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).     

Fabrication of TiC/Fe–Ni cermet coatings by reactive thermal spraying of Fe–Ti–Ni–C composite powder

Abstract

A mixture of ferrotitanium, nickel powders and sucrose was heated with an intention of carbonising the sucrose. The tiny ferrotitanium, nickel particles are bound by the carbon obtained from pyrolysis of the sucrose to form a unique structure of Fe–Ti–Ni–C composite powder for reactive thermal spraying. The carbon is a reactive constituent as well as the binder in the composite powder. TiC/Fe–Ni cermet coating was prepared by reactive plasma spraying of this powder. A mass of TiC particles were in situ synthesised and uniformly distributed in the Fe–Ni alloy matrix without residuals of raw materials in the coating. The coating is consisted of two different areas: one is the composite area, where lots of spherical fine TiC particles (100–500 nm) are homogeneously distributed within the Fe–Ni alloy matrix; the other is a small fraction of TiC accumulation. The volume fraction of composite area is >87%.     

Synthesis of nanocrystalline titanium carbide alloy powders by mechanical solid state reaction

A high-energy ball mill operated at room temperature has been used for preparing titanium carbide (TiC) alloy powders, starting from elemental titanium (Ti) and carbon (C) powders. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) have been used to follow the progress of the mechanical solid state reaction of Ti and C powders. A complete single phase of fcc-Ti₄₄C₅₆ alloy powders is obtained after a very short milling time (20 ks). The lattice parameter (a ₀ ) of the end product of Ti₄₄C₅₆ was calculated to be 0.4326 nm. The presence of excess starting reactant materials (Ti and/or C atoms) in the final product of the alloy powders could not be detected. The end product of Ti₄₄C₅₆ alloy powders possesses homogeneous, smooth spherical shapes with an average particle diameter of less than 0.5 μm. The internal structure of the particles is marked by fine cell-like features of about 3 nm. On the basis of the results of the present study, the mechanical alloying (MA) process appears to provide a powerful tool for the fabrication of Ti₄₄C{im56} alloy powders at room temperature. The mechanism of mechanical solid state reaction for formation of Ti₄₄C₅₆ alloy powders is discussed. Formerly Lecturer of Materials Science, Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City 11884, Cairo-Egypt.     

Phase transitions in reactive formation of Ti5Si3/TiAl in situ composites

A new method is developed for preparing Ti₅Si₃/TiAl in situ composites by incorporating metastable phases (called metastable precursors) into TiAl (a mixture of elemental Ti and Al) matrix powders. Metastable precursors with a starting composition of Ti-14Al-21Si are prepared by mechanical alloying (MA). They have been proven through X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) observations to be mainly consisting of mixtures of nanostructured solid solutions and milling-formed TiAl compound. Particularly, phase reactions and transitions in the precursors and the composites during heating have been investigated in detail by using diffraction thermal analysis (DTA) in conjunction with XRD. It has been found that Ti₅Si₃ is in situ formed through a phase transition chain, TiSi₂ → Ti₅Si₄ → Ti₅Si₃. When the composite powder (precursor, Ti and Al) is heated, a combustion reaction first occurs in the matrix, which results in the formation of TiAl₃ and/or TiAl followed by the completion of the previously mentioned silicide transitions in a very short time. Scanning electron microscope (SEM) observations indicated the locations of reinforcements in the reaction-formed composite, and TEM observation provided some details of the structures for the reinforcements and their neighborhood. This method is intriguing because a designed phase hierarchy is possible.     

水解沉淀-碳热还原氮化法制备碳氮化钛粉末

以四氯化钛、炭黑为原料,利用水解沉淀-碳热还原氮化法制备了碳氮化钛粉末.利用差热分析、X射线衍射及扫描电镜等表征手段,研究了合成工艺对粉末物相、组成及形貌等的影响.结果发现:前驱体粉末经350 ℃煅烧2h后,钛以TiO2的形式存在,TiO2与炭黑形成了混合均匀的团聚体:在碳热还原氮化反应时,钛氧化物向TiCxNyOz转...     

氢化脱氢制备钛粉的研究进展

钛粉、氢化钛粉及其预合金粉是钛材加工的重要原料。综述了氢化制取钛粉末的研究现状,并分析了氢化制取钛粉末的主要工艺。氢化球磨法制粉具有操作简单、成本低等优点,但氧含量高是此法制粉的一大缺点。随着制粉工艺的发展,能够制备出低价优质的钛粉、氢化钛粉。制粉新工艺开发与应用也将极大推动钛材的发展。     

层状可加工陶瓷Ti3SiC2自蔓延高温合成反应机理的研究

采用燃烧波淬熄法,以Ti粉、Si粉和C粉为原料研究了层状可加工陶瓷Ti3SiC2在自蔓延高温合成(SHS)中的反应机理.淬熄试样中保留未反应区、反应区和已反应区,用扫描电子显微镜观察燃烧反应中显微组织的转变过程,用能谱仪分析各微区的成分变化,并通过差热分析(DSC-TGA)和XRD分析考察了从600℃到1500℃ Ti-C-Si系统的反应合成过程和相形成规律.结果表明:层状可加工陶瓷Ti3SiC2自蔓延高温合成的机理为溶解-析出机制,Ti粉与Si粉的固态扩散导致低熔点Ti-Si溶液形成,Ti、Si、C粉粒逐渐向Ti-Si溶液中溶解,当溶液中的Ti、Si、C浓度饱和时,从中析出TiC、SiC颗粒,最后形成最终产物Ti3SiC2.     

Inorganic interfacial engineering: processing of hard materials

Abstract

Inorganic interfacial engineering may be regarded as the core of powder metallurgical processing of hard materials. The present paper reviews recent results from an interdisciplinary research effort, BRIIE (the Brinell Centre for Inorganic Interfacial Engineering), a joint effort between five industrial companies, three universities, two research institutes and VINNOVA (the Swedish Agency for Innovation Systems). The research involves experimental work on the aqueous processing of powders and the use of surface actants is reviewed as well as the colloidal processing of ceramics. Pressing and sintering of agglomerated powders have been studied both theoretically and experimentally. Models for the simulation of pressing and sintering of hard metal powders are developed. Results on ceramic materials obtained by spark plasma sintering and their resistance to thermal shock are reported.     

TiH2对TiB2自蔓延高温合成过程的影响

采用纯Ｔｉ粉或氢化钛粉、Ａｌ粉及Ｂ粉混合后进行整体加热，使其发生自蔓延高温反应，合成了ＴｉＢ２粉末。研究了氢化钛粉及合成温度对ＴｉＢ２合成反应过程的影响，探讨了ＴｉＢ２粒子的形成机理。     

Wear-resistant amorphous and nanocomposite steel coatings

In this article, amorphous and nanocomposite thermally deposited steel coatings have been formed by using both plasma and high-velocity oxy-fuel (HVOF) spraying techniques. This was accomplished by developing a specialized iron-based composition with a low critical cooling rate (≈10⁴ K/s) for metallic glass formation, processing the alloy by inert gas atomization to form micron-sized amorphous spherical powders, and then spraying the classified powder to form coatings. A primarily amorphous structure was formed in the as-sprayed coatings, independent of coating thickness. After a heat treatment above the crystallization temperature (568 °C), the structure of the coatings self-assembled (i.e., devitrified) into a multiphase nanocomposite microstructure with 75 to 125 nm grains containing a distribution of 20 nm second-phase grain-boundary precipitates. Vickers microhardness testing revealed that the amorphous coatings were very hard (10.2 to 10.7 GPa), with further increases in hardness after devitrification (11.4 to 12.8 GPa). The wear characteristics of the amorphous and nanocomposite coatings were determined using both two-body pin-on-disk and three-body rubber wheel wet-slurry sand tests. The results indicate that the amorphous and nanocomposite steel coatings are candidates for a wide variety of wear-resistant applications.     

Fabrication of micro-fine spherical Ti–6Al–4V alloy powders based on hydrogen decrepitation and plasma spheroidisation

A novel process was developed for scalable fabrication of micro-fine spherical Ti–6Al–4V alloy powders. The hydrogenation-treated Ti–6Al–4V alloy ingot was mechanically crushed into particles and then sieved into three size grades. The powders were separately sent through the radio frequency (RF) argon plasma system for spheroidisation. The fabrication process and powder characteristics were investigated. The results indicate the alloy ingot upon hydrogenation treatment can be efficiently crushed into fine particles with size of 5–76?μm. During RF plasma processing, the powders are found to be greatly refined due to hydrogen decrepitation with subsequent transformation into spherical morphology. The effect of hydrogen decrepitation on particle refinement is impaired with decreasing particle size of feed powders. The spherical powders exhibit a narrow particle size distribution and the average size is in the range of 8.2–27.9?μm. The spheroidised powders mainly consist of β-Ti and TiH_1.5.     

Fabrication of in situ TiB2–TiC reinforced steel matrix composites by spark plasma sintering

Abstract

In situ TiB₂ and TiC particulates reinforced steel matrix composites have been fabricated using cheap ferrotitanium and boron carbide powders by spark plasma sintering (SPS) technique. The sintering behaviour and the formation mechanism of the composite were studied. The results show that when the composite was sintered at 1050°C for 5 min, the maximum relative density and hardness of the composite are 99·2% and 83·8 HRA respectively. The phase evolution of the composite during sintering indicates that the TiB₂ and TiC reinforcements were formed in situ as follows: first, the solid/solid interface reaction between Fe₂Ti and B₄C, resulting in the formation of a small amount of TiB₂ and TiC below 950°C; second, the solid–liquid solution precipitation reaction in the Fe–Ti–B–C system, resulting in the formation of the main TiB₂ and TiC reinforcements at ～1000°C.     

<Emphasis Type="Italic">In-situ</Emphasis> reactive processing of nickel aluminides by laser-engineered net shaping

Nickel aluminide intermetallics (e.g., Ni₃Al and NiAl) are considered to be attractive materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a rapid prototyping process, which involves laser processing fine metal powders into three-dimensional shapes directly from a computer-aided design (CAD) model. In this work, an attempt has been made to fabricate aluminide intermetallic compounds via reactive in-situ alloying from elemental powders using the LENS process. In-situ reactive alloying was achieved by delivering elemental Ni and Al powders from two different powder feeders, eliminating segregation observed in the samples deposited by using the premixed elemental powders. Nickel aluminides of various compositions were obtained easily by regulating the ratio of their feed rates. The aluminide deposits exhibited a high solidification and subsolidus cracking susceptibility and porosity formation. The observed porosity resulted from a water-atomized Ni powder and can be minimized or eliminated by the use of a N₂-gas-atomized Ni powder of improved quality. Cracking was due to the combined effect of the high thermal stresses generated from the LENS processing and the brittleness of the intermetallics. Crack-free deposits were fabricated by preheating the substrate to a temperature of 450 °C to 500 °C during LENS processing. Compositionally graded Ni-Al deposits with a gradient microstructure were also produced by the in-situ reactive processing.     

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.     

Pressure-assisted reactive synthesis of titanium aluminides from dense 50Al-50Ti elemental powder blends

In the present research, dense γ-TiAl based intermetallic samples were fabricated by reactive synthesis of fully dense elemental 50 at. pct Al-50 at. pct Ti powder blends. Two different processing routes were attempted: thermal explosion under pressure (combustion consolidation) and reactive hot pressing. In both approaches, relatively low processing or preheating temperatures (900 °C) were used. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The application of a moderate external pressure (≤250 MPa) during synthesis was shown to be enough to accommodate the negative volume change associated with TiAl formation from the elemental components and, thereby, to ensure full density of the final product. Microstructure and phase composition of the materials obtained were characterized employing X-ray diffraction and scanning electron microscopy with energy dispersive analysis. It was found that at elevated temperatures(e.g., 900 °C), the equiatomic 50Al-50Ti alloy lies beyond the homogeneity range of the y-TiAl phase in the Ti-Al binary and contains, in addition to γ-TiAl, Al-rich Ti₃Al. Mechanical properties of the materials synthesized were evaluated in compression tests at different temperatures and by microhardness measurements. Due to its very fine microstructure, the Ti-Al material synthesizedvia reactive hot pressing exhibited superplastic behavior at temperatures as low as 800 °C. Formarly with the Department of Materials Engineering, Drexel University. 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).     

射频等离子体制备球形Ti—6Al—4V粉末性能表征

以不规则形状的Ti-6Al-4V(TC4)粉末为原料,通过射频等离子体球化处理制备了球形TC4粉末,并研究了球化处理对粉末特性及加料速率对粉末球化率的影响.利用扫描电子显微镜、激光粒度分析和霍尔流速计分别对其粉末微观结构、粒度分布和粉体性能进行了测试和分析.结果表明:TC4粉末经等离子球化处理后得到表面光滑、球形度好及球化率可达到100%的球形粉末;球化处理后,粉末的松装密度、振实密度和粉末流动性得到明显改善,粒度略微增大;随着加料速率的增加,TC4粉的球化率逐渐降低.      

A flame process for synthesis of unagglomerated, low-oxygen nanoparticles: Application to Ti and TiB2

A gas-phase flame process for synthesizing unagglomerated nanoparticles of metals, intermetallics, ceramics, and composites is described. Employing this process, titanium and titanium boride have been synthesized by the vapor-phase reaction of sodium with titanium tetrachloride and a 1:2 mixture of titanium tetrachloride and boron trichloride, respectively. To minimize agglomeration and protect the particles from postflame oxidation, the NaCl by-product is allowed to condense onto the particles in situ, yielding NaCl-encapsulated particles. In this way, stable, unagglomerated Ti and TiB₂ nanoparticles have been produced and the encapsulated powders have been handled in air without oxidation. Particle size has also been varied with the encapsulation process, and titanium particles with mean sizes of 10 and 60 nm have been produced by varying operating conditions. The NaCl has been removed by water washing as well as vacuum annealing. Thermodynamic results show that the sodium/halide process is applicable for synthesis of many materials, with yields approaching 100 pct under a wide range of operating conditions. Similarly, the encapsulation process is generally applicable, making the sodium/halide flame and encapsulation process a viable one for large-scale synthesis of environmentally insensitive nanopowders.     

Synthesizing of nanocomposite WC/MgO powders by mechanical solid-state reduction and subsequent plasma-activated sintering

A ceramic/ceramic nanocomposite powder of WC/MgO has been fabricated by high-energy ball milling a mixture of elemental Mg and powders of C with WO₃ under an argon gas atmosphere at room temperature. During the early stage of milling (at 1.8 ks), the WO₃ and C powders are embedded into the soft matrix of Mg (the reducing agent) particles to form coarse composite powders of the reactant materials. Increasing the milling time (to 22 ks) leads to the formation of fresh active surfaces of Mg, which have a high reducing potential and react with the WO₃ in a typical oxidation/reduction reaction. At the end of this stage (at 43 ks), the Mg powders are oxidized to MgO, whereas the WO₃ is reduced completely to metallic W. During the last stage of milling (86 to 173 ks), a solid-state reaction takes place between W and the unreacted C powders to yield nanocomposite WC/MgO particles. This end-product was consolidated in vacuum at 1963 K with a pressure of 19.6 to 38.2 MPa for 0.3 ks, using a plasma-activated sintering (PAS) method. The sintered sample is fully dense (above 99.5 pct of the theoretical density) and contains nanocrystalline grains of less than 50 nm in diameter. This fine grain structure offers an opportunity for the composite material to combine high values of two opposite properties, i.e., hardness and fracture toughness (K _c ), of 15 GPa and 14 MPa √m, respectively. Here, we propose this nanocomposite material for a wide range of industrial applications, including tips for cutting tools and tips for oil drilling equipment.     

Synthesis of ductile titanium-titanium boride (Ti-TiB) composites with a beta-titanium matrix: The nature of TiB formation and composite properties

The study focused on the in-situ synthesis of titanium (Ti)-titanium boride (TiB) composites with β phase in the matrix by reaction sintering of TiB₂ with Ti and alloying element powders. The goal was to examine the nature of TiB whisker formation in three different kinds of powder mixtures: (1) β-Ti alloy powders and TiB₂; (2) α-Ti powder, a master alloy (Fe-Mo) powder containing the β-stabilizing elements, and TiB₂; and (3) α-Ti powder, a β-stabilizing elemental powder (Mo or Nb), and TiB₂. The effects of powder packing and the relative locations of powder particles on the morphological changes in TiB whisker formation and their growth were studied at processing temperatures ranging from 1100°C to 1300°C. The morphology, size, and distribution of whiskers were found to be influenced by the powder-packing conditions. A large particle-size ratio in bimodally packed mixtures led to the formation of a TiB monolithic layer around β grains. With a relatively finer starting powder, smaller size ratio, and trimodal packing arrangement, the TiB whiskers were found to be distributed more homogeneously in the matrix. The study also used the X-ray direct comparison method and the structure factor for the β phase to determine the volume fraction of TiB phase from X-ray data. Tensile tests and fractographic investigations were carried out on selected composites. The evolution of the composite microstructure, the influence of powder-packing variables, and the morphology and growth of TiB whiskers and their effect on mechanical properties are discussed.