In situ combustion synthesis of dense ceramic and ceramic-metal interpenetrating phase composites

A model exothermic reaction is used to demonstrate the application of simultaneous combustion synthesis, conducted under a consolidating pressure, as a one-stepin situ synthesis technique for the production of dense ceramic and ceramic-metal interpenetrating phase composites (IPC). The addition of an excess amount of metal,e.g., Al, and/or a diluent,e.g., Al₂O₃, lowers the combustion temperature and aids in the refinement of the microstructure, facilitating an increase in compressive strength and elastic modulus. The effects of the important process parameters,e.g., reaction stoichiometry and diluents, green density, pressure, and temperature, on microstructure and properties of these high-performance composites are discussed.     

Combustion synthesis of Ni3Al and Ni3Al-matrix composites

The self-propagating mode of combustion synthesis (SHS) of Ni₃Al starting from compacts of stoichiometrically mixed Ni and Al powders readily forms fully reacted structures with about 3 to 5 pct porosity, if green density of the compacts is greater than about 75 pct of theoretical. SHS-produced Ni₃Al matrix composites with up to 2 wt pct A1₂O₃ whiskers also have relatively low porosity levels. Porosity increases rapidly with lower green densities, higher Al₂O₃, or SiC whisker contents, and the degree of reaction completeness diminishes. The SiC whiskers undergo reaction with the matrix, while Al₂O₃ whiskers are nonreactive. All of these observations correlate well with temperature measurements made during the course of the reaction. The SHS mode can be achieved with agglomerated particle size ratioD _Al/D _Ni ≥ 1, larger than the limit established from studies of the thermal explosion mode of combustion synthesisD _Al/D _Ni ≃ 0.3. 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.     

Combustion characteristics of the Ni3Ti-TiB2 intermetallic matrix composites

Combustion synthesis (SHS) of Ni₃Ti-TiB₂ metal matrix composites (MMCs) was selected to investigate the effect of gravity in a reaction system that produced a light, solid ceramic particle (TiB₂) synthesized in situ in a large volume (>50 pct) of the liquid metallic matrix (Ni₃Ti). The effects of composition, green density of pellets, and nickel particle size on the combustion characteristics are presented. Combustion reaction temperature, wave velocity, and combustion behavior changed drastically with change in reaction parameters. Two types of density effects were observed when different nickel particle sizes were used. The structures of the combustion zones were characterized using temperature profile analysis. The combustion zone can be divided into preflame, reaction, and after-burning zones. The combustion mechanism was studied by quenching the combustion front. It was found that the combustion reactions proceeded in the following sequence: formation of liquid Ni-Ti eutectic at 940 °C → Ni₃Ti+NiTi phases → reduction of NiTi with B→TiB₂+Ni₃Ti.     

Microstructure and properties of Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites formed byin situ reaction of Al with aluminosilicate ceramics

Al₂O₃-Al(Si) and Al₂O₃-Al(Si)-Si composites have been formed byin situ reaction of molten Al with aluminosilicate ceramics. This reactive metal penetration (RMP) process is driven by a strongly negative Gibbs energy for reaction. In the Al/mullite system, Al reduces mullite to produce α-Al₂O₃ and elemental Si. With excess Al (i.e., x > 0), a composite of α-Al₂O₃, Al(Si) alloy, and Si can be formed. Ceramic-metal composites containing up to 30 vol pct Al(Si) were prepared by reacting molten Al with dense, aluminosilicate ceramic preforms or by reactively hot pressing Al and mullite powder mixtures. Both reactive metal-forming techniques produce ceramic composite bodies consisting of a fine-grained alumina skeleton with an interpenetrating Al(Si) metal phase. The rigid alumina ceramic skeletal structure dominates composite physical properties such as the Young’s modulus, hardness, and the coefficient of thermal expansion, while the interpenetrating ductile Al(Si) metal phase contributes to composite fracture toughness. Microstructural analysis of composite fracture surfaces shows evidence of ductile metal failure of Al(Si) ligaments. Al₂O₃-Al(Si) and Al₂O₃-Al(Si)-Si composites produced byin situ reaction of aluminum with mullite have improved mechanical properties and increased stiffness relative to dense mullite, and composite fracture toughness increases with increasing Al(Si) content. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).     

Interfacial criterion of spontaneous and forced engulfment of reinforcing particles by an advancing solid/liquid interface

The sign of the interfacial force acting between a ceramic particle and a solidification front through the thin layer of a liquid metal is determined by the sign of the quantity Δσ _cls. A new, generally valid equation has been developed for this parameter: Δσ _cls = 2σ _cs− σ _cl− σ _sl(where σ _cs, σ _cl, and σ _slare the interfacial energies in the ceramic/solid metal, in the ceramic/liquid metal, and in the solid metal/liquid metal systems, respectively). The interfacial force is attractive, i.e., spontaneous engulfment of reinforcing particles by the front is expected, if Δσ _cls < 0. A new estimation method has also been developed for the quantity σ _cs. Combining this equation with the new equation for Δσ _cls, the approximated expressions with easily available parameters for the parameter Δσ _cls have been obtained for normal metals (Δσ _cls = σ _cv− σ _lv· (0.08 + 1.22 · cos Θ_clv)) and for Si and Ge (Δσ _cls = σ _cv− σ _lv· (0.57 + 1.66 · cos Θ_clv), where σ _cvand σ _lvare the surface energy of the ceramic and the surface tension of the liquid metal, respectively, while Θ_clv is the contact angle of the liquid metal on the ceramics). Calculations performed with these equations are in good qualitative agreement with all known pushing/engulfment experiments for metal/ceramic systems. Particularly, it has been theoretically predicted that, while in the majority of normal metal/ceramic and Ge/ceramic systems pushing (and, at higher solidification rates, forced engulfment) is expected, primary Si crystals (crystallizing from hypereutectic Al-Si and other alloys) will spontaneously engulf the majority of ceramic phases. The so-called “pushing/spontaneous engulfment” (PSE) diagrams have been constructed to help make a quick judgement as to whether spontaneous engulfment or pushing is expected in a given metalceramic system. For systems with Δσ _cls > 0, a new equation (similar to that derived earlier by Chernov et al.) has been derived to estimate the critical velocity of the pushing-engulfment transition (PET). Calculations with this equation show excellent quantitative agreement with the critical interface velocity of the PET in the Al/ZrO₂ (250 μm) system, measured recently under microgravity conditions by Stefanescu et al.     

The effect of gravity on the combustion synthesis of metal-ceramic composites

The effects of gravity on the combustion characteristics and microstructure of metal-ceramic composites (HfB₂/Al and Ni₃Ti/TiB₂ systems) were studied under both normal and low gravity conditions. Under normal gravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was carried out on a DC-9 aircraft at the NASA-Lewis Research Center. It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was directly opposed to the gravitational force. The SHS of 70 vol pct Al (in the Al-HfB₂ system) was changed from unstable, slow, and incomplete when ignited from the top to unstable, faster, and complete combustion when ignited from the bottom. The hydrostatic force (height × density × gravity) in the liquid aluminum was thought to be the cause of formation of aluminum nodules at the surface of the pellet. The aluminum nodules that were observed on the surface of the pellet when reacted under normal gravity were totally absent for reactions conducted under low gravity. Buoyancy of the TiB₂ particles and sedimentation of the Ni₃Ti phase were observed for the Ni₃Ti/TiB₂ system. The possibility of liquid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave velocity were lower than those under normal gravity. The distribution of the ceramic phase, i.e., TiB₂ or HfB₂, in the intermetallic (Ni₃Ti) or reactive (Al) matrix was more uniform.     

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.     

Synthesis ofin situ TiB2/TiN ceramic matrix composites from dense BN-Ti and BN-Ti-Ni powder blends

In the present research, near-net-shapein situ TiB₂/TiN and TiB₂/TiN/Ni composites were fabricated from cold-sintered BN/Ti and BN/Ti/Ni powder blends by pressureless displacement reaction synthesis or thermal explosion under pressure. In both approaches, the processing or preheating temperatures (≤1200 °C) were considerably lower than those typical of current methods used for the processing/consolidation of ceramic matrix composites. Microstructural characterization of the materials obtained was performed using X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical properties were evaluated by measuring microhardness, fracture toughness, and three-point bending strength. Application of a moderate external pressure (≤250 MPa) during self-propagating synthesis (SHS) synthesis was shown to be sufficient to ensure full density of the TiB₂/TiN/Ni composite. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The high fracture toughness of thein situ synthesized TiB₂/TiN/Ni composite (20.5 MPa√m) indicated that the finely dispersed ductile Ni phase was effective in dissipating the energy of cracks propagating in the ceramic matrix. Formarly Postdoctoral Student, Department of Materials Engineering, Technion. Formerly Fulbright Postdoctoral Fellow, Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).     

The use of an electric field as a processing parameter in the combustion synthesis of ceramics and composites

The imposition of an electric field is shown to activate self-propagating combustion reactions and thus makes possible the synthesis of a variety of ceramic and composite phases. Experimental observations and modeling studies indicated that activation is accomplished by the localized effect of the current. The relationship between wave propagation and the direction of the applied field was investigated. The synthesis of composites by field-activated combustion synthesis (FACS) was demonstrated. It was shown that the imposition of a field during the combustion synthesis of MoSi₂ results in a decrease in the product particle size. The results suggest that the field can be used as a processing parameter in self-propagating combustion synthesis. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).     

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 and subsequent

Explosive densification following combustion synthesis of titanium and graphite powder mixtures has been used to fabricate bulk compacts (100-mm diameter × 20-mm thick) of TiC ceramics. A model rocket ignitor was used to initiate the combustion reaction in ≈65 pct dense green pressed reactants of titanium and carbon powder mixtures. Upon completion of reaction, the reacted mass was allowed to cool. After the desired time delay (t _d ) and while the reacted mass was still above the ductile-brittle transition temperature, an explosive charge was detonated in contact with a steel driver plate to transmit the pressure into the reacted mass and consolidate it to solid density. Temperature-time cooling profiles for the reacted material were developed using calculations based on a heat flow model. The explosive loading conditions, namely, the densification pressure controlled by the ratio of explosive charge mass (C) to driver plate mass (M) ratio (C/M) and thet _d between the combustion reaction completion and explosive detonation, were observed to critically affect the density and the microstructure of the final compacted reaction product. 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.     

Numerical modeling of solidification combustion synthesis

A numerical study of self-propagating combustion synthesis is carried out to determine the effect of the heat of reaction(Q), the activation energy (E), the frequency factor (K₀), thermal conductivity(K*), and initial temperature(T ₀ on the combustion velocity and combustion temperature in the presence of cooling at one end. The numerical procedure allows for the formation and solidification of nonstoichiometric combustion products. This includes the phase change of the product through its solidification and eutectic range. Calculations are carried out for the Ti-C system with an objective of predicting solutions which are comparable to previously reported, experimentally determined values. Calculations are also compared with the thin zone analytical solution to the combustion problem. The use of lowK ₀ values to bring the solutions close to the experimentally determined numbers is discussed. Solutions are presented to elucidate the effect of relevant parameters on the thickness of the combustion and preheat zones. Conditions where extinction is expected to occur are identified. The effect of the thermal conductivity on the velocity may be to increase or decrease the velocity, depending on the value ofK ₀. At lowK ₀ values, an increase in the thermal conductivity may lead to a decrease in the combustion velocity. The effect of the initial temperature on the combustion velocity and temperature is to increase both; however, the increase in the combustion temperature may not be proportional to the increase in the initial temperature. The activation energyE has a pronounced effect on reducing the combustion velocity while not influencing the combustion temperature. The time rate of the solidification process which determines the final microstructure is discussed. Formerly Fellow, Department of Materials Science and Engineering, University of Cincinnati, is Scientist, Los Alamos National Laboratory, Los Alamos, NM 87545. 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.     

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.     

Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites

The attributes of an orthorhombic Ti aluminide alloy, Ti-21Al-22Nb (at. pct), and an alpha-two Ti aluminide alloy, Ti-24Al-11Nb (at. pct), for use as a matrix with continuous SiC (SCS-6) fiber reinforcement have been compared. Foil-fiber-foil processing was used to produce both unreinforced (“neat”) and unidirectional “SCS-6” reinforced panels. Microstructure of the Ti-24A1-11Nb matrix consisted of ordered Ti₃Al (α ₂) + disordered beta(β), while the Ti-21 Al-22Nb matrix contained three phases: α₂, ordered beta (β ₀), and ordered orthorhombic(O). Fiber/ matrix interface reaction zone growth kinetics at 982 °C were examined for each composite system. Although both systems exhibited similar interface reaction products(i.e., mixed Ti carbides, silicides, and Ti-Al carbides), growth kinetics in theα ₂ +β matrix composite were much more rapid than in theO +β ₀ +α ₂ matrix composite. Additionally, interfacial reaction in theα ₂ +β} composite resulted in a relatively large brittle matrix zone, depleted of beta phase, which was not present in theO +β ₀+α ₂ matrix composite. Mechanical property measurements included room and elevated temperature tensile, thermal stability, thermal fatigue, thermo-mechanical fatigue (TMF), and creep. The three-phase orthorhombic-based alloy outperformed the α₂+β alloy in all of these mechanical behavioral areas, on both an absolute and a specific(i.e., density corrected) basis.     

Interfacial kinetics of hydrogen with liquid slag containing iron oxide

Interfacial kinetics on the hydrogen reduction of liquid Fe _t O in Fe _t O-M _x O _y slag (M _x O _y = CaO, SiO₂, Al₂O₃, and TiO₂) has been studied at 1673 K. Because the rate of hydrogen reduction was very fast, the rate was controlled by gas-phase mass transfer under most of the experimental conditions. The effect of CaO or SiO₂ addition on the interfacial chemical reaction rate of hydrogen reduction was empirically evaluated as a function of the ferrous-ferric ratio in the slag. The observed interfacial chemical reaction rates in Fe _t O-CaO and Fe _t O-SiO₂ slags showed reasonable agreement with the estimated values. Most of the available literature data on the reduction rate of liquid iron oxide by solid carbon, hot metal, and reducing gases were also reviewed and compared with the results of the present work. It was found that the rate of hydrogen reduction of liquid iron oxide slag is much faster than that with other reducing agents such as solid carbon, carbon dissolved in the liquid iron, and CO gas. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Multistage sintering approach to prepare Ni3Al/α-Al2O3 composites

The nickel aluminide intermetallic matrix composites (IMC), Ni₇₆Al₂₄B_0.1 with either 5 or 10 vol pct α-Al₂O₃, were synthesized through a multistage sintering approach from the elemental powders of Ni, Al, and oxide of α-Al₂O₃. An electroless nickel-boron (Ni-B) plating process was adopted to improve the contacted interface between the reinforced oxide ceramics and the metal matrix, as well as to supply the atomic scale boron in the metallic matrix of the IMCs. The entire process comprises steps involving preparing a powdery starting material, sealing it within a metal sheath or can, compacting or cold deforming it, preliminarily heating the compacted material at a relatively low temperature, executing a pore-eliminating (mechanical deforming) process to eliminate the pores resulting from the preceding heating step, and sintering the material at a relatively high temperature to develop a transient liquid phase to heal or to eliminate any microcracks, crazes, or collapsed pores from the previous steps. Most of all, it is important that contact with a heat absorbent material, such as a metal sheath, produces the Ni₂Al₃ phase during preliminary heating. This new phase is a brittle and crispy material with a low melting point (1135 °C). It has been found to play an important role in preventing any significant cracks during the pore-eliminating process and in developing a transient liquid phase in the following 1200 °C sintering step. This multistage sintering with a heat absorbent process is beneficial for producing a product that has large dimensions, a desirable shape, good density, and excellent mechanical properties. The resulting elongation of tensile tests in air reaches 14.6 and 8.9 pct for the present 5 and 10 vol pct powder metallurgy IMCs, respectively.     

X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part I. Experimental setup and processing of spectra

An energy dispersive X-ray (EDX) detector mounted on a laboratory scale electron beam furnace (30 kW) was employed to assess the potential use of X-rays as a means of on-line liquid alloy composition monitoring during electron beam (EB) melting of alloys. The design and construction of the collimation and protection systems used for the EDX are described in Part I. X-ray spectra are obtained from a sample of AISI 316 stainless steel at both beam idle (in the absence of liquid metal) and high power (in the presence of liquid metal). Two different types of molds are employed: (1) a water-cooled copper mold and (2) a ceramic lined water-cooled copper mold. Various strategies for signal processing and filtration are presented and compared. Correction factors for beam voltage were developed and applied in order to develop correlations between the mole fraction and normalized X-ray intensity for Ni−K _α, Cr−K _α, and Fe−K _α based on an analysis of the vapor condensate. Correlations were also developed relating the change in the X-ray intensities to time for (a) Mo−L, (b) Cr−K _α, (c) Fe−K _α, and (d) Ni−K _α. The stability of the electron beam was found to be the principal source of error, and suggestions for further improvements are also discussed. The study confirms the feasibility of the method and is the first reported study of on-line analysis of a high-temperature liquid alloy. In Part II, the technique is applied to the study of the complex evaporation processes occurring during EB melting.     

Last-stage solidification of alloys: Theoretical model of dendrite-arm and grain coalescence

Hot tearing in castings is closely related to the difficulty of bridging or coalescence of dendrite arms during the last stage of solidification. The details of the process determine the temperature at which a coherent solid forms; i.e., a solid that can sustain tensile stresses. Based on the disjoining-pressure concept used in fluid dynamics, a theoretical framework is established for the coalescence of primary-phase dendritic arms within a single grain or at grain boundaries. For pure substances, approaching planar liquid/solid interfaces coalesce to a grain boundary at an undercooling (ΔT _b ), given by

Microstructural characterization of self-propagating high-temperature synthesis/ dynamically compacted and hot-pressed titanium carbides

Titanium-Carbide produced by combustion synthesis followed by rapid densification in a high-speed forging machine was characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The density of the combustion synthesized/dynamically compacted TiC reached values greater than 96 pct of theoretical density, based on TiC_0.9, while commercially produced hot-pressed TiC typically exceeded 99 pct of theoretical density. The higher density of the hot-pressed TiC was found to be attributable to a large volume fraction of heavy element containing inclusions. The microstructure of both TiCs consists of equiaxed TiC grains with some porosity located both at grain boundaries and within the grain interiors. In both cases, self-propagating high-temperature synthesis (SHS)/dynamically compacted (DC) and hot-pressed, the TiC is ordered cubic (NaCl-structure,B ₁; Space Group Fm3m) with a lattice parameter of ≈0.4310 nm, indicative of a slightly carbon deficient structure; stoichiometric TiC has a lattice parameter of 0.4320 nm. For the most part, the grains were free of dislocations, although some dislocation dipoles were found associated with the voids within the grain interiors. In one SHS/DC specimen, a new, complex Ti-Al-O(C) phase was observed. The structure could not be matched with any previously published phases but is believed to be hexagonal, with a c-axis/a-axis ratio of ≈6.6, similar to the AlCTi₂ phase which has a point group 6 mmm. In all other SHS/DC TiC samples, the grains and grain boundaries were devoid of any second-phase particles. The hot-pressed TiC exhibited a greater degree of porosity than the SHS/densified specimens and a large concentration of second-phase particles at grain boundaries and within grains. The structure and composition of these second-phase particles were determined by con-vergent beam electron diffraction (CBED) and X-ray microanalysis. 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.     

Processing and Characterization of Dual Phase Steel Foams Featured by Different Pore Distribution

Porous materials featuring cellular structures are known to have many interesting combinations of physical and mechanical properties. Some of them have been extensively used in structural applications (i.e. balsa wood), as well as in functional applications (heat exchangers, filters, etc.). Steel foams present promising theoretical properties for both functional and structural applications, but processing such kind of foams is complex due to their high melting temperature. Starting from a technique based on molten metal infiltration into a ceramic space holder, a new process is presented here for open‐cell steel sponges processing. Using a SiC cellular preforms as a space holder, dual phase steels foams with different porosity and steel microstructure were successfully developed. This technique is suitable to obtain steel alloy sponges featured by a relative density equal to 0,6 and interconnected pores. The compression tests indicate that the resulting material features the typical stress‐strain behaviour of classical cellular metals. Moreover mechanical properties, such as Elastic Modulus, σ_plateau, ε_{densification} and E_absorbed, depended on porosity and on martensite fraction, which is a function of the applied intercritical temperature.