Preferential formation of Al self-interstitial defects in γ-TiAl under irradiation (original) (raw)
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Plastic deformation of the intermetallic Al 3 Ti
Philosophical Magazine A, 1991
X-ray diffraction and high resolution electron microscope techniques have been used to examine the degree of order, atom site location, and precipitation in ternary titanium trialuminide alloys. For some of the alloys studied it is shown that precipitation of AI2Ti occurs, essentially because of an excessively high Ti content. For all the alloys there are subtle changes occurring on annealing at temperatures of 600-800°C which affect both hardness and toughness of the materials. In an AI-Ti-Fe alloy there are signs of incipient, fine-scale precipitation, probably of A12Ti. In all these intermetallics there is partial chemical disorder, which is taken to suggest an inherent structural tendency to tetragonal atomic arrangement, even though the crystal retains the ordered LI 2 cubic structure. The improved ductility of some of these interrnetallics may follow from the reduced order obtained as a result of alloying. R~sum~-43n utilise la diffraction des rayons X et la microscopic 61ectronique ~ haute r~solution pour examiner le degr6 d'ordre, la localisation des sites atomiques et la precipitation dans des alliages temaires de trialuminiure de titane. Pour certains alliages ~tudi~s, on montre qu'une pr~cipiatation de A12Ti se produit essentiellement par suite d'une teneur en Ti excessivement ~levte. Pour tousles alliages, il existe de subtiles variations qui se produisent pendant le recuit/t des temperatures de 600-800°C et qui affectent la fois la duret~ et la ttnacit~ des mat~riaux. Dans un alliage AI-Ti-Fe, il y a des signes d'une precipitation naissante ~ tr~s petite ~ehelle, probablement de A12Ti. Dans tous ces interm~talliques, il existe un d~sordre chimique partiel qui sugg~r¢ une tendance structurale inh~rente ~i un arrangement atomique quadratique, m~me lorsque le cristal garde la structure cubique L12 ordonn~e. La ductilit~ am~liorte de certains de ces alliages intermttalliques peut r~sulter de l'ordre r~duit rtsultant de la composition de l'alliage. Zusammenfassung-Ordnungsgrad, Atomorte und Ausscheidungen werden in tem/iren Trialuminid-Legierungen mittels R6ntgenbeugung und hochaufltsender Elektronenmikroskopie untersucht. Bei einigen untersuchten Legierungen scheidet sich AI2Ti aus, im wesentlichen wegen des auBergewfhnlich hohen Ti-Gehaltes. Bei allen Legierungen treten feine .~nderungen beim Ausheilen zwischen 600-800°C auf, die sowohl H~irte als auch Z/ihigkeit des Materials beeinflussen. In einer A1-Ti-Fe-Legierung gibt es Hinweise auf entstehende feine Ausscheidungen, wahrscheinlich A12Ti. In allen diesen intermetallischen Legierungen besteht eine chemische Teilentordnung, welehe als der Hinweis auf eine inh/irente struktureUe Tendenz zu einer tetragonalen Atomanordnung gewertet wird, auch wenn der Kristall seine geordnete kubische Ll:Struktur beibeh/ilt. Die verbesserte Duktili~t einiger dieser intermetallischen Legierungen kann durch die verringerte Ordnung, die sich durch die Legierungsbildung ergibt, verursacht sein.
Development and microstructure of the Al-depleted layer of oxidized TiAl
Materials and Corrosion/Werkstoffe und Korrosion, 1997
The microstructure and development of the scale of oxidized y-TiAl were studicd in cross section by electron microscopy and metallographic techniques. Ti-SO(at.%)Al samples wcre oxidized at 900°C in air for various times and especially the evolution of the scale/metal interface was investigated. The formation of the aluminium depleted subsurface layer could be observed after 1 h and develops from a single phase to a two phase region with longer exposure times. The single phase region is visible up to I00 h and consists of a simple cubic phase with lattice parameter a = 0.692 nm. After an oxidation time of 140 h the formation of a two phase region consisting of az-Ti3Al and the cubic phase occurred. Both phases are depleted in aluminium compared to the base metal and contain some amount of oxygen whereby the cuhic phase contains more Al and 0 than a2-Ti3Al. The metallographic cross sections suggest that the U2-Ti3AI phase forms at the y-TiAli cubic-phase interface and grows into the cubic phase.
Growth Characterization of Intermetallic Compound at the Ti/Al Solid State Interface
Materials
Ti-Al diffusion couples, prepared by resistance spot welding, were annealed up to 112 hours at 823, 848, and 873 K in ambient atmosphere. The interfacial microstructure was observed and analyzed using SEM and TEM. The growth characterization of intermetallic compound formed at the Ti/Al solid state interface was investigated. Only TiAl3 phase was detected in the interfacial zone, and its growth was governed by reaction-controlled mechanism in the previous period and by diffusion-controlled mechanism in the latter period. The activation energies were 198019 and 122770 J/mol for reaction-controlled and diffusion-controlled mechanism, respectively.
Metallurgical and Materials Transactions A, 2014
Metallic-intermetallic laminate composites are promising materials for many applications, namely, in the aerospace industry. Ti/TiAl 3 laminates are one of the interesting laminate composites, which are mostly used in aerospace applications. In this work, commercially pure aluminum and titanium sheets were explosively joined. The multilayer samples were annealed between 1 and 260 hours at 903 K (630°C) in ambient atmosphere, and the formation and growth of the intermetallic compound at the Ti/Al interface were monitored. Microstructural investigations were carried out using optical and scanning electron microscopes equipped with energy-dispersive spectroscopy and the X-ray diffraction technique. The microhardness profile of the layers was also determined. The thickness and type of Al-Ti intermetallics were determined. It was found that the only intermetallic phase observed in the interfaces was TiAl 3 . It was also shown that two mechanisms for TiAl 3 growth exist: reaction and diffusion controlled. The growth exponent was equal to 0.94 for the reaction-controlled mechanism (first step) and 0.31 for the diffusion-controlled mechanism (second step). These values were in good agreement with theoretical values (1 and 0.5 for the first and second steps, respectively). Based on the results of this research, a kinetic model for the formation and growth of TiAl 3 intermetallic phase was proposed.
Interdiffusion Behavior of Aluminide Coated Two-Phase α2-Ti3Al/γ-TiAl Alloys at High Temperatures
Journal of Engineering and Technological Sciences
Lower density materials of TiAl based intermetallic alloys have recently attracted intensive attention for the replacement of nickel-based superalloys used at high temperatures. As aluminium-rich titanium aluminide intermetallic compounds are normally brittle, two-phase α 2-Ti 3 Al/γ-TiAl alloys have been developed. To increase the corrosion resistance of these alloy systems, an aluminide coating of TiAl 3 layer is normally applied. During operation at high temperatures, however, interdiffusion between the coating and the alloy substrate can occur and decrease the TiAl 3 layer thickness of the coating. The effects of temperature exposure on the growth of the TiAl 2 interdiffusion zone layer on two-phase α 2-Ti 3 Al/γ-TiAl alloys with a chemical composition of Ti-47Al-2Nb-2Cr-0.5Y-0.5Zr are presented in this paper. The exponents for kinetics and rate constant of the TiAl2 interdiffusion layer growth of this multi-component system were found under variation of temperature. The results were compared with those from other researchers.
Materials Letters, 2019
Intermetallic compound (IMC) layer growth at dissimilar metal interfaces, controlled by diffusion, usually follows a parabolic law. However, in the present study, the growth of Al 3 Ti layer on the Al-Ti interface at a low annealing temperature (T A , 500 °C) obviously didn't follow a parabolic law. A very long incubation stage of Al 3 Ti layer growth was observed in the Al-Ti diffusion couple annealed at 500 °C. Residual oxide layers and an oxygen rich layer were experimentally observed on Al-Ti and Al 3 Ti-Ti interface regions using high resolution STEM-EDS. The very long incubation stage is probably caused by the oxide layers and the oxygen rich layer on the interface region, which act as a diffusion barrier and as a source of oxygen atoms. The length of the incubation stage became shorter and even unobservable with increasing temperature probably due to the higher reaction rate between titanium oxides and aluminum at higher temperatures.
Phase Reactions and Processing in the Ti-Al based Intermetallics
ISIJ International, 1991
In the developmentof high temperature intermetallics, it has becomeevident that it is essential to consider the strong influence of materials processing. Among the fundamental data needed for effective processing are the relevant phasediagrams, the characteristic diffusivities and possible solidification reaction pathways. In the Ti-AI system recent advances in the clarification of the phase diagram have had a direct impact on the analysis of phase stability and crystal growth processes. Building on the binary phase equilibria, it has been possible to develop new insight into the Ti-Al-Nb ternary system and the identification of ternary intermetallic phase reactions. Simiiarly, diffusion couple studies have allowed for an analysis of reaction rates that is a necessary bases for an effective microsttuctural control and design strategy especially in the case of intermetallic matrix composites.
The Kinetics of TiAl 3 Formation in Explosively Welded Ti-Al Multilayers During Heat Treatment
Metallic-intermetallic laminate (MIL) composites, including Ti/TiAl 3 composite, are promising materials for many applications, namely, in the aerospace industry. One method to produce Ti/ TiAl 3 laminate composite is to provide close attachment between desired number of titanium and aluminum plates, so that by applying heat and/or pressure, the formation of intermetallic phases between the layers becomes possible. In this work, explosive welding was used to make a strong bond between six alternative Ti and Al layers. The welded samples were annealed at three different temperatures: 903 K, 873 K, and 843 K (630 °C, 600 °C, and 570 °C) in ambient atmosphere, and the variation of the intermetallic layer thickness was used to study the growth kinetics. Microstructural investigations were carried out on the welded and annealed samples using optical microscopy and scanning electron microscopy equipped with energy-dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) technique was used to identify the formed intermetallic phases. It was found that at each temperature, two different mechanisms govern the process: reaction controlled and diffusion controlled. The calculated values of activation energies for reaction-controlled and diffusion-controlled mechanisms are 232.1 and 17.4 kJ, respectively.
Estimation of point defect formation energies in the D019-type intermetallic compound Ti3Al
Solid State Sciences, 2002
A statistical-thermodynamic model for binary nonstoichiometric intermetallic A 3 B compounds with D0 19 -structure was developed based on a mean-field approximation. Vacancies and anti-structure atoms are allowed on the two sublattices as possible point defects. Due to the identical stoichiometry and the analogous coordination around A and B atoms it turned out that the same approach is valid as for A 3 B compounds with L1 2 -structure, and identical expressions were obtained, both for the concentrations of the different point defects and for the thermodynamic activities. The energies of formation of the four types of point defects were used as parameters. The model equations were applied to the intermetallic compound Ti 3 Al using experimental aluminum activities from the literature. By a simple curve fitting procedure the following defect formation energies were obtained: E f (V Ti ) = E f (V Al ) = 1.5 eV, and E f (Ti Al ) = E f (Al Ti ) = 0.6 eV. This results in very low vacancy concentrations which means that the thermal disorder and the deviation from stoichiometry in Ti 3 Al is caused almost entirely by anti-structure atoms. Their concentrations (referred to the total number of lattice sites) are found to be about 0.0009 at 1123 K, i.e., 0.12% of the Ti sites are occupied by Al atoms and 0.36% of the Al sites by Ti atoms. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.