Influence of Aluminum Additives on the Content and Parameters of the Fine Structure of Titanium Silicon Carbide in SHS Powders

The dependence of the phase composition and parameters of a fine structure of titanium silicon carbide in powders formed by the self-propagating high-temperature synthesis on the aluminum concentration in the 5Ti/2Si/1C reaction mixture is investigated. The aluminum content is varied in a range of 0.1–0.4 mole fraction with the conservation of the total carbon content. It is established that the additives of aluminum not only affect the yield of titanium silicon carbide, but also promote the preferential formation of Ti5Si3 in synthesis products instead of TiSi2 identified in powders containing no aluminum. The introduction of a small amount of aluminum (0.1 mole fraction) leads to the formation of the Ti3Si1 – xAlxC2 solid solution and makes it possible to decrease the content of impurity phases in SHS powders by 6%. The silicon carbide concentration in SHS powders decrease at a higher aluminum content in the reaction mixture, while that of binary compounds (TiC, Ti5Si3, TiAl) correspondingly increases. No noticeable effect from the introduction of aluminum on the parameters of the crystal lattice of titanium silicon carbide in SHS powders is found in concentration limits of 0.1–0.25 mol %. A noticeable increase in parameters of a and c for Ti3Si1 – xAlxC2 (from a = 3.067 Å, c = 17.67 Å to a = 3.07 Å, c = 17.73 Å) with the conservation of the c/a ratio in limits of known values (c/a = 5.78) is observed only with the aluminum concentration of 0.4 mole fraction. The crystallite size of titanium silicon carbide depends, first and foremost, on the combustion parameters. At the same time, the deformation of the crystal lattice of Ti3Si1 – xAlxC2 in SHS powders increases monotonically with an increase in the aluminum content in the reaction mixture in the concentration range under study.     

Synthesis of TiC–NiCrBSi Binder Alloy Composite Powders for Cladding and Deposition of Wear-Resistant Coatings

Russian Journal of Non-Ferrous Metals - TiC–NiCrBSi binder metal matrix composites are fabricated by self-propagating high-temperature synthesis (SHS) in reaction powder mixtures of titanium,...     

Structure and Phase Composition of SHS Products in Powder Mixtures of Titanium,Carbon, and Aluminum

Russian Journal of Non-Ferrous Metals - TiC + Al binder metal-matrix composites are fabricated by self-propagating high-temperature synthesis (SHS) in reaction power mixtures of titanium, carbon...     

Formation of the structure and phase composition of the Ti–Al–Ta-based materials

The experiments on the fabrication of materials based on the Ti–3Al–0.5Ta and 3Ti–2Al–Ta systems by self-propagating high-temperature synthesis (SHS) are performed. The influence of the composition of the initial mixture, dispersity of powders, and preliminary mechanical activation on the phase composition and structure of the SHS product is investigated. The optimal ratio between the mechanically activated and initial powder in a mixture for the synthesis of materials is determined. The dependence of the structure of final products on the structure of initial powders is established. The use of porous tantalum leads to the formation of the intermetallic matrix based on titanium aluminide with the uniform distribution of Ta particles. It is noteworthy that tantalum powders of both studied series (which differ by dispersity and morphology) partially reacted already at the stage of mechanical activation with the formation of the Al₂Ta phase. It is shown that aluminum plays the leading role in processes of mechanical activation in Ti–Al–Ta reaction mixtures. Indeed, a considerable rise of unreacted tantalum particles in the microstructure of sintered samples is observed with a decrease in the amount of aluminum in the reaction mixture.     

Thermal analysis of self-propagating high-temperature reactions in titanium, boron, and aluminum powder compacts

This article focuses on the characterization of self-propagating high-temperature synthesis (SHS) reactions that occur in powder compacts containing titanium, boron, and aluminum. Interest in this powder system is based on the critical need to develop new joining techniques for bonding ceramics to metals. The exothermic reactions of particular interest in this study include those that generate TiB₂, TiB, Ti₃Al, and TiAl from their elemental powders. Data from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and X-ray diffractometry are presented. These results demonstrate that the gas phase surrounding the SHS powders plays an important role in initiating the SHS reaction and in determining which reaction products will form in the final bond.     

Microstructure and tensile properties of Fe3Al produced by combustion synthesis/hot isostatic pressing

Combustion synthesis was carried out in a hot isostatic press (HIP) in order to prepare near-theoretical density Fe₃Al and Fe₃Al + Cr alloys from elemental powder mixtures. The microstructures and room-temperature tensile properties of these materials were studied in the as-synthesized condition and after heat treatment. As-synthesized materials exhibit a fine, equiaxed grain structure with grain sizes typically less than 10 μm. Yield and ultimate tensile strengths were found to be significantly higher than what has been reported for conventionally processed materials having similar composition. Although lower ductility was generally observed, elongations exceeding 5 pct were obtained in heat-treated Cr-containing alloys. Fracture occurred predominantly by transgranular cleavage. The strengthening of these materials is attributed to the fine grain size resulting from combustion synthesis/HIP processing. Transmission electron microscopy (TEM) revealed the presence of two distinct populations of aluminum oxide particles in the material. Nanometer-sized oxides exist within grains that likely formed at prior iron particle boundaries, and a high density of larger oxides exist along grain boundaries that probably originated from surface oxides on the aluminum powder. The presence of the grain boundary oxides was qualitatively observed to provide resistance to grain growth. 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.     

Al-TiC Composites Fabricated by a Thermally Activated Reaction Process in an Al Melt Using Al-Ti-C-CuO Powder Mixtures. Part I: Microstructural Evolution and Reaction Mechanism

Al matrix composites reinforced with TiC particles are fabricated by a thermally activated reaction of Al-Ti-C powder mixtures in an Al melt. In the presence of CuO, reactant mixtures in the form of a pellet added to molten Al at temperatures higher than 1093 K (820 °C) instantly reach the peak temperature over 1785 K (1512 °C), followed by combustion wave propagation with in situ synthesizing TiC with a size of approximately 1 μm. Incomplete reaction products such as unreacted C, Al₃Ti, and TiC aggregates are also observed. The pellet microstructure evolution upon the combustion reaction indicates that preheating temperature, i.e., the initial melt temperature, affects both the thermodynamic and kinetic characteristics of the reaction, and thereby influences the final microstructure of the Al/TiC composites. Based on the experimental and theoretical results, a sequence of the reaction leading upto the in situ synthesis of TiC is illustrated and the corresponding mechanism for the present process is proposed.     

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.     

Effect of the oxygen content of titanium powder on the synthesis of titanium carbide by the self-propagating high-temperature method

Conclusions An investigation was carried out into the possibility of employing various grades of titanium powder for the production of titanium carbide by the SHS process. The relationship between the chemical composition of the end product and synthesis parameters has been established. The completeness of chemical transformation, degree of purification of the titanium carbide from oxygen, and velocity of propagation of the synthesis wave depend on the oxygen content and particle size of the titanium powder as well as on the synthesis (combustion) temperature and time of residence of the reaction mass in the inert atmosphere at high temperature. Raising the CO concentration in synthesis increases the contamination of the titanium carbide with oxygen and the amount of free carbon in the synthesis product. The authors wish to thank Professor A. G. Merzhanov for helpful advice and discussion.The authors wish to thank Professor A. G. Merzhanov for helpful advice and discussion.Translated from Poroshkovaya Metallurgiya, No. 12(228), pp. 49–54, December, 1981.     

自蔓延高温合成铝碳氮材料

采用Al粉和CNx前驱体粉末为原料,通过自蔓延高温合成技术,制备铝碳氮材料。利用燃烧波淬熄实验并结合XRD、SEM和EDS测试技术,研究自蔓延反应合成铝碳氮的机理。研究结果表明,自蔓延高温合成铝碳氮材料可分为3个阶段,第一阶段为Al融化和蔓流,形成的Al熔体包覆CNx前驱体粉末;第二阶段,Al与CNx分解的产物C和N2发生反应,生成AlN和Al4C3等二元相;反应放出大量的热,引发其它未反应的原料发生自蔓延反应,使反应自维持;第三阶段,AlN和Al4C3反应生成三元铝碳氮化合物。     

Thermal analysis of self-propagating high-temperature reactions in titanium, boron, and aluminum powder compacts

This article focuses on the characterization of self-propagating high-temperature synthesis (SHS) reactions that occur in powder compacts containing titanium, boron, and aluminum. Interest in this powder system is based on the critical need to develop new joining techniques for bonding ceramics to metals. The exothermic reactions of particular interest in this study include those that generate TiB₂, TiB, Ti₃Al, and TiAl from their elemental powders. Data from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and X-ray diffractometry are presented. These results demonstrate that the gas phase surrounding the SHS powders plays an important role in initiating the SHS reaction and in determining which reaction products will form in the final bond. L.H. CHIU, formerly with the Material Science and Engineering Department, Johns Hopkins University. L.A. BONNEY, formerly with the Materials Science and Engineering Department, Johns Hopkins University.     

Reaction Synthesis of Titanium Matrix Composites Through Powder Metallurgy Processing Technique

Titanium matrix composites reinforced with 10, 20 and 30 vol.% of in situ TiB–TiC reinforcements were fabricated using powder metallurgy method which offers economical solution for production of high performance materials. The in situ composites were produced by reaction of sintering of uniformly blended and compacted powders of titanium and boron carbide in required proportions. XRD analysis confirmed the completion of in situ reaction. SEM studies confirmed uniform distribution and the shape of the reinforcement, TiB as whiskers and TiC as equiaxed particles. The effect of morphology and reinforcement volume on properties of the composites were evaluated by measurement of modulus, hardness, three-point bend test and compression test at ambient temperature along with unreinforced titanium. The study has revealed that the composites with increasing percentage of the reinforcements exhibited improved properties due to the presence of TiB whiskers distributed uniformly. Fractography of the three-point bend tested samples revealed the plastic mode of fracture for unreinforced titanium and the composites exhibiting mixed mode of fracture.     

Combustion synthesis and mechanical properties of dense NiTi-TiC intermetallic-ceramic composites

Combustion synthesis (SHS) coupled with a quasi-isostatic densification step was employed to produce dense NiTi-TiC composites. The synthesis and characterization of five composites are presented, including ceramic-intermetallic (≥50 pct ceramic) composites and intermetallic-ceramic (≥50 pct intermetallic) composites. Particle size, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis was conducted to characterize the microstructure of the composites. Refractory TiC and NiTi intermetallic phases become more stoichiometric and the TiC particle size decreases with increasing intermetallic content. Micro- and nanoindentation and quasi-static compression tests were performed, to determine mechanical and material properties. The Vickers hardness decreases as the matrix shifts from ceramic to intermetallic. Modulus and compressive strength decreases with increasing amounts of Ni-Ti intermetallic. The SEM photomicrographs of fractured surfaces are included.     

Combustion of Powder Mixtures Forming Reaction Products - Synthesis of NiAl

Abstract

Combustion of powder mixtures under adiabatic conditions -Self-propagating High-temperature Synthesis (SHS)-is an efficient and attractive technique to synthesize a host of materials for high technology applications, namely, ceramics, intermetallics, composites and functionally graded materials. Conditions necessary for and general features of this technique are briefly reviewed.

Experimental investigations on synthesis of nickel monoaluminide from stoichiometric mixtures of component powders by this technique using plane-wave propagation mode is carried out. Effects of reaction parameters namely, bulk density of compacts, amount of inert dilutions and atmospheres on velocity of combustion wave propagation and combustion peak temperature are investigated. From the dependence of propagation velocity on combustion temperature an activation energy of 170.99 kJ/mole is obtained for SH-synthesis of NiAl. The synthesized products showed equiaxed large grained microstructures.     

气固反应原位生成TiC颗粒增强钛基复合材料

以氢化脱氢钛粉为原料, 经冷等静压成型, 在一定温度下通过CH4和钛粉颗粒间的气固反应在钛粉表面原位生成均匀的TiC颗粒, 采用真空烧结技术制备得到氧含量(体积分数)低于0.2%的TiC颗粒增强钛基复合材料。研究表明, TiC颗粒体积分数比可通过气固反应温度和时间控制, 可获得较高体积分数(> 30%)的TiC颗粒增强钛基复合材料。TiC首先在钛粉颗粒表面形成, 烧结过程中, 钛粉颗粒明显阻碍TiC晶粒长大, 细化TiC晶粒; 同时, 过多的TiC颗粒也阻碍烧结过程中钛的自扩散, 降低烧结相对密度。钛粉压坯在700℃、CH4气氛下发生气固反应(30 min), 再经1300℃烧结后获得的相对密度为98.6%的烧结试样, 试样的综合力学性能较好, 抗拉强度为606 MPa, 延伸率达14.4%, 硬度为HV 442。值得注意的是, 较短时间的气固反应不能够保证压坯内外整体实现原位生成均匀TiC颗粒, 导致烧结试样内外组织的不均性。     

Texture and residual strain in two SiC/Ti-6-2-4-2 titanium composites

Residual strain and texture variations were measured in two titanium matrix composites reinforced with silicon carbide fibers (Ti/SiC) of similar composition but fabricated by different processing routes. Each composite comprised a Ti-6242 α/β matrix alloy containing vol 35 pct continuous SiC fibers. In one, the matrix was produced by a plasma sprayed (PS) route, and in the other by a wiredrawn (WD) process. The PS and WD composites were reinforced with SCS-6 (SiC) and Trimarc (SiC) fibers, respectively. The texture in the titanium matrices differed significantly. The titanium matrix for the PS material exhibited random texture pre and post fabrication of the composite. For the WD material, the starting texture of the monolithic titanium matrix was ≈17 times random, but after consolidation into composite form, it was ≈6 times random. No significant differences were noted in the fiber-induced matrix residual strains between the composites prepared by the two procedures. However, the Trimarc (WD) fibers recorded higher (≈1.3 times) compressive strains than the SCS-6 (PS) fibers. Stresses and stress balance results are reported. Plane-specific elastic moduli, measured in load tests on the unreinforced matrices, showed little difference. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

In situ synthesis of TiC particulate-reinforced aluminum matrix composites

Particulate TiC-reinforced aluminum composite specimens were processed by compacting a mixture of titanium, carbon, and aluminum powders into preforms that were infiltrated with molten aluminum and subsequently heated in a differential thermal analyzer to about 1573 K under argon atmosphere. The onset of formation of TiC particles began at about 1150 K by reaction of TiAl₃ with Al4C₃. Subsequent formation of TiC particles at higher temperatures to approx-imately 1265 K occurred by direct reaction of carbon with TiAl₃. Above this temperature, the TiC particles coarsened with increasing temperature from an initial size of about 0.15μm. TiC particles were also produced in preforms that were not infiltrated; however, the presence of liquid aluminum in infiltrated specimens inhibited particle agglomeration and sintering. Infil-trated preforms could, therefore, serve as excellent "master alloys" for subsequent dilution in an aluminum melt and processing of metal-matrix composites (MMCs) reinforced with sub-micron TiC particulates. Formerly Research Scientist, Massachusetts Institute of Technology, Cambridge, MA 02139     

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.     

Texture and residual strain in two SiC/Ti-6-2-4-2 titanium composites

Residual strain and texture variations were measured in two titanium matrix composites reinforced with silicon carbide fibers (Ti/SiC) of similar composition but fabricated by different processing routes. Each composite comprised a Ti-6242 α/β matrix alloy containing vol 35 pct continuous SiC fibers. In one, the matrix was produced by a plasma sprayed (PS) route, and in the other by a wiredrawn (WD) process. The PS and WD composites were reinforced with SCS-6 (SiC) and Trimarc (SiC) fibers, respectively. The texture in the titanium matrices differed significantly. The titanium matrix for the PS material exhibited random texture pre and post fabrication of the composite. For the WD material, the starting texture of the monolithic titanium matrix was ≈17 times random, but after consolidation into composite form, it was ≈6 times random. No significant differences were noted in the fiber-induced matrix residual strains between the composites prepared by the two procedures. However, the Trimarc (WD) fibers recorded higher (≈1.3 times) compressive strains than the SCS-6 (PS) fibers. Stresses and stress balance results are reported. Plane-specific elastic moduli, measured in load tests on the unreinforced matrices, showed little difference. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.