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.     

Preparation of γ-TiAl intermetallic compounds through self-propagating high-temperature synthesis and compaction

Compact materials based on titanium and aluminum are prepared using self-propagating high-temperature synthesis and compaction. The parameters of the self-propagating high-temperature synthesis and compaction are optimized for the purpose of producing a dense intermetallic compound, namely, γ-TiAl.     

Oxidation resistance of intermetallic compounds Al3Ti and TiAl

The oxidation kinetics and morphological features of Al₃Ti and TiAl were investigated. The oxidation resistance of Al₃Ti is much better than that of TiAl, for example, by a factor of about 30 at 1000° C for 48 h. The big difference in the oxidation resistance is related to the characteristics of the external oxide scales of a protective Al₂O₃ or a mat of crystalline TiO₂ formed on Al₃Ti or TiAl, respectively. Sufficient aluminium transport from Al₃Ti assists the formation of the Al₂O₃ scale which acts as a protective film against oxidation. The poor aluminium content of TiAl produces Ti₃Al phase at the interface of TiAl and oxide scales and increases the diffusivity of titanium in the Al₂O₃ scale. The external crystalline TiO₂ scale produced by the diffusion of titanium through the Al₂O₃ scale enhances oxidation of TiAl.     

Mathematical simulation and structural macrokinetics of the high-temperature synthesis of intermetallic compounds

In extending the results of [1], a theoretical study is made of the ignition temperature of a nickel-aluminum powder mixture as a function of the power of the external heating source, the dispersity of the refractory component, and the porosity of the powder mixture in the case of a volume reaction. The initial mixture is modeled by a set of spherical elementary cells whose dimension is determined by the range of nickel particle sizes, the mixture composition, and the porosity.Institute of the Physics of Strength and Material Science, Siberian Branch of the Russian Academy of Sciences, Novosibirsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 4, pp. 451–454, October, 1993.     

Rapidly quenched intermetallic compounds,TiAl and Al3Ti

Two types of molten intermetallic compounds, with stoichiometric compositions TiAl and Al₃Ti, are rapidly solidified at a cooling rate ranging from 10⁴ to 10⁵ Ksec^–1 using a melt-spinning method. Solidified specimens are analysed by X-ray diffractometry and observed by a high-voltage electron microscope and an analytical transmission electron microscope. Extra phases other than stoichiometric composition phases were not detected for either TiAl or Al₃Ti specimens by X-ray diffractometry. Both specimens have very small grains from 1 to 3 m in diameter. TEM observation reveals that very fine precipitates (100 to 300 nm), which are not detected by X-ray diffractometry, are present within grains. They are Ti₃Al in the TiAl specimens, and aluminium in the Al₃Ti specimens, respectively. Electron micrographs of the Al₃Ti specimens show the presence of pair dislocations (super dislocations) and anti-phase boundaries. They are believed to have been formed at high temperature.     

A numerical study of the heterogeneous porosity effect on self-propagating high-temperature synthesis (SHS)

Self-propagating high-temperature synthesis (SHS), or the so-called micropyretic/combustion synthesis, is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Heterogeneous distributions of porosities are common during self-propagating high-temperature synthesis when powders are pressed and the conventional modeling treatments thus far have only considered uniform systems. Heterogeneities in the porosity are thought to result in local variations of such thermophysical/chemical parameters for the reactants as density and thermal conductivity further changing the combustion temperature, the propagation velocity, and the propagation pattern of a combustion front. This study investigates the impact of porosity variations during self-propagating high-temperature synthesis with Ti + 2B. In addition, the simulations for the propagation of combustion fronts across a non-uniform compact where the porosity is monotonically decreased or increased along the specimens due to die wall friction are also carried out.     

Formation mechanism of Ti2AlC under the self-propagating high-temperature synthesis (SHS) mode

Ti₂AlC was fabricated by the self-propagating high-temperature synthesis (SHS) technique from a compacted powder mixture consisting of Ti:Al:C = 2:1:1 (molar ratio) of mortar mixing, planetary mixing and various packing densities. A thermocouple was placed directly into the green compact body in order to monitor the combustion temperature during the SHS process. When the green compact with planetary mixing treatment and a packing density of 17% as well as 60% was used, propagation of the reaction exothermicity could occur, and the starting compact completely changed to Ti₂AlC. Formation mechanism of Ti₂AlC using a SHS technique was discussed. In addition, the melting point of the resultant sample was determined to be 1570 °C.     

Specific features of the behavior of synthetic diamond in a self-propagating high-temperature synthesis (SHS) combustion wave

The behavior of synthetic diamond powder in an SHS combustion wave is considered by using the model Ti-B-diamond system as an example. The results presented show that the diamond can retain its properties when exposed to the extreme thermal conditions realized in an SHS combustion wave.Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 5, pp. 577–582, November, 1992.     

Development of natural convection in a liquid heated by the front of self-propagating high-temperature synthesis (SHS)

The process of development of free convection in a liquid heated by a moving combustion front is considered. The emphasis is on the dynamics of development of the process.Moscow Power Institute, Kazan' Branch. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 4, pp. 400–403, October, 1992.     

Formation of pores and amorphous-nanocrystalline phases in porous TiNi alloys made by self-propagating high-temperature synthesis (SHS)

The objective of this study was to examine the mechanism how the surface of porous TiNi compounds produced by SHS method evolves. The prepared samples were investigated using light-microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS). The results indicated that the surface of all pores is represented by a granular stratum due to dendrite liquation by peritectic crystallization mechanism. The voids of 2–15?μm in size are formed owing to a capillary spreading of the liquid. Reaction gases with dissociated carbon, nitrogen, and oxygen are responsible for heat-and-mass transfer through the forming pores. High pressure-temperature effect of reaction gases on the melt causes the forming voids to coalesce, as well as transfers the peritectic liquid (PL) throughout the open pores catalyzing a distinctive spitted topography. It is through the chemisorption of gasiform nonmetallics by the pore surface melt, where these impurities are chemically bound, that it was formed a massive corrosion-resistant amorphous-nanocrystalline stratified shell deduced as an intermetallic oxycarbonitride layer.     

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.     

Reaction chemistry during self-propagating high-temperature synthesis (SHS) of H3BO3-ZrO2-Mg system

In the present investigation, the ZrB₂ powder is produced by SHS of mixture containing H₃BO_3, ZrO₂ and Mg. The thermal analysis and XRD study reveal the reaction mechanism of ZrB₂ formation by SHS process. Synthesis of H₃BO₃-Mg system results in formation of Mg₃(BO₃)₂ and MgB₄ phases, whereas ZrO₂ is partially reduced to Zr₃O and Zr during synthesis of ZrO₂-Mg system. The reaction between elemental Zr and MgB₄ results in ZrB₂ formation. The particle size of ZrB₂ is found to decrease with the addition of SHS diluent.     

The self-propagating high-temperature synthesis (SHS) of ultrafine high-purity TiC powder from TiO2+Mg+C

The self-propagating high-temperature synthesis (SHS) method has been applied to the preparation of TiC powder from mixtures of TiO₂, Mg, and C. The TiC/MgO product is leached by hydrochloric acid to remove the MgO. The optimum mixing ratio was TiO₂MgC=12.21.5. The final product TiC had > 99.9% purity and relatively uniform particle size of 0.3–0.4 m.     

Effect of HCl concentration on TiB2 separation from a self-propagating high-temperature synthesis (SHS) product

This paper presents a synthesis of TiB₂ powder via self-propagating high-temperature synthesis (SHS) method using a mixture of TiO₂, B₂O₃ and Mg followed by acid leaching. In the acid leaching step, the MgO content in the SHS product was leached in different HCl concentrations. X-ray diffraction (XRD) analysis showed that when 9.3 M HCl was used, the leached SHS product was found similar to that of the commercial TiB₂ powder. However, scanning electron microscopy (SEM) and BET surface area analysis revealed that the leached product was agglomerated and exhibited very high surface area.     

Layer-by-layer laser synthesis of Cu–Al–Ni intermetallic compounds and shape memory effect

We have studied conditions for the synthesis of intermetallic phases in the Cu–Al–Ni system by selective laser sintering/melting, in particular by heating a powder mixture to 300°C. The effects of laser synthesis and heating on the microstructure of the intermetallic phases in the samples obtained have been studied using electron microscopy, optical metallography, and X-ray diffraction analysis. The results demonstrate high sinterability of stoichiometric mixtures. Resistivity measurements indicate that the samples exhibit a shape memory effect. We discuss the feasibility of producing biomicroelectromechanical systems using layerby- layer synthesis.     

Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials

Self-propagating high-temperature synthesis (SHS) of powder compacts is a novel processing technique currently being developed as a route for the production of engineering ceramics and other advanced materials. The process, which is also referred to as combustion synthesis, provides energy- and cost-saving advantages over the more conventional processing routes for these materials. At the same time, the rapid heating and cooling rates provide a potential for the production of metastable materials with new and, perhaps, unique properties. This paper reviews the research that has been, and is being, undertaken in this exciting new processing route for high-technology materials and examines the underlying theoretical explanations which will, eventually, lead to improved control over processing parameters and product quality.     

金属间化合物Ni3Al薄膜的制备及性能研究

采用直流磁控共溅射法,在衬底温度为450℃的SiO2基体上制备了厚度为500nm的Ni3Al薄膜,X射线衍射(XRD)和透射电子显微镜(TEM)等测试表明,薄膜为(111)取向的L12型晶体结构金属间化合物。采用纳米压痕方法测试了薄膜的力学性能,其硬度为8.00GPa,弹性模量为200GPa。为克服亚微米级薄膜氧化增重难以测量的困难,采用四探针测试金属薄膜电阻的方法,间接给出了薄膜的腐蚀性能和高温氧化程度。结果表明Ni3Al金属间化合物薄膜的氧化速率为2.28×10-13g2/(cm4.s),薄膜具有良好的高温抗氧化性能。     

Assessment and modelling of isothermal forging of intermetallic compounds. Part 3 – Ni3Al

Abstract

An alloy based on Ni₃(Al, Cr), IC­264, has been isothermally forged in the temperature range 950–1100°C and strain rate range 3×10^-4 to 3.5×10^-2 s^-1. Stress–strain curves show flow softening for most forging conditions, suggesting dynamic recrystallisation, as does microstructural examination. Deformation modelling has been carried out using a simple constitutive relationship.