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-pressaided exothermic synthesis (HPES) technique. Portions of the HPES-processed samples were hot isostatically pressed (“hipped”) at 1165 °C/150 MPa for 4 hours or annealed at 1400 °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 °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 °C of the composite were considerably higher than that of the monolithic NiAl. At 980 °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.     

Enhanced Electrical and Mechanical Properties of Alumina-Based TiC Composites by Spark Plasma Sintering

Alumina composites incorporating with 0, 5, 10 15, 20, and 25 vol pct of TiC were consolidated by the spark plasma sintering at 1673 K (1400 °C). The effects of increasing TiC compositions on electrical and mechanical properties of the composites were investigated at room temperature. The dc electrical conductivity behavior demonstrates a transition from insulator to conductor around 12.5 vol pct of TiC in the framework of percolation theory. The conductivity attains a maximum value of ≈230 S/m at 25 vol pct of TiC sufficient to machine the composite by electro discharging machining. The Vickers hardness and fracture toughness of the composites increase with the addition of TiC vol pct, whereas elastic modulus decreases. The results indicate that crack deflection, crack bridging, and crack branching by the TiC particles are responsible for the significantly improved fracture toughness of the composites.     

Microstructure/processing relationships in reaction-synthesized titanium aluminide intermetallic matrix composites

Near-γ TiAl- and Al₃Ti-based intermetallic matrix composites have been produced using in-situ reaction-synthesis techniques. The intermetallic matrices have been reinforced with relatively high loadings (e.g., 20 to 50 vol pct) of dispersed TiB₂ particulates. It is shown that the as-synthesized TiB₂ size is strongly dependent on the specific alloy formulation; specifically, the TiB₂ size tends to increase as the nominal volume percent of TiB₂ in the composite increases. The observed size effect is determined to be associated with the temperature that is attained during the synthesis event, which is established primarily by the net exothermicity of the participating synthesis reaction(s). The exothermicity of the reactions can be assessed through the calculation of a formulations’s adiabatic temperature, which is found to increase with the percentage of TiB₂ over the range of approximately 10 to 60 vol pct. The coupling of a composite’s characteristic adiabatic temperature with the resulting reinforcement size provides direct links among composition, processing, and mechanical performance, since the size of a reinforcing particle is influential in establishing the interparticle spacing, which, in turn, establishes the strengthening potency of the dispersed phase within the composite.     

Preparation and casting of metal-particulate non-metal composites

A new process for the preparation and casting of metal-particulate non-metal composites is described. Particulate composites of ceramic oxides and carbides and an Al-5 pet Si-2 pct Fe matrix were successfully prepared. From 10 to 30 wt pct of A1₂O₃, SiC, and up to 21 wt pct glass particles, ranging in size from 14 to 340 ώ were uniformly distributed in the liquid matrix of a 0.4 to 0.45 fraction solid slurry of the alloy. Initially, the non-wetted ceramic particles are mechanically entrapped, dispersed and prevented from settling, floating, or agglomerating by the fact that the alloy is already partially solid. With increasing mixing times, after addition, interaction between the ceramic particles and the liquid matrix promotes bonding. Efforts to mix the non-wetted particles into the liquid alloy above its liquidus temperature were unsuccessful. The composite can then be cast either when the metal alloy is partially solid or after reheating to above the liquidus temperature of the alloy. End-chilled plates and cylindrical slugs of the composites were sand cast from above the liquidus temperature of the alloy. The cylindrical slugs were again reheated and used as starting material for die casting. Some of the reheated composites possessed “thixotropy.” Distribution of the ceramic particles in the alloy matrix was uniform in all the castings except for some settling of the coarse, 340ώ in size, particles in the end-chilled cast plates.     

Combustion synthesis of ceramic-metal

One of the main disadvantages of combustion synthesis of ceramic and composite materials is the relatively high levels of porosity,e.g., ≥50 pct, present in the product. This article discusses a novel application of combustion synthesis for producing ceramic-metal composites with reduced levels of porosity by allowing an excess amount of liquid metal, generated by the exothermic reaction, to infiltrate the pores. This application of combustion synthesis of ceramic-metal composite materials is discussed with respect to a model reaction system that utilizes an inexpensive oxide,i.e., TiO₂, reacted with carbon and an excess stoichiometric amount of alu-minum. The aluminum is in the liquid state at the ignition temperature and is intentionally allowed to infiltrate the porous ceramic matrix,i.e., TiC-Al₂O₃, produced from the combustion synthesis reaction. Thisin situ process for producing ceramic-metal composites by the simul-taneous liquid metal infiltration of the pores in a ceramic matrix using the combustion synthesis approach provides considerable advantages over conventional processes which involve two stages,i.e., sintering followed by liquid metal infiltration. However, there are also certain limitations with respect to total penetration of the liquid metal into the porous ceramic matrix and main-taining a stable propagation of the combustion reaction. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.     

Fabrication of metal matrix composites of

Fine fibrous titanium carbide (TiC) was processed through the self-propagating high-temperature synthesis (SHS) method and employed to fabricate aluminum matrix composites. Two consol-idation methods were investigated: (1) combustion synthesis of TiC fiber/Al composites directly using titanium powders and carbon fibers ignited simultaneously with varying amounts of the matrix metal powder and (2) combustion synthesis of TiC using titanium powders and carbon fibers followed by consolidation into different amounts of the metal matrix powder, Al,via hot isostatic pressing (HIP). In the former method, when the amount of the Al in the matrix was increased, the maximum temperature obtained by the combustion reaction decreased and the propagation of the synthesis reactions became difficult to maintain. Preheating was required for the mixture of reactants with more than approximately 5 mole pct aluminum matrix powders in order to ignite and maintain the propagation rate. Microstructural analysis of the products from the Al/C/Ti reaction without preheating shows that small amounts of an aluminum carbide phase (AI4C3) are present. In the second method, following separation of the individual fibers in the TiC product, dense composites containing the SHS products were obtained by HIP of a mixture of the TiC fibers and Al powders. No ternary phase was formed during this procedure. Formerly Graduate Research Assistant, Department of Chemical Engineering, Michigan Technological University, is with Particle Technology, Inc., Hanover, MD 21076. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Evaluation of the wettability of liquid aluminum with ceramic particulates (SiC,TiC, AI2O3) by means of pressure infiltration

One of the major problems in the fabrication of aluminum-ceramic composites is the wettability of ceramic reinforcements by liquid aluminum. In this article, the wettability at the liquid metal/ ceramic particulates interface has been evaluated by means of an infiltration technique that permits a determination of the threshold pressure and the infiltration kinetics. The aim of the present article is to check the reliability of the experimental technique. The work was carried out on pure aluminum and A1₂O₃, SiC, and TiC particulates of average diametersD in the range 7.5 to 150 μm. An Al-1 wt pct Pb alloy was also used to investigate the effects of surface tension. All infiltrations were carried out in air. The results for the infiltrated heightvs applied pressure or time follow Darcy’s law. On the other hand, threshold pressures are proportional to 1/D (capillary law) and some deviations can be ascribed to differences in the shape of the particulates (especially TiC). More than 75 pct of the reduction in the threshold pressure promoted by 1 wt pct of lead can be related to the decrease in the surface tension of pure Al. The results indicate that effects due to changes in wetting should be very small, as expected when aluminum is covered with an oxide layer.     

Grain refining of Al-4.5Cu alloy by adding an Al-30TiC master alloy

A particulate Al-30 wt pct TiC composite was employed as a grain refiner for the Al-4.5 wt pct Cu alloy. The composite contains submicron TiC particles. The addition of the TiC grain refiner to the metal alloy in the amount of 0.1 Ti wt pct effected a remarkable reduction in the average grain size in Al-4.5 wt pct Cu alloy castings. With the content of over 0.2 Ti wt pct, the grain refiner maintained its refining effectiveness even after a 3600-second holding time at 973 K. The TiC particles in the resulting castings were free of interfacial phases. It is concluded that the TiC are the nucleating agents and that they are resistant to the “fading effect” encountered with most grain refiners.     

Transformation superplasticity of iron and Fe/TiC metal matrix composites

Unreinforced iron was thermally cycled around the α/γ phase field under an externally applied uniaxial tensile stress, resulting in strain increments which could be accumulated, upon repeated cycling, to a total strain of 450 pct without failure. In agreement with existing theory attributing transformation superplasticity to the biasing of the internal allotropic strains by the external stress, the measured strain increments were proportional to the applied stress at small stresses. However, for applied stresses higher than the nominal yield stress, strain increments increased nonlinearly with stress, as a result of strain hardening due to dissolved carbon and iron oxide dispersoids. Also, the effects of transient primary creep and ratchetting on the superplastic strain increment values were examined. Finally, partial cycling within the α/γ phase field indicated an asymmetry in the superplastic strain behavior with respect to the temperature cycling range, which is attributed to the different strengths of ferrite and austenite. Transformation superplasticity was demonstrated in iron-matrix composites containing 10 and 20 vol pct TiC particles: strain increments proportional to the applied stress were measured, and a fracture strain of 230 pct was reached for the Fe/10TiC composite. However, the strain increments decreased with increasing TiC content, a result attributed to the slight dissolution of TiC particles within the matrix which raised the matrix yield stress by solid-solution strengthening and by reducing the transformation temperature range.     

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC _p ) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10⁻⁴ and 3.7×10⁻³ s⁻¹). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10⁻⁴ s⁻¹ to 3.7×10⁻³ s⁻¹ lowered the yield strength, particularly in 10 vol pct TiC _p -Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC _p composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ ⁿ ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiC _p composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC _p -Al composites, with the effect being minor at room temperature.     

Chemically induced reduction: A viable process for synthesizing γ-TiAl based intermetallic matrix composite powders containing nanocrystalline TiC

A chemically induced reduction process has been developed for synthesizing intermetallic matrix composites (IMCs) consisting of titanium aluminide and titanium carbide. The process involves the reduction of metal chlorides (TiCl₄ and AlCl₃) with metallic lithium in polar organic solvents such as acetonitrile (MeCN) and tetrahydrofuran (THF) to form a colloidal precursor. The as-prepared precursors have been either directly heat treated in ultra-high-purity argon (UHP-Ar) or pretreated in hydrogen (H₂) followed by further heat treatment in UHP-Ar. The powders have been characterized primarily using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results of the structural analyses conducted on the heat-treated precursors derived using MeCN as a solvent indicate the formation of either single-phase titanium carbide (TiC) or a composite mixture of γ-TiAl and nanocrystalline TiC, depending on the heat-treatment conditions. The formation of TiC is related to the strong interaction between TiCl₄ and the polar organic solvents, resulting in the formation of adducts which contain primary Ti-C linkages. Pretreatment of the precursors derived using MeCN as a solvent in H₂ promotes the removal of carbon and results in the formation of the composite mixture of γ-TiAl and TiC after subsequent Ar treatment at 1200 °C. At this stage, washing the pretreated powders in water helps to minimize and even eliminate any impurity phases to a large extent, leaving behind phase-pure composites containing γ-TiAl and TiC after the final Ar treatment. However, extended pretreatment in H₂ appears to be ineffective toward removal of additional carbon and leads to formation of hydride-phase impurities. On the other hand, the reductive reaction conducted using THF as a solvent results in minimizing the amount of carbon while inducing the formation of γ-TiAl during direct Ar treatment of the precursors. This is because of the weaker interaction between TiCl₄ and THF. Transmission electron microscopy was used to characterize the size distribution of the constituent phases. The analysis shows that the composite synthesized using these chemical approaches consist of discrete nanocrystalline TiC particles (<20 nm) that are uniformly distributed intermixed with submicron sized γ-TiAl (0.1 to 0.2 μm). Thus, the new chemical process proposed in this study demonstrates the potential for synthesizing in situ composites containing fine distribution of γ-TiAl and nanocrystalline TiC. Such composites could potentially exhibit unique mechanical properties and deformation behavior useful for high-temperature structural applications.     

The Effect of Iron Content on the Iron-Containing Intermetallic Phases in a Cast 6060 Aluminum Alloy

The effect of iron content, ranging from 0.1 to 0.5 wt pct, on the formation of Fe-containing intermetallic phases in a cast 6060 aluminum alloy was investigated. Various characterization techniques, including optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were used to examine the identity, morphology, and prevalence of the Fe-Al and Fe-Al-Si intermetallic phases. The predominant phase is found to be β-Al₅FeSi at lower Fe levels, but this is replaced by α-AlFeSi (bcc structure) with increasing Fe content. The Fe containing intermetallic phases observed are compared to those predicted using the Scheil module of THERMO-CALC software, and the similarities and discrepancies are discussed.     

Deformation mechanisms in Ti-6Al-4V/TiC composites

The compressive behavior at room temperature of Ti-6Al-4V/TiC composites was examined at strain rates from 0.1 to 1000 s⁻¹. As little as 1 vol pct TiC particulates provided greater than a 20 pct increase in strength over that of the monolithic Ti-6Al-4V, while further additions of TiC did not provide proportional benefits. Microstructural examination before and after compression testing was instrumental in understanding the relative importance of the primary strengthening mechanism in the composites as compared to the monolithic material. A comparison of the various possible mechanisms clearly showed that the dominant mechanism was due to carbon in solid solution. At low strain rates, the failure process consisted of a progression of damage in the matrix and at particle-matrix boundaries, while at high strain rates, failure occurred along adiabatic shear bands. The composites had a greater susceptibility to adiabatic shear-band formation than did the monolithic material.     

In situ formation of three-dimensional TiC reinforcements in Ti-TiC composites

Three-dimensional, single-crystal reinforcements of TiC were producedin situ during manufacture of Ti-TiC composites. The composites, containing 40 to 50 vol pct TiC, were produced using standard casting procedures. The presence of aluminum in Ti-TiC composites showed enhanced strength without loss of ductility at room and elevated temperatures. Aluminum additions were found to solid solution strengthen the Ti matrix and increase the strength of the TiC phase. The morphology of the TiC, which was controlled by processing parameters, influenced the properties of the Ti-TiC composites investigated. Refinement of the secondary dendrite arm spacing of the three-dimensional (3-D) TiC particles was found to dramatically improve the ultimate tensile strength (UTS) and ductility of the Ti-TiC composites.     

Fatigue crack growth through alloyed niobium, Nb-Cr2Nb, and Nb-Nb5Si3 in situ composites

Fatigue cracks were grown through several niobium-based materials. For Nb-Cr-Ti composition materials, the single-phase alloy represented the matrix of two in situ composites with about 22 and 38 vol pct Cr₂Nb. Grain boundaries were coated with intermetallic in the lower-volume fraction material, while the 38 vol pct Cr₂Nb composite consisted of mainly spherical, dispersed intermetallic. The Nb-10Si composite was composed of about 28 vol pct primary Nb₅Si₃, with most of the matrix alloy in “fiberlike” shapes due to extrusion. Crack growth rates through the composites were generally faster than for unalloyed Nb, roughly in proportion to the volume fraction of intermetallic, although differences in microstructure make this comparison difficult. The presence of intermetallic greatly alters deformation of material near the crack tip. Particles of Cr₂Nb were broken during the crack growth process, leading to increased crack growth rates. These results suggest microstructural modifications that could be expected to enhance fatigue crack growth resistance.     

Influence of calcium on the microstructure and properties of an Al-7Si-0.3Mg-xFe alloy

The effect of Ca addition on the microstructure, physical characteristics (density/porosity), and mechanical properties (tensile and impact strength) has been investigated in an Al-7Si-0.3Mg-xFe (x=0.2, 0.4, and 0.7) alloy. The size of Al-Fe intermetallic platelets (β-Al₅FeSi) increased with increasing Fe content. The addition of Ca modified the eutectic microstructure and also reduced the size of intermetallic Fe-platelets, causing improved elongation and impact strengths. A low level of Ca addition (39 ppm) reduced the proosity of the alloys. The tensile strength was decreased marginally with Ca addition. However, Ca addition improved the ductility of the alloy by 18.3, 16.7, and 44 pct and the impact strength by 44, 48, and 15.8 pct for Fe contents of 0.2, 0.4, and 0.7 pct, respectively.     

Effective elastic moduli of fiber-matrix interphases in high-temperature composites

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature composites. The interphasial moduli are calculated from the ultrasonically measured composite modulivia inversion of multiphase micromechanical models. Explicit equations are obtained for determination of interphasial stiffnesses for an interphase model with spring boundary conditions and multiphase fiber. The results are compared with the exact multiphase representation. The method was applied to ceramic and intermetallic matrix composites reinforced with SiC SCS-6 fibers. In both composites, the fiber-matrix interphases include approximately 3-μm-thick carbon-rich coatings on the outer surface of the SiC shell. Although the same fiber is used in both composite systems, experimental results indicate that the effective interphasial moduli in these two composite systems are very different. The interphasial moduli in intermetallic matrix composites are much greater than those in ceramic matrix composites. After taking the interphase microstructure into account, we found that the interphasial moduli measured for the intermetallic matrix composites are very close to the estimated bulk moduli of the pyrolytic carbon with SiC particle inclusions. Our analysis shows that the lower effective interphasial moduli in the reaction-bonded Si₃N₄ (RBSN) ceramic matrix composites are due to imperfect contact between the interphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effective moduli give information on the quality of mechanical contact between fiber and matrix. Possible errors in the interphasial moduli determined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for assessment of interphasial damage and interphase reactions is discussed.     

Tensile and Fracture Properties of Cast and Forged Composite Synthesized by Addition of Al-Si Alloy to Magnesium

Cast Mg-Al-Si composites synthesized by addition of Al-Si alloy containing 10, 15, and 20 wt pct of Si, in molten magnesium, to generate particles of Mg₂Si by reaction between silicon and magnesium during stir casting has opened up the possibility to control the size of these particles. The microstructure of the cast composite consists of relatively dark polyhedral phase of Mg₂Si and bright phase of β-Al₁₂Mg₁₇ along the boundary between dendrites of α-Mg solid solution. After hot forging at 350 °C, the microstructure has changed to relatively smaller sizes of β-Al₁₂Mg₁₇ and Mg₂Si particles apart from larger grains surrounded by smaller grains due to dynamic recovery and recrystallization. Some of the Mg₂Si particles crack during forging. In both the cast and forged composite, the Brinell hardness increases rapidly with increasing volume fraction of Mg₂Si, but the hardness is higher in forged composites by about 100 BHN. Yield strength in cast composites improves over that of the cast alloy, but there is a marginal increase in yield strength with increasing Mg₂Si content. In forged composites, there is significant improvement in yield strength with increasing Mg₂Si particles and also over those observed in their cast counterpart. In cast composites, ultimate tensile strength (UTS) decreases with increasing Mg₂Si content possibly due to increased casting defects such as porosity and segregation, which increases with increasing Mg₂Si content and may counteract the strengthening effect of Mg₂Si content. However, in forged composite, UTS increases with increasing Mg₂Si content until 5.25 vol pct due to elimination of segregation and lowering of porosity, but at higher Mg₂Si content of 7 vol pct, UTS decreases, possibly due to extensive cracking of Mg₂Si particles. On forging, the ductility decreases in forged alloy and composites possibly due to the remaining strain and the forged microstructure. The initiation fracture toughness, J _IC , decreases drastically in cast composites from that of Mg-9 wt pct. alloy designated as MA alloy due to the presence Mg₂Si particles. Thereafter, J _IC does not appear to be very sensitive to the increasing presence of Mg₂Si particles. There is drastic reduction of J _IC on forging of the alloy, which was attributed to the remaining strain and forged microstructure, and it is further lowered in the composites because of cracking of Mg₂Si particles. The ratio of the tearing modulus to the elastic modulus in cast composites shows a lower ratio, which decreases with increasing Mg₂Si content. The ratio decreases comparatively more on forging of cast MA alloy than those observed in forged composites.