Processing and Characterization of Fiber Reinforced Intermetallic Matrix Composites

A series of intermetallic matrix composites reinforced with Al₂O₃ based fibers were fabricated by pressure casting. The Al₂O₃ based fibers used were DuPont's 20 μm diameter Fiber FP and PRD-166 fiber, Mitsui's 10 μm diameter Almax fiber, and Saphikon's 125 μm diameter single crystal Al₂O₃ fiber. The intermetallic matrices employed were alloys based on Ni₃Al, NiAl, Fe₃Al, Ti₃Al+TiAl, and Nb₂Al+NbAl₃. Optical, scanning and transmission electron microscopy were used to investigate the microstructure of the composites and the fibers. Tensile testing was conducted to determine the Weibull mean strength of the fibers in the as-received and heat treated conditions. The effect of fiber gage length on the Weibull mean strength of the PRD-166 and Fiber FP was evaluated. Indentation tests were performed to determine the effect of alloying additions on the fiber/matrix bond strength in shear in Saphikon fiber reinforced Ni₃Al composites.     

Boron in polycrystalline Ni3Al-mechanism of enhancement of ductility and reduction of environmental embrittlement

A model is proposed to explain on a unified basis the role of boron in enhancing ductility and reducing environmental embrittlement in polycrystalline Ni₃Al. The grain boundaries in strongly ordered Ni₃Al have a porous structure with crack-like microcavities, which can open up under small stresses. Furthermore, atomic hydrogen, generated by the reaction of environmental moisture with Al, can diffuse to the tips of the microcavities under stress and can cause embrittlement. It is argued that strong bonding between interstitial B atoms and Ni atoms in B-doped Ni₃Al reduces the strength of directional bonding between Ni and Al atoms in the interior of the grains. When two such grains with weakened Ni---Al bonding meet each other, the atoms near the grain boundary can relax easily and close up the microcavities. As a result, the ductility is enhanced in B-doped Ni₃Al. The environmental embrittlement is also reduced, because a small amount of environmental hydrogen cannot nucleate microcracks. Numerous experimental observations have been explained with the proposed mechanism.     

FABRICATION OF A Ni3Al/Al2O3UNIDIRECTIONAL COMPOSITE BY PRESSURE CASTING

A gas pressure liquid metal infiltration technique for producing fiber reinforced Ni₃Al/Al₂O₃ and Ni₃Al/Al₂O₃ composites has been described. Composite bars of 9.5 mm diameter, 150 mm long were produced. It was found necessary to add 0.5 at.%Ti to both Ni₃Al and NiAl alloys to obtain adequate wetting. There was a strong tendency for liquid metal infiltration along one side of the fibers causing a shift of the fibers to the opposite side. No interface reaction zone was observed at optical magnifications up to 1000X. A composite of Ni₃Al containing 33 volume percent of FP fibers fractured at 890 MPa which is approximately 3 times the yield strength of the matrix IC-50(21.73A1-0.34Zr-0.IB-Balance Ni at.%) + 0.5 at. %Ti.     

Nickel Enriched Diffusion Layers on a NiAl Alloy

The intermetallic phase NiAl is a perspective material for high-temperature and shape memory effect applications. Formation of Ni₅Al₃, Ni₂Al, Ni₃Al phases which influence the extent of martensitic transformation in NiAl have been studied up to now with controversial results. We have investigated (using SEM and local elemental analyses) the microstructure of nickel enriched surface layers on a Al-79 wt.% Ni alloy. The layers were prepared by diffusion annealing and subsequently given two different heat treatments: at 930°C outside the Ni₅Al₃ region and at 500°C within the Ni₅Al₃ region of the phase diagram. In the specimen which was only diffusion annealed separate islands of Ni₅Al₃ phase elongated in the direction of the concentration gradient could be recognized within the nickel enriched surface layer. In the samples additionally annealed at 500°C, a well defined continuous layer of the Ni₅Al₃ phase situated 0.4 mm below the specimen surface was found. In the samples annealed at 930°C, isolated Ni₃Al precipitates were observed. Their number and size gradually increased with increasing nickel content.     

NOVEL PROCESSING APPROACHES TO INTERMETALLIC MATRIX COMPOSITES

Intermetallic compounds have been recognized as potentially useful structural materials. They form the basis for high temperature materials of the future. Intermetallic compounds based on aluminum have attractive characteristics of high strength at elevated temperatures, good corrosion and oxidation resistance, and low densities. This research focuses on the use of the nickel aluminide (Ni₃Al) intermetallic compound as a matrix for high temperature composites. The powder processing approach has been adopted for the consolidation of these composites. This paper describes research on powder injection molding (PIM) of an alumina fiber reinforced nickel aluminide intermetallic. Major problems are associated with the debinding process. They have been resolved and composite structures formed. Preliminary investigations on a reactive hot isostatic pressing approach for forming these nickel aluminide matrix composites, are briefly discussed.     

An atomistic structural study of the mechanism responsible for boron-enhanced ductility in Ni3Al

By now significant progress has been achieved in the study of ductility improvement of Ni₃Al alloys. There is, however, a lack of general consensus on the mechanism responsible for the ductilizing effect of boron in Ni₃Al. In this paper, atomistic simulations have been carried out to study the mechanism from the structural aspect, and the focus has been placed on the interaction between the grain boundary (GB) and dislocations. It was found that the absorption and emission of dislocations at the Ni-rich GB under an applied stress are facilitated by the boron segregation, which eases the slip transfer across the GB.     

Effect of Ion-Plated Yttrium on the Oxidation Behavior of Nickel Aluminide Intermetallic Compound

The Ni₃Al specimens were coated with yttrium (Y) by ion plating method. Post heat treatment was performed after Y ion plating to improve adherence of the Y-coating layer to the substrate. Performance of the Y-coated Ni₃Al was evaluated by isothermal oxidation and cyclic oxidation tests. A simple deposition of Y on Ni₃Al did not change the oxidation kinetics, but the post heat treatment after Y ion plating altered the oxidation kinetics of Ni₃Al significantly. The test results suggest that the corrosion resistance of Ni₃Al can be improved by a proper method of Y-deposition.     

In-situ SEM observation of tensile deformation in rapidly solidified Ni3Al ribbons

Mechanical properties of Ni₃Al ribbons have been studied using in-situ SEM tensile tests between room temperature and 400°C, The onset of plastic strain of the annealed material is characterized, after exceeding the yield stress, by the appearance of a plastic instability region followed by the homogeneous deformation region. The slope of the serrated zone decreases with annealing temperature and increases with test temperature. However, ribbons in the as-rapidly solidified (RS) condition do not show the plastic instability zone. This different behavior is related to distribution, arrangement and density of dislocations generated during rapid solidification and to their evolution during thermal treatments. Serrated yielding takes place due to the nucleation and propagation of Lüders bands, which are nucleated in high stress concentration regions and subsequently propagate through the gauge length. A model is proposed in order to explain strain hardening in the serrated zone and its dependence on annealing temperature and test temperature.     

Ni3Al金属间化合物多孔材料的制备及抗腐蚀性能

使用粉末冶金模压成形和无压反应烧结方法制备出Ni3Al金属间化合物多孔材料,研究了反应过程中Ni3Al金属间化合物多孔材料的体积膨胀、孔结构参数、组织形貌,以及在KOH溶液中的抗腐蚀性能.结果表明:烧结后Ni3Al金属间化合物多孔材料发生了显著膨胀,最大孔径和开孔隙度都随着温度的升高而升高,当温度到达750℃时体积膨胀...     

The effect of segregation and partial order on the thermodynamics of (1 1 1) antiphase boundaries in Ni3Al

Thermodynamic properties of thermal and a-thermal (1 1 1) antiphase boundaries (APB) in Ni₃Al are computed from first-principles. The effect of off-stoichiometry, partial disordering and segregation are evaluated and a rough estimate of the vibrational contribution to the antiphase boundary energy is given. Although the vibrational effect is found to be small, the configurational effects are large, so that at non-zero temperature and in off-stoichiometric Ni₃Al the antiphase boundary energy (APBE) may be only half of that in perfectly ordered stoichiometric Ni₃Al at zero temperature. This result points to a discrepancy between electronic structure calculations and experimental measurements.     

Abnormal precipitation behavior of Ni3Al phase

Evolution of Ni₃Al phase in a nickel-base directionally solidified superalloy has been investigated during creep rupture testing (1253 K/150 MPa). Besides the splitting, rafting and Ostwald ripening cases, many finer Ni₃Al particles have been firstly observed to precipitate from the rafted Ni₃Al particles. A simple qualitative interpretation has been proposed.     

Microstructure and strength of brazed joints of Ti3Al-base alloy with NiCrSiB

Brazing of Ti₃Al alloys with the filler metal NiCrSiB was carried out at 1273–1373 K for 60–1800 s. The relationship of brazing parameters and shear strength of the joints was discussed, and the optimum brazing parameters were obtained. When products are brazed, the optimum brazing parameters are as follows: brazing temperature is 1323–1373 K, brazing time is 250–300 s. The maximum shear strength of the joint is 240–250 MPa. Three kinds of reaction products were observed to have formed during the brazing of Ti₃Al alloys with the filler metal NiCrSiB, namely, TiAl₃ (TiB₂) intermetallic compounds formed close to the Ti₃Al alloy. TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds layer formed between TiAl₃ (TiB₂) intermetallic compounds and the filler metal and a Ni[s,s] solid solution formed in the middle of the joint. The interfacial structure of brazed Ti₃Al alloy joints with the filler metal NiCrSiB is Ti₃Al/TiAl₃ (TiB₂)/TiAl₃+AlNi₂Ti (TiB₂)/Ni[s,s] solid solution/TiAl₃+AlNi₂Ti (TiB₂)/TiAl₃ (TiB₂)/Ti₃Al, and this structure will not change with brazing time once it forms. The formation of over many intermetallic compounds TiAl₃+AlNi₂Ti (TiB₂) results in embrittlement of the joint and poor joint properties. The thickness of TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) in the brazed joints of Ti₃Al alloys with the filler metal NiCrSiB are 349 kJ/mol and 24.02 mm²/s, respectively, and the growth formula was y²=24.04exp(−41977.39/T)t. Careful control of the growth of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) can influence the final joint strength.     

Rapidly Solidified and Annealed Powders of Ni3Al

Rapidly solidified powders of stoichiometric Ni₃Al (B,Ti) were characterized, both as received and after short anneals. Powders generally exhibit spherical morphologies; deviations arise from particle collisions. In the as-received state the stoichiometric Ni₃Al exhibits both lamellar and dendritic structures but the Ni₃Al (B,Ti) contains only dendrites. Only small compositional variations exist across lamellae or dendrites. The as-received powders are only partially ordered. Annealing homogenizes the microstructure and produces strongly ordered structures in which most particles develop large grains. Hardness decreases during annealing. No cracks were found around microhardness indentations on any powders, indicating that Ni₃Al exhibits ductility under compression.     

Interfacial microstructure and strength of partial transient liquid-phase bonding of silicon nitride with Ti/Ni multi-interlayer

Partial transient liquid-phase bonding (PTLP bonding) of silicon nitride (Si₃N₄) ceramic has been performed using Ti/Ni multi-interlayer in vacuum at 1273–1423 K. Interfacial microstructures were examined by scanning electron microscope, electron probe micro-analysis, and X-ray diffraction. The joint strength has been measured by four-point bending tests from room temperature up to 1000 °C. Interfacial structure of Si₃N₄/TiN/Ti₅Si₃ + Ti₅Si₄ + Ni₃Si/(NiTi)/Ni₃Ti/Ni is formed after bonding process. The NiTi layer is gradually consumed with simultaneous growth of the reaction layer and the Ni₃Ti layer. The room temperature joint strength is significantly affected by the reaction layer thickness, whereas the elevated temperature joint strength significantly depends on whether the low melting point NiTi layer exists in the joint. The joint strength of more than 100 MPa is retained up to 800 °C as the NiTi layer is completely consumed. A model is proposed to optimize the PTLP bonding parameters for optimizing joint strength at both room temperature and elevated temperature.     

Self-Propagating High Temperature Synthesis (SHS) of Intermetallic Compounds Titanium and Nickel Aluminides

The self-propagating high temperature synthesis (SHS) of titanium and nickel aluminides was investigated. Effect of experimental variables on SHS temperature, burning rate, product composition, and microhardness are discussed. The densities of the products are 70.80% of theoretical values and the microhardness of Ni₃Al produced is similar to that of low carbon steel. It is also shown that the SHS process is affected by the reactant ratios. Small reactant particle size and moderate preheating result in high SHS rates. A small addition of boron and a small change in the green density do not significantly affect the process.     

Grain growth mechanisms in a spray-formed Ni3Al/Al2O3 composite in the presence of a liquid phase

Grain growth in the two-phase (liquid + solid) region of Ni₃Al reinforced with 0.8 vol.% Al₂O₃ participates synthesized by a spray atomization and co-injection technique was investigated. The grain growth of the as-sprayed and hot isostatically pressed (HIPed) materials in the two-phase region was found to be consistent with cube law kinetics, i.e., grain growth exponent was approximately 3. The activation energy for grain growth for the as-sprayed material was determined to be 308 ± 19 kJ mol⁻¹ while that of the HIPed material was calculated to be 327 ± 23 kJ mol⁻¹. The activation energy for grain growth was not a function of the amount of liquid phase or the composition of the liquid. Furthermore, the activation energy for grain growth was higher than that for diffusion through the liquid phase, suggesting that the mechanism for grain growth of the as-sprayed and HIPed Ni₃Al composite in the two-phase region was controlled by an interface reaction. The role of the second-phase Al₂O₃ particles on grain growth for the as-sprayed and HIPed Ni₃Al materials was not significant.     

Creep of fibrous composite materials

Models are presented for the creep behaviour of fibrous composite materials with aligned fibres. The models comprise both cases where the fibres remain rigid in a creeping matrix and cases where the fibres are creeping in a creeping matrix. The treatment allows for several contributions to the creep strength of composites. The advantage of combined analyses of several data sets is emphasized and illustrated for some experimental data. The analyses show that it is possible to derive creep equations for the (in situ) properties of the fibres. The experiments treated include model systems such as Ni + W-fibres, high temperature materials such as Ni + Ni₃Al + Cr₃C₂-fibres, and medium temperature materials such as Al + SiC-fibres. For the first two systems reasonable consistency is found for the models and the experiments, while for the third system too many unquantified parameters exist and further studies seem necessary.     

Growth of copper and vanadium on a thin Al2O3-film on Ni3Al(111)

The growth of different metals on thin Al₂O₃-films on Ni₃Al(111) was investigated using scanning tunneling microscopy (STM). These thin alumna films are well ordered showing two superstructures, which appear in the STM images at different bias voltages. These superstructures, with periodicities of 2.6 and 4.5 nm, respectively, are shown here to govern the nucleation of the deposited metals. Copper clusters grow on these nucleation centers only at room temperature. Higher temperatures lead to an increase of the cluster size and the loss of order. In turn, vanadium forms ordered cluster arrays at room and higher temperature. Due to the stronger metal–oxide interaction compared to copper vanadium forms smaller clusters at low and high coverages, which do not show any ripening after annealing. Based on these observations, Al₂O₃-films on Ni₃Al(111) prove to be an interesting template for the fabrication of periodic cluster arrays.     

Microstructure and strength of diffusion-bonded joints of TiAl base alloy to steel

Diffusion bonding of TiAl-based alloy to steel was carried out at 850–1100 °C for 1–60 min under a pressure of 5–40 MPa in this paper. The relationship of the bond parameters and tensile strength of the joints was discussed, and the optimum bond parameters were obtained. When products are diffusion-bonded, the optimum bond parameters are as follows: bonding temperature is 930–960 °C, bonding pressure is 20–25 MPa, bonding time is 5–6 min. The maximum tensile strength of the joint is 170–185 MPa. The reaction products and the interface structures of the joints were investigated by scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA) and X-ray diffraction (XRD). Three kinds of reaction products were observed to have formed during the diffusion bonding of TiAl-based alloy to steel, namely Ti₃Al+FeAl+FeAl₂ intermetallic compounds formed close to the TiAl-based alloy. A decarbonised layer formed close to the steel and a face-centered cubic TiC formed in the middle. The interface structure of diffusion-bonded TiAl/steel joints is TiAl/Ti₃Al+FeAl+FeAl₂/TiC/decarbonised layer/steel, and this structure will not change with bond time once it forms. The formation of the intermetallic compounds results in the embrittlement of the joint and poor joint properties. The thickness of each reaction layer increases with bonding time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reacting layer Ti₃Al+FeAl+FeAl₂+TiC in the diffusion-bonded joints of TiAl base alloy to steel are 203 kJ/mol and 6.07 mm²/s, respectively. Careful control of the growth of the reacting layer Ti₃Al+FeAl+FeAl₂+TiC can influence the final joint strength.