Room temperature tensile tests were performed on annealed melt-spun ribbons of a 50Ni-20Al-30Fe alloy. The ribbons were found to possess a duplex structure consisting of fcc γ and B2 β grains and exhibited tensile elongations in excess of 10% while still maintaining good strength. The tested specimens were found to contain high dislocation densities in both the γ and β grains, with no indications of stress-induced martensite formation. Dislocation analysis revealed that the vast majority of dislocations in the β have <100> Burgers vectors; however, <111> dislocations were also observed. Slip transfer was often facilitated by specific orientation relationships between the γ and β grains.

Intermetallic compound Ni3Al(γ′) is one of the major constituents in Ni-base superalloys. But it is not used by itself for structural use because of its low ductility and low strength.

In the present study, the improvement of elevated temperature mechanical properties of Ni3Al intermetallic compound is pursued by addition of such elements as Fe, Ti, Cr, V, Nb, Ta, Mo, W, Zr and Hf. These results show that Ni 3AI which contains some elements substituting for Al has better mechanical properties and creep rupture properties than commercial Inconel 713C.

Stress-controlled fatigue tests and fatigue crack growth rate tests respectively have been carried out on two Ni3Al Cr/Zr alloys, IC 218 at 600°C and 800°C, and IC 221 at 800°C, in vacuum, at various test frequencies. Decreasing the test frequency and/or increasing the temperature leads to a decrease in the number of cycles to failure, and a gradual disappearance of a fatigue fracture zone. In fatigue crack propagation tests, the crack growth rate only decreases at the lowest frequency and remains constant in the major part of the frequency range investigated. The fatigue propagation mode in all cases is intergranular. These trends are shown in both cases to be related to a true creep component but, under fatigue crack growth test conditions, crack blunting intervenes gradually as the frequency is decreased, leading therefore to a less severe frequency effect.

Unlubricated sliding tests were performed at room temperature and at 400°C on polycrystalline Ni3Al with and without boron and on IC218, an alloy containing nickel plus 8.94 Al, 7.55 Cr, 0.88 Zr, 0.015 B and 0.01 Hf (all in wt. %). A pjn/disk geometry was used with self-mated samples in air and vacuum (1.3×10-3 Pa). Normal load was 1.5 kg (14.7 N), sliding speeds were 15–19 mm/s and sliding distances were 145–175 m.

It had been expected that materials with and without boron would behave differently during sliding because of their well-known differences in ductility and fracture resistance. However, sliding behavior was similar for materials with and without boron. Environmental effects were more dramatic. For tests in vacuum, the friction trace was very noisy and the average coefficient of friction varied erratically during most of each test. Wear debris ranged from 100 μm to 1 mm in size. Wear scars were smooth, but with signs of heavy deformation. All test materials exhibited smoother sliding behavior in air than in vacuum, but the wear rate was usually higher in air. Debris particles were finer, most of them were at least partially oxidized, and they included at least one ferromagnetic component, probably arising from selective oxidation. It is suggested that the increased wear rate in air is related to oxygen-enhanced fracture of these nickel aluminides.

The isothermal oxidation behavior of Ni3A1 + 8Cr + 0.1B alloy containing zirconium addition was studied over a temperature range 850 to 1150°C and for periods up to 100 h. The results show that isothermal oxidation kinetics of the alloys at various temperatures obey a parabolic law. The addition of zirconium to Ni3A1-8Cr-0.1B alloy improves oxidation resistance at 850 to 950°C. However, the additive results in a considerable weight gain at 1050 to 1150°C. The structure of scale and phase transformation of the surface zone beneath the scale have been investigated. The addition of zirconium leads to the formation of adherent scale at all temperatures, since the internal oxides, like “tree roots” between the oxide scale and the substrate, are developed during oxidation.

IC218 contains about 8.4% aluminum and about 7.8% chromium by weight and forms a tenacious alumina scale. In this work the ability of the scale to protect against aqueous corrosion and abrasive and adhesive wear was evaluated. Wrought IC218 was heat treated at 1150°C, yielding a microstructure of ordered FCC islands in a disordered FCC matrix. This material was more resistant to corrosion than common stainless steels in 96% or 98% sulfuric acid and in 5% hydrochloric acid. It was approximately equivalent to stainless steels in 10% sulfuric acid, 50% acetic acid, 50% sodium hydroxide, 5% sodium chloride, and in a sodium chloride + ferric sulfate + hydrochloric acid mixture used to test for pitting resistance. The IC218 performed poorly in 45% sulfuric acid, 20% or 65% nitric acid, 20% phosphoric acid, and in electrolytic 10% oxalic acid. It did not exhibit sensitivity to intergranular corrosion. It resisted stress corrosion cracking at a stress level equal to its yield strength, 645 MPa, in 45% magnesium chloride and in NACE TM0177, a test that simulates an oil well environment. IC218 wore more quickly than austenitic stainless steels do in a dry-sand-rubber-wheel test, but resisted galling as well as or better than most of them do.

Several nickel aluminide matrix composites were prepared using vacuum hot pressing techniques. The matrix compositions, based on Ni3Al, Ni3Al+B and NiAl, were reinforced with 10 volume % TiB2 particles. Both smooth and notched bend tests were conducted at room temperature on the monolithic as well as the reinforced materials in order to determine the effects of TiB2 reinforcement on both the smooth bend and notched bend properties. TiB2 additions were shown to improve the smooth bend strengths regardless of the matrix composition while notched bend tests, conducted to provide estimates of fracture toughness, revealed somewhat lower values for the composites in comparison to the monolithic materials. Fractographic analyses and in-situ fracture observations of the composites revealed that preferential fracture in regions of clustered TiB2 particles may significantly affect the measured toughnesses.

The objective of this study is to develop a metal-matrix composite based on the intermetallic alloy Ni3Al reinforced with Al2O3 fibers, with improved high-temperature strength and lower density compared to the matrix material. This paper summarizes results of initial fabrication and mechanical tests on specimens produced using IC-15 [Ni-24% Al-0.24% B (at.%)] and IC-218 [Ni-16.5% Al-8% Cr-0.4% Zr-0.1% B (at.%)], with 20 vol. % Al2O3 fibers. Fabrication methods include both hot-pressing and hot-extrusion. Mechanical tests include four-point bending and tensile tests. The integrity of the fiber-matrix interface was studied and correlated with mechanical properties. Tensile ductilities of approximately 10% at room temperature were achieved for Ni3Al/Al2O3 composites with controlled material processing and interfacial structure. Fabrication of composites by hot-extrusion produced better tensile properties at room temperature, but superplastic behavior (i.e., low strengths, high ductilities) at 1000°C.

The tensile properties of a powder processed intermetallic matrix composite (IMC) and a similarly processed matrix alloy were investigated in the temperature range 298–1273K. The matrix alloy utilized in the study was the advanced nickel-aluminide IC-221 (Ni-16Al-8Cr-1Zr-0.05B). The composites contained 25 vol% TiC particulate reinforcement. The specific modulus of the composites was 20–30% higher than that of typical Ni-base superalloys. The yield strength of the composites were 10–20% greater than that of the matrix at all test temperatures. However, the composites exhibited lower ductility than the matrix. SEM fractography revealed that tensile failure occurred at the matrix/particulate interface. However, remnants of the matrix was observed on TiC particles on the fracture surface, suggesting good matrix/particle bonding. The strengths of the composites were very competitive with Ni-base superalloys.

Owing to their superior strength in comparison to other single phase intermetallics, NiAl-Ni2AlTi alloys have potential for the matrix material in high temperature composites. An investigation of two compositions, Ni-40Al-10Ti and Ni-45Al-5Ti with and without 20 vol. pct. TiB2 in the form of −1μm diameter particles, was initiated with materials produced by the Martin Marietta XDTM process and compacted by hot pressing. The as fabricated materials were fully dense and polycrystalline where the grain sizes measured −8μm for Ti-poor and about 15μm for the Ti-rich unreinforced materials but could not be determined for ei her TiB2-containing composite. Elevated temperature compression testing was conducted to approximately 8 percent deformation between 1200 and 1400 K with strain rates varying from −10−4 to −10−7 s−1. While the majority of the tests exhibited diffuse yielding over approximately one percent strain followed by negative strain hardening, a few experiments resulted in steady state behavior where deformation continued under a constant stress. The flow strengths on yielding of both forms of Ni-40Al-1OTi were higher than those for the Ni-45Al-5Ti versions. Although for each matrix composition the addition of 20 vol. pct. TiB2 decreased the strength at the higher strain rates in comparison to the TiB2-free forms, the particles do provide reinforcement during slow deformation conditions. Optical microscopy of tested specimens revealed that these materials are generally quite brittle as numerous longitudinal and transverse cracks were found irrespective of the type of stress-strain behavior.

The martensitic transformation and the effect of cobalt additions on important martensitic toughening parameters are being studied as a means of toughening NiAl alloys. Cobalt additions to NiAl martensite are seen to lower the Ms temperature, reduce the transformation strain anisotropy, and reduce the transformation temperature hysteresis (an indicator of interfacial mobility). Optimization of these parameters should allow martensitic transformation toughening processes to aid in overcoming the ambient temperature brittleness of NiAl alloys.

As part of a study aimed at developing a ductile NiAl-based alloy, ingots of Ni-Fe-Al alloys were cast and hot extruded to rods. The purpose of the iron additions was two-fold viz; to produce a change in the slip vector from <001> to <111> and, in one alloy, to add a L12-structured ductile second phase. Extruded Ni-20A1–3OFe was two-phase, containing a pro-eutectic B2 phase in a fine lamellar structure (< 0.5 μm spacing) of B2+L12 phases. Room temperature tensile testing of both single extruded and double extruded alloys resulted in 8% and 22% plastic elongation and yield stresses of 850 and 760 MPa respectively. Fracture in both cases occurred by ductile tearing of the eutectic and transgranular cleavage of the proeutectic phase at 1350 MPa. The ductility in double extruded condition is higher than that reported earlier in rapidly solidified wires by Inoue et al., J. Mat. Sci., 19, (1984), 3097. By comparison, extruded single-phase B2-structured Ni- 30AI-2OFe exhibited a fracture strength of 780 MPa, no plasticity and a mixture of intergranular fracture and transgranular cleavage. This is in contrast to earlier work by Inoue et al. (ibid) where a yield stress of 400 MPa, 5% plastic strain and a mixture of dimple and intergranular fracture was reported.

The influence of various melt spinning parameters and the effect of consolidation on the microstructure of melt spun NiAl and NiAl + W alloys have been examined by optical and electron microscopy techniques. It was found that the addition of 0.5 at.% W to NiAl results in a fine dispersion of W particles after melt spinning which effectively controls grain growth during annealing treatments or consolidation at temperatures between 1523 and 1723 K. Increased wheel speeds are effective at reducing both ribbon thickness and grain size, such that proper choice of both composition and casting parameters can produce structures with grain sizes as small as 2 μ m. Finally, fabrication of continuous fiber reinforced composites which used pulverized ribbon as the matrix material was demonstrated.

Ingots with a nominal composition of Ni–34a/o Al–15a/o Ti + O.Ow/o B, 0.05w/o B, 0.1w/o B or 0.3w/o B were prepared and compression tested at a strain rate of 1.7 x 10–7/s at room temperature, 650°C, 870°C, and 1000°C in air. Alloys were single-phase Ni2 AITi and borides were contained in all compositions containing boron. The compressive yield strength and the ultimate compressive strength of NA2AITi containing boron is strongly affected by the type and distribution of borides. The formation of TiB2 at 0.3w/o B increased the strength at room teperature, 650°C, 870°C, ang 1000°C, while formation of a TiB-Ni2 AITi eutectic at 0.05w/o and 0.lw/o B decreased strength at all temperatures. Crack deflection by TiB2 is an important strengthening mechanism at room temperature and 650°C. However, at 1000°C, dispersion strengthening may be an important strengthening factor.

Alloys of Al75-xFexTi25, where x = 0 to 12 at. % Fe, were fabricated by splat cooling and examined primarily with X-ray diffraction to determine the crystallographic phases present, their composition, volume fraction, lattice parameters, and sublattice occupation. The tetragonal DO22 structure of Al3Ti dissolved 0.8 ± 0.4 at. % Fe, with additional iron partitioned to the Ll2 structure containing 6.2 ± 0.4 at. % Fe, and completed the transformation to the Ll2 structure with the addition of about 6 at. % Fe. The lattice parameters of both phases were independent of iron concentration up to about 6 at. %. At 8 at. % Fe, the iron was found to be predominantly distributed on the nominal aluminum sublattice which also contained about 5 at. % Ti. The Al3 Ti composition was similarly found to contain about 5 at. % Ti on the aluminum sublattice. All compositions, including a single crystal of Al67Fe8 Ti25, were found to be brittle.

An investigation has been made on the constitution of the Ti-Al-C system in the range of 15–55 at% Al and of 0.1–1.0 at% C and the temperature range of 1250–450 °C. Equilibria including three compounds TiC, Ti3AlC, and Ti2AlC have been studied and transmission electron microscopy (t.e.m.) observations of ordering reactions in az have been made.

Transmission electron microscope investigation of extruded and rolled XD™ titanium aluminide sheet has shown the development of recrystallized TiAl regions adjacent to grains which exhibit submicron scale TiAl twins. Detailed diffraction analysis confirmed that the TiAl twins were separated by a layer of Ti3Al. This is consistent with previous observations of the microstructure of extruded XD™ titanium aluminide. In the extruded material, however, the scale of the twins was larger, but the extent of the recrystallized region was smaller. The evidence suggests that the recrystallization of the TiAl regions may result from the impingement of twins from adjacent grains.

Samples of polycrystalline Ti3Al-base alloys, and a two phase Ti3Al/TiAl mixture have been deformed at room temperature and at elevated temperatures and examined subsequently by transmission electron microscopy in order to determine the influence of temperature and alloy content on the relative activity of the various slip systems. In particular, the detailed influence of covalent bonding on dislocation mobility in Ti3Al has been identified.

The effect of adding boron to Ti3Al on the microstructure and high temperature tensile properties has been studied. Boron caused a large grain refinement that dominated the tensile properties at all temperatures. Particles of Ti2B were found in all of the boron containing alloys. TiB was found only at concentrations of 0.1% B or more.

Ternary Ti-Al-V (Llo) alloys containing vanadium up to 10 at.% and aluminum in the range of 50–55 at.% were prepared. Miniature specimens machined from these alloys were deformed in uniaxial compression at room temperature.

The yield stress, and fracture stress and strain were determined with respect to vanadium and aluminum concentration. The deformed alloys were electropolished into thin foils and studied by TEM. In particular, the types of dislocations, stacking faults and twins in the Ti-Al-V alloys were investigated by TEM.