The diffusion bonding behaviour of Ti-48at.%Al-2at.%Mn-2at.%Nb has been studied as a function of temperature (in the range 1200–1350°C), time (15–45 minutes) and starting microstructure (lamellar, duplex and near γ) at constant bonding pressure of 10 MPa. It was found, that under the above conditions, small twin related γ grains, approximately 10–20 μm in size, nucleated at the original interface and grew into the matrix forming a double necklace grain structure. Particles of α2 were observed around the interface, the formation of α2 particles was believed to be related to oxygen partitioning and stabilisation effects from dissolved oxide films during the bonding process. Evidence for this mechanism was obtained from parallel electron energy loss spectroscopy (PEELS), which identified oxygen partitioning in the α2 particles. For the fully lamellar structure bonded at 1250°C for 45 minutes the failure strength of the bond was found to be 250 MPa, approximately 50 MPa lower than the failure strength of the base material.

Alloys based on the intermetallic compound γ-TiAl are rapidly being developed for elevated temperature structural applications. In order to produce an alloy, by either I/M or P/M techniques, with useful engineering properties, it is necessary to control the microstructure by heat treatment and/or hot working. By combining consolidation with the heat treatment, unique microstructures can be produced in powder metallurgy (P/M) materials. The effect of processing on the microstructures and tensile and creep properties are discussed.

Reactive synthesis directly forms compounds from their elemental constituents. In this U.S. Bureau of Mines study the microstructure, hot-hardness and tensile properties of alloys and composites based on TiAl and Ti2AlNb were studied. Ti2AlNb sheets were processed from elemental Ti, Al and Nb foils. The elemental foils were consumed during processing, leaving a microstructure consisting of three phases: O (orthorhombic Ti2AlNb), B2 (ordered cubic Ti), and α2 (Ti3Al). The composite sheet structures were heat-treated to produce a microstructure consisting of O lathes in a B2 matrix. A significant advantage of reactive synthesis is the ease of formation of complex shapes prior to synthesis of the aluminide. TiAl-based alloys and in situ TiAl/boride and TiAl/Ti5Si3 composites were produced from elemental mixtures of Ti and Al with either additions of B or Si powders. The hot-hardness and tensile properties of these alloys and composites are reported. The Kirkendall porosity associated with reactive sintering of TiAl can be eliminated by simultaneously reactive sintering TiAl and Ti5Si3.

Single crystals of single-phase TiAl alloy (Ti-56 at. % Al) have been grown at lOmm/h solidification rate in an ASGAL FZ-SS35W Optical Floating Zone Furnace. The orientations of the resulting single crystals have been determined using the Laue X-ray diffraction, and Electron Backscattering Pattern (EBSP) methods. A correlation between orientations of the crystal growth and grown-in facets has been established.

Erbium additions of 0.25, 0.5, 1.0 and 2.0 atomic percent were incorporated into base alloys of 40 at.% titanium - 60 at.% aluminum by arc melting. Samples of 0.30g were electromagnetically levitated and melted and then rapidly solidified by double anvil splat quenching with liquid temperatures ranging from the liquidus temperature to near the maximum undercooling temperature for each alloy. Microstructures of TiAl with 0.25 and 0.5 at.% Er showed the presence of small erbium rich particles within γ grains as well as antiphase domain boundaries, while TiAl with 1.0 and 2.0 at.% Er showed no evidence of these features. These observations were correlated with solidification velocity measurements on levitated samples. Evidence for disordered primary solidification and a solid state disorder/order reaction is presented.

The notch-sensitivity of the ductile L12 aluminides Ni-rich Ni3A1 doped with boron and Zr3A1 is attributed to the triaxial tensile state of stress induced by a notch.

The effect of simultaneous alloying of boron (B) and the substitutional elements M on mechanical properties of Ni3Al was investigated by the tensile test at room temperature. The yield strength of Ni3Al+B increases by alloying with M except for Fe and Ga. In particular, it increases by alloying with Hf, Nb, W, Ta, Pd and Si. The fracture strength of Ni3Al+B increases by alloying with Pd, Ga, Si and Hf, but decreases with the other elements. Elongation of Ni3Al+B increases by alloying with Ga, Fe and Pd, but decreases with other elements. Hf and Pd is the effective element for the increase of the yield strength and the fracture strength of Ni3Al+B, respectively. Alloying with Hf leads to the increases of the yield strength and the fracture strength of Ni3Al+B, but to the lowering of elongation. On the other hand, alloying with Pd improves all mechanical properties, i.e. the yield strength, the fracture strength and elongation. On the contrary, alloying with Ti, V and Co leads to the lowering of mechanical properties of Ni3Al+B. The reason why ductility of Ni3Al+B is reduced by alloying with some elements M is discussed.

Ductility of binary and ternary Ni3Al alloys with conventional purity, fabricated by isothermal hot-forging, was examined by tensile tests in dry oxygen to eliminate environmental embrittlement. Re crystallized Ni-23A1 doped with approximately 0.4 mol% X(X=Hf, Nb, and Zr) alloys exhibits pronounced ductility, whereas recrystallized Ni-23A1–2X alloys exhibits elongation of less than 3%. Ductility of a highly purified binary Ni-23A1 alloy was also examined and showed remarkable ductility, resulting in trans granular fracture. This indicates that a Ni-rich Ni3Al alloy inherently exhibits marked ductility without intergranular fracture if impurity atoms and other extrinsic factors(e.g., environmental effect and internal microcracks) are eliminated in the alloy. Thus it is found that Hf, Nb and Zr in the significant ductilization of Ni-23A1 can act as a scavenger of harmful impurities when added in a small amount. With further increasing amount of additions of Hf, Nb and Zr, elongation decreases rapidly and reaches almost no ductility at 2 mol% additions, indicating that Hf, Nb and Zr lower the intrinsic ductility of Ni3Al alloy.

The Co3Ti phase hardens appreciably by the fine precipitation of disordered fee Co-rich phase upon aging after quenching from solution annealing temperature. Transmission electron microscope(TEM) observations revealed that the precipitates are platelet in shape, lying nearly parallel to the {100} planes of the L12-ordered matrix, and perfectly coherent with the matrix lattice at the beginning of aging. The high temperature strength increases appreciably with the fine precipitation of disordered Co-rich phase over the whole temperature range investigated. TEM observations of the underaged and deformed alloys revealed that superdislocations are pinned by precipitates indicating an attractive interaction between dislocations and precipitates. At the overaged state, thin twins are introduced in the fee Co-rich precipitates during deformation.

Intermetallic alloys based on the Ll2 Ni3(Al,Be) phase in the ternary Ni-Al-Bc system are prepared so that the alloys are multi-phase with the B2 intermetallic compound NiBe and a Ni primary solid solution denoted as (Ni). Such three-phase alloys, Ni-16 to 20 at%Al-10 at%Be, exhibit good room temperature ductility as measured by four-point bending. In order to examine the phase stabilities and relations among constituent phases, a vertical section of the ternary system is constructed at a constant 10 at%Bc mainly by differential thermal analysis. It is found that improvement in room temperature ductility can be achieved by the formation of a fine mixture of constituent phases during invariant reactions during solidification, which is further enhanced by the co-existence of the Ll2 phase formed as the primary solidification phase.

An oxide-dispersion strengthened (ODS) Ni3Al-based alloy has been fabricated and creep tested. Previously reported data for minimum creep rate as a function of stress indicated that two creep mechanisms operate at intermediate temperatures of 732 and 816°C [1]. This paper reports the results of recent interrupted creep tests and fractographic studies which serve to identify the two creep mechanisms. Creep at low stresses or low creep-rates occurs by constrained growth of cavities on transverse grain boundaries. In this low stress region an apparent stress exponent of 5.1 is observed. Creep at high stresses or high creep-rates results from the bulk deformation of grains by power law creep with a much smaller contribution due to grain boundary cavitation. The stress exponents of 13 and 22 observed in this high stress region are typical of ODS alloys. In both regions fracture is observed to be mixed mode with a large transgranular component due to the high grain aspect ratio developed in this material. Limited data at 982°C indicate the occurrence of only one mechanism which can be described by a stress exponent of 9.1. It was not possible, based on fractographic studies, to associate the creep mechanism at 982°C with either of those observed at the intermediate temperatures. No fractographic studies were performed at 649°C due to lack of valid specimens; however, the stress exponent of 13.5 observed at 649°C suggests that creep occurs by deformation of the grains.

Nickel aluminide Ni3Al in single phase form, with grain size ∼ 10 μm, porosity ∼ 5%, tensile strength 425 MPa, modulus 92 GPa and ductility 9.5% at room temperature, was fabricated by reactive infiltration at 800˚C of liquid aluminum into a porous preform containing 78 vol.% nickel and made by sintering 3–7 μm size nickel particles.

A process for successful melting of Ni3Al- and Fe3Al-base alloys is described. Casting and welding of these alloys are also briefly described.

A potential method to produce Ni3Al+0. 1B PM products of large dimension and as sheets, rods, tubes or other shapes with simple laboratory facilities is presented. The method includes multi-stage sintering and interstage cold deformation to collapse and to eliminate the sintering pores; most important of all, a thermally absorbing material must keep contact with the powder compacts during the preliminary reactive sintering stage to decrease the maximum temperature of the specimen and to develope the Ni2Al3 phase. The found Ni2Al3 regions with some special characteristics can be used to sustain the deformation of cold rolling more than 30% without failure. The monolithic Ni76Al24B0.1 phase is obtained after the second stage of sintering at 1200°C for 4h. At meantime, common reactive sintered specimens without a thermally absorbing material contacting were also studied and compared side by side.

The effects of zirconium addition on tensile properties and grain boundary (GB) chemistry in the Ni3Al alloys were studied. The results showed that with the increasing amount of Zr, the yield and ultimate strength increase, the elongation increases below 0.7 at. % Zr content and decreases above that content. Zirconium segregation to grain boundaries (GBs) is found by Auger electron spectroscopy (AES) study and the amount of Zr at GB is three times greater than that in the bulk. In situ transmission electron microscopy (TEM)/ scanning electron microscopy (SEM) investigation showed that slip can transfer across the GB to the adjacent grain in Zr-doped Ni3Al alloys. These results indicate that the effect of Zr on improving ductility of Ni3Al alloys had a strong relationship with the segregation of Zr on the GBs.

The dislocation substructure of polycrystalline MoSi2 deformed in compression at temperatures ranging from 900°C to 1300°C has been investigated. Slip is found to occur primarily by <100> Burgers vectors. A quantitative characterization of the <100> dislocation substructure is developed for several deformation temperatures, including the slip systems present and the relative occurrence of each. Orientation distributions showing the screw/edge character of the <100> dislocations are generated at each deformation temperature. Variations in these distributions with temperature are noted, and the implications of these variations to the deformation behavior of MoSi2 are discussed. Notable observations include the onset of dislocation climb between 900°C and 1100°C, a strong preference for dislocations of mixed character at 900°C, and the complete absence of pure screw dislocations from 900°C to 1300°C.

The deformation behavior of MoSi2 single crystals has been studied in the temperature range of -196<1500°C. While [001]-oriented crystals can be plastically deformed only above 1300°C, plastic flow is possible from temperatures as low as room temperature for single crystals with orientations other than [001]. Five slip systems, {110)<111], {011)<100], {010)<100], {023)<100] and {013)<331], are identified to be operative, depending on crystal orientation. An anomalous increase in CRSS is observed in the intermediate temperature range for the former three slip systems. Schmid’s law is generally valid for the soft modes, {110)<111], {011)<100] and {023)<100]. In contrast, the CRSS for the hard mode, {013)<331], strongly depends on crystal orientation with the higher values for orientations closer to [001].

Various morphologies of ductile Nb refractory metal reinforcement are incorporated into a MoSi2 matrix using powder metallurgy, including single-ply laminates, continuous metal ribbons and sections of 2-dimensional wire mesh. Hot forging techniques are used to redistribute the reinforcement and change the dimensions and the aspect ratio of the reinforcing metal ligaments. Work-of-rupture measurements are conducted on bend test specimens and precracked tensile specimens of the composite so that the toughness contribution from the various ductile metal morphologies can be assessed according to its effectiveness. Accompanying microstructural examination of crack bridging interaction with the reinforcement is conducted.

Solidification microstructures and phase selection in ingots and melt-spun ribbons of MoSi2, MoSi2-W-Ta and MoSi2-Al alloys were studied. Rapid quenching from the melt refined the grain size of all alloys by two orders of magnitude compared to the ingots. Zone A microstructures were formed in the wheel side of all alloy ribbons. Addition of W and Ta and of Al suppressed the formation of Mo5Si3 in the ingots and ribbons. Tungsten exhibited complete solid solubility in MoSi2. In the ribbons the solubility of Ta in MoSi2 was extended to 2 at% in Zone A, and in the ingots and zone B of the ribbons hexagonal (C40) TaSi2 and tetragonal (C11b) MoSi2 structures were formed. A fine lamellar microstructure between the Cllb and C40 phases was formed at 1350°C after 48h. Addition of Al changed dramatically the solidification microstructure which consisted of two phases C40 Mo(Al0.5Si0.5)2 and C54 MoAl1.3Si0.7. In the ingots the room temperature hardness decreased with addition of W and Ta and of Al. Addition of Al increased the thermal stability of the MoSi2-Al alloy.

Exothermicity associated with the synthesis of aluminides was utilized to obtain nickel, iron, and cobalt aluminides. Combustion synthesis, extrusion, and hot pressing were utilized to obtain intermetallics and their composites. Extrusion conditions, reduction ratios, and hot-pressing conditions of the intermetallics and their composites are discussed.