Effects of powder processing on densification of titanium diboride based cermets

Abstract

The sinterability of TiB₂-Ni₃(Al,Ti) based cermets has been significantly improved by aggressive milling of the starting TiB₂-Ni-TiAl₃ powder mixtures. This technique improves not only liquid spreading by reducing TiAl₃ particle size but also eliminates alumina agglomerates and the associated porosity found after vacuum sintering. Liquid phase sintering of TiB₂-Ni-TiAl₃ powder mixtures involves the presence of Ni based secondary borides at low temperatures (1200°C), which react afterwards with TiAl₃ particles leading to the formation of the final TiB₂-Ni₃(Al,Ti) eutectic liquid. Apart from improving liquid spreading around TiB₂ grains, aggressive milling is also found to disperse alumina agglomerates, which reduces the porosity associated to these particles. By this refined procedure, the amount of binder phase required for full densification of TiB₂ cermets by sinter hipping has been reduced from a previous limit of 16 vol.-% to 10 vol.-%. The hardness of these TiB₂-10 vol.-%Ni₃(Al,Ti) cermets is in the range of ultrafine WC-Co hardmetals in spite of their much coarser microstructure.     

The Interface of TiB<Subscript>2</Subscript> and Al<Subscript>3</Subscript>Ti in Molten Aluminum

In the grain refinement of aluminum, Al₃Ti and TiB₂ particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB₂ particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al₃Ti particles in the master alloys. The titanium in solution either forms Al₃Ti layers on the surface of TiB₂ particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB₂/Al₃Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al₃Ti on the surface of TiB₂ particles, a sign of the formation of an Al₃Ti layer on the surface of TiB₂ particles in grain refinement practice. The Al₃Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al₃Ti and TiB₂ particles leading to the loss of efficiency in grain refinement.     

Development of TiB<Subscript>2</Subscript> reinforced in-situ Ti aluminide matrix composite through reaction synthesis

TiB₂ reinforced in-situ titanium aluminide matrix composite was made through reaction synthesis process using high purity elemental powders of Ti, Al, Cr, Nb and B. XRD of the synthesized block showed presence of mainly Al₃Ti and TiB₂ phases. To obtain γ Ti aluminide based matrix, the material was homogenized in two phase region (α₂+γ). Presence of γ phase matrix alongwith α₂ was confirmed through XRD, SEM and TEM. Uniform distribution of TiB₂ phase was confirmed through elemental mapping and by analyzing specimens of different locations. Differential scanning calorimetry of powder mixture showed presence of endothermic peak for Al melting and exothermic peak of Ti aluminide and TiB₂ formation.     

The Fabrication and Characterization of a TiB<Subscript>2</Subscript>/Ni Composite Using Spark Plasma Sintering

Ni matrix composites synergistically reinforced by TiB₂ particles were prepared by spark plasma sintering. According to the Nelsone Riley method and Debye–Scherrer formula, the driving force for the growth of crystallite in sintered TiB₂/Ni composites was discussed by using X-ray diffraction technique to analyse the dislocation density and lattice strain of composite powders. TiB₂/Ni composites not only showed great increase in tensile strength but also possessed perfect ductility. Tensile results showed that Ni–3TiB₂ (3 vol% TiB₂) showed an increase in tensile strength, furthermore, contained an acceptable elongation of ~?45%. Besides, the strengthening mechanism was discussed later. The corrosion resistance test of TiB₂/Ni composites was performed by electrochemical method in 3.5 wt% NaCl solution. Results showed that the corrosion resistance of pure Ni sample was superior to that of other specimens. SEM of tensile fracture revealed that debonding phenomenon between TiB₂ and Ni was hindered.     

Thermodynamic effect of alloying addition on <Emphasis Type="Italic">in-situ</Emphasis> reinforced TiB<Subscript>2</Subscript>/Al composites

From the viewpoint of thermodynamics, using the Wilson equation and an extended Miedema model, the effect of the alloying element on the stability of the precipitated phases during the fabrication of in-situ reinforced TiB₂/Al composites was evaluated. The result shows that additions of alloying elements, such as Mg, Cu, Zr, Ni, Fe, V, and La, can promote the formation of Al₃Ti and TiB₂ phases. Particularly, Zr has the most pronounced effect among these alloying elements. In addition, alloying elements can hinder the formation of AlB₂ to a small extent. The calculation results also show that it is easier for magnesium to react with the salts to form TiB₂ than aluminum during the fabrication of in-situ reinforced TiB₂/Al using the flux-assisted synthesis (FAS) technology.     

Quantitative characterization of the microstructure of two-phase TiB<Subscript>2</Subscript>+Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramics using mean integral curvature

Two-phase TiB₂+Al₂O₃ ceramics with an interconnected or dispersed TiB₂ (minor)-phase microstructure can be produced by variations in processing parameters. A standard method of quantitative characterization of the microstructural bias, i.e., the degree of TiB₂ phase connectivity relative to its dispersion, is necessary to comprehend the mechanism(s) controlling the evolution of microstructure during processing. In this work, techniques derived from stereology were used to quantitatively characterize the microstructural bias on the basis of the connectivity and dispersion of the minor phase (TiB₂), in addition to the size of the TiB₂- and Al₂O₃-phase regions. The mean integral curvature calculated using the area particle-count and area tangent-count methods was determined to quantitatively describe the connectivity of the TiB₂ minor phase around the Al₂O₃ major phase. The results illustrate that, in spite of partial and mixed bias, integral curvature measurements (particularly those based on the area tangent-count method) provide a reliable and reproducible means for quantitative characterization of the two-phase biased microstructure.     

Reaction synthesis of the Ti<Subscript>2</Subscript>AlN MAX-phase

The Ti₂AlN MAX-phase is synthesized by reaction sintering from Ti–AlN powder mixtures. The optimal synthesis mode of the compound containing less than 1% of the TiN impurity phase is established. It includes isothermal annealing at 1300°C for 2 h in argon at a pressure of 3 atm. The influence of the preliminary mechanical activation of the powder mixture and the reaction synthesis medium on the yield of the Ti₂AlN phase are investigated. It is shown that the activation leads to an increase in the content of the secondary TiN phase. It is revealed that the vacuum synthesis also does not make it possible to form the singlephase Ti₂AlN material.     

Development of Wear-Resistant Cu-10Cr-3Ag Electrical Contacts with Nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Dispersion by Mechanical Alloying and High Pressure Sintering

Cu-10Cr-3Ag (wt pct) alloy with nanocrystalline Al₂O₃ dispersion was prepared by mechanical alloying and consolidated by high pressure sintering at different temperatures. Characterization by X-ray diffraction and scanning electron microscopy or transmission electron microscopy shows the formation of nanocrystalline matrix grains of about 40 nm after 25 hours of milling with nanometric (<20 nm) Al₂O₃ particles dispersed in it. After consolidation by high pressure sintering (8 GPa at 400 °C to 800 °C), the dispersoids retain their ultrafine size and uniform distribution, while the alloyed matrix undergoes significant grain growth. The hardness and wear resistance of the pellets increase significantly with the addition of nano-Al₂O₃ particles. The electrical conductivity of the pellets without and with nano-Al₂O₃ dispersion is about 30 pct IACS (international annealing copper standard) and 25 pct IACS, respectively. Thus, mechanical alloying followed by high pressure sintering seems a potential route for developing nano-Al₂O₃ dispersed Cu-Cr-Ag alloy for heavy duty electrical contact.     

Solid-state reactions during heating mechanically milled Al/TiO<Subscript>2</Subscript> composite powders

Solid-state reactions between Al and TiO₂ during heating high-energy mechanically milled Al/TiO₂ composite powders have been investigated by using a combination of thermal analysis, X-ray diffraction (XRD), and various microstructural characterization techniques. When the TiO₂ particles and their interparticle spacing in the Al/TiO₂ composite powder particles are sufficiently large, the reaction between Al and TiO₂ proceeds by two steps. The low-temperature step is an interfacial reaction, which starts at a temperature close to 660 °C. The high-temperature step is a reaction facilitated by bulk diffusion and starts at a temperature above 820 °C. The first phase formed from the reaction is always Al₃Ti irrespective of the starting powder composition or milling time. Al₂O₃ is difficult to form at temperatures below 800 °C. The formation of the α-Ti(Al,O) phase proceeds slowly and requires either continuous heating to a temperature above 1000 °C or holding at a temperature close to 1000 °C for a period of time. Mechanical milling of the Al/TiO₂ powder enhances the interfacial reaction between Al and TiO₂. This enhancement is originated from the establishment and refinement of Al/TiO₂ composite microstructure.     

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.     

An Experimental and Modeling Study of Synthesis,Consolidation and Aging Behavior of AA2014 Composite Reinforced by TiB<Subscript>2</Subscript> via Powder Metallurgy Method

Aluminum 2014 alloy composite reinforced with TiB₂ particulates with different volume% of TiB₂ (5, 10 and 15%) has been successfully synthesized by P/M route. The composite powders were consolidated by cold uniaxial compaction pressure followed by sintering at 590 °C in N₂ atmosphere. The Al 2014–TiB₂ composites were aged at 160 °C between 0 and 8 h followed by microstructural characterization and hardness evaluation. Scheil cooling and equilibrium calculations were performed using FactSage for qualitative understanding of the microstructural evolution during sintering and aging operations. In addition, the thermo-physical properties such as hardness, density and transverse rupture strength of the sintered samples were evaluated.     

Fatigue crack growth in a particulate TiB<Subscript>2</Subscript>-reinforced powder metallurgy iron-based composite

Fatigue crack growth behavior has been examined in a particulate titanium diboride (TiB₂)-reinforced iron-based composite that had been produced via a mechanical alloying process. Comparison with equivalent unreinforced material indicated that fatigue crack growth resistance in the composite was superior to monolithic matrix material in the near-threshold regime. The composite exhibited relatively low crack closure levels at threshold, indicative of a high intrinsic (effective) threshold growth resistance compared to the unreinforced iron. The lower closure levels of the composite were consistent with reduced fracture surface asperity sizes, attributable to the reinforcement particles limiting the effective slip distance for stage I-type facet formation. The observed shielding behavior was rationalized in terms of recent finite-element analysis of crack closure in relation to the size of crack wake asperities and the crack-tip plastic zone. The different intrinsic fatigue thresholds of the composite and unreinforced iron were closely consistent with the influences of stiffness and yield strength on cyclic crack-tip opening displacements. Cracks in the composite were generally seen to avoid direct crack-tip-particle interaction.     

Synthesis,Characterization and Mechanical Properties of AA7075 Based MMCs Reinforced with TiB<Subscript>2</Subscript> Particles Processed Through Ultrasound Assisted In-Situ Casting Technique

AA7075/TiB₂ composites have been synthesized through both in situ salt-melt reaction method and ultrasound assisted in situ process. Microstructural studies reveals that ultrasound assisted in situ method improves the dispersion of TiB₂ particles and reduces the porosity level. Moreover, the ultrasonic treatment refines the reinforcement particle size along with improvement in particle dispersion. The mechanical property assessment confirms that ultrasonic treatment improves the mechanical properties of composite. The hardness of the AA7075 alloy is increased from 55 H_V to 74 H_V by the addition of 5% TiB₂ particles and it further increased to 82 H_V by ultrasonic treatment. A similar trend is also observed when weight percentage of particles increases to 7.5%. Addition of 5% in situ TiB₂ particles increases the ultimate tensile strength of AA7075 alloy by 60 MPa and it is further enhanced by 80 MPa upon ultrasound assisted process. Composites have shown a small reduction in ductility when compared to un-reinforced alloy, though 81% ductility of matrix alloy has been retained. Similar trend has been observed in composites fabricated using ultrasonic assisted casting.     

Calciothermic reduction of titanium oxide in molten CaCl<Subscript>2</Subscript>

Titanium oxides were reduced to metallic titanium using the liquid calcium floating on the molten CaCl₂. A part of Ca dissolved into CaCl₂ and reacted with TiO₂ settled below CaCl₂. The by-product CaO also dissolved by about 20 mol pct into CaCl₂, which was effective in reducing the oxygen concentration in the obtained Ti particles. The compositional region in the Ca-CaCl₂-CaO system was examined for the less oxygen contamination in Ti and the better handling in leaching. A large amount of the residual calcium oxidized the titanium powder in leaching. The metallic Ti powder less than 1000 mass ppm oxygen could be obtained only for 3.6 ks using 5 to 7 mol pct Ca-CaCl₂ at 1173 K. The powder was slightly sintered like sponge, and contained approximately 1500 ppm Ca. The anatase phae, the intermediate product in the refining process of TiO₂, could be also supplied as raw material as well as rutile.     

Formation of a TiB2-reinforced copper-based composite by mechanical alloying and hot pressing

The microstructural changes during mechanical alloysing and subsequent hot pressing of Cu, Ti, and B powder mixtures were studied using scanning electron microscopy, X-ray diffraction, differential thermal analysis, and electron microprobe analysis. In particular, the changes in the Cu grain size, lattice parameter, and lattice strain of the powder mixtures and the formation of new phases (TiCu₄ during mechanical alloying and TiB₂ during hot pressing) were investigated. The mechanism of the in-situ formation of TiB₂ particles in the resultant copper composite was also studied.     

Mechanisms of Hydrogen-Assisted Magnesiothermic Reduction of TiO<Subscript>2</Subscript>

Direct reduction of TiO₂ powder has been attempted for decades by researchers in an effort to decrease titanium (Ti) metal production costs. The main objective has been to avoid energy-intensive steps involved in production of primary Ti by the Kroll process. The emerging hydrogen-assisted magnesiothermic reduction process, which uses Mg to directly reduce TiO₂ powder under a H₂ atmosphere, has been shown to have the potential to compete directly with the Kroll process. The present studies represent an effort to understand the reduction reaction mechanisms of this process. Phase transformations and the reaction pathways are examined by SEM/EDX analysis of partially reduced powder cross-sectional, X-ray diffraction, and other analytical techniques. The results show important morphological changes, the prominent intermediate and final phases under the H₂ atmosphere, as well as the local deposition behavior of the MgO byproduct. The effect of the specific surface areas of the initial particles is also discussed.     

Microstructural characterisation and mechanical properties of spark plasma-sintered TiB2-reinforced titanium matrix composite

Titanium and titanium matrix composites, reinforced with TiB₂ particles, have been synthesised by the spark plasma sintering method at 1050°C under 50?MPa pressure, using mixtures of 2.4?wt.-% TiB₂ and 97.6?wt.-% Ti powders. The changes in microstructural features and mechanical properties were investigated. XRD results and SEM observations confirm the formation of TiB whiskers as a result of the reaction between Ti and TiB₂. However, some unreacted TiB₂ particles have remained in the composite owing to the incomplete chemical reaction between matrix and additives. The measured mechanical properties demonstrate that the increase in hardness and tensile strength with TiB₂ addition is mainly attributed to the generation of TiB whiskers, increase in relative density and decrease in grain size, while the reduction in bending strength is possibly due to the plastic restraint imposed on the matrix by the TiB whiskers and unreacted TiB₂ particles.