Dispersion and reaction of TiB2 in liquid iron alloys

Abstract

The degree of reaction and dispersion achieved when TiB₂ powders are melted in contact with liquid iron based alloys has been assessed via a levitation dispersion test which had been developed earlier. Both Fe₂B and TiC were observed to form as a result of dissolution and reaction of TiB₂. The formation of TiC occurs during the reaction of commercial grade TiB₂ with liquid iron alloys containing as little as 0·08 wt-%C. The reaction of high purity TiB₂ with liquid iron alloys containing 0·24 wt-%C does not however lead to TiC formation. The formation of Fe₂B was observed for all conditions tested, owing to the effectively zero solubility of boron in solid iron. The TiB₂ remaining after dissolution and reaction was found to produce relatively good dispersions in the iron matrix and therefore additions of TiB₂ to liquid iron alloys may provide a means of producing Fe–TiB₂ composite materials. However, the brittle properties of Fe₂B will mean that, whereas such materials may be very hard, they are likely to lack toughness.

MST/1477     

TiB2 and TiC stainless steel matrix composites

Stainless steel matrix composites reinforced with TiB₂ or TiC particulates have been in situ produced through the reactive sintering of Ti, C and FeB. X-ray diffraction analysis confirmed the completion of reaction. The TiB₂, TiC and steel were detected by X-ray diffraction analysis. No other reaction product or boride was found, indicating the stability of TiB₂ and TiC in steel matrix. The SEM micrographs revealed the morphology and distribution of in situ synthesized TiB₂ and TiC reinforcements in steel matrix. During sintering the reinforcements TiB₂ and TiC grew in different shapes. TiB₂ grew in hexagonal prismatic and rectangular shape and TiC in spherical shape.     

In situ reacted TiB2-reinforced mullite

In situ formation of TiB₂ in mullite matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed and pressureless-sintered samples, in addition to TiB₂, TiC was also found to be dispersed phases in mullite matrix. However, in the case of pressurelesssintered samples, mullite/TiB₂ composite with 98% relative density can be obtained through a preheating step held at 1300 °C for longer than 3 h and then sintering at a temperature above 1600 °C. Hot-pressed composite containing 30 vol% TiB₂ gives a flexural strength of 427 MPa and a fracture toughness of 4.3 MPam^1/2. Pressureless-sintered composite containing 20 vol% TiB₂ gives a flexural strength of 384 MPa and a fracture toughness of 3.87 MPam^1/2.     

Self-propagating high temperature synthesis and dynamic compaction of titanium diboride/titanium carbide composites

Self-propagating High-temperature Synthesis (SHS) of titanium and boron carbide (B₄C) combined with explosively driven Dynamic Compaction (DC) was employed for the fabrication of composite TiB₂/TiC compacts. A 2³ factorially designed experiment set was used to examine the effects of the TiB₂/TiC ratio, delay time and C/M ratio on the consolidation and properties of the compacts. The delay time is the time between completion of the SHS reaction and compaction. The C/M ratio, the ratio of the explosive mass to that of the flyer plate, influences the pressure applied to the samples during compaction. Composites with molar TiB₂/TiC ratios of 2:1 or 1:2 were prepared using Ti and B₄C or Ti, C and B₄C, respectively, as reactants. The SHS/DC of Ti and B₄C resulted in high quality, near fully dense TiB₂/TiC composite compacts. Under best conditions, the densities were greater than 98% of the theoretical maximum. While the microhardness and densities of the compacts with TiB₂/TiC ratio of 2:1 were comparable to those of monolithic TiB₂ and TiC, compacts with TiB₂/TiC ratios of 1:2 were poorly consolidated and contained extensive cracks. Given the high energy and time efficiency, high product quality and inexpensive reactants, the SHS/DC of Ti and B₄C represents an attractive technique for the economical fabrication of TiB₂/TiC composites.     

Fabrication of (TiB<Subscript>2</Subscript> − TiC)<Subscript>p</Subscript>/AZ91 magnesium matrix hybrid composite

Magnesium MMCs reinforced with TiB₂−TiC particulates were fabricated successfully via a master alloy route using a low cost Al-Ti-B₄C system as starting material system. Microstructural characterization of the (TiB₂−TiC)/AZ91 composite shows relatively uniform distribution of TiB₂ and TiC particulates in the matrix material. Moreover, the results show that the hardness and wear resistance of the composites are higher than those of the unreinforced AZ91 alloy.     

An experimental study on synthesizing TiC-TiB2-Ni composite coating using electro-thermal explosion ultra-high speed spraying method

The synthesis of TiC-TiB₂-Ni composite coating on steel substrate using electro-thermal explosion ultra-high speed spraying (EEUSS) method has been investigated. The coating exhibits a compact microstructure and good interface bonding. TiC-TiB₂-Ni coating consists of TiC, TiB₂, Ni, as well as the residual C. Microstructure characterization reveals micro-sized clubbed TiB₂ particles with the length of 5-10 µm and submicron-sized spherical TiC particles with the average diameter of 1 µm and these different morphologies are due to their respective crystal structure. The average hardness value for TiC-10TiB₂-Ni and TiC-20TiB₂-Ni are HV0.3 1800 and HV0.3 2000.     

Fabrication of TiB2/TiC composites by the directional reaction of titanium with boron carbide

Dense TiB₂/TiC composites were fabricated by the directional reaction of molten titanium with boron carbide preform. The reaction between pure molten titanium and boron carbide preform could not progress due to reaction choking. However, when a few weight per cent of nickel were added in the titanium, the reaction progressed continuously and resulted in TiB₂/TiC composites. A gradient of grain sizes was observed in the reaction products. The processing temperature affected the microstructure of the reaction products rather than the reaction rate. The degree of grain-size gradient in the reaction product increased with the processing temperature.     

In situ reacted TiB2-reinforced alumina

In situ formation of TiB₂ in Al₂O₃ matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed samples, in addition to TiB₂, TiC and Al₂TiO₅ were also found to be dispersed phases in Al₂O₃ matrix. However, in the case of pressureless-sintered samples, pure Al₂O₃/TiB₂ composite with > 99% relative density can be obtained through a preheating step held at 1300°C for longer than 30 min and then sintering at a temperature above 1500°C. Pressureless-sintered composite containing 20vol% TiB₂ gives a flexural strength of 580 MPa and a fracture toughness of 7.2 MPa m^1/2.     

The microstructure and mechanical properties of TiC and TiB2-reinforced cast metal matrix composites

TiC and TiB₂ particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB₂ and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC and TiB₂ particle additions are greater than those for Al₂O₃ and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB₂ systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB₂-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.     

Microstructural study and densification analysis of hot work tool steel matrix composites reinforced with TiB2 particles

Hot work tool steels are characterized by good toughness and high hot hardness but are less wear resistant than other tooling materials, such as high speed steel. Metal matrix composites show improved tribological behavior, but not much work has been done in the field of hot work tool steels. In this paper TiB₂-reinforced hot work tool steel matrix composites were produced by spark plasma sintering (SPS). Mechanical alloying (MA) was proposed as a suited process to improve the composite microstructure. Density measurements and microstructure confirmed that MA promotes sintering and produces a fine and homogeneous dispersion of reinforcing particles. X-ray diffraction patterns of the sintered composites highlighted the formation of equilibrium Fe₂B and TiC, as predicted by thermodynamic calculations using Thermo-Calc® software. Scanning electron microscopy as well as scanning Kelvin probe force microscopy highlighted the reaction of the steel matrix with TiB₂ particles, showing the formation of a reaction layer at the TiB₂-steel interface. Phase investigations pointed out that TiB₂ is not chemically stable in steel matrix because of the presence of carbon even during short time SPS.     

The mechanism of thermal explosion (TE) synthesis of TiC-TiB2 particulate locally reinforced steel matrix composites from an Al-Ti-B4C system via a TE-casting route

TiC-TiB₂ particulate locally reinforced steel matrix composites were fabricated by a novel TE-casting route from an Al-Ti-B₄C system with various B₄C particle sizes. The formation mechanism of TiC and TiB₂ in the locally reinforced regions was investigated. The results showed that TiC and TiB₂ are formed and precipitated from Al-Ti-B-C melt resulting from the dissociation of B₄C into Al-Ti melt when the concentrations of B and C atoms in the Al-Ti-B-C melt become saturated. However, in the case of coarse B₄C powders (≥40 μm) used, the primary reaction in the Al-Ti-B-C melt is quite limited due to the poor dissociation of B₄C. The poured steel melt infiltrates into the primary reaction product and thus leads to the formation of Al-Fe-Ti-B-C melt, thanks to the favorable reaction of molten Fe with remnant B₄C, and then TiC and TiB₂ are further formed and precipitated from the saturated Al-Fe-Ti-B-C melt. The relationship between the mechanisms of thermal explosion (TE) synthesis of TiC and TiB₂ in the electric resistance furnace and during casting was proposed.     

Structure and mechanical properties of TiB2/TiC – Ni composites fabricated by pulse plasma sintering method

TiB₂/TiC – Ni composites were synthesized starting from the powders of Ti, B₄C and Ni, using Pulse Plasma Sintering (PPS) method. Typically used one-step (1100?°C–10?min.) and novel double-step sintering processes (900?°C–10?min. +1100?°C–5?min.) were applied and compared. XRD studies showed that the composite obtained by double-step sintering consisted of TiB₂, TiC and Ni phases. For one-step processing additionally undesired Ni₃B and graphite were detected. SEM observations revealed dark-grey grains of TiB₂, light-grey grains of TiC (both around 25?µm in size) and Ni areas surrounded by TiC. The composites synthesized in one- and double-step processes revealed the hardness and relative density of 2335 HV5 (±110) and 97.8% and 2470 HV5 (±70) and 99.8%, respectively. Novel double-step sintering process allowed to avoid undesired phases (graphite, Ni₃B) and only TiB₂, TiC and Ni were present in the structure. Additionally it was possible to decrease the temperature of the process comparing to other fabrication methods.     

Understanding the reaction mechanism of in-situ synthesized (TiC–TiB2)/AZ91 magnesium matrix composites

Magnesium matrix composites reinforced with a network of TiC and TiB₂ compounds have been successfully synthesized via an in-situ reactive infiltration technique. In this process, the ceramic reinforcing phases, TiC and TiB₂, were synthesized in-situ from the starting powders of Ti and B₄C without any addition of a third metal powder such as Al. The molten AZ91 magnesium alloy infiltrates the preform of 3Ti–B₄C by capillary forces. Furthermore, adding different weight percentages of MgH₂ powder to the 3Ti–B₄C preforms was used in an attempt to increase the Mg content in the fabricated composites. The results reveal a relatively uniform distribution of the reinforcing phases in the magnesium matrix with very small amounts of residual Ti, boron carbide and intermediate phases when they are fabricated at 900 °C for 1.5 h using a 3Ti–B₄C preform with 70% relative density. On the other hand, after adding MgH₂ to the 3Ti–B₄C preform, TiC_x and TiB₂ formed completely without any residual intermediate phases with the formation of the ternary compound (Ti₂AlC) at the expense of TiC. The percentage of reinforcing phases can be tailored by controlling the weight percentages of MgH₂ powder added to the 3Ti–B₄C preform. The results of the in-situ reaction mechanism investigation of the Ti–B₄C and Mg–B₄C systems show that the molten magnesium not only infiltrates through the 3Ti–B₄C preform and thus densifies the fabricated composite as a matrix metal, but also acts as an intermediary making the reaction possible at a lower temperature than that required for solid-state reaction between Ti and B₄C and accelerates the reaction rate. The investigation of the in-situ reaction mechanism with or without the addition of MgH₂ powder to the 3Ti–B₄C preforms reveals similar mechanisms. However, the presence of the MgH₂ in the preform accelerates the reaction resulting in a shorter processing time for the same temperatures.     

The effect of processing parameters in the carbothermal synthesis of titanium diboride powder

The mechanism of the carbothermal method for synthesizing titanium diboride (TiB₂) powder has been studied. Mixtures of TiO₂, H₃BO₃ and carbon were heated in an argon atmosphere at 1000–1600 °C. The effect of the molar ratio and holding time on the phase evolution was studied by X-ray diffraction. The products were also characterized by scanning electron microscopy observations and particle size measurements.For a composition with a molar ratio of TiO₂:H₃BO₃:C = 1:2.4:5 heated for 1 h, the simultaneous presence of TiC and TiB₂ phases at 1100 °C and the transformation of TiO₂ to Ti₂O₃ at 1200 °C and higher confirms that TiB₂ synthesis is based on a TiC formation mechanism, in which TiC may be formed from a reaction between TiO₂ or Ti₂O₃ and carbon. Then TiC may react with liquid B₂O₃ and/or gaseous B₂O₂ to form the TiB₂ phase. The reaction is completed at 1500 °C. Also by increasing the molar ratio of boric acid to 3, the impurities decreased considerably and pressing of the material had an obvious effect on decreasing the impurities, due to an increase of the surface contact of particles, which causes an effective inhibition of boron escape from the reaction chamber. Under these experimental conditions, a relatively narrow size distribution of TiB₂ particles was produced. When the reaction time increased to 1.5–2 h, grain growth of particles occurred. Therefore, a wider distribution of particle size was obtained.     

In-Situ TiB2–TiC–Al2O3 Composite Coating on Aluminum by Laser Surface Modification

To improve the surface hardness of aluminum, in-situ TiB₂–TiC–Al₂O₃ composite coating was deposited on it by pre-placed laser coating process using precursor mixture of (TiO₂ + B₄C) and (TiO₂ + B₄C + Al). Pulsed Nd:YAG laser was used to produce coating track by scanning a laser beam in overlapped condition. Multiple tracks again overlapped to get a wider coating area. Phase constituents and microstructure of the deposited coating were studied by XRD and FESEM analysis. Vickers micro-hardness tester was used to measure micro-hardness of the coating. Results indicate that, in appropriate laser processing condition, coating was obtained with metallurgical bonding to aluminum substrate. XRD and microstructure analysis confirms the formation of TiB₂, TiC, and Al₂O₃ in the coating layer through in-situ reaction of reactant powders. Micro-hardness of the coating was found appreciably higher in comparison to the as-received aluminum substrate, due to presence of hard ceramic particles produced during in-situ reaction and their grain refinement for rapid cooling.     

Fabrication of Co-Based Coatings on Titanium Alloy by Laser Cladding with CeO2 Addition

In this study, Co-based laser cladding coatings reinforced by multiple phases were fabricated on titanium alloy. Co42 Co-based self-fluxing alloy, B₄C, and CeO₂ mixed powders were used as the precursor materials. The coatings were mainly composed of γ-Co/Ni, CoTi₂, CoTi, NiTi, TiC, Cr₇C₃, TiB₂, and TiB phases. A typical TiB₂/Cr₇C₃/TiC composite structure was chosen. It was found that CeO₂ did not influence the phase types of the coating significantly, but was effective in refining the microstructure and enhancing the microhardness and dry sliding wear resistance. Compared with the Ti-6Al-4V titanium alloy, the microhardness and wear resistance of the composite coatings were enhanced by 3.44–4.21 times and 14.26–16.87 times, respectively.     

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.     

Structural and mechanical properties of TiB2 and TiC prepared by self-propagating high-temperature synthesis/dynamic compaction

Titanium-diboride and titanium-carbide compacts with diameters of 100 mm and thicknesses of 25 mm were fabricated by self-propagating high-temperature synthesis/dynamic compaction (SHS/DC) of the elemental powders. Under the best conditions, the densities were greater than 99% and 96.8% of the theoretical densities for TiB₂ and TiC, respectively. The microhardness, compressive strength, and elastic modulus of the TiB₂ prepared by the SHS/DC method were comparable to reported values for hot-pressed TiB₂. While the microhardness and elastic modulus of the TiC compacts were comparable to those for hotpressed TiC, the compressive strength was lower due to extensive cracks in the compacts. The TiB₂ prepared using a low-purity boron powder (1–5% carbon impurity) compacted to higher densities and had less cracking than that prepared using a high-purity boron powder (0.2% carbon). This result could have an impact on the cost of producing TiB₂/TiC structural components by the SHS/DC process.     

Effect of different reinforcements on composite-strengthening in aluminium

In the development of metal-matrix composites, reinforcements of aluminium and its alloys with ceramic materials has been pursued with keen interest for quite sometime now. However, a systematic comparison of the effect of different reinforcements in powder-processed aluminium and its alloys is not freely available in the published literature. This study examines the influence of SiC, TiC, TiB₂ and B₄C on the modulus and strength of pure aluminium. B₄C appears slightly superior as a reinforcement when comparing the effect of SiC, TiC, B₄C and TiB₂ on specific modulus and specific strength values of composites. However, TiC appears to be a more effective reinforcement, yielding the best modulus and strength values among those considered in this study. The differences in thermal expansion characteristics between aluminium and the reinforcements do not seem to explain this observation. The other advantage of TiC is that it is economically a more viable candidate as compared to B₄C and TiB₂ for reinforcing aluminium alloys. It is suggested that the superior effect of TiC as a reinforcement is probably related to the high integrity of the bond at the Al-TiC interface.     

Plasma-sprayed coatings for fusion reactor applications

A series of plasma-sprayed coatings has been given a preliminary evaluation to assess the potential of this class of materials in fusion reactor applications. TiC, TiB₂, beryllium and VBe₁₂ coatings on copper and stainless steel (SS) were tested for coating adherence, ion erosion resistance and susceptibility to arc erosion. The coatings, in general, display a good resistance to thermal shock failure. The TiC and TiB₂ coatings exhibit favorable ion erosion characteristics and the resistance of the coatings to arc erosion was, in general, superior to that of SS.