Synthesis of Ti3AlC2/Al2O3 nanopowders by mechano-chemical reaction

Ti₃AlC₂/Al₂O₃ nanopowders were synthesized by the combination of mechanically-induce self-propagating reaction (MSR) of Ti, C, Al and TiO₂ powder mixtures and subsequently heat treatment. Effects of high energy milling and heat treatment temperatures on the phase transformation were investigated in detail. X-ray diffraction (XRD) was used to characterize the powders of milled and annealed, respectively. The morphology and microstructure of as fabricated products were also studied by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Results show that TiC, Ti_xAl_y and Al₂O₃ transitional phases were formed when the initial powder mixtures were milled for 24 h. The desired Ti₃AlC₂/Al₂O₃ nanopowders with high purity were obtained when annealed the as-milled powders at 1100 °C. SEM image confirmed that the as fabricated Ti₃AlC₂/Al₂O₃ particles has nanocrystalline layered structural matrix of Ti₃AlC₂, and the second phase of nanosized Al₂O₃ disperses uniformly in the Ti₃AlC₂ matrix.     

In Situ Synthesis of Ti5Si3 Matrix Nanocomposites Reinforced with Nanoparticles by High‐Energy Mechanical Alloying

In the present work, in situ TiN/Ti₅Si₃ nanocomposite powder was prepared by high‐energy mechanical alloying of a Ti and Si₃N₄ powder mixture via the following route: 9Ti + Si₃N₄ = Ti₅Si₃ + 4TiN. Constitution phases and microstructural features of the milled powders at different milling times were studied by XRD, SEM, and TEM. The operative formation mechanisms behind the microstructural developments were disclosed. It showed that the original Si₃N₄ and Ti constituents demonstrated two different reaction mechanisms during milling, i.e., a progressive mechanism of Si₃N₄ (≤20 h) and a speedy mechanism of Ti (≤10 h). The morphologies of the milled composite powders experienced a successive change: pre‐refining – coarsening – re‐refining on increasing the applied milling time. The variation of the operative mechanisms was ascribed to the existence/exhaustion of the ductile Ti constituent in the milling system due to the nonoccurrence/initiation of the in situ reaction. The 20 h milled powder was the typical nanocomposites featured by the nanocrystalline Ti₅Si₃ matrix reinforced with in situ TiN nanoparticles. The grain sizes of the in situ formed Ti₅Si₃ and TiN phases were generally ≤15 nm, exhibiting coherent interfacial structure between reinforcement and matrix.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Thermal stability and phase formation of mechanically alloyed Ti-Al-Si-C powders

Two quarternary Ti-Al-Si-C powder mixtures, 55Ti-27Al-12Si-6C and 55Ti-36Al-6Si-3C, were mechanically alloyed. The as-alloyed and heated powders have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). XRD patterns showed diffuse halos of amorphous like phase for 20-40 h milled powders, but TEM examinations demonstrated that the 40 h milled powders were mainly composed of Ti solid solutions, with some amount of amorphous phase. SEM observations displayed that the lamellar structures of Ti and Al formed at the early stage of milling process subsequently led to the formation of nano- or sub-micrometer particles of homogeneous composition after prolonged milling to 40 h. It is deduced that the solid-stated reaction by inter-diffusion of components should be responsible for phase formation during mechanical alloying. DSC curves of the 40 h milled powders exhibited two sharp exothermal peaks, and the investigation on thermal stability of the 40 h milled powders indicated that Ti₅Si₃ was first formed at lower temperature, followed by Al₂Ti₄C₂and TiC at intermediate temperature (820°C), and these phases were stable at elevated temperatures. These results raise the possibility of synthesizing TiAl-based composite with titanium silicides and titanium carbides as reinforcements by proper selection of powder compositions.     

The synthesis of high purity Ti3AlC2 MAX phase via molten salt method

Many methods have been used to synthesize the Ti₃AlC₂ MAX phase, but obtaining high-purity (>99.0 %) Ti₃AlC₂ is still a challenge. In this work, nano/submicro-sized Ti₃AlC₂ powder was prepared using Ti/Al/C elemental powders by the molten salt method in a vacuum quartz tube system. The effects of reaction temperature and dwell time, mechanical activation, the ratio of flux to reactants, and the ratio of Al on the synthesis of Ti₃AlC₂ were systematically investigated. The results show that Ti₃AlC₂ powder with a high purity of up to 99.0 % was prepared using ball milled 3Ti/1.1Al/2C mixture as the reaction material, and NaCl-KCl as the molten salt at 1150 °C for 2 h. The reaction mechanisms of Ti₃AlC₂ in molten salt have been discussed.     

Sintering behavior of mechanically alloyed Ti-48Al-2Nb aluminides

Ti-Al intermetallics have been produced using mechanical alloying technique. A composition of Ti-48Al-2Nb at % powders was mechanically alloyed for various durations of 20, 40, 60, 80 and 100 h. At the early stages of milling, a Ti (Al) solid solution is formed, on further milling the formation of amorphous phase occurs. Traces of TiAl and Ti₃Al were formed with major Ti and Al phases after milling at 40 h and beyond. When further milled, phases of intermetallic compounds like TiAl and Ti₃Al were formed after 80 hours of milling and they also found in 100 h milled powders. The powders milled for different durations were sintered at 785°C in vacuum. The mechanically alloyed powders as well as the sintered compacts were characterized by XRD, FESEM and DTA to determine the phases, crystallite size, microstructures and the influence of sintering over mechanical alloying.     

Structural studies on Ti–Cu alloy obtained by high-energy ball milling

ABSTRACT

Ti₅₀Cu₅₀ (at.-%) alloy has been produced from elemental Ti and Cu powders by high-energy ball milling in a planetary ball mill. Structural evolution of the alloy during milling and after subsequent heat treatment have been studied. It has been stated that high-energy ball milling of the investigated powder produces two nanocrystalline solid solutions: Cu(Ti) and Ti(Cu), both characterised by the fcc (Fm-3 m) structure. The transition of Ti structure from hcp (P63/mmc) to fcc (Fm-3 m) is observed during milling. Heat treatment of the milled powder leads to recrystallisation of Cu(Ti) and Ti(Cu) solid solutions.

This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.     

Role of intensive milling in mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research a nano-composite structure containing of an intermetallic matrix with dispersed Al₂O₃ particles was obtained via mechanical activation of TiO₂ and Al powder mixture and subsequent sintering. The mixture has been milled for different lengths of time and then as a subsequent process it has been sintered. Phase evolutions in the course of milling and subsequent sintering of the milled powder mixture were investigated. Samples were characterized by XRD, SEM, DTA and TEM techniques.The results reveal that the reaction begins during milling by formation of Al₂O₃ and L1₂ Al₃Ti and further milling causes partial amorphization of powder mixture. DTA results reveal that milling of the powder mixture causes solid state reaction between Al and TiO₂ rather than liquid–solid reaction. Also, it was observed that the exothermicity of aluminothermic reduction is reduced by increasing the milling time and the exothermic peak shifts to lower temperatures after partial amorphization of powder mixture during milling. Phase evolutions of the milled powders after being sintered reveal that by increasing the milling time and formation of L1₂ Al₃Ti in the milled powder, intermediate phase formed at 500 °C changes from D0₂₂ Al₃Ti to Al₂₄Ti₈ phase.     

Microstructure and oxidation behaviour of TiAl(Nb)/Ti<Subscript>2</Subscript>AlC composites fabricated by mechanical alloying and hot pressing

TiAl-based intermetallic matrix composites with dispersed Ti₂AlC particles and different amounts of Nb were successfully synthesized by mechanical alloying and hot pressing. The phase evolution of Ti–48 at%. Al elemental powder mixture milled for different times with hexane as a process control agent was investigated. It was found that after milling for 25 h, a Ti(Al) solid solution was formed; also with increase in the milling time to 50 h, an amorphous phase was detected. Formation of a supersaturated Ti(Al) solid solution after 75 h milling was achieved by crystallization of amorphous phase. Addition of Nb to system also exhibited a supersaturated Ti(Al,Nb) solid solution after milling for 75 h, implying that the Al and Nb elements were dissolved in the Ti lattice in a non-equilibrium state. Annealing of 75 h milled powders resulted in the formation of equilibrium TiAl intermetallic with Ti₂AlC phases that showed the carbon that originates from hexane, participated in the reaction to form Ti₂AlC during heating. Consolidation of milled powder with different amounts of Nb was performed by hot pressing at 1000°C for 1 h. Only the presence of γ-TiAl and Ti₂AlC was detected and no secondary phases were observed on the base of Nb. Displacement of γ-TiAl peaks with Nb addition implied that the Nb element was dissolved into TiAl matrix in the form of solid solution, causing the lattice tetragonality of TiAl to increase slightly. The values for density and porosity of samples indicated that condition of hot pressing process with temperature and pressure was adequate to consolidate almost fully densified samples. The isothermal oxidation test was carried out at 1000°C in air to assess the effect of Nb addition on the oxidation behaviour of TiAl/Ti₂AlC composites. The oxidation resistance of composites was improved with the increase in the Nb content due to the suppression of TiO₂ growth, the formation and stabilization of nitride in the oxide scale and better scale spallation resistance.     

Synthesis of titanium carbide and TiC–SiO2 nanocomposite powder using rutile and Si by mechanically activated sintering

High-energy ball milling was applied with subsequent heat treatment for synthesizing nanoparticles of TiC powders by the carbothermic and carbosilisisothermic reduction of titanium oxide (rutile type). The milling procedure involved milling of TiO₂/C and TiO₂/Si/C powders at room temperature in an argon atmosphere. The progress of the mechanically induced solid state reaction was monitored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results showed that TiC nanoparticles were duly synthesized in the TiO₂/C system at 1700 °C in 60-h milled samples. In the non-milled samples, although heated at the same temperature, only a minor amount of a lower degree of titanium oxide (Ti₃O₅) was observed to form. Further, in other non-milled samples, but with Si initially present, despite heating to 1550 °C no TiC phase was detected. However, using Si as a reducing agent accompanied by graphite, after 60 h ball milling, only Si remained as a distinguishable crystalline phase. Further, heat treatment of activated powders by forming the interphase compounds (such as Ti₃Si₅ and Ti₅Si₃) remarkably decreased the synthesis temperature to 900 °C for the 60 h milled samples.     

Microstructural characterization of ball-milled metal matrix nanocomposites (Cr,Ni, Ti)-25 wt% (Al2O3np,SiCnp)

The microstructure of high-temperature metals such as Ti, Ni, and Cr can be modified using ceramic nanoparticles to form metal matrix nanocomposites (MMNCs). Such materials are generally prepared via powder metallurgy routes. In this study, 25?wt% SiC_np and Al₂O_3np were separately ball milled as a reinforcement of Ti, Cr, and Ni matrices to investigate their effects on the phase formation and morphology of the MMNCs. The x-ray diffraction (XRD), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FESEM) results indicated that the alumina–metal system could not be thermodynamically stable in a high-energy ball mill, while the SiC reinforcement could be retained and milled with the metals even after 24?h. It was further observed that the distribution of nanoparticles was not affected by the type of metal, ceramic, and milling time. Finally, it was determined that the nanoparticles significantly reduced the average particle size of composite powders.     

Microstructure Evolution of Ti/BN Powder Blend during Ball Milling and Heat Treatment

Ball milled Ti/BN composite powder was prepared by high energy ball milling for 40 h, using Ti and BN (the molar ratio of Ti/BN is 3:2) as starting materials. The as-milled composite powder consists of TiN, Ti and amorphous phase. TiN formed while the milled powder was annealed at 400℃. The heat treatment at 700℃ led to the formation of TiB2 and TiB. The nanocrystalline Ti and amorphous phase converted to TiN and TiB2 when the powder was heated to 1300℃.     

Synthesis of Ti2SnC from Ti/Sn/TiC powder mixtures by pressureless sintering technique

Ti/Sn/TiC powder mixtures were first employed to synthesize Ti₂SnC powder by pressureless sintering in the temperature range of 950–1250 °C at vacuum atmosphere. Ti₂SnC began to form at 950 °C, its content increased with increasing temperature. High purity of Ti₂SnC was obtained by sintering the mixtures with deficient Sn and TiC at 1200 °C for 15 min. A reaction mechanism was proposed to explain the formation of Ti₂SnC. The Ti₂SnC powder was characterized by scan electron microscopy (SEM) and X-ray diffraction (XRD). Using the above mixtures and process, the Ti₂SnC ceramic powder can be obtained on a larger scale.     

Fabrication of Ti2AIC by hot pressing of Ti, TiC, Al and active carbon powder mixtures

Polycrystalline Ti₂AlC samples were synthesized by hot pressing of Ti, Al, TiC and active carbon powder mixtures. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase identification and microstructure evaluation. No other phase except Ti₂AlC was detected in samples synthesized by hot pressing of the 0.5TiC/1.5Ti/1.0Al/0.5C powder mixtures at 1400°C for 1 and 3 h under a pressure of 30 MPa. The densities of these two samples were 96.1 and 98% of the theoretical value of pure Ti₂AlC, respectively. The reason that the densities of these two samples were lower than the theoretical density of pure Ti₂AlC is that pore existed in these two samples. At lower temperature of 1300°C, the speed of the reaction forming Ti₂AlC was slow. While at higher temperature of 1500°C, Ti₂AlC transformed to Ti₃AlC₂. So these two temperatures are not suitable for the fabrication of Ti₂AlC.     

Formation mechanism of B4C–TiB2–TiC ceramic composite produced by mechanical alloying of Ti–B4C powders

In this study, the B₄C–TiB₂–TiC composite powder was synthesized by mechanical alloying (MA) of Ti–B₄C powder mixture. For this purpose, four powder mixtures of Ti and B₄C powders with different molar ratios were milled. In order to study the mechanism of Ti–B₄C reaction during milling, structural changes and thermal analysis of powder particles were studied by X-ray diffractometry (XRD) and differential thermal analysis (DTA). Morphology and microstructure of powder particles during milling were studied by scanning electron microscopy (SEM). It was found that during MA, after decomposition of the outer layers of B₄C particles, first, C reacted with Ti and after that, B was diffused in Ti structure and TiC and TiB₂ phases were formed in gradual reaction mode. Also, the results of DTA and thermodynamic analysis confirmed the suggested mechanism for Ti–B₄C reaction.     

Low temperature synthesis of Ti3 SiC2 from Ti/SiC/C powders

Abstract

Ternary carbide Ti₃ SiC₂ was first synthesised through a pulse discharge sintering (PDS) technique from mixtures of Ti, SiC, and C with different molar ratios. Sintering processes were conducted at 1200 – 1400°C for 15 – 60 min at a pressure of 50 MPa. The phase constituents and microstructures of the synthesised samples were analysed by X-ray diffraction (XRD) technique and observed by scanning electron microscopy (SEM). The results showed that, for samples sintered from 3Ti/SiC/C powder at 1200 – 1400°C, TiC is always the main phase and only little Ti₃ SiC₂ phase is formed. When the molar ratios Ti : SiC : C were adjusted to 3 : 1.1 : 2 and 5 : 2 : 1, the purity of Ti₃ SiC₂ in the synthesised samples was improved to about 93 wt-%. The optimum sintering temperature for Ti₃ SiC₂ samples was found to be in the range 1250 – 1300°C and all the synthesised samples contain platelike grains. The relative density of Ti₃ SiC₂ samples was measured to be higher than 99% at sintering temperatures above 1300?C. It is suggested that the PDS technique can rapidly synthesise ternary carbide Ti₃ SiC₂ with good densification at lower sintering temperature.     

Thermodynamics and kinetics of Ti/CdTe contact metallurgy

The ternary phase diagram Ti-Cd-Te was investigated by using approximate thermodynamic calculations as well as experimental methods (XRD, SEM, EDX, DTA). Isothermal sections at 500 C and 600 C and a tentative liquidus surface are given. Massive Ti/CdTe diffusion couples have been used to study the reaction mechanism and the kinetics of interface reactions in cross-sections with optical microscopy and SEM/EDX and also by qualitative X-ray depth profiling. Both the phase equilibrium and diffusion study reveal that Ti forms a thermodynamically unstable contact on CdTe. The diffusion path with the reaction layer sequence Ti/Ti₂Cd/Ti₅Te₄/Ti₃Te₄/CdTe indicates that local equilibria prevail in the reaction diffusion. The thermal stability of metallization structures alternative to Ti/CdTe and based on the intermediate phases in the ternary Ti-Cd-Te system is discussed.     

Synthesis of LaB<Subscript>6</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite powders via ball milling-assisted annealing

In this study, LaB₆–Al₂O₃ nanocomposite powders were synthesized via ball milling-assisted annealing process starting from La₂O₃–B₂O₃–Al powder blends. High-energy ball milling was conducted at various durations (0, 3, 6 and 9 h). Then, the milled powders were annealed at 1200 °C for 3 h under Ar atmosphere in order to obtain LaB₆ and Al₂O₃ phases as reaction products. X-ray diffractometry (XRD), scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) and transmission electron microscopy (TEM) techniques were utilized to carry out microstructural characterization of the powders. No reaction between the reactants was observed in the XRD patterns of the milled powders, indicating that high-energy ball milling did not trigger any chemical reactions even after milling for 9 h. LaAlO₃ and LaBO₃ phases existed in the annealed powders which were milled for 0, 3 and 6 h. LaBO₃ phase was removed after HCl leaching. 9-h milled and annealed powders did not exhibit any undesired phases such as LaAlO₃ and LaBO₃ after leaching step, and pure nanocrystalline LaB₆–Al₂O₃ composite powders were successfully obtained. TEM analyses revealed that very fine LaB₆ particles (~?100 nm) were embedded in coarse Al₂O₃ (~?500 nm) particles.     

Composites prepared by multistage wet ball milling of Ti and Cu powders: Phase composition and effect of surfactant addition

Titanium carbohydride-based composites were produced by one-, two- and three-stage ball milling of titanium-copper powder mixture in liquid hydrocarbon and subsequent annealing at 600 °C for 1 h in argon. The phase composition and morphology of the milled and annealed powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Single-stage milling of titanium-copper mixture followed by its annealing was shown to result in the formation of the titanium carbohydride and intermetallic phases which may deteriorate the composite properties because of their high brittleness. Addition of copper at the second stage of the two-stage milling lowered the fraction of intermetallic phases. Octadecylamine added as a surfactant in the three-stage ball milling aided to prevent the formation of intermetallic phases under both milling and subsequent annealing owing to the formation of a barrier adsorption layer on the particle surface.     

Preparation of NixTiy compound via Ni plating followed by thermal treatment of Ti powder

In this research, Ni_xTi_y compound was prepared by thermal treatment of Ni-plated Ti powder. For this purpose, Ti powder was plated in an electroless Ni bath for various times (120, 225, 300, and 720?min). Hydrazine hydrate was used as a reductant for the deposition of pure Ni on the Ti particles. The plated powder (225?min) was heat treated under argon atmosphere to achieve Ni_xTi_y powder. Finally, the heated/plated powder was pressed by CIP followed by sintering at 980°C for prepare the Ni_xTi_y bulk sample. The plated powders as well as sintered one were characterized using scanning electron microscopy, energy dispersive spectrometer, X-ray fluorescence, X-ray diffraction and differential scanning calorimetric. The NiTi₂, NiTi, and Ni₃Ti phases were detected in the XRD patterns of heated/plated Ti powder. According to DSC data, the heated/plated Ti powder showed reversible martensitic transformation at temperature range of ?38.0°C to +38.1°C, while sintered/heated/plated Ti powder displayed reversible transformation at temperature range of 16.0°C–15.4°C.