NiAl Matrix Composites Research Articles

The Influence of Binders on Interfacial Failure in Sapphire Fiber-reinforced NiAl Composites

The influence of organic binders on fiber/matrix bonding during the powder metallurgy fabrication of sapphire fiber-reinforced NiAl matrix composites (sapphire/NiAl) was investigated. One composite panel was fabricated using a poly(methyl methacrylate) (PMMA) fiber binder and a teflon matrix powder binder; another panel was fabricated by binderless powder metallurgy consolidation. The effect of the binders on fiber/matrix bonding was evaluated by fiber push-out testing from room temperature to 900 °C. Examination of mating fiber and matrix-trough fracture surfaces by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) revealed differences in interfacial morphology and chemistry, depending on the use of binders in fabrication. The primary difference between the two composites was the much higher concentration of carbon at the fiber/matrix interface in sapphire/NiAl fabricated with binders. This carbon residue from binder burnout prevented clean contact between the sapphire fiber and NiAl matrix surfaces, resulting in a weak, thermomechanically clamped fiber/matrix interface, in contrast to the stronger, less temperature dependent, interfacial bonding observed without binders.

Fabrication and fracture behavior of metallic fiber reinforced NiAl matrix composites

NiAl intermetallic is recently of considerable interest as the high temperature structure material because of its high melting point, high specific stiffness, better oxidation and creep resistance. However, the low-temperature brittleness of the NiAl intermetallic remained a main reason for its unpopularity for industrial applications. Composite ductile phase toughening approaches have been utilized by many researchers to improve the fracture toughness of intermetallics. In liquid metallurgy, pressure casting or infiltration of molten nickel aluminide into a preform is the usual method for the fabrication of nickel aluminide intermetallic composites. But generally, it is not useful for metallic reinforcements because of the drastic reactions between the molten nickel aluminide and the metallic preform, and the difficulty in sustaining the performance of the metallic preform at a high temperature. In solid metallurgy, this process is based on reactive powder metallurgy and hot pressing, hot extrusion and hot isostatic pressing (HIP). High processing temperature and pressure, generally at a temperature of at least 1,200 C, are necessary conditions for hot pressing, hot extrusion and HIP. Hence the processes require sophisticated manufacturing equipment and considerable energy and render the application of nickel aluminide intermetallic composites unpopular. Work on reactive hot pressing(RHP) at a lowmore » temperature near the melting point of aluminum is reconsidered again. Efforts indicated that by combining the spontaneous reaction of the electrically coated nickel film and the aluminum foils, and hot pressing at a temperature about 500 C lower than previously accomplished by HIP, would overcome the fabrication problem of NiAl intermetallic composites reinforced with the uniformly distributed metallic fibers.« less

Reinforcement/matrix interaction in Al 2O 3 and Al 2O 3ZrO 2 fiber-reinforced Ni 3Al matrix composites

The interfacial reaction characteristics of fibers with nickel aluminide (Ni 3Al) matrices have been investigated after consolidation by hot isostatic pressing (HIPing) and after post-consolidation heat treatment. Two matrix alloys have been studied: Ni 3Al alloyed with 0.6 wt.% Zr-(IC-50) and Ni 3Al alloyed with 8 wt.% Cr, 2.8 wt.% Mo and 0.6 wt.% Zr (IC-396). Pure alumina (FP) and alumina-zirconia (PRD-166) reinforcements, in the form of discontinuous fibers, were added to the Ni 3Al alloys. Elemental compositions across the reaction zone have been analyzed quantitatively using scanning electron microscopy (SEM), electron microprobe analysis (EMPA) and Auger electron spectroscopy (AES). The results show that all composites HIPed at 900°C have residual porosity, but no chemical interactions between matrix and reinforcements were detected. At the other extreme, while full density was achieved at 1200°C, significant formation of zirconium rich precipitates was observed at the fiber/matrix. The extent of fiber/matrix reaction varied significantly among the various fiber/matrix combinations. At intermediate temperatures (1050°C), non observable porosity was detected by optical microscopy and precipitation of zirconium rich phases was reduced or eliminated, depending on the fiber/matrix combination. Fiber clustering, fiber breakage and fiber/matrix debonding occurred during HIPing processes. Solid-state reactions between fibers and matrices were studied after heat treating (vacuum annealing) the HIPed composites at 950°C, 1050°C and 1150°C for times ranging from 1 to 100 hours. The reaction zone, i.e. precipitation at the fiber matrix interface, increased with time and temperature. Nickel and zirconium (and, when present, chromium and molybdenum) diffused from the matrix toward the reaction zone. Aluminum diffused outward from the reaction zone. The thickness of the reaction zone varied from 1 to 10 μm according to composition and experimental conditions. Zirconium is believed to be responsible for the strong chemical reaction between fiber and matrix.

Fabrication of Al2O3-reinforced Ni3Al composites by a novel in situ route

A novel in situ technique has been used to fabricate an Al2O3-reinforced Ni3Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al2O3 fibers in a Ni3Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction of the Al2O3 fiber by the Zr present in the matrix and the formation of a layer of ZrO2 on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100 °C for 10 days. Air annealing led to the formation of networks of Al2O3 around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni3Al particles in a matrix of disordered α-Ni(Al). The Al2O3 networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile ductility of 14 pct.

Investigation of NiAl–TiB2in situcomposites

A hot-pressing aided exothermic synthesis (HPES) technique to fabricate NiAl matrix composites containing 0 and 20 vol.% TiB2 particles was developed. The conversion of mixtures of elements to the product was complete after processing, and TiB2 particles in the matrix were uniformly dispersed. The microstructure and interfaces were very thermally stable. The interfaces between NiAl and TiB2 were atomically flat, sharp, and generally free from interfacial phases. In some cases, however, thin amorphous layers existed at NiAl/TiB2 interfaces. At least three kinds of orientation relationships between TiB2 and NiAl were observed. The compressive yield strengths at room temperature and at 1000 °C of the composite were approximately three times as strong as those of the unreinforced NiAl. The tensile yield strength at 980 °C of the composite was about three times stronger than that of NiAl. The ambient fracture toughness of the composite was slightly greater than that of the monolithic NiAl.

Microstructure and mechanical behavior of the NiAl-TiC In situ composite

The microstructure, interfaces, and mechanical properties of NiAl-matrix composites reinforced by 0 and 20 vol pct TiC particles have been examined. The composites were prepared by the hot-press-aided exothermic synthesis (HPES) technique. Portions of the HPES-processed samples were hot isostatically pressed (''hipped'') at 1165 degrees C/150 MPa for 4 hours or annealed at 1400 degrees C for 48 hours. In the as-fabricated state, TiC particles were generally polygonal and faceted, and the interfaces' between TiC and NiAl were atomically flat, sharp, and generally free from any interfacial phase. At least two orientation relationships between TiC and NiAl were observed. In some cases, thin amorphous layers existed at NiAl/TiC interfaces. After ''hipping,'' the TiC particles tended to become round and the TiC/NiAl interfaces became overlapped. Annealing at 1400 degrees C for 48 hours did not affect the microstructure or the interfacial structure of the composite in most cases. The compressive yield strengths (YSs) from room temperature to 1100 degrees C of the composite were considerably higher than that of the monolithic NiAl. At 980 degrees C, the tensile YS of the composite was approximately 3 times that of the monolithic NiAl. In addition, the ambient fracture toughness of the composite was 50 pct higher than that of the monolithic NiAl.

Processing stainless steel fibre reinforced NiAl matrix composites by reactive hot pressing

Stainless steel fibres (SSF) were first wound and then plated with electrical nickel film. Alternative layers of these properly spaced Ni-coated stainless steel fibres (SSF–Ni) and aluminum foils were then stacked and diffusion bonded at 500 °C, 100 MPa in vacuum for 10 min to produce an SSF–Ni–Al precursor. Lower-temperature reactive hot pressing (RHP) at 700 °C, a process employed in this work, allowed the nickel and aluminum to react to form an NiAl matrix at a temperature approximately 500 °C lower than that accomplished by hot pressing, hot extrusion or hot isostatic pressing (HIP). RHP at 650 °C was also undertaken in order to investigate the mechanism leading to the formation of a nickel aluminide matrix composite. A dense specimen with complete NiAl reaction was obtained by hot pressing at 900 °C. The variance in microstructure that resulted from the different conditions of heat treatment and hot pressing was analysed by optical microscopy, scanning electron microscopy and X-ray diffraction (XRD).

Improvement in mechanical properties of NiAl matrix composites fabricated by reaction compocasting

With Ni, Al, Fe, Ti and C elemental powders, the NiAl(Fe) matrix composites have been successfully fabricated by the RC technique. The composites consist of three phases, which are β′ and γ′ and TiC, respectively. The TiC particles, with the particle size of 0.2−3μm, tend to be uniformly dispersed in the matrix. The composites not only offer the high elevated temperature strength, but also exhibit the satisfied ambient ductility, which indicates that the shortcomings of the insufficient elevated temperature strength and the poor ambient temperature ductility of stiochiometric NiAl are overcome, and which makes it possible for NiAl(Fe) matrix composites to be used in the high temperature structural applications.

Mechanical Behavior of Reactively Hot-Pressed Aluminide Matrix Composites

ABSTRACTFeAl and Ni3Al matrix composites containing various fine particles were fabricated successfully by reactive hot-pressing. The strength and the fracture toughness of these composites at ambient temperatures were evaluated. The addition of β-SiC particles was effective for strengthening of the Fe-40at%Al matrix, however, an extreme decrease of fracture toughness occurred due to the suppression of stress relaxation effect by plastic deformation at a crack tip. The fracture toughness of the reactively hot pressed Fe-40at%Al and its composites was also affected by the environmental embrittlement effect. TiB2 and ZrB2 particles in the Fe-40at%Al matrix composites were clarified to play a role in the reduction of the environmental effect. For the Ni-25at%Al matrix, higher flexural strength was achieved by the addition of TiB2, TiC and TiN particles. TiB2 particles reacted with the matrix during hot-pressing. The Ni-25at%Al/TiB2 composite had a fracture strength of 1.5 GPa in spite of large grain size of the matrix. TiC and TiN were the best choices as effective reinforcing matrials for the Ni3Al matrix among the chemical compatible ones.

Influence of Cr and W alloying on the fiber-matrix interfacial shear strength in cast and directionally solidified sapphire nial composites

Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys reinforced with sapphire fibers.

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.

Influence of fabrication technique on the fiber pushout behavior in a sapphire-reinforced NiAl matrix composite

Directional solidification (DS) of “powder-cloth” (PC) processed sapphire-NiAl composites was carried out to examine the influence of fabrication technique on the fiber-matrix interfacial shear strength, measured using a fiber-pushout technique. The DS process replaced the fine, equiaxed NiAl grain structure of the PC composites with an oriented grain structure comprised of large columnar NiAl grains aligned parallel to the fiber axis, with fibers either completely engulfed within the NiAl grains or anchored at one to three grain boundaries. The load-displacement behavior during the pushout test exhibited an initial “pseudoelastic” response, followed by an “inelastic” response, and finally a “frictional” sliding response. The fiber-matrix interfacial shear strength and the fracture behavior during fiber pushout were investigated using an interrupted pushout test and fractography, as functions of specimen thickness (240 to 730 μm) and fabrication technique. The composites fabricated using the PC and the DS techniques had different matrix and interface structures and appreciably different interfacial shear strengths. In the DS composites, where the fiber-matrix interfaces were identical for all the fibers, the interfacial debond shear stresses were larger for the fibers embedded completely within the NiAl grains and smaller for the fibers anchored at a few grain boundaries. The matrix grain boundaries coincident on sapphire fibers were observed to be the preferred sites for crack formation and propagation. While the frictional sliding stress appeared to be independent of the fabrication technique, the interfacial debond shear stresses were larger for the DS composites compared to the PC composites. The study highlights the potential of the DS technique to grow single-crystal NiAl matrix composites reinforced with sapphire fibers, with fiber-matrix interfacial shear strength appreciably greater than that attainable by the current solid-state fabrication techniques.

Melt-Processed Ni3Al matrix composites reinforced with TiC particles

TiC-reinforced Ni3Al metal matrix composites have been investigated. The composites were prepared by in situ precipitation of either 0.01, 0.05, or 0.1 vol fraction TiC in molten Ni3Al. These precipitates are thermodynamically stable in the Ni3Al at 1000 °C up to 50 hours. Although almost ideal precipitate distribution was achieved at low volume fraction of TiC, at higher volume fraction, there was a tendency toward agglomeration. Room and hightemperature mechanical testing showed significant improvement in modulus, yield strength, and ultimate tensile strength, while still maintaining a 2.5 pct elongation at 0.1 vol. fraction TiC and 500 °C. The density of precipitates (number/m3) is strongly dependent on the fraction of TiC, on temperature, and on the holding time at that temperature. The critical nucleation temperature for these precipitates was determined to be at 1475 °C. Experimentally evaluated growth rate constants indicate a diffusion-controlled coarsening mechanism along with coalescence and multiparticle interaction at higher volume fractions of TiC. This is in accordance with either the Lifshitz Slyozov Encounter Modified (LSEM) model or with the Voorhees and Glicksman (VG) model.

Development and characterization of interface coatings in molybdenum-reinforced NiAl matrix composites

The use of simple processing schemes to modify the interface in molybdenum-reinforced NiAl is described. Initial studies revealed that at high temperatures, intermediate barrier layers are required to prevent interdiffusion between NiAl and molybdenum. Two different processing schemes to produce interface layers of Mo2C are described. It is shown that Mo2C can be formed by alloying NiAl with small additions of carbon or by carburizing molybdenum externally to form a layer of Mo2C on the surface prior to bonding it with NiAl. The advantages, disadvantages, and generality of these approaches are presented.

Mechanistic studies in combustion synthesis of Ni3Al and Ni3Al-matrix composites

Nickel aluminides exhibit limited ductility and toughness at room temperature. One way to improve these characteristics is by adding ceramic reinforcements to the matrix. In this paper, we have studied the combustion synthesis of Ni3Al and Ni3Al-matrix composites, using the self-propagating high-temperature synthesis (SHS) mode. First, studies of the Ni3Al synthesis were carried out by quenching the reaction during its progress, which revealed the mechanism of the synthesis. The influence of Al2O3and SiC whiskers or particulates, and B4C particulates added to the reaction mixture prior to combustion synthesis, was investigated next. It was found that, in general, reinforcements are heat sinks and limit the propagation of the reaction. Also, whiskers impede the flow of formed liquid to a larger extent than do particulates. Al2O3is inert and matrices reinforced with up to 2 wt. % Al2O3are composed essentially of Ni3Al grains. However, both B4C and SiC react with the Ni-Al matrix and lead to complex phases. In particular, B4C readily forms a Ni-Al-B liquid phase and disrupts dramatically the progress of the Ni3Al matrix synthesis.

Processing and Heat Treatment Effects on an Al2O3/Ni3Al Composite

Abstract Various processing routes may be used to fabricate Ni3Al matrix composites. In the present study, vacuum hot pressing and hot extrusion were used to fabricate an Al2O3 Composite particulate-reinforced Ni3Al composite. Relatively low extrusion temperatures promoted gamma-phase formation as did extended high temperature anneals. The room-temperature bend strength was found to increase as the gamma-phase content increased. Furthermore, the composite creep rate was affected by the. processing parameters and their effect on the amount of gamma/pgamma-prime interface. The matrix and particulates did not exhibit any reaction after normal processing. However, a minor reaction was noted after annealing near 1000°C, while extensive reaction resulting in Ni-Al spinel formation occurred above 1200°C in air

Elevated Temperature Tensile Properties of Ni3Al Alloyed with Chromium and Cerium Prepared from Elemental Powders.

Ni3Al matrix composites are potential materials for high temperature application because of their marvelous temperature dependence of yield strength. We have tried to make Ni3Al composites reinforced with alumina fibers that have excellent compatibility with Ni3Al matrix by means of powder metallurgy technique. As a result, we feel keenly that ductility of Ni3Al matrix composites at elevated temperature must be made better by alloying element. In this sence, chromium and cerium are added to Ni3Al with boron and tensile properties of these alloys were investigated.

Strength of Single Crystal Al2O3 Fibers in Ni-Based Intermetallic and Superalloy Composites

AbstractStrength degradation of single crystal Al2O3 fibers due to the effect of fiber/matrix interaction and processing of NiAl and superalloy matrix composites, was investigated. Strength loss was quantified by tensile testing fibers that were exposed to the matrix alloy using two different methods. In one method, the fibers were incorporated into a composite by either the Powder Cloth (P-C) or binderless powder technique. The fibers were then extracted from the composite by chemical dissolution of the matrix and subsequently tensile tested and examined by scanning electron microscopy. In the other method, fibers were sputter-coated with a similar matrix composition and heat-treated to simulate conditions similar to those experienced during composite powder fabrication methods. In the sputter coating method, the contribution of fiber-matrix reaction on fiber strength loss was isolated from the effects of the various mechanical loads which are present during powder fabrication. For all matrices studied, significant strength loss was observed both in fibers extracted from composites and in fibers sputter-coated and annealed. Although surface ridges and pores were observed on the degraded fibers, it remains uncertain whether these features were responsible for the strength loss.

The Athermal Strengthening of Discontinuous Reinforced NiAl Composites

AbstractAn increase in the athermal component of the yield stress at low temperatures is the caused of the high temperature strengthening of discontinuously reinforced NiAl matrix composites. The reinforcements stabilize the microstructure. The strengthening in the temperature range of 300–1273K, is believed to be related to the grain size refinement and effective pinning of grain boundaries. This conclusion was obtained from analysis of data from AlN, TiB2 and Al2O3/NiAl particulate composites.

Investigation of Ni3Al-matrix composites strengthened by TiC

In this paper, the possibilities of preparing TiC-reinforced Ni3Al-matrix composites by SM (self-propagating high temperature synthesis and melting process) were examined. Two kinds of composites, namely, commercial TiC-reinforcement and synthesized TiC-reinforcement Ni3Al-matrix composites, were fabricated. The effects of particle size of the commercial TiC on the mechanical properties of the Ni3Al-matrix composites were studied. The results show that the mechanical properties of the composites decrease with increasing particle size of the commercial TiC. The microstructures of 35 wt. % TiC + 65 wt. % Ni3Al composites produced by SM technology from the four elements Ti, C, Ni, and Al were examined. The results show that in these composites, the particle size of TiC synthesized in situ is fine and that the materials have considerable high-temperature bending strength and fracture toughness.