiation, compared to the thick ones, is a surprising result for which we have no satisfactory explanation. Examination by transmission electron microscopy showed that Zr3Al begins to undergo chemical disorder at a dose of only 0.02 dpa and Is nearly completely disordered at 0.41 dpa. Electron diffraction indicates the presence of satellite reflections along 〈110〉 in {001} diffraction patterns. The nature of the defects responsible for these has not yet been determined, although rather similar effects have been observed in electron and Kr+-ion irradiated Zr3Al and attributed to elastic softening of the lattice. The possible roles of these defects, as well as disordering, are discussed.

Interstitial boron atoms are known to increase room temperature ductility in Ni-rich Ni3Al and Ni3Si. Inerstitial carbon atoms increase ductility in Ni-rich Ni3Si, but not in Ni3Al. It is argued that grain boundary segregation of B or C is not the cause of the increase in ductility. Instead, strong Ni-B bonding and antisite Ni defects reduce the strength of directional Ni-Al bonding in ordered Ni3Al so that Ni and Al atoms can relax more easily to fill up the microcavities at the grain boundaries, thereby strengthening the boundaries. The reduced Ni-Al bonding will also enhance the ductility in the interior of the grains so that dislocations can be easily generated to shield the tips of the cracks at the grain boundary. In Ni3Si, strong Si-C bonds distort the directionality of Ni-Si bonding. Evidence for this model is presented. Estimates of the concentration of interstitial atoms to achieve maximum ductility are made and the estimate agrees well with the experimental value.

Gamma titanium aluminide alloys of current interest are two-phase alloys consisting of γ-TiAl phase as the matrix and a α2-Ti3Al phase as the second phase. The properties of these alloys depend on alloy composition, processing, microstructure, and their combination. Two major microstructural constituents are gamma grains and lamellar grains, the latter of which contain alternate layers of gamma (γ) and alpha-2 (α2) thin plates. The relative amounts and distribution of these two constituents are the main factors controlling mechanical properties. This paper reviews our current understanding of the composition/microstructure/property relationships. An extended discussion will be made on the fundamental aspects of the formation of lamellar structure during cooling and the evolution of microstructure occurring during thermomechanical treatments.

A Microstructurally controlled γ(TiAl)-based alloy containing β(bcc) has shown remarkable tensile properties at high temperatures. It exhibits over 450% tensile elongation at 1473K. The strain rate sensitivity factors are calculated to be over 0.3 above 1273K. A specific texture is not observed from transmission X-ray photograph. TEM observation reveals that the precipitated β phase located at grain boundaries elongates uniformly along γ grain boundaries and causes large ductility. We propose β/γ dual phase alloys as high temperature deformable materials.

The possibility of modifying the complex crystal structure of selected refractory metal aluminides and silicides of AB3 stoichiometry by interstitial alloying additions was investigated. The compounds Nb3Si, V3Si, Mo3Si, Cr3Si, and Nb3Al were alloyed with oxygen or carbon up to about 20 atomic %. Alloying with oxygen or carbon in V3 Au was studied as a model for the A15 to Ll2 (or L'l2) structural change induced by interstitial alloying. Oxygen and carbon stabilized the perovskite, L'l2, structure in V3Au. Oxygen stabilized a L12 phase in Nb3Si while carbon induced a hexagonal structure phase. No L12 or L'12 phases were stabilized with the addition of oxygen or carbon in V3Si, Mo3Si, Mo3A1, Cr3Si, or Nb3Al. The Pettifor structural map for AB3 compounds was used to predict the structural changes in these compounds on alloying of oxygen or carbon.

Vacuum plasma spraying has been used to fabricate dense MoSi2 and TiB2-reinforced MoSi2-based composites. The spray-formed deposits have high hardness and fracture toughness as compared to that achieved by other processing techniques. Annealing of the spray formed MoSi2 leads to a decrease in hardness and an increase in fracture toughness. Reinforcement with TiB2 leads to toughening and strengthening of the spray-formed MoSi2.

Thermal spray technologies provide a method for depositing high temperature intermetallics. Deposits of Shape Memory Effect NiTi were produced by Electric-Arc Spraying (EAS) of NiTi wire, Vacuum Plasma Spraying (VPS) of NiTi powder, and codeposition of Ni and Ti using VPS. NiTi deposits displayed shape memory effect properties, which are highly sensitive to processing conditions. They were found to also exhibit superior cavitation-erosion resistance. Under identical cavitation erosion conditions, weight losses of 0.41, 1.65 amd 7.71 mg were found VPS, arc-sprayed and blended Ni-Ti deposits respectively. Minimal amounts of NiTi was formed from co-spraying both Ni and Ti powders onto steel substrates.

Prealloyed powders of NiAl, NiAl-5Ti, NiAl-5Nb and NiAl-5Ti-5Nb (at.%) were gas atomized with an average particle size of 90 μm and cooling rates of > 104 K/s. The powders were attritor milled under Ar atmosphere reducing the average particle size to 1.5 μm. Sintering CIP compacts developed microstructure and density distributions, which allowed subsequent containerless HIPing to near full density (>98.5%) and a final grain size < 8 μm. Alloying of NiAl with 5at.% Nb or Ti increased its hardness, Young's modulus, toughness and creep resistance. The room temperature fracture path changed from primarily intergranular (NiAl) to primarily transgranular (Nb, Ti alloyed), increasing the Klc, values from 3.8 to 14.4 and 15.3 MPa√m, respectively. For comparison, HIPed materials from the as-atomized powders were also tested.