In the Nb-Si system, it is possible to produce in-situ composites consisting of a brittle Nb5Si3 intermetallic matrix and ductile Nb particles. The two phases are thermochemically stable up to ∼ 1500∼C and are amenable to wide microstructural variations including morphology, volume fraction, and the size of the individual microconstituents. This paper presents microstructures and phase transformations in these composites as a function of composition and heat treatments and bend properties from room-temperature to 1400°C.

The relationship between the tensile properties and microstructure of Ti-TiC composite materials with Al additions was investigated. Composites containing 40–50% TiC by volume were produced using standard casting procedures from the melt. Ultimate tensile strengths of 1113 MPa with 1–2 % ductility were found in these materials. Results show that the tensile properties are controlled by the size and distribution of TiC particles.

The interfacial reaction characteristics of two different types of SiC fibers with Ni3A1 (Ni-Al-Cr-Zr-B) matrix have been investigated. The microstructure and chemical compositions across the reaction zone have been analyzed quantitatively using microscopy and electron probe microanalysis. In both types of SiC/Ni3Al composites, it was found that Ni was the dominant diffusing species responsible for the overall reaction. The C-rich layer outside the SCS-6 fiber provided an incubation period, but could not stop the inward diffusion of Ni. It could, however, effectively stop the diffusion of Al, Zr and Cr. No significant increase in reaction zone thickness after exposure at temperatures below 900 °C for up to 100 hours was observed. When the C-rich layer was depleted, a rapid increase in reaction zone thickness and the formation of multilayer reaction products occurred. In the case of Sigma/Ni3A1 composite, extensive reaction between the fiber and the matrix occurred at all the temperatures studied. Diffusion barrier coating for both types of fibers is required to develop nickel aluminide matrix composites.

A series of Du Pont PRD-166 and FP fiber reinforced Ni and nickel aluminide matrix composites, containing Ti or Y as a wetting agent, were produced by pressure casting. Modulus values determined under compression varied from 273 to 287 GPa in a direction parallel to the fibers and ranged from 148 to 159 in a direction normal to the fibers. Modulus values parallel to the fiber direction were approximately 5% lower than that predicted by the rule-of-mixtures. The much lower values of modulus normal to the fibers has been attributed to a number of sources which include fiber/matrix and fiber/fiber debonding and stress concentration resulting from voids existing at fiber/fiber bonds. It was observed, qualitatively, that good bonding between matrix and fiber occurred when Ti diffused from the matrix into fibers. The presence of Al in the matrix tended to inhibit Ti diffusion, increasingly as Al content increased. Thus fiber/matrix bond strength was poorer in the aluminides than in an alloy containing no Al. Y was not found to improve bonding, and an interface reaction zone was obserced in some areas where liquid metal initially entered the fiber preform.

Recent weak-beam electron microscopic studies of Ni3Al provided evidence indicating that glide of Kear-Wilsdorf locked dislocation segments may occur on {111} planes, accompanied by APB dragging, during deformation at high temperature. A direct implication of those studies is that a mechanism other than the onset of {010} slip may be important in controlling the peak in yield strength in Ni3Al. The present weak-beam investigations of Ni3Al have centered on the possibility that such configurations result from thermally induced dislocation kink and jog activity rather than the APB dragging mechanism. previously proposed.

An X-ray powder diffraction method was used to determine the location of tantalum atoms in Ni3Al. A series of Ni3 (Al,Ta) alloys were produced with tantalum content ranging from 0.1 to 3.0 at%. Fine powders with average particle sizes less than 80 pm were made from melt-spun ribbons by a grinding process. The values of the superlattice peak intensities normalized to the intensity of the (200) fundamental peak agreed with the calculated values assuming that tantalum atoms substitute on aluminum sites, not on nickel sites. Thus it is concluded from our experiments that tantalum atoms substitute for aluminum in Ni3Al, except for Ta contents around 0.2 at% where the data can not be interpreted in terms of a simple substitution scheme.

This study was undertaken to characterize the intermediate temperature creep properties of Ni3Al. Itfocuses on the mechanisms controlling creep deformation and their relationship to the anomalous yielding behavior of this alloy. Constant stress creep tests were conducted for temperatures between 713–973 K, and the following observations were made. The creep curves exhibited two distinct regions. Primary creep was followed by inverse creep. Specimens cooled under constant stress strained an additional 20% during cooling. Temperature drop experiments indicate that Ni3Al is weakened by the addition of creep deformation.

Glide on the primary octahedral plane appears to be exhausted during primary creep. Slip trace and TEM studies indicate that inverse creep is controlled by slip on the cube cross slip plane and a secondary octahedral plane. Primary octahedral slip is observed in the specimens that are cooled and deformed under constant stress.

Compositional effects on hot and cold workability of ductile-ordered nickel aluminides are described. Compositional effects studied include variations of chromium, aluminum, iron, and zirconium. The hot and cold workability of various nickel-aluminide alloys was ranked with respect to workability of stainless steel. The nickel-aluminide alloy containing chromium and low zirconium (IC-218LZr) had the best combination of hot and cold workability. This alloy also offered a good combination of strength and ductility values over a range of test temperatures.

Explosive compaction and HIPping of stoichiometric mixtures of Ni and Al (and B) powder is optimized in order to obtain fine grained NiAl− and Ni3Al(+B) intermetallics.

Starting from an appropriate mixture of pure fine Ni− and fine Al− (and B) powders fine nickel-aluminides are produced due to a selfpropagation high temperature synthesis during processing. HIPping experiments are done also on spherical Ni-coated Al-powders. The specimens are characterized by metallography and the homogeneity of the final product is controlled by microanalytical methods.

Ordered Ni3Al containing 24 at.% Al was irradiated by 2 MeV protons at −45 °C to a fluence of 1.34 × 1016 H+/mm2. The induced microstructural changes were investigated by transmission electron microscopy, X-ray diffraction, scanning electron fractography and microhardness measurements. Proton irradiation produced a large volume fraction of small disordered zones residing in the ordered matrix. The morphology of the disordered zones bears some striking similarities to “tweed” microstructures. X-ray diffraction reveals distinct splitting of the higher order fundamental peaks toward low angles, and broadening of the displaced peaks, indicating that the zones observed are small clusters of tetragonally distorted disordered Ni3Al. The microhardness measurements suggest that the disordered zones strengthen the remaining ordered matrix considerably, most likely through a precipitation strengthening mechanism. The fractographic observations suggest that an increase in ductility accompanies disordering, as evidenced by the appearance of dimples, slip markings and river patterns on the fracture surface of the irradiated region of the sample.

A series of fine-grain and amorphous Ni3Al-X alloys, where X is a solute of 1 to 45 at.% Hf, C or Hf+C in continuously graded compositions, was formed simultaneously into a 0.2 mm-thick disk by high-rate sputter co-deposition. The as-deposited Ni3Al-X alloys exhibited a disordered Ll2 structure and extended solid solubility limits beyond which amorphous phases were formed. Heat-treatment of the alloys at 900°C resulted in fine grain size of 0.5 to 1.5 μm. Ultra-fine dispersoids less than 500 Å in size were observed and they seemed resistant to coarsening.

The recrystallization kinetics, structure development, stored energy of cold work, and rolling and recrystallization textures were studied on boron doped Ni76Al24. The recrystallization kinetics were very slow compared to pure metals or disordered solid solutions, but similar to other intermetallic compounds. The development of the deformed and recrystallized microstructure is similar to metals with intermediate stacking fault energy. Despite a lack of massive deformation twinning the rolling texture is a typical brass type texture. The recrystallization texture is not a brass type recrystallization texture, but consists of components, which are related by a 25°<100> rotation among each other and with the deformation texture. The observed phenomena are attributed to the constraints of grain boundary motion in long range ordered structures. It is further shown that Ni3Al does undergo dynamic recrystallization during high temperature deformation.

Creep strengths and ductilities of boron-doped nickel aluminides are significantly improved by small additions of hafnium. Since creep failure in these alloys proceeds by growth and coalescence of grain boundary cavities, effects of Hf additions on the compositions of grain boundaries and creep cavity surfaces were examined using Auger electron spectroscopy (AES). In order to facilitate intergranular fracture in the AES analysis chamber, gage sections from creep specimens were hydrogen charged and copper plated. Grain boundaries in crept specimens were not flat, and the grains did not exhibit a normal polyhedral appearance. The overall levels of boron detected on the fracture surfaces of the creep specimens were significantly higher than those from uncrept specimens. Additionally, boron was found to segregate more strongly to the grain boundaries while higher concentrations of residual sulfur and phosphorus were detected on the cavity surfaces in both alloys. These results are discussed in terms of the effects of impurity segregation on high temperature grain boundary cavitation.

The notched mechanical properties of Ni3AI and Ni3Al+B prepared by powder metallurgy techniques have been determined in both tension and bending at room temperature. Ten- sile tests performed using double notched specimens containing relatively blunt notches produced intergranular fracture in both Ni3Al and Ni3AI+B, with evidence of fracture initiating in an intergranular manner ahead of the blunt notch in both cases. Estimates of notched fracture toughness from bend tests and of local grain boundary fracture stress from the notched tensile tests suggest an increase in these values with boron addition.

In a recent MRS symposium, the effect of boron on grain boundary hardness (GBH) in Ni3Al was reported. Addition of 0.2 at.% of boron decreased the GBH in all three sets of alloys tested (24, 25, and 26 at.% Al). The present paper summarizes the study of the effect of boron on hardness within grains. Boron-induced softening was again observed in grain interiors. However, it occurred only in alloys with 24 and 25 at.% Al, but not in the alloy with 26 at.% Al. An explanation for this behavior is presented.

Tensile tests on splat-quenched Ni3Al-Cr alloys showed a sharp decrease in ductility with long-time annealing. The growth of the initially very-fine size anti-phase domains showed a tenuous correlation with ductility up to a critical size, where ductility was lost. The grain size was relatively unaffected by these annealing treatments, but the grain-boundary curvature decreased, implying less toughness. An important observation was that for the longest annealing time a chromium-rich precipitate formed, which our data indicate could be a boride. Miniaturized tensile tests were performed on samples which were all obtained from the same splat-quenched foil, and the various domain sizes were controlled by subsequent annealing treatments.

Ni and Ni-base alloys have been simultaneously chromized-aluminized by a halide-activated pack cementation coating process. The computer program SOLGASMIX was used to calculate the equilibrium partial pressures of the gaseous species. Chloride-activated packs are predicted to be chromizingaluminizing while fluoride-activated packs are not. Codeposition of Cr and Al occurs in packs containing 90–95 w/o Cr in the masteralloy and either NaCl, CrCl2 or NH4 C1 as the halide salt activator at 1273K.

The alloying effects of Cr, Zr, Ta, Nb and B on the microstructure and properties of single crystal Ni3Al alloys have been studied. The strength of the alloys has been improved markedly with promising plasticity. The 0.2% yield stress of some of these alloys is higher than that of IN713C, a typical cast nickel-base superalloy, in the temperature range from 25 to 950°C and some of them reach the strength level of advanced cast nickel-base alloys such as IN100 in the high temperature range. The creep strength of these alloys is also improved markedly. The single crystal Ni3Al alloy with a high Cr content ( 7 at-%) obviously shows an intermediate temperature low plasticity behavior. Although heat treatment can only slightly improve the strength, it can double the elongation of the alloy from 25% to 50%. In the Ni3Al monocrystal containing 1.2at%Zr there exist great amounts of the low melting point Ni5 Zr-γ eutectics around the ordinary γ/γ′ eutectics. This makes it difficult to homogenize the coarse casting structures completely.

A series of cast Ni3Al-base alloys with the addition of alloying elements, such as Hf, Zr, Ti, Nb etc., have been studied, in which the total amount of Al and alloying elements substituting for Al was controlled in the range of 18 to 23 at%. It was found the alloying elements change remarkably the morphology, distribution and amount of γ phase as well as the morphology and size of primary γ′ intermetallic compound. The size of primary γ′ can be decreased to micron order. The brittle γ′-γ′ boundary (refer to primary γ′) can be substituted by tough γ′-γ-γ′ boundary. As a result, the mechanical properties of the cast Ni3Al-base alloys, especially high temperature ductility, can be enhanced. In addition, the effect of Cr is discussed when tested in air environment, and considered that chromium addition is not very effective to improve the high temperature embrittlement.

The effects of alloying additions on the mechanical properties of the B2 intermetallic NiAl have been investigated in both the melt-spun ribbon and consolidated, bulk form. The study is based on a matrix of NiAl-based alloys with up to 20 a/o Co and Fe additions and with reduced Al levels in the range of 30 – 40 a/o. Characterization of the melt-spun ribbon by optical and scanning electron microscopy indicates a range of microstructures: single phase β γ, necklace phase surrounding either martensitic or β grains, and a mixture of equiaxed martensitic and γ grains. Bend ductility is present in melt-spun and annealed ribbons exhibiting the γ necklace structure and in a single phase β material containing 20 a/o Fe.

The analysis of compressive flow behavior on consolidated, bulk specimens indicates that the single phase γ alloys exhibit a continuous decrease in yield stress with increasing temperature and profuse microcracking at grain boundaries. In contrast, multiphase (γ + either martensite or β) alloys tend to display a peak in flow stress between 600 and 800K with little or no signs of microcracking. In general, heat treatments which convert the martensitic grains to β + γ result in improved strength at temperatures above 600K and better resistance to crack initiation. These results are discussed in terms of the effects of β, martensite and γ on the yield stress and flow behavior of NiAl-based alloys.