Influence of interfacial reactions on the fiber-matrix interfacial shear strength in sapphire fiber-reinforced Nial(Yb) composites

The influence of microstructure of the fiber-matrix interface on the interfacial shear strength, measured using a fiber-pushout technique, has been examined in a sapphire-fiber-reinforced NiAl(Yb) matrix composite under the following conditions: (1) as-fabricated powder metallurgy (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM com-posites after directional solidification (DS). The fiber-pushout stress-displacement behavior con-sisted of an initial “pseudoelastic” region, wherein the stress increased linearly with displacement, followed by an “inelastic” region, where the slope of the stress-displacement plot decreased until a maximum stress was reached, and the subsequent gradual stress decreased to a “fric-tional” stress. Energy-dispersive spectroscopy (EDS) and X-ray analyses showed that the inter-facial region in the PM NiAl(Yb) composites was comprised of Yb₂O₃,O-rich NiAl and some spinel oxide (Yb₃Al₅O₁₂), whereas the interfacial region in the HT and DS composites was comprised mainly of Yb₃Al₅O₁₂. A reaction mechanism has been proposed to explain the pres-ence of interfacial species observed in the sapphire-NiAl(Yb) composite. The extent of inter-facial chemical reactions and severity of fiber surface degradation increased progressively in this order: PM < HT < DS. Chemical interactions between the fiber and the NiAl(Yb) matrix resulted in chemical bonding and higher interfacial shear strength compared to sapphire-NiAl composites without Yb. Unlike the sapphire-NiAl system, the frictional shear stress in the sap-phire-NiAl(Yb) composites was strongly dependent on the processing conditions. Formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

Interfacial shear strength of cast and directionally solidified NiAl-sapphire fiber composites

The feasibility of fabricating intermetallic NiAl-sapphire fiber composites by casting and zone directional solidification has been examined. The fiber-matrix interfacial shear strengths measured using a fiber push-out technique in both cast and directionally solidified composites are greater than the strengths reported for composites fabricated by powder cloth process using organic binders. Microscopic examination of fibers extracted from cast, directionally solidified (DS), and thermally cycled composites, and the high values of interfacial shear strengths suggest that the fiber-matrix interface does not degrade due to casting and directional solidification. Sapphire fibers do not pin grain boundaries during directional solidification, suggesting that this technique can be used to fabricate sapphire fiber reinforced NiAl composites with single crystal matrices.     

Structure and strength of Al,Cu-Al3Ni directionally solidified composites

A series of Al₃Ni fiber reinforced composites with a matrix composition varying from pure aluminum to Al-3.3 wt pct Cu were prepared by directional solidification of Al-Ni-Cu alloys. The solidification conditions were kept constant in all cases atG/R ≃ 10⁴ °C · s/mm² (G is the temperature gradient andR is the growth rate). The mechanical properties of the composites were studied in the as grown and in the heat treated conditions and the results were discussed in terms of the structure and composition. With the techniques used, it was possible to preserve the Al-Al₃Ni eutectic composite structure while strengthening the matrix by copper addition. The addition of 1 wt copper to the matrix caused a considerable increase in the mechanical strength, especially after heat treatment, without affecting the ductility. Strength values of the order of 530 MN/m² were reached in the heat treated composites which is higher than predicted by the rule of mixtures. This is attributed to the high work hardening capacity of the matrix especially in the presence of θ’ phase. Massive Al₃Ni rods and dendrites caused premature fracture and reduction in the strength of the composites containing 2 and 3 wt pct copper. Eliminating these defects by using higherG/R values can produce composites with exceptionally high strength.     

Structure and strength of Al, Cu-Al3Ni directionally solidified composites

A series of Al₃Ni fiber reinforced composites with a matrix composition varying from pure aluminum to Al-3.3 wt pct Cu were prepared by directional solidification of Al-Ni-Cu alloys. The solidification conditions were kept constant in all cases atG/R ≃ 10⁴ °C. s/mm² (G is the temperature gradient andR is the growth rate). The mechanical properties of the composites were studied in the as grown and in the heat treated conditions and the results were discussed in terms of the structure and composition. With the techniques used, it was possible to preserve the Al-Al₃Ni eutectic composite structure while strengthening the matrix by copper addition. The addition of 1 wt copper to the matrix caused a considerable increase in the mechanical strength, especially after heat treatment, without affecting the ductility. Strength values of the order of 530 MN/m² were reached in the heat treated composites which is higher than predicted by the rule of mixtures. This is attributed to the high work hardening capacity of the matrix especially in the presence of θ′ phase. Massive Al₃Ni rods and dendrites caused premature fracture and reduction in the strength of the composites containing 2 and 3 wt pct copper. Eliminating these defects by using higherG/R values can produce composites with exceptionally high strength.     

Tailoring interfacial shear strength of SiC fiber-reinforced brittle matrix composites

The interfacial shear strength of Nicalon SiC fiber-reinforced glass-ceramic matrix composites was aimed to be tailored via two methods: (1) varying of the thickness of the carbon-rich interfacial layer between the fiber and the matrix by controlling hot pressing period and (2) formation of the secondary interfacial layer, TaC, at the carbon/matrix boundary by doping the Ta₂O₅ matrix addition. In the series of composites with varying carbon-rich layer thickness, fiber/matrix debonding mostly occurred at the carbon/matrix boundary and hence the increase in the carbon-rich layer thickness did not cause any apparent changes in the interfacial shear strength. In the TaC formed series of composites, the interfacial shear strength was affected considerably by the presence of the TaC phase at carbon/matrix boundary. The Ta₂O₅ addition to control the quantity of the TaC phase has shown to be a useful method to tailor the interfacial shear strength of SiC fiber/glass-ceramic composites.     

The effect of fiber-matrix debond energy on the matrix cracking strength and the debond shear strength

The matrix cracking model given by Budiansky, Hutchinson and Evans [J. Mech. Phys. Solids34, 167 (1986)] is extended to include the effect of fiber-matrix bonding in a unidirectional fiber-reinforced composite. By use of an energy balance analysis, the work of debonding Γ_d is related to a characteristic critical interface shear stress τ_d and to the frictional interface sliding resistance τ_f by the expression τ_d − τ_f = √4G_mΓ_d/av, where G_m is the matrix shear modulus, a, the fiber radius, and v the function of fiber volume exceeds the background fiber tensile stress far from the crack by a fixed calculable amount. Finally, the combined effect of τ_f and adhesion on the intercrack spacing is predicted for the case where multiple cracks develop in the matrix.     

The influence of interfacial energies and gravitational levels on the directionally solidified structures in hypermonotectic alloys

Several Cu-Pb-Al alloys were directionally solidified under one-g conditions and alternating high-g/low-g conditions in order to determine the influence of interfacial energies and gravitational levels on the resulting microstructures. The low-g conditions were obtained through use of NASA's KC-135 aircraft. In the Cu-Pb-Al system, changes in the Al content are known to result in variations in the interfacial energy relationships between the phases. Theory predicts that this should lead to a transition from an irregular to a regular, aligned microstructure in monotectic composition alloys. Four different hypermonotectic alloy compositions were used in this study in order to vary systematically the interfacial energies between the phases. Preliminary results indicate microstructural variations between control and flight samples and samples processed at different rates under both one-g and high-g/low-g conditions. In addition, directional solidification of low Al content alloys resulted in samples with coarse, irregular microstructures, as compared to finer, more aligned microstructures in alloys with high Al contents. This was seen in samples processed under both one-g and high-g/low-g conditions. The resulting structures have been related to interfacial energies, growth rates, and gravitational levels. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Effect of shape variations on the structure of directionally solidified Al-Al3Ni composites

The effect on structure of some of the possible changes in shape of directionally solidified Al-Al₃Ni eutectic composites has been studied for two growth conditions. The shape changes investigated included both contraction and divergence in cross-section of the grown part, as well as 90 deg bends in the center-line of the composites. The experimental results showed that contractions in the solidifying cross section do not seriously affect the growth of composites, apart from some coarsening of the structure near the surface. Fiber branching took place in the case of gradual divergence in the solidifying cross section with the fibers deviating from the general growth direction by an angle determined by the shape of equitemperature contours during solidification. Sharp changes in growth direction, 90 deg bend, gave rise to nucleation of new grains of considerable misorientation and hypoeutectic alloys nucleated primary aluminum phase before the eutectic structure was established. The relatively large under cooling needed for nucleation gave rise to high local growth rates in the 90 deg bend area. As the Al₃Ni fibers grow at right angles to equitemperature contours during solidification, it is concluded that control of the composite structure can be achieved by controlling the growth conditions and mold design. Molten alloy-graphite reactions resulted in the formation of aluminum carbides which were more extensive at higher temperatures and longer exposure times.     

Influence of microgravity on the morphology of the directionally solidified front in an alSi alloy

One aim of the experiments carried out in the GFQ during the German Spacelab Mission D1 was to study the influence of convection on the coarsening of secondary dendrite arm spacing in an AlSi 7.0 alloy during normal crystallization at constant velocitiesv _SF and the temperature gradientG _SF with quenching of the residual melt. When, under μg and 1 g conditions, the same temperature gradientG _SF ≈ 16 K/mm and two different velocities (5 and 8 mm/min) were used, dendrite arm coarsening was shown to be lower than in the 1 g reference experiments atv _SF ≈ 5 mm/min and nearly identical with the reference results atv _SF ≈ 8 mm/min. A separation of the different kinds of convection, gravity-driven convection and convection driven by the volume jump, was tried using the coarsening factorM. The influence of gravity convection on the dendrite spacings seems to be high, if the velocity of crystallization is low. This influence fades away, if the velocity is high(e.g., >8 mm/min). This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Effects of composite processing on the strength of sapphire fiber-reinforced composites

The current interest in tough, high-temperature materials has motivated fiber coating development for sapphire fiber-reinforced alumina composites. For this system, it has been demonstrated that the interfacial properties can be controlled with coatings which can be eliminated from the interface subsequent to composite consilidation. However, these fugitive coatings can contribute to the high temperature strength degradation of sapphire fibers. Such degredation,which compromises the composite strength and toughness, is the focus of the current investigation. It has been observed that, in some cases, by selecting appropriate composite processing conditions, such effects can be minimized. But overcoming fiber strength loss remains an important issue.     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C. The transition in morphology of the carbide phase is discussed in terms of the relative volume fraction of the phases, growth rate, and orientation relationships. The carbide morphology is influenced by the growth rate and carbon content. For constant growth rate increasing the volume fraction of the carbide phase favors the lamellar morphology. At low growth rates the lamellar morphology is favored, and at high growth rates the rod-like morphology is favored. Growth crystallography has no direct influence on the transition in carbide morphology.     

Evaluation of interfacial shear strength,residual clamping stress and coefficient of friction for fiber-reinforced ceramic composites

The shear strength, the residual clamping stress, the coefficient of friction and the frictional stress at the fiber/matrix interface are evaluated for fiber-reinforced ceramic composites by using the theoretical analysis for fiber push-out and the corresponding experimental results. The shear strength is evaluated from the load at which debonding initiates. Sliding occurs at the interface after complete debonding. For a fiber with a Poisson's ratio greater than zero, the characteristics of the nonlinear relationship between the load required to push out the fiber and the sample thickness enable the residual clamping stress, the coefficient of friction and the interfacial frictional stress to be evaluated in the present analysis.     

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Stability of the directionally solidified eutectics NiAl-Cr and NiAl-Mo

The eutectics NiAl-Cr with cylindrical chromium fibers and NiAl-Mo with faceted molybdenum fibers were heated at 1400°C to determine the stability of the composite structure and to compare the stability of the nonfaceted fibers with that of the faceted fibers in the NiAl matrix. Fiber size and size distribution and number of fibers per unit area were measured as a function of time at temperature. The number of fibers in the NiAl-Cr eutectic decreased continuously reaching half the initial value in about 30 h at temperature. Spheroidization of the fibers occurred and was complete in 160 h. In the NiAl-Mo eutectic, the number of fibers per unit area remained constant to 150 h and the fiber size was constant to 331 h at 1400°C. In NiAl-Cr, the cylindrical chromium fibers first formed pinchedoff segments at random diameter variations along the length of the fibers. The segments gradually shortened and thickened and finally spheroidized. The faceted molybdenum fibers remained stable because the Mo-NiAl interface is constrained to lie in the facet plane which inhibits the formation of faults leading to pinching off of the fibers.     

Elevated temperature strength and room-temperature toughness of directionally solidified Ni-33Al-33Cr-1Mo

The eutectic composition Ni-33Al-33Cr-1Mo has been directionally solidified (DS) via a modified Bridgman technique at rates ranging from 7.6 to 508 mm/h to determine if the growth rate affects the mechanical properties. Microstructural examination revealed that all DS rods had grain/cellular microstructures containing alternating plates of NiAl and Cr alloyed with Mo. At slower growth rates (≤12.7 mm/h), the grains had sharp boundaries, while faster growth rates (≥25.4 mm/h) led to cells bounded by intercellular regions. None of the growth conditions resulted in either dendrites or third phases. Compressive testing between 1200 and 1400 K indicated that alloys DS at rates between 25.4 and 254 mm/h possessed the best strengths, while room-temperature toughness exhibited a plateau of about 16 MPa√m for growth rates between 12.7 and 127 mm/h. Thus, a growth rate of 127 mm/h represents the best combination of fast processing and mechanical properties for this system.     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C.