Titanium aluminides from cold-extruded elemental powders with Al-contents of 25–75 at% Al

Titanium aluminide alloys were prepared from cold-extruded elemental Ti and Al powders with compositions ranging from 25 to 75 at % Al. The production route in this study includes four essential steps: mixing, precompaction, cold-extrusion and reaction treatment. During reaction treatment, the intermetallic phases Ti₃Al, TiAl and TiAl₃ are formed by interdiffusion of Ti and Al, and pore formation takes place because of the different diffusivities of Ti and Al. The processes of phase formation, as well as pore formation, were studied by means of calorimetric, dilatometric and metallographic methods. In order to obtain nearly fully dense specimens, the technique of hot isostatic pressing was applied. The sintering behaviour and microstructures of the prepared alloys are reported.     

Explosive shock-compression processing of titanium aluminide/titanium diboride composites

Explosive shock-compression processing is used to fabricate Ti₃Al and TiAl composites reinforced with TiB₂. The reinforcement ceramic phase is either added as TiB₂ particulates or as an elemental mixture of Ti + B or both TiB₂ + Ti + B. In the case of fine TiB₂ particulates added to TiAl and Ti₃Al powders, the shock energy is localized at the fine particles, which undergo extensive plastic deformation thereby assisting in bonding the coarse aluminide powders. With the addition of elemental titanium and boron powder mixtures, the passage of the shock wave triggers an exothermic combustion reaction between titanium and boron. The resulting ceramic-based reaction product provides a chemically compatible binder phase, and the heat generated assists in the consolidation process. In these composites the reinforcement phase has a microhardness value significantly greater than that of the intermetallic matrix. Furthermore, no obvious interface reaction is observed between the intermetallic matrix and the ceramic reinforcement.     

Fabrication of multilayered titanium aluminide sheets by self-propagating high-temperature synthesis reaction using hot rolling and heat treatment

This study is concerned with the fabrication of multilayered and bulk Ti aluminide sheets by self-propagating high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. A multilayered Ti/Al sheet was prepared by stacking thin Ti and Al sheets alternatively. When this sheet was hot-rolled and heat-treated at 1000°C, a multilayered sheet composed of Ti₃Al and TiAl was made through the process of formation and growth of intermetallic phases at Ti/Al interfaces and porosity reduction. A bulk Ti aluminide sheet having a lamellar structure of TiAl and Ti₃Al was also fabricated successfully by heat treatment at 1400°C.     

Reaction synthesis of titanium aluminides

The formation of titanium aluminides from the elemental powders has been investigated. A traditional powder metallurgy route of compaction (by cold isostatic pressing, hot pressing or hot extrusion) followed by heat treatment was compared with the novel technique of hot extrusion reaction synthesis (HERS). The products from these different production methods were characterised by x-ray diffraction and microscopy (light and scanning electron). The intermetallic compound formed under most processing conditions wasTiAl₃. Only when there was a rapid increase in temperature to high temperatures, as found in induction heating of compacts or in HERS, were the compounds Ti₃Al and TiAl formed.     

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.     

Effect of heating rate on the combustion synthesis of Ti-Al intermetallic compounds

Titanium aluminide compounds were synthesized by the thermal explosion mode of self-propagating high-temperature synthesis (SHS). The effects of heating rate on the combustion characteristics and the microstructures of the products were studied. It was found that the low density of the reacted sample was due to the outgassing of water vapour and other gases, which were released by dissociation of hydrated aluminium oxides. Higher heating rates resulted in a product of higher density and single-phase microstructure. At lower heating rates, the reaction product was a mixture of phases for TiAl and Ti₃Al reactions. A liquid (Al)-solid (Ti) reaction mechanism is predicted for slow heating while a solid-solid mechanism is expected for high heating rates. The origin of porosity in the product is also discussed.     

Joining of TiAl intermetallic by self-propagating high-temperature synthesis

This study reports on a novel approach for joining TiAl by self-propagating high-temperature synthesis (SHS). The Ti, Al and C powders were applied in the joining process with the assisted electromagnetic field. The microstructure analysis revealed that a dark TiAl₃ reaction layer existed at the interface. The TiC particles and multi-layer granular structures of the Ti–Al system were found in the reaction products. It was noted that the porosity was inevitable in direct SHS joining. In order to improve the joining quality, Ag-based brazing foil was inserted between the powder compacts and the TiAl substrate. Molten brazing alloy during SHS reaction improved the wettability of the interlayer to TiAl substrate and filled well into the holes in the reaction products. With the application of Ag-based brazing alloy, the density increased and the joining quality improved.     

Microstructure and mechanical properties of NiAl produced in the SHS process induced by low-temperature hydrostatic extrusion

A stoichiometric nickel aluminide intermetallic alloy produced by self-sustained high-temperature synthesis (SHS) induced to the green compacts of the mixture of elemental Ni and Al powders by mechanical heavy deformation during low-temperature hydrostatic extrusion has been investigated. The process was performed with the high strain rate (>10² s^?1) at the low temperature (～500°C). To reduce the porosity, grain size and non-homogeneity of the obtained material a further high-temperature (～950°C) hydrostatic extrusion was applied. Mechanical properties of the material were measured on the samples cut out from extruded rods in the longitudinal and transverse directions. Elastic anisotropy (elastic tensor) was determined using a resonant ultrasound spectroscopy (RUS) method applied to small cylindrical samples of the material. Its value described as the ratio of the coefficients of linear compressibility in transverse and longitudinal directions was at the level of 2 for extruded material even after annealing. Plastic anisotropy of the material was determined in uniaxial compression tests using an acoustic emission (AE) method for monitoring early stages of microcracking process in the samples under investigation. The phenomenon of the yielding point was observed for the samples compressed in the direction of extrusion.     

电子束粉末床熔融制备钛铝基金属间化合物研究进展

钛铝基金属间化合物是一种理想的高温结构材料,但因存在室温塑性差、加工困难等不足而限制了其发展与应用。电子束粉末床熔融(Electron Beam Powder Bed Fusion, EB?PBF)技术能够实现近净成形,其加工中的低热应力特点适宜脆性材料的制备,是近年来广受关注的新型钛铝基金属间化合物成形方法。对用电子束粉末床熔融制备的钛铝基金属间化合物进行了介绍,并对近年来发表的以EB?PBF钛铝材料为研究对象的相关文献进行了综述。从工艺、后处理和性能表征等角度对目前的研究现状进行了分析总结,并对未来的研究工作提出了展望。     

Titanium aluminide (Ti-48Al) powder synthesis,size refinement and sintering

Titanium aluminide based alloys have shown significant potential in high temperature applications, but the high production cost of TiAl considerably limits its utilisation. Although the use of powder metallurgy processes can reduce the cost by minimising post-machining, an economical powder production route is still required. Therefore, in the present study a pre-alloyed Ti-48Al powder is developed using an elemental Ti and Al powder blend prepared using a simple vacuum heat treatment. A formation model of the intermetallic phases (i.e. TiAl, Ti₃Al, TiAl₂, TiAl₃) during powder synthesis is proposed. In order to improve the sinterability, various milling methods (i.e. ball, attrition and shatterbox milling) are examined to reduce the particle size. The sintered microstructures, particularly the two-phased (α₂-Ti₃Al γ-TiAl) lamellar structures are also investigated. Improved densification is achieved at 1300 °C, held for 2 h, using the manufactured powder, compared to the elemental powder blend (～55%). With higher sintering temperatures or longer hold periods, increased density TiAl components are possible.     

Solid-state processing of oxidation-resistant molybdenum borosilicide composites for ultra-high-temperature applications

The high-temperature capabilities of multi-phase composites based on Mo₅Si₃B_x are examined after solid-state processing and pulsed laser deposition (PLD) coating fabrication approaches. These composites are prepared by mechanical alloying of elemental powders and densified by vacuum hot pressing, which is a scalable processing approach. Chemical analyses of the hot-pressed compacts reveal a consistent 15–22 percent loss of boron, which is primarily due to the high-temperature hot-pressing step. Composites possessing sufficient amounts of boron are evaluated by thermogravimetric studies in temperatures up to 1650 °C in air. One composition demonstrates oxidative stability after long-term (100 h) isothermal conditions, as well as thermal cycling to simulate solar-thermal operation. Hot-pressed samples of composites consisting of Mo₅Si₃B_x + MoSi₂ + MoB are also employed as deposition targets for PLD trials. X-ray diffraction analysis of the resulting films indicates the absence of long-range crystallographic order.     

Interfaces in nickel aluminide/alumina fibre composites

Metal matrix composites based on the intermetallic alloy Ni₃Al and fibres of Al₂O₃ were fabricated by hot-pressing nickel aluminide powders and alumina fibres. Two matrix alloys were used in this investigation: Ni₃Al microalloyed with boron and Ni₃Al alloyed with 8 at% chromium and smaller amounts of zirconium and boron. The materials were studied using optical and transmission electron microscopy with particular emphasis placed on the characteristics of the matrix-fibre interface. The base Ni₃Al/Al₃O₃ composite displayed no evidence of chemical reaction at the interface, an intimate bond between matrix and fibre was observed, and the material exhibited 10% ductility at room temperature. Composites with the more complex matrix alloy were brittle, a phenomenon attributed to the formation of zirconia particles at the interface.     

Shock synthesis and synthesis-assisted shock consolidation of suicides

Shock-induced chemical synthesis and synthesis-assisted consolidation of high-temperature materials (suicides) were investigated. Niobium, molybdenum, and titanium powders mixed with silicon powders were chosen as reactant materials for shock-induced synthesis of silicides. In parallel experiments, these reactant materials were also respectively mixed with inert intermetallic compound powders of NbSi₂, MoSi₂, and Ti₅Si₃ in different proportions and were shock consolidated. Shock processing was carried out using a modification of the experimental set-up developed by Sawaoka and Akashi. The shock waves were generated in the materials by the impact of a flyer plate at a velocity of 2 km sec^–1. An explosive plane-wave generator was used to initiate the main explosive charge to accelerate the flyer plate. The passage of shock waves of sufficient pressure and temperature induced a highly exothermic and self-sustaining reaction between reactant materials. The shock-synthesized intermetallic compounds and the heat of reaction enhanced bonding between inert matrix materials. The proportion of reactant powder mixtures blended with inert intermetallic materials plays a very important role in the synthesis-assisted consolidation process. Characterization of compacts was done by optical microscopy, scanning electron microscopy, and X-ray diffraction. A preliminary analysis of shock-induced chemical reactions is conducted; it predicts a 30% increase in shock pressure and shock-wave velocity over those in unreacted powders. For shock synthesis, the profuse formation of voids indicates that melting of the material occurred; in contrast, unreacted regions did not exhibit porosity.     

Synthesis of titanium diboride TiB2 and Ti-Al-B metal matrix composites

Titanium diboride TiB₂ and TiAl aluminide composites reinforced with in situ borites have been synthesized from the elemental powders of Ti and B, and Ti, Al and B respectively using mechanical alloying technique. No progressive diffusion between Ti and B was observed. The formation of TiB₂ was found to be governed by strong and fast exothermic heat release. This indicates that the formation of TiB₂ compound in local area of mechanically alloyed powder generated high energy which in turn ignited and promoted the formation of new compound in the rest of the area. Because of the presence of Al in Ti-Al-B system, the concentration of Ti or B was diluted. The exothermic reaction between Ti and B was consequently delayed. However, grain refinement of Ti and Al in this system down to nanometer scale is faster than that in Ti-Al system due to the contribution of B. Using X-ray analysis, strong but broad TiAl, and weak TiB and TiB₂ peaks had been detected at 50 h of mechanical alloying indicating the formation of nano TiAl composite reinforced by TiB and TiB₂. However, TiB was, however, not a stable phase; it later was transformed into equilibrium phase of TiB₂ after annealing at 800 °C.     

Synthesis,microstructure and mechanical properties of Al2O3 reinforced Ni3Al matrix composite

A new method to synthesize alumina reinforced Ni₃Al intermetallic matrix composites has been described. The powder mixture of nickel and aluminium was mechanically alloyed. The powder mixture was excessively heated during mechanical alloying and then exposed to atmosphere for oxidation. The oxidized powder mixture was transformed into alumina reinforced nickel aluminide matrix composite on subsequent pulse current processing. Alumina reinforcements were generated in the nickel aluminide matrix by in situ precipitation. The microstructure of the composite showed that the alumina reinforcements were 50–150 nm in size. The fine alumina reinforcements were homogeneously distributed in the matrix phase. The mechanical properties of the alumina reinforced nickel aluminide matrix composite fairly exceeded the nickel aluminide alloys. This novel synthesis approach allowed the rapid and facile production of high strength alumina reinforced Ni₃Al matrix composites.     

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.     

Self-assembling of atomic vacancies at an oxide/intermetallic alloy interface

Oxide layers grown on the surface provide an effective way of protecting metallic materials against corrosion for sustainable use in a broad range of applications. However, the growth of cavities at the metal/oxide interface weakens the adherence of the protective layer and can promote its spallation under service conditions, as observed for alumina layers formed by selective oxidation of aluminide intermetallic alloys used in high-temperature applications. Here we show that direct atomic-scale observations of the interface between an ultrathin protective oxide layer (alumina) grown on an intermetallic titanium aluminide substrate (TiAl) can be performed with techniques sensitive to the topmost atomic layers at the surface. Nanocavities resulting from the self-assembling of atomic vacancies injected at the interface by the growth mechanism of the protective oxide are observed for the first time, bringing new insight into the understanding of the fate of injected cavities in oxidation processes.     

Effect of partial mechanical alloying on the self-propagating high-temperature synthesis of Ni3Si

Self-propagating high-temperature synthesis (SHS) is a new method for economical processing of intermetallic compounds and ceramic materials, as well as composites based on them. On the other hand, mechanical alloying is an effective method for producing highly metastable and, therefore, reactive metal powders. In this paper an overview of partial mechanical alloying is given. The effect of partial mechanical alloying on the self-propagating high-temperature synthesis of Ni₃Si-compounds is studied. The influence of alloying time on powder characteristics, e.g. particle size distribution, is given. The effect of alloying time on the properties of Ni–Si composite powders and on the characteristics of the SHS process, e.g. propagation rate, is reported. Ni₃Si was chosen as the object for this study because of its corrosion and high-temperature oxidation resistance. Like other L1₂-type compounds, the strength of Ni₃Si shows an anomalous behaviour as a function of temperature, therefore, it has potential for high-temperature applications.     

In situ formation of titanium carbide in titanium powder compacts by gas–solid reaction

Porous titanium compacts of fine and coarse sponge titanium powders were reacted with methane gas to produce Ti–TiC in situ composites. The kinetics of titanium carbide formation during the reaction were studied in relation to powder size, reaction temperature and time, and methane flow rate. The titanium carbide was initially formed as a layer around each titanium powder and the rate of formation was found to be diffusion-controlled. Titanium hydride was also formed during the reaction but was easily removed by post-vacuum annealing. A significant reduction of chlorine content in the compact also resulted during the processing. High temperature vacuum sintering could densify the reacted compacts to over 95% theoretical density and, at the same time, alter the layered titanium carbide phase into rounded particles. It was possible to produce fully dense titanium base composites containing up to 30 vol% by the present gas–solid reaction-based processing.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.