Synthesis of ultrafine particles of intermetallic compounds by the vapor-phase magnesium reduction of chloride mixtures: Part I. Titanium aluminides

A new chemical synthesis process for the preparation of intermetallic compounds has been developed. It involves the vapor-phase reduction of mixtures of constituent metal chlorides by magnesium vapor to produce intermetallic compounds in the form of fine powder. The advantages of the process include (a) the use of inexpensive raw materials, (b) low reaction temperatures, and (c) products in the form of fine particles. Part I describes the synthesis of titanium aluminide particles by this method, whereas Part II presents the synthesis of nickel aluminides particles. Although nickel aluminides can also be prepared by the hydrogen reduction of nickel chloride and aluminum chloride vapor mixtures, titanium aluminides cannot be produced by hydrogen reduction because of unfavorable thermodynamics. The effect of AlCl₃/TiCl₃ partial pressure ratio on the formation of different titanium aluminides was studied. A two-phase mixture containing 80 mol pct of TiAl+20 mol pct of TiAl₃ formed at an AlCl₃/TiCl₃ ratio of 10. The amount of TiAl₃ was maximized to 72 mol pct at an AlCl₃/TiCl₃ ratio of 16. The maximum conversion of the limiting chloride TiCl₃ was 94 pct. The product particles were very fine in the size range of 0.2 to 0.3 μm.     

Titanium powder prepared by magnesiothermic reduction of Ti2+ in molten salt

A process to produce titanium powder from Ti²⁺ in the molten salt was confirmed experimentally. It consists of two steps. In the first step, titanium (IV) chloride gas dissolves in the molten salt as the titanium (II) chloride by reacting chemically with the metallic titanium or magnesium. In the second step, this molten salt is exposed to the metallic reductant Mg for the final reduction to the metallic Ti powder. Experimentally, the feed of TiCl₄ gas and the subsequent reaction with Ti prepared the molten salt containing 5 to 10 mass pct Ti²⁺. By Mg reduction of this salt, the well-isolated fine Ti particles were recovered. The powder morphology and particle size depended on the Ti²⁺ concentration, reduction temperature, time, and concentration of the by-product MgCl₂. The stirring by argon gas bubbling effectively grew the fine and round particles to a few tens of microns in size.     

Titanium powder production by TiCl4 gas injection into magnesium through molten salts

A process to produce titanium powder continuously is proposed and its applicability is examined experimentally. The method is based on the chemical reaction in the conventional Kroll reduction process; however, TiCl₄ gas is injected into molten salt on which a molten magnesium layer is floated as the reductant. Bubbles of gaseous TiCl₄ can be reacted at the lower surface of the liquid Mg layer, while TiCl₄ gas reacts on the upper surface in the Kroll process. The fine Ti particles produced in this study were well separated from magnesium and could be recovered from the bottom of the molten salts. The particles were small and fine enough for use in powder metallurgy, while congregated lumps of about 20 μm in size are obtained by the Kroll process. The composition of molten salts and an operation temperature above 1073 K did not affect the morphology of the Ti particles, if suitable material for the reaction vessel was chosen.     

In Situ Observation and Investigation of Mold Flux Crystallization by Using Double Hot Thermocouple Technology

The crystallization processes of mold fluxes for casting low-carbon (LC) and medium-carbon (MC) steels were investigated by using double hot thermocouple technology (DHTT) in this article. The results showed that the glass phase was first formed at the cold side thermocouple (CH-2), when the LC mold flux (mold flux for casting low-carbon steel) was exposed to the temperature gradient of 1773 K (1500 °C) to 1073 K (800 °C); then, the fine crystals were precipitated at the liquid/glass interface and grew toward glass and later on to liquid phase. However, the crystals were directly formed at CH-2 when MC flux (mold flux for casting medium-carbon steel) was under the same thermal gradient. The growth rate of MC flux crystals was much faster than that of LC ones. Scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS) analyses suggested that the crystals formed in LC mold flux were mainly dendritic cuspidine Ca₄Si₂O₇F₂, and the crystals formed from the liquid phase were larger than those from the glass. For MC mold flux, the earlier precipitated crystals were large dendritic Ca₄Si₂O₇F₂, whereas the later ones were composed of equiaxed Ca₂Al₂SiO₇ crystals. The results of DHTT measurements were consistent with the time-temperature-transformation (TTT) diagrams and X-ray diffraction (XRD) analysis.     

Deformation Behavior and Damage Evaluation in a New Titanium Diboride (TiB2) Steel‐Based Composite

Deformation behavior and damage evaluation of a new composite steel has been investigated by means of in situ three‐point bend tests in the scanning electron microscope. The titanium diboride (TiB₂)‐reinforced steel composite is produced by in situ precipitation of the TiB₂ particles during eutectic solidification. This production process developed by ArcelorMittal leads to a steel composite with a significant increase in specific stiffness (>20%), and good strength/ductility compromise. The microstructures obtained consist of primary TiB₂ crystals surrounded by a eutectic mixture of ferrite and TiB₂ particles. The primary mode of damage is particle fracture and inhomogeneous plastic deformation in the matrix. In contrast with other production process, particle fracture was more common than interfacial debonding indicating that interfacial strength is not the limiting factor in damage accumulation and fracture in this composite. Crack growth occurred by particle fracture ahead of the crack tip, producing large microvoids, which then link up to the growing crack by ductile failure of the remaining matrix ligaments. The results suggest also that the cracks tended to avoid direct particle interactions.     

Development of Al-Ti-C grain refiners containing TiC

Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A1₄C₃ and Ti₃AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB₂ commercial grain refiner. TiC particles introducedvia the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals. On leave from Regional Research Laboratory (CSIR), Bhopal, is Research Associate.     

Chlorination Kinetics of Titanium Nitride for Production of Titanium Tetrachloride from Nitrided Ilmenite

The kinetics of chlorination of titanium nitride (TiN) was investigated in the temperature range of 523 K to 673 K (250 °C to 400 °C). The results showed that the extent of chlorination slightly increased with increasing temperature and decreasing particle size of titanium nitride at constant flow rate of N₂-Cl₂ gas mixture. At 523 K (250 °C), the extent of chlorination was 85.6 pct in 60 minutes whereas at 673 K (400 °C), it was 97.7 pct investigated by weight loss measurement and confirmed by ICP analyses. The experimental results indicated that a shrinking unreacted core model with mixed-control mechanism governed the chlorination rate. It was observed that the surface chemical reaction of chlorine gas on the surface of TiN particles was rate controlling in the initial stage and, during later stage, internal (pore) diffusion through the intermediate product layer was rate controlling step. Overall the process follows the mixed-control model incorporating both chemical reaction and internal diffusion control. The activation energy for the chlorination of TiN was found to be about 10.97 kJ mol^?1. In processing TiCl₄ from TiN and TiO_0.02C_0.13N_0.85, the solids involved in the chlorination process were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectrometer (EDX). The SEM/EDX results demonstrated the consumption of TiN particles with extent of chlorination that showed shrinking core behavior.     

Understanding the Effects of CoAl2O4 Inoculant Additions on Microstructure in Additively Manufactured Inconel 718 Processed Via Selective Laser Melting

The effect of varying amounts of CoAl₂O₄ inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl₂O₄ resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl₂O₄ content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al₂O₃ particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl₂O₄ as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

     

Increasing the filtration rate of phospho-gypsum using surfactant

Phospho-gypsum is a by-product of processing phosphate rock for phosphoric acid production by acidulation with sulfuric acid. The size distribution of phosphor-gypsum is a major factor for the economics of the process as it greatly affects the filterability of the acid. Surface active agents proved to be very effective additive for growth of gypsum crystals. Two phosphate concentrates, one from Egypt (New Valley) and the other from USA (South Florida) were tested for phosphoric acid production with modification of gypsum crystals using non-ionic surfactant (CMR-100) containing a mixture of C₆–C₂₂ sorbitan esters. The studies were carried out using a semi-continuous laboratory-scale unit simulating the dihydrate process conditions.The mean diameter of the phospho-gypsum crystals increases in the presence of surfactant for both phosphate concentrates. The surfactant leads to decreasing the viscosity and modification of gypsum crystals through decreasing the fine size fractions and attainment of large and uniform crystal shape. The mean diameter of New Valley phosphor-gypsum increases from about 30.0 μm to 36.6 μm while the mean diameter of South Florida phospho-gypsum increases from about 30.3 μm to about 38.4 μm. On the other hand, the average surface area of both New Valley and South Florida phosphor-gypsum were decreased from 4461 and 8069 cm²/g without surfactant to 3284 and 3995 cm²/g with surfactant, respectively. In addition, the surfactant leads to an increase in P₂O₅ recovery of 1–2%, which consequently improves plant productivity.     

Strengthening of steel by the method of spraying oxide particles into molten steel stream

A new technique, Spray-Dispersion Method, which produces a steel containing homogeneously dispersed fine oxide particles sprayed from outside into the molten steel stream has been developed. The conditions for the distribution of particles in solid steel, and the mechanical properties of A1₂O₃- or ZrO₂-dispersed steels were studied. The homogeneous distribution of fine oxide particles is obtained by the addition of a certain suitable controlling element which lowers the contact angle of molten steel on various oxides and the interfacial tension at the oxide—molten steel interface. Among others columbium (niobium) was found to be the most effective on decreasing the average particle size of sprayed oxides. Because of fine dispersion of particles of less than 120 nm, the strength of steels increased with their volume fractions; for example, 1.15 vol pct ZrO₂ causes an increment of 79 MN/m² in proof strength and 94 MN/m² in tensile strength. For practical applications, the Spray Dispersion Method makes it possible to produce 18 chromium-8 nickel austenitic stainless steels dispersion-strengthened by A1₂O₃ or ZrO₂ particles, and ThO₂-dispersed nickel with an average particle size of 61.4 nm and a volume fraction of 5.1 pct.     

Effect of TiN particles and microstructure on fracture toughness in simulated heat-affected zones of a structural steel

Thermally stable TiN particles can effectively pin austenite grain boundaries in weld heat-affected zones (HAZs), thereby improving toughness, but can also act as cleavage initiators. The HAZs simulated in a GLEEBLE 1500 TCS using two peak temperatures (T _p ) and three cooling times (Δ∼ _8/5) have determined the effects of matrix microstructure and TiN particle distribution on the fracture toughness (crack tip opening displacement (CTOD)) of three steels microalloyed with 0.006, 0.045, and 0.1 wt pct Ti. Coarse TiN (0.5 to 6 μm) particles are identified in steels with the two higher levels of Ti, and fine Ti(C, N) (35 to 500 nm) particles were present in all three steels. Large prior austenite grain size caused by higher T _p decreased fracture toughness considerably in steels containing coarse TiN particles but had little effect in their absence. Fracture toughness was largely independent of matrix microstructure in the presence of coarse particles. Cleavage fracture initiation was observed to occur at coarse TiN particles in the samples with a large prior austenite grain size. Alloy thermodynamics have been used to rationalize the influence of Ti content on TiN formation and its size.     

Production of Titanium Powder by Sodiothermic Reduction in CaCl2 Molten Salts

A new sodiothermic reduction process of TiO₂ in CaCl₂ melt was proposed aimed at fine Ti powder preparation. The chemical analysis and direct potentiometric methods were used to investigate the reaction pathway of sodiothermic reduction in CaCl₂ melt. The as-prepared samples were characterized by X-ray diffraction and scanning electron microscopy. It was found that when reductant of Na was added into the CaCl₂ melt, Ca²⁺ was reduced to Ca by Na and Ca dissolved in the CaCl₂ melt. The whole melt would have the reducing power with dissolved Ca. Using this melt as a reaction medium, fine and uniform Ti powder with a purity of around 99 mass pct was successfully produced at 1173 K (900 °C). In addition, as the CaCl₂ melt could dissolve about 20 mol pct CaO, it was found that the molar ratio of TiO₂ and CaCl₂ should be 1:20 to eliminate the by-product CaO from the reaction interface within the experimental period to continue the reduction.     

Flux Entrapment and Titanium Nitride Defects in Electroslag Remelting of INCOLOY Alloys 800 and 825

Electroslag remelted (ESR) ingots of INCOLOY alloys 800 and 825 are particularly prone to macroscale slag inclusions and microscale cleanliness issues. Formation of these structures near the ingot surface can cause significant production yield losses (~10 pct) due to the necessity of extensive surface grinding. Slag inclusions from near the outer radius of the toe end of alloy 800 and 825 ingots were found to be approximately 1 to 3 mm in size and have a multiphase microstructure consisting of CaF₂, CaTiO₃, MgAl₂O₄, MgO, and some combination of Ca₁₂Al₁₄O₃₂F₂ and/or Ca₁₂Al₁₄O₃₃. These inclusions were often surrounded by fields of 1- to 10-μm cuboidal TiN particles. A large number of TiN cuboids were observed in the ESR electrode with similar size and morphology to those observed surrounding slag inclusions in the ESR ingots, suggesting that the TiN particles are relics from the ESR electrode production process. Samples taken sequentially throughout the AOD processes showed that the TiN cuboidals that are found in ESR ingots form between tapping the AOD vessel into the AOD ladle and the casting of ESR electrodes.     

Mechanical properties of laser-deposited composite boride coating using nanoindentation

Nanoindentation proves to be an effective technique to measure mechanical properties of “composite” materials, as it has high spatial resolution that enables estimation of properties even from fine grains, particles, and precipitates. The elastic modulus, E, of the composite boride coating deposited on AISI 1010 steel using the laser surface engineering (LSE) process has been computed using the nanoindentation technique. The highest E value of 477.3 GPa was achieved for coating in a sample that contained 0.69 volume fraction of TiB₂ particles in the coating after processing with the highest laser traverse speed of 33 mm/s. A comparison between the theoretical and experimental computation of the elastic modulus suggests that theoretical elastic modulus values are lower than computed elastic modulus, as the latter includes the effect of dissolution of fine TiB₂ particles in Fe matrix and metastable phase formation such as Fe _a B _b and Ti _m B _n . Dissolution of fine TiB₂ particles in the Fe matrix in the coating region has been corroborated by transmission electron microscope (TEM) micrographs and corresponding energy-dispersive spectroscope (EDS) analysis and selected area diffraction (SAD) pattern.     

Viscosity of a CaO-MgO-Al2O3-SiO2 melt containing spinel particles at 1646K

This article reports an experimental investigation into the effect of solid suspension on the viscosity of molten slags. Up to about 20 vol pct of spinel (MgAl₂O₄) particles of three size ranges (fine: 0.10 to 0.21 mm; medium: 0.21 to 0.44 mm; and coarse: 0.44 to 0.99 mm) were added to a CaO-MgO-Al₂O₃-SiO₂ melt at 1646 (±10)K. A Brookfield DVII+ viscometer was used. The viscosity determined for the solid-free melts was in good agreement with the results of published work. The viscosity for the solid-containing melt was found to increase with the addition of the particles. With more than 10 vol pct solid particles, particularly the fine and the coarse ones, the melt showed an apparent “Bingham” behavior, i.e., the shear stress increased linearly with the shear rate but had a residual shear stress (up to 3 Pa depending on the amount and size of solid added) at zero shear rate. The viscosity of the solid-containing slag, η, was found to fit an Einstein-Roscoe type equation, η=η ₀ (1−af)⁻ⁿ, where η ₀ is the viscosity of the solid-free melt, f is the volume fraction of solid particles in the melt, and a and n are parameters taking the value of 4.24, 3.29, and 3.56 and 1.28, 2.36, and 2.24 for the fine, medium, and coarse particles, respectively, for best fit.     

Investigation of Precipitation Behavior in Age-Hardenable Cu-Ti Alloys by an Extraction-Based Approach

We investigated the precipitation processes in Cu-4 mol pct Ti alloy specimens aged at 723 K (450 °C), by means of X-ray diffraction and chemical analyses of the precipitates extracted from the parent alloy specimens. Aging-induced precipitate particles of a spinodally decomposed disorder, α′; those of a metastable order, β′-Cu₄Ti; and those of a stable order, β-Cu₄Ti, were continuously formed in the aged specimens. The extraction of the precipitate particles from the aged specimens by submergence in a nitric solution allowed for not only the structural analyses of the constituent precipitate phases but also the quantitative evaluation of their chemical compositions and volume fractions. Early during the aging process, the supersaturated Cu solid solution decomposes spinodally in a continuous manner, and an unstable disorder, α′, appears. Then, fine needle-shaped β′-Cu₄Ti particles, which have a Ti content of approximately 37.5 mol pct, form in the Cu matrix. During prolonged aging, coarse cellular components composed of the terminal Cu solid solution and stable β-Cu₄Ti particles which have a Ti content of 20.5 mol pct nucleate and grow, primarily in the grain boundaries, at the expense of the metastable β′-Cu₄Ti particles. The volume fraction of the β′-Cu₄Ti particles in the alloy reaches a maximum of approximately 1.7 pct after aging for 24 hours, while that of the β-Cu₄Ti particles increases steadily to more than 18 pct after 480 hours. The volume fraction of the fine β′-Cu₄Ti particles in the alloy specimens remained constant throughout the age-hardening, indicating that the hardening is primarily owing to the fine dispersion of the β′-Cu₄Ti particles and not because of the large volume fraction of coarse β-Cu₄Ti particles.     

Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering

The dependence of the magnetic properties on the particle size of recycled HDDR Nd-Fe-B powders was investigated,with the aim to assess the reprocessing potential of the end-of-life scrap magnets via spark plasma sintering(SPS).The as received recycled HDDR powder has coercivity(H_(ci))=830 kA/m and particles in the range from 30 to 700 μm(average 220 μm).After burr milling,the average particle size is reduced to 120 μm and subsequently the H_(ci) of fine(milled) powder was 595 kA/m.Spark plasma sintering was exploited to consolidate the nanograined HDDR powders and limit the abnormal grain coarsening.The optimal SPS-ing of coarse HDDR powder at 750℃for 1 min produces fully dense magnets with H_(ci)=950±100 kA/m which further increases to 1200 kA/m via thermal treatment at 750℃for 15 min.The burr milled fine HDDR powder under similar SPS conditions and after thermal treatment results in H_(ci)=940 kA/m.The fine powder is further sieved down from 630 to less than 50 μm mesh size,to evaluate the possible reduction in H_(ci) in relation to the particle size.The gain in oxygen content doubles for 50 μm sized particles as compared with coarser fractions(200 μm).The XRD analysis for fractionated powder indicates an increase in Nd_2O_3 phase peaks in the finer(100 μm)fractions.Similarly,the H_(ci) reduces from 820 kA/m in the coarse particles(200 μm) to 460 kA/m in the fine sized particles(100μm).SPS was done on each HDDR powder fraction under the optimal conditions to measure the variation in H_(ci) and density.The H_(ci) of SPS-ed coarse fraction(200 μm) is higher than 930 kA/m and it falls abruptly to just 70 kA/m for the fine sized particles(100 μm).The thermal treatment further improves the H_(ci) to1000 kA/m only up to 100 μm sized fractions with90% sintered density.The full densification(99%) is observed only in the coarse fractions.The loss of coercivity and lack of sinterability in the fine sized particles(100 μm) are attributed to a very high oxygen content.This implies that during recycling,if good magnetic properties are to be maintained or even increase the HDDR powder particles can be sized down only up to≥100 μm.     

Dynamic deformation behavior of an oxide-dispersed tungsten heavy alloy fabricated by mechanical alloying

The objective of this study is to investigate the dynamic deformation and fracture behavior of an oxide-dispersed (OD) tungsten heavy alloy fabricated by mechanical alloying (MA). The tungsten alloy was processed by adding 0.1 wt pct Y₂O₃ powders during MA, in order to form fine oxides at triple junctions of tungsten particles or at tungsten/matrix interfaces. Dynamic torsion tests were conducted for this alloy, and the test data were compared with those of a conventional liquid-phase sintered (LPS) specimen. A refinement in tungsten particle size could be obtained after MA and multistep heat treatment without an increase in the interfacial area fraction between tungsten particles. The dynamic test results indicated that interfacial debonding between tungsten particles occurred over broad deformed areas in this alloy, suggesting the possibility of adiabatic shear-band formation. Also, oxide dispersion was effective in promoting interfacial debonding, since the fine oxides acted as initiation sites for interfacial debonding. These findings suggest that the idea of forming fine oxides would be useful for improving self-sharpening and penetration performance in tungsten heavy alloys.     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y₂O₃ produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ～0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y₂O₃ powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y₂O₃ added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y₂O₃ interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y₂O₃ lattice and to the formation of submicroscopic particles of (Fe,Al)₅Y₃O₁₂-type oxides.     

Large size rare earth iron garnet single crystals grown by the flux–Bridgman method

Rare earth garnet (R₃Fe₅O₁₂, RIG) single crystals are the most ideal magneto-optical medium for optical isolators for wavelength longer than 1.1 μm, which has been commercially used in optical fiber communications. However, it is still a great challenge to grow large size RIG single crystals. In this work, high-quality Y₃Fe₅O₁₂ (YIG) and Bi_0.9Tb_2.1Fe₅O₁₂ (Bi:TIG) single crystals were successfully grown by the flux–Bridgman method for the first time. The as-grown crystals up to 20 mm in diameter was obtained using the PbO–B₂O₃ flux. The transmittance of YIG crystals is over 75% in the region of 1100–2500 nm. TIG crystals also have good transmittance in the range of 1100–1700 nm, and show typical Tb absorption from 1700 to 2500 nm. The specific Faraday rotations of YIG and Bi_0.9Tb_2.1Fe₅O₁₂ crystals are 185 (°)/cm and –1250 (°)/cm at 1550 nm, which are comparable to the commercial RIG crystals grown by LPE method. The present results indicate that flux–Bridgman method shows great potential to grow large size and high-quality RIG magneto-optical crystals.