Composite materials with an intermetallic matrix based on nickel and titanium monoaluminides hardened by oxide particles or fibers

Hardening phase/intermetallic matrix pairs are chosen for composite materials (CMs) intended for long-term high-temperature operation. These materials must have high and stable mechanical properties during a long time at high temperatures and loads. The compatibility of the physicochemical and mechanical properties of CM components is estimated to choose hardening phase/intermetallic matrix pairs in which the matrix is represented by an alloy based on NiAl or TiAl monoaluminide and the hardening phase is a refractory thermodynamically stable oxide of a Group III transition metal M ₂O₃. The following two schemes are used to perform hardening of a CM with a matrix consisting of a TiAl or NiAl alloy by the most thermodynamically stable interstitial phases, i.e., refractory oxides, at temperatures higher than the operating temperature (T _op) of the IMM. The first scheme consists in creating Al₂O₃/TiAl CMs hardened by continuous single-crystal sapphire fibers using the impregnation of a bundle of single-crystal fibers with a matrix melt followed by directional solidification. The TiAl-based matrix in these CMs serves as a binder connecting oxide phase fibers and preventing them from fracture due to high adhesion forces between oxide fibers and the matrix and a high fiber/matrix interface strength. In the second scheme, Y₂O₃/NiAl CMs are produced by powder metallurgy methods, which include severe deformation by extrusion accompanied by the formation of deformation texture and subsequent recrystallization annealing. In these CMs, disperse refractory oxide particles stabilize grain boundaries in a recrystallized matrix material and lead to the formation of directional structures with coarse elongated grains and a low fraction of transverse boundaries. Al₂O₃/TiAl CMs containing 20–25 vol % hardening single-crystal sapphire Al₂O₃ fibers can operate at temperatures of 1000–1050°C (∼0.7T _m of matrix), which is 250–300°C higher than the maximum values of T _op of a TiAl-based matrix and 400-450°C higher than the maximum values of T _op of a Ti-based matrix. An Y₂O₃/NiAl composite with a directionally recrystallized structure of a NiAl-based matrix hardened by 2.5 vol % Y{ia2}O₃ particles can be recommended for operation at temperatures of 1400–1500°C ((0.8–0.9)T _m of matrix), which are higher by 100–400°C than not only T _op but also T _m of Ni superalloys.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

Elevated temperature compressive properties of Zr-modified NiAl

Small Zr additions are known to substantially affect the deformation behavior and strength of po-lycrystalline NiAl, yet little information is currently available regarding the high-temperature prop-erties of such alloys. Utilizing prealloyed powder technology, a series of four NiAl alloys have been produced containing from 0.05 to 0.7 at. pct Zr. The creep behavior of these alloys was characterized in compression between 1000 and 1400 K at strain rates ranging from ∼0.1 to 10^-9 s^-1. All the Zr-modified alloys were significantly stronger than binary NiAl under lower temperature and faster strain-rate conditions; however, the single-phase materials (Zr ≤ 0.1 at. pct) and binary NiAl had similar strengths at high temperatures and slow strain rates. The two-phase NiAl-Ni₂AlZr alloys containing 0.3 and 0.7 at. pct Zr had nearly identical strengths. While the two-phase alloys were stronger than the single-phase materials at all test conditions, the degree of microstructural damage in the two-phase alloys due to internal oxidation during testing appeared to increase with Zr level. Balancing the poor oxidation behavior with the consistent strength advantage of the two-phase alloys, it is concluded that optimum elevated-temperature properties could be obtained in Heusler-strength-ened NiAl containing between 0.1 and 0.3 at. pct Zr.     

High-Temperature deformation of B2 NiAl-base alloys

The high-temperature deformation behavior of three rapidly solidified and processed NiAl-base alloys-NiAl, NiAl containing 2 pct TiB₂, and NiAl containing 4 pct HfC—have been studied and their microstructural and textural changes during deformation characterized. Compression tests were conducted at 1300 and 1447 K at strain rates ranging from 10^-6 to 10^-2 s^-1. HfC-containing material showed dispersion strengthening as well as some degree of grain refinement over NiAl, while TiB^-2 dispersoid-containing material showed grain refinement as well as secondary recrystallization and did not improve high-temperature strength. Hot-pack rolling was also performed to develop thin sheet materials (1.27-mm thick) from these alloys. Without dispersoids, NiAl rolled easily at 1223 K and showed low flow stress and good ductility during-the hot-rolling operation. Rolling of dispersoid-containing alloys was difficult due to strain localization and edge-cracking effects, resulting partly from the high flow stress at the higher strain rate during the rolling operation. Sheet rolling initially produced a (111)< 112) texture, which eventually broke into multiple-texture components with severe deformation. Formerly with Marko Materials, Inc. Formerly Graduate Student, The University of Michigan.     

The synthesis of nickel aluminides by multilayer self-propagating combustion

The synthesis of Ni-Al intermetallic thin films by self-propagating combustion reactions was investigated for the 1:1 and 3:1 Ni/Al stoichiometries. The dependence of the combustion wave velocity on the individual layer thickness was determined. The marked decrease in velocity with layer thickness was consistent with results of modeling studies on multilayer systems. Activation energies for the synthesis of NiAl were determined to be in the range 127.9 to 149.8 kJ · mol₋₁, and those for the synthesis of Ni₃Al were found to be in the range 133.8 to 146.3 kJ · mol₋₁. In the case of NiAl, the experimental value is attributed to a diffusion process of Al in NiAl. Differential thermal analysis (DTA) showed the sequence of steps in the formation of NiAl and Ni₃Al. The dependence of the thermal peaks on the heating rate for both cases was found to be consistent with theory. The activation energies obtained from the DTA analysis were compared to previous results obtained with relatively thin layers.     

The effect of the precipitation of coherent and incoherent precipitates on the ductility and toughness of high-strength steel

The effect of the coexistence of coherent and incoherent precipitates, such as M₂C and NiAl, on the ductility and plane strain fracture toughness of 5 wt pct Ni-2 wt pct Al-based high-strength steels was studied. In order to disperse coherent and incoherent precipitates, the heat treatments were carried out as follows: (a) austenitizing at 1373 K, (b) tempering at 1023 or 923 K for dispersing the incoherent precipitates of M₂C and NiAl, and then (c) aging at 843 K for 2.4 ks to disperse the coherent precipitate of NiAl into the matrix, which contains incoherent precipitates, such as M₂C and NiAl. The results were obtained as follows: (a) when the strengthening precipitates consist of coherent ones, such as M₂C and/or NiAl, the ductility and toughness are extremely low, and (b) when the strengthening precipitates consist of coherent and incoherent precipitates, such as M₂C and NiAl, the ductility and fracture toughness significantly increase with no loss in strength. It is shown that the coexistence of coherent and incoherent precipitates increases homogeneous deformation, thus preventing local strain concentration and early cleavage cracking. Accordingly, the actions of coherent precipitates in strengthening the matrix and of incoherent precipitates in promoting homogeneous deformation can be expected to increase both the strength and toughness of the material.     

Interfaces in XD processed TiB2/NiAl composites

Interfaces of TiB₂−NiAl and α-Al₂O₃−NiAl in TiB₂/NiAl composites have been investigated by analytical electron microscopy. Although no consistent crystallographic orientation relationships have been found between NiAl and TiB₂ or Al₂O₃, semicoherent interfaces between α-Al₂O₃ and NiAl have been observed by high-resolution electron microscopy (HREM) in areas where the low indexed crystallographic planes of α-Al₂O₃ aligned with that of NiAl. No semicoherent interfaces between NiAl and TiB₂ have been observed. Silicon segregation was consistently detected by X-ray energy-dispersive spectroscopy (EDS) at the TiB₂/NiAl interface region. Segregation has not been detected in the α-Al₂O₃−NiAl interface region. The segregation layer observed at the TiB₂−NiAl interface is too thin to absorb any of the thermal residual stress.     

Microstructure characterization and creep deformation of an Al-10 Wt Pct Ti-2 Wt Pct Cu nanocomposite

The creep behavior of a cryomilled Al-10Ti-2Cu nanocomposite has been studied at temperatures of 533, 588, and 644 K at initial applied stresses ranging from 55 to 117 MPa. Although the strain rates fall within the 10⁻¹⁰ to 10⁻⁹ S⁻¹ regime, we observe no evidence of threshold-type creep behavior in this material. We attribute this to the unique microstructure of the present material combined with the mechanism of dislocation slip in ultrafine grain size materials. In particular, the very fine AIN precipitates present within the microstructure are ineffective as obstacles to dislocations during high-temperature deformation. The coherent nature of these fine particles along with their extremely small size prevents a strong dislocation-particle attraction. The inability of the activation energy for self-diffusion in Al to successfully collapse the present creep data onto a single slope combined with the fact that the true activation energy for creep exceeds the value for lattice self-diffusion are both features found in materials containing second-phase particles, which deform simultaneously with the matrix during high-temperature deformation. In the present case, these particles are likely to be Al₃Ti.     

Kinetics of spreading in initial stages in metallic systems with different types of interaction between components. IV. Effect of chemical reaction on the kinetics of spreading in tin—Iron group metal systems

A droplet flowing over onto a plate introduced from above has been used to study the kinetics of spreading and to describe the observable characteristics of spreading of tin on iron, cobalt, nickel, and the intermetallic compounds Ni₃Sn, Ni₃Sn₂ under a vacuum of (2–4)·10⁻³ Pa at 400–1000°C, droplet mass 0.01–0.06 g. We show by a formal kinetic analysis of experimental data that in the low-temperature range (400–500°C) the kinetic regime dominates, and in the high-temperature range (600–1000°C) the inertial—kinetic regime dominates. In spreading of tin on iron, cobalt, nickel, and the intermetallic compounds Ni₃Sn and Ni₃Sn₂, the nature of the interaction corresponds to the phase equilibrium in the studied systems. The results for the kinetics of spreading of tin on nickel and the intermetallic compound Ni₃Sn showed that spreading of the main bulk is preceded by spreading of a precursor film. Deceased. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 65–72, July–August, 1998.     

Processing and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5

In this article, we report on the fabrication and characterization of Ti₂AlC, Ti₂AlN, and Ti₂AlC_0.5N_0.5. Reactive hot isostatic pressing (hipping) at ≈40 MPa of the appropriate mixtures of Ti, Al₄C₃ graphite, and/or AlN powders for 15 hours at 1300 °C yields predominantly single-phase samples of Ti₂AlC_0.5N_0.5, 30 hours at 1300 °C yields predominantly single-phase samples of Ti₂AlC. Despite our best efforts, samples of Ti₂AlN (hot isostatic pressed (hipped) at 1400 °C for 48 hours) contain anywhere between 10 and 15 vol pct of ancillary phases. At ≈25 μM, the average grain sizes of Ti₂AlC_0.5N_0.5 and Ti₂AlC are comparable and are significantly smaller than those of Ti₂AlN, at ≈100 μm. All samples are fully dense and readily machinable. The room-temperature deformation under compression of the end-members is noncatastrophic or graceful. At room temperature, solid-solution strengthening is observed; Ti₂AlC_0.5N_0.5 is stronger in compression, harder, and more brittle than the end-members. Conversely, at temperatures greater than 1200 °C, a solid-solution softening effect is occurring. The thermal-expansion coefficients (CTEs) of Ti₂AlC, Ti₂AlN, and Ti₂AlC_0.5N_0.5 are, respectively, 8.2 × 10⁻⁶, 8.8 × 10⁻⁶, and 10.5 × 10⁻⁶ °C⁻¹, in the temperature range from 25 °C to 1300 °C. The former two values are in good agreement with the CTEs determined from high-temperature X-ray diffraction (XRD). The electrical conductivity of the solid solution (3.1 × 10⁶ (Θ m)⁻¹) is in between those of Ti₂AlC and Ti₂AlN, which are 2.7 × 10⁶ and 4.0 × 10⁶ Θ ⁻¹ m⁻¹, respectively.     

Thermal expansion behavior of silver matrix composites

Silver matrix composites containing 10 to 40 vol pct ceramic reinforcements of different types, shapes, and sizes were processed by electroless silver plating and hot pressing. The thermal expansion behavior of the silver matrix composites has been studied from room temperature to 300 °C. The coefficients of thermal expansion (CTEs) of the composites are effectively lowered to below 10 × 10⁻⁶/°C by the addition of 40 vol pct reinforcements. The CTEs of composites decrease as the content of reinforcements increases due to the constraint effect provided by the ceramic reinforcements with low CTEs; this effect is more pronounced with the addition of whiskers and fibrous reinforcements. At the beginning of measurements, large residual thermal stresses existing in the water-quenched composites restrict the expansion of the silver matrix, resulting in lower CTEs than those of furnance-cooled composites. As the temperature increases, the residual thermal stresses are gradually released, and the CTEs of composites reach higher and stable values. At temperatures above 250 °C, the CTEs of composites increase at a higher rate due to the matrix yielding and interfacial debonding.     

Thermodynamics of the zinc-sulfur dioxide-water system

The known thermodynamic data for the aqueous zinc and sulfur dioxide systems have been gathered, evaluated, and presented in the form of potential -pH diagrams. Empirical relationships describing the entropy of ions at 298 K combined with the extrapolation method of Criss and Cobble[6] have been used in the absence of high-temperature free-energy data. The free energy of formation of zinc sulfite (ZnSO₃ · 2.5H₂O) has been experimentally determined to be −1256 ± 4 kJ mol⁻¹ and has been incorporated into the diagrams along with the various metastable polythionate species of sulfur.     

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a high magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] direction of the parent Heusler phase can promote the premartensitic transition of Ni₂MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (T _PM ) increases from 233 to 250 K (−40 to −23 °C). The martensite transition start temperature (M _s ) decreases from 175 to 172 K (−98 to −101 °C), while the reverse transition start temperature (A _s ) increases from 186 to 189 K (−87 to −84 °C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (M _f ). The real transition process of Ni₂MnGa single crystal under the high magnetic field was in-situ traced.     

Cavitation erosion of NiAl

Vibratory cavitation erosion tests were carried out on as-cast NiAl intermetallic compounds containing 46.5 to 62.1 at pct Ni. The erosion rate decreased with increasing nickel content by over two orders of magnitude, from a high of 16.4 to 0.11 mg·h⁻¹. These low erosion rates exhibited by the nickel-rich alloys containing 58 and 62.1 at. pct Ni, the interruptions in their mass loss with time, and the unusual effects associated with surface finish and intensity of cavitation were found to be associated with the stress-induced martensitic transformation. Alloys containing 58 to 62 at. pct Ni have the potential for use as materials for the cavitation protection of hydraulic machinery.     

Time-temperature-precipitation characteristics of high-nitrogen austenitic Fe−18Cr−18Mn−2Mo−0.9N steel

The time-temperature-precipitation (TTP) and corresponding mechanical properties in high-nitrogen austenitic Fe−18Cr−18Mn−2Mo−0.9N steel (all in weight percent) were investigated using electron microscopy and ambient tensile testing. The precipitation reactions can be categorized into three stages: (1) high-temperature region (above 950°C)—mainly coarse grain-boundary (intergranular) Cr₂N; (2) nose-temperature region—integranular Cr₂N→cellular Cr₂N→intragranular Cr₂N+ sigma (σ); and (3) low-temperature region (below 750°C)—intergranular Cr₂N→cellular Cr₂N→ intragranular Cr₂N+σ+chi(χ)+M₇C₃ carbide. After cellular Cr₂N precipitation became dominant above 800°C, yield and tensile strength gradually decreased, whereas elongation abruptly deteriorated with aging time. On the contrary, prolonged aging in the low-temperature regime increased tensile strength, caused by the precipitation of fine χ and M₇C₃ within grains. Based on the analyses of selected area diffraction (SAD) patterns, the crystallographic features of the second phases were analyzed.     

Preferred orientations in extruded nickel and iron aluminides

Preferred orientations in both powder-extruded and cast and extruded binary NiAl (≃45 at. pet Al), FeAl (≃40 at. pet Al), and Ni₃Al (≃24 at. pet Al) have been characterized by plotting inverse pole figures. The preferred orientation, [111], was observed along the extrusion direction in both powder-extruded and cast and extruded NiAl. Powder-extruded FeAl also exhibited [111] as the preferred orientation in the as-extruded condition. However, [110] was observed to be the preferred orientation in the cast and extruded FeAl and was replaced by a [211] orientation preference upon annealing. Annealing did not change the preferred orientations in NiAl or in powder-extruded FeAl. In contrast to the B2 NiAl and FeAl alloys, the Ll₂ Ni₃Al alloy exhibited nearly random orientations with only a minor preference for a [111] orientation in the as-extruded condition. Formerly Research Associate, Case Western Reserve University.     

Mechanical properties of NiAl-Y2O3-based powdered alloys produced by directional recrystallization

The mechanical properties of NiAl-Y₂O₃-based powdered composite alloys (0.5–7.5 vol %), including those with an NiAl intermetallic matrix alloyed with 0.5 wt % Fe and 0.1 wt % La have been studied. Structures with various aspect ratios (AR, the ratio of the grain length to the grain diameter) are formed using deformation and subsequent annealing. A combination of the optimum amount of strengthening phase (2.5 vol % Y₂O₃) and a quasi-single-crystalline structure with a sharp axial texture with the (100) main orientation and AR ≈ 20–40 provides the maximum short-term strength and life at temperatures up to 1400–1500°C. An NiAl-Y₂O₃ alloy (2.5 vol %) has the best strength properties among all known nickel superalloys at temperatures higher than 1200°C and can operate under moderate loads at temperatures higher than the working temperatures of nickel superalloys (by 100–400°C) and their melting points. Additional alloying with 10 wt % Co and 2 wt % Nb makes it possible to increase the ultimate tensile strength of an intermetallic NiAl matrix at 1100°C by a factor of 1.3–1.4.     

Processing and mechanical properties of AI2O3 fiber-reinforced NiAl composites

The mechanical properties of NiAl-matrix composites reinforced with 125-μm diameter single-crystal A1₂O₃ (sapphire) fibers have been examined over the temperature range of 300 to 1200 K. Composites were fabricated with either a strong or weak fiber-matrix interfacial bond strength. During fabrication, a fiber-matrix interaction occurred such that fibers extracted from the NiAl matrix were fragmented and significantly weaker than the as-received fibers. Tensile results of the weakly bonded composite demonstrated that the composite stiffness was greater than the monolithic at both 300 and 1200 K in spite of the weak bond. Room-temperature strengths of the composite were greater than that of the monolithic but below rule-of-mixture predictions (even when the degraded fiber strengths were accounted for). At 1200 K, the ultimate strength of the composite was inferior to that of the monolithic primarily because of the poor fiber properties. No tensile data was obtained on the strongly bonded material because of the occurrence of matrix cracking during fabrication. Primarily because of the fiber strength loss, sapphire-NiAl composite mechanical properties are inferior to conventional high-temperature materials such as superalloys and are currently unsuitable for structural applications.     

The thermodynamics of tetrataenite and awaruite: A review of the Fe-Ni phase diagram

There are two well-known ordered ferromagnetic intermetallic compounds in the iron-nickel system: tetrataenite, FeNi, and awaruite, FeNi₃. Their thermodynamic properties are established from the various stable and metastable equilibria in which they are involved, and a review to make this data accessible is appropriate. The Fe-Ni phase diagram shows a paramagnetic face-centered cubic phase (gamma) with well-measured properties over the entire composition range from 1184 to 1662 K. Its properties can be extrapolated to lower temperatures, and known stable and metastable equilibria are used to establish models for the thermodynamic properties of the ferromagnetic alpha phase, a high-temperature paramagnetic alpha phase, a paramagnetic gamma-ordered Fe₃Ni phase, the ferromagnetic disordered gamma phase, and both ordered ferromagnetic gamma phases, tetrataenite, FeNi, and awaruite, FeNi₃. The overestimate of Chuang et al. for the entropy of ordering in awaruite is corrected. The standard enthalpies at 298.15 K of tetrataenite (0.5 FeNi) and awaruite (0.25 FeNi₃) are −1170 and −3560±1000 J mol⁻¹. The corresponding standard entropy values are 35.7 and 33.3±3 J mol⁻¹ K⁻¹. All the second-order transitions are accurately modeled and the full phase diagram is calculated. The spinodal regions and metastable equilibria responsible for the complex behavior on cooling of alloys with mole fractions of nickel between 0.26 and 0.40 are described in detail.     

Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 vol pct aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 °C to 1000 °C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al particle interfaces and concentrically around the original Al particles. Upon heating to the 500 °C to 600 °C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (600 °C to 1000 °C), diffusional reaction between NiAl and nickel produces (γ′)Ni₃Al. The final equilibrium microstructure consists of blocks of (γ′)Ni₃Al in a γ(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminides.