The use of titanium aluminides to form electric-spark coatings

Titanium aluminides (TiAl₃, TiAl, Ti₃Al) fabricated by powder metallurgy were used as alloying electrodes for the formation of electric-spark coatings. Intermetallic coatings were deposited on steel substrates in argon or nitrogen. The microstructure and composition of fabricated coatings were investigated by scanning electron microscopy, X-ray structural analysis, and electron probe microanalysis. It is established that initial Ti–Al intermetallic phases are present in fabricated coatings; however, the ratio between Ti and Al concentrations is shifted to aluminum compared with the stoichiometric one. When depositing titanium aluminide in the nitrogen medium, titanium nitride is additionally formed in surface layers. Thermal and tribotechnical tests showed that the Ti₃Al coating deposited in nitrogen possesses high wear resistance and heat resistance.     

Microstructural and Phase Composition Differences Across the Interfaces in Al/Ti/Al Explosively Welded Clads

The microstructure and phase composition of Al/Ti/Al interfaces with respect to their localization were investigated. An aluminum-flyer plate exhibited finer grains located close to the upper interface than those present within the aluminum-base plate. The same tendency, but with a higher number of twins, was observed for titanium. Good quality bonding with a wavy shape and four intermetallic phases, namely, TiAl₃, TiAl, TiAl₂, and Ti₃Al, was only obtained at the interface closer to the explosive material. The other interface was planar with three intermetallic compounds, excluding the metastable TiAl₂ phase. As a result of a 100-hour annealing at 903 K (630 °C), an Al/TiAl₃/Ti/TiAl₃/Al sandwich was manufactured, formed with single crystalline Al layers. A substantial difference between the intermetallic layer thicknesses was measured, with 235.3 and 167.4 µm obtained for the layers corresponding to the upper and lower interfaces, respectively. An examination by transmission electron microscopy of a thin foil taken from the interface area after a 1-hour annealing at 825 K (552 °C) showed a mixture of randomly located TiAl₃ grains within the aluminum. Finally, the hardness results were correlated with the microstructural changes across the samples.

     

Layered composites made from bimetallic strips produced by plasma spraying of TiAl on niobium

The production and structure of a multilayer TiAl/Nb composite material made from bimetallic TiAl/Nb strips fabricated by plasma spraying of TiAl granules onto niobium plates are studied. Here, 3-mm-and 2-mm-thick plates of a layered composite material (LCM) are produced by hot isostatic pressing of a stack of 35 bimetallic plates followed by hot rolling (the total degree of reduction is 78.6 and 85.7%, respectively). The LCM consists of discontinuous TiAl layers separated by niobium layers, and the adhesion between the layers is good. Diffusional intermediate layers form at the TiAl/Nb interfaces in the 3-mm-thick LCM and consist of the following two solid solutions: an α₂-Ti₃Al-based solid solution contains up to 28 at % Nb, and a niobiumbased solid solution contains up to 27 at % Ti and 32 at % Al. The diffusional intermediate layers in the 2-mmthick LCM plates consist of an α₂-Ti₃Al-based solid solution with up to 16.0 at % Nb; a τ-Ti₃Al₂Nb-or Ti₄Al₃Nb-based solid solution with 51.5 at % Ti, 32 at % Al, and 16.5 at % Nb; and a niobium-based solid solution with up to 22 at % Ti and 30.5 at % Al. When a bimetallic TiAl/Nb strip is fabricated by plasma spraying of granules of the Ti-48 at % Al alloy, this alloy is depleted of aluminum to 42–45 at %, and the fraction of the α₂-Ti₃Al phase in the sprayed layer increases. When the LCM is produced by hot isostatic pressing followed by hot rolling, the layer of plain niobium (Nb1) dissolves up to 5 at % Ti and 7 at % Al.     

Composition and properties of the diffusion layer formed upon heat treatment of an aluminum gas-thermal coating on titanium

An aluminum gas-thermal coating 200–300 μm thick is deposited onto the surface of titanium samples with an arc discharge. After heat treatment of the samples at 900 or 1000°C for 2, 4, or 6 h, the chemical composition and properties of the diffusion layer formed as a result of titanium and aluminum interdiffusion are studied. The main phase of the diffusion layer is found to be TiAl₃. The fractions of the phases TiAl₂, TiAl, and Ti₃Al are insignificant. The adhesion of the diffusion layer to the titanium substrate, the wear resistance, the friction coefficient, the hardness, the corrosion resistance, and the thermal and electrical conductivities are studied as functions of the heat-treatment temperature and time.     

Fabrication of aluminum–ceramic skeleton composites based on the Ti<Subscript>2</Subscript>AlC MAX phase by SHS compaction

A one-stage manufacturing technology of aluminum–ceramic skeleton composites by combining the processes of self-propagating high-temperature synthesis (SHS) of a porous skeleton formed by the MAX phase of the Ti₂AlC composition and its impregnation by the aluminum melt under pressure (SHS compaction) is considered. A composition of the exothermic charge 2Ti + C + 22.5 wt % Al + 10 wt % TiH₂, which provides the formation of a porous skeleton of the Ti₂AlC phase without impurity phases by the SHS technology, is selected. It is shown that, when impregnating the hot SHS skeleton with aluminum, new phases are formed such as the MAX phase (Ti₃AlC₂), titanium carbide (TiC), and titanium aluminide (Al₃Ti). However, the content of the basic MAX phase remains high, and the ceramic component of the material consists of Ti₂AlC by 76%. When analyzing the microstructure, it is revealed that the composite has certain residual porosity after impregnation and cooling. The influence of the impregnation pressure (q = 22, 28, and 35 MPa) on the distribution of the aluminum content over the height and radius of the diametral sample section is investigated experimentally. It is shown that the nonuniform Al distribution over the sample bulk is caused by the nonuniform pressure and temperature fields, as well as the different compactibility of hot inner and colder outer sample parts. The degree of compaction of characteristic zones is leveled as the impregnation pressure increases, and the composition inhomogeneity over the sample bulk decreases. The difference in aluminum concentration over the sample bulk at q = 35 MPa does not exceed 5%. The SHS-compacted aluminum–ceramic skeleton composite based on the Ti₂AlC MAX phase corresponds to high-strength Al-Zn–Mg–Cu aluminum alloys by the hardness level (HB ≈ 150 kg/mm²).     

Effect of Coatings on the Heat Resistance of VT-41 and VIT1 Alloys during Isothermal Oxidation

The influence of aluminide coatings on high-temperature α + β VT-41 alloy and a VIT1 alloy based on the Ti₂NbAl orthophase on the isothermal heat resistance at 700°C is studied during oxidation in still air. NiCrAlY and Al(Si) coatings are found to substantially increase the heat resistance of the alloys at 700°C. The phase composition of the aluminosilicide layer (TiAl₃ + TiSi₂ + Ti₅Si₃) under the NiCrAlY coating significantly retards the titanium diffusion to the NiCrAlY layer and can be used as a diffusion barrier and a aluminum source for the NiCrAlY layer during high-temperature oxidation.     

Effect of Hot Rolling on Grain Refining Performance of Al–5Ti–1B Master Alloy on Hot Tearing on Al–7Si–3Cu Alloy

It has been known experimentally that TiAl₃ acts as a powerful nucleant for the solidification of aluminum from the melt; however, a full microscopic understanding is still lacking. To improve microscopic understanding, hot rolling technique has been performed to the Al–5Ti–1B alloy and the effect of shape and size of the particles on grain refinement has been studied. The effect of hot rolling of Al–5Ti–1B master alloy on its grain refining performance and hot tearing have been studied by OM, XRD, and SEM. Hot rolling improves the grain refining performance of this master alloy, which is required to reduce hot tearing in Al–7Si–3Cu alloy. The improvement in grain refining performance of Al–5Ti–1B master alloy on rolling is due to the fracture of larger TiAl₃ particles into fine particles during rolling. The presented results illustrate that the morphology of TiAl₃ particles alter from the plate-like structure in the as-cast condition Al–5Ti–1B master alloy to the blocky type after rolling due to the fragmentation of plate-like structures. The grain refining response and effect on hot tearing of Al–7Si–3Cu alloy have been studied with as-cast and rolled Al–5Ti–1B master alloys. The results display hot-rolled master alloys revealing enhanced grain refining performance and minimizing hot tear tendency of the alloy at much lower addition level as compared to as-cast master alloys.     

Preparation of TiAl15Si15 intermetallic alloy by mechanical alloying and the spark plasma sintering method

This work is devoted to the preparation of alloys based on intermetallic compounds in the Ti–Al–Si system by powder metallurgy using mechanical alloying and the spark plasma sintering (SPS) method. The aim was to describe the formation of intermetallic phases during mechanical alloying of TiAl15Si15 (wt-%) alloy and to consolidate the powder prepared by optimised conditions. Phase composition, microstructure and hardness of compacted alloy were determined. Four hours of mechanical alloying is sufficient time for preparation of pure elements free material composed only of intermetallic phases. After consolidation, the TiAl15Si15 alloy has a homogeneous structure composed of silicide (Ti₅Si₃) in aluminide (TiAl) matrix. The hardness of the material reaches 865?±?42 HV 5.     

Influence of Nb and Cu Elements on the Intermetallic Particles Morphology in Ti/Al Multilayer Composite Processed Through Hot Pressing and Rolling

Ti–Al–Nb composites were produced by solid state diffusion bonding through hot pressing and rolling followed by annealing at 700 °C for 0.5, 1, 1.5 and 2 h. The morphologies of TiAl₃ intermetallics were investigated by Scanning Electron Microscopy combined with Energy-dispersive X-ray spectroscopy. Titanium tri-aluminide (TiAl₃) particles with blocky morphology were dispersed into Aluminum matrix. In the presence of niobium and copper, TiAl₃ particles were produced in different sizes and morphologies. The presence of Nb in the composite led to the formation of irregular angular morphology, while the copper resulted in cubic morphology of the intermetallic particles. The EDS results indicated that TiAl₃, (Ti, Nb)Al₃ and (Ti, Nb, Cu)Al₃ intermetallic compounds appeared near Ti zone, Nb Zone and in the presence of Cu, respectively.     

Investigation of the reaction zone between TiAl and Mo

Pure Mo was incorporated in TiAl matrixvia two different routes: (1) hot pressing of alternately sandwiched Ti-Al sheets and Mo foils; and (2) coextrusion and heat treatment of Ti-Al green compact and Mo rod. The reaction zone between TiAl and Mo is found to contain two intermetallic phases: β-(Mo,Ti)Al andp-(Mo,Ti)₃Al. The β-p boundary is incoherent, whereas the TiAl-β andp-Mo boundaries are semicoherent. The reaction zone grows with increasing heat-treatment time in a parabolic form. The incorporated Mo exhibits lower hardness than the TiAl matrix, implying that ductilizing and toughening of TiAl by introducing Mo as a ductile reinforcement are possible.     

Phase transitions in reactive formation of Ti5Si3/TiAl in situ composites

A new method is developed for preparing Ti₅Si₃/TiAl in situ composites by incorporating metastable phases (called metastable precursors) into TiAl (a mixture of elemental Ti and Al) matrix powders. Metastable precursors with a starting composition of Ti-14Al-21Si are prepared by mechanical alloying (MA). They have been proven through X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) observations to be mainly consisting of mixtures of nanostructured solid solutions and milling-formed TiAl compound. Particularly, phase reactions and transitions in the precursors and the composites during heating have been investigated in detail by using diffraction thermal analysis (DTA) in conjunction with XRD. It has been found that Ti₅Si₃ is in situ formed through a phase transition chain, TiSi₂ → Ti₅Si₄ → Ti₅Si₃. When the composite powder (precursor, Ti and Al) is heated, a combustion reaction first occurs in the matrix, which results in the formation of TiAl₃ and/or TiAl followed by the completion of the previously mentioned silicide transitions in a very short time. Scanning electron microscope (SEM) observations indicated the locations of reinforcements in the reaction-formed composite, and TEM observation provided some details of the structures for the reinforcements and their neighborhood. This method is intriguing because a designed phase hierarchy is possible.     

Phase equilibria investigations on the aluminum-rich part of the binary system Ti-Al

The binary system Ti-Al has been reinvestigated in the composition range of 50 to 76 at. pct Al by X-ray diffraction, metallography, electron probe microanalysis (EPMA), and differential thermal analysis (DTA). Heat-treated alloys (600°C to 1300°C) as well as the as-cast alloys were investigated. Seven stable intermetallic phases were observed: TiAl, Ti_1−x Al_1+x , Ti₃Al₅, TiAl₂, Ti₅Al₁₁, TiAl₃ (h), and TiAl₃ (1); two metastable phases, TiAl₂ (m) and TiAl₃ (m), were also found. For each of these phases, the homogeneity range and the crystal chemical parameters were determined. The temperatures of the solid-state phase reactions were re-established. On the basis of the experimental results, an improved version of the equilibrium phase diagram has been drawn and critically compared with earlier versions presented in the literature.     

Reaction products of Al/TiC composites fabricated by the pressureless infiltration technique

The interfacial reaction products of the Al-Mg/TiC _p composite fabricated by the pressureless infiltration method were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). During the fabrication of composites, reaction products with various morphologies and sizes were formed in the A1 matrix as well as in the vicinity of the TiC particles by the interfacial reaction between the Al alloy and the TiC particles. From the EDS and selected-area diffraction pattern (SADP) analysis, Al₄C₃, Al₁₈Ti₂Mg₃, Ti₂AlC, Al₃Ti, and TiAl could be identified to form as interfacial reaction products. Both the size and the amount of the reaction products were increased with increasing fabrication temperature as well as fabrication time. Coarse Al₃Ti was barely observed in water-quenched composites, while it was observed at all fabrication temperatures (700 °C to 1000 °C) in furnace-cooled conditions. An erratum to this article is available at .     

Diffusion of oxygen in the Al2O3 oxidation product of TiAl3

The oxidation behavior of TiAl₃ was investigated. The studies were carried out using thermogravimetric analysis (TGA) in the temperature range from 1123 to 1273 K in a 1 atm pure oxygen environment. Samples were analyzed using the X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersion X-ray analysis techniques. The oxidation product was determined to be Al₂O₃ over the temperature range of the investigation. The parabolic-rate constant for TiAl₃ was deduced and compared with those for Ti, TiAl, and Ti₃Al. The electronprobe microanalysis technique was used to obtain concentration profiles for O, Al, and Ti in the oxide layer and the matrix alloy. The parabolic-rate data were used to calculate the diffusivities of oxygen at various temperatures. The activation energy for diffusion was determined to be 337.66 kJ/mol, while the frequency factor (D ₀) was 167.2 × 10⁻⁴ m²/s.     

Modeling creep deformation of a two-phase TiAI/Ti3Al alloy with a lamellar microstructure

A two-phase TiAl/Ti₃Al alloy with a lamellar microstructure has been previously shown to exhibit a lower minimum creep rate than the minimum creep rates of the constituent TiAl and Ti₃Al single-phase alloys. Fiducial-line experiments described in the present article demonstrate that the creep rates of the constituent phases within the two-phase TiAl/Ti₃Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single-phase TiAl and Ti₃Al alloys tested in compression at the same stress and temperature. Additionally, the fiducial-line experiments show that no interfacial sliding of the phases in the TiAl/Ti₃Al lamellar alloy occurs during creep. The lower creep rate of the lamellar alloy is attributed to enhanced hardening of the constituent phases within the lamellar microstructure. A composite-strength model has been formulated to predict the creep rate of the lamellar alloy, taking into account the lower creep rates of the constituent phases within the lamellar micro-structure. Application of the model yields a very good correlation between predicted and experimentally observed minimum creep rates over moderate stress and temperature ranges. Formerly with the Department of Materials Science and Engineering, University of Virginia     

Self-propagating high-temperature synthesis of composite targets based on titanium carbonitride,silicide, and aluminide for ion-plasma deposition of multifunctional coatings

The macrokinetic features of combustion of the mixtures in the Ti-Al-Si₃N₄-C system calculated for the formation of compact ceramic materials (CCMs), the composition of which is described by the general formula X(TiAl₃) + (100 − X)(0.448TiC_0.5 + 0.552(Ti₅Si₃ + 4AlN) with mixture parameter X = 10–50%, are investigated. Compact CCM samples with the main structural components in the form of TiC _x N _y grains and binding phases TiAl3 and Ti5Si3 are fabricated by the technology of forced self-propagating high-temperature synthesis (SHS) compaction. An increase in X promotes the formation of the M _n +1AX _n phase with the composition Ti₃SiC₂ in the synthesis products. Complex investigations into the physicomechanical properties of the obtained ceramics are performed. Based on their results, the inverse dependence of the density and hardness of compact materials on parameter X is established. Tests of the samples for oxidation resistance showed that the obtained CCMs based on titanium carbonitride, silicide, and aluminide possess excellent resistance to high-temperature oxidation, and their oxidation rate in air at t = 900°C for 30 h does not exceed 7.8 × 10⁻⁵ g/(m² s).     

The deposition of aluminide and silicide coatings on γ-TiAl using the halide-activated pack cementation method

The halide-activated pack cementation method (HAPC) was utilized to deposit aluminide and silicide coatings on nominally stoichiometric γ-TiAl. The deposition temperature was 1000°C and deposition times ranged from 2 to 12 hours. The growth rates of the coatings were diffusion controlled, with the rate of aluminide growth being about a factor of 2 greater than that of silicide growth. The aluminide coating was inward growing and consisted of a thick, uniform outer layer of TiAl₃ and a thin inner layer of TiAl₂, with the rate-controlling step being the diffusion of aluminum from the pack into the substrate. Annealing experiments at 1100 °C showed that the interdiffusion between the aluminide coating and the γ-TiAl substrate was rapid. In contrast to the aluminide coating, the silicide coating was nonuniform and porous, consisting primarily of TiSi₂, TiSi, and Ti₅Si₄, with the rate-controlling step for the coating growth believed to be the diffusion of aluminum into the γ-TiAl ahead of the silicide/γ-TiAl interface. The microstructural evolution of the aluminide and silicide coating structures is discussed qualitatively.     

Reactions and diffusion between an Al film and a ti substrate

The reaction between a 0.5 to 1.0 micron Al film and a thick Ti substrate to form TiAl₃ occurs very rapidly on heating to 635 °C and causes the Al to be confined to the surface region. After heating to 900 °C, Ti₃Al is formed with little release of Al into α-Ti. Further annealing at 900 °C eventually causes the Ti₃Al phase to decompose and a substantial amount of Al is released into α-Ti. The interdiffusion coefficient for Al in α-Ti at 900 °C was found to increase by less than one order of magnitude as Al is varied from 0 to 20 at. pct. These data were obtained from the (101) X-ray diffraction intensity band using polycrystalline samples. Improvements in the analysis of X-ray diffraction data for the determination of composition profiles are discussed.     

Effect of phase morphology on the mechanical behavior of two titanium aluminide composites

Two binary titanium aluminide alloys, Ti-43A1 and Ti-47A1 (atomic percent), were discontinuously reinforced with 6 vol pct titanium diboride, resulting in a two-phase Ti₃Al and TiAl (α₂ and γ, respectively) matrix with a dispersion of TiB₂ particulate. Cast material was successfully ex-truded and subjected to a series of single-step and duplex-step heat treatments. Thermo-mechanical processing was correlated with microstructural changes, and the ambient temperature mechanical properties were measured for the various heat-treated conditions using tensile and hardness testing. Yield stress and plastic elongation to failure and hardness were found to cor-relate well with the fraction of proeutectoid, or primary, TiAl formed during heat treatment within the α/γ phase field. Precipitation of y within proeutectoid α grains during subsequent aging treatments within the α₂ phase field was seen to increase the room-temperature ductility with negligible debits in yield stress. Enhanced ductility and decreases in yield stress and hard-ness are associated with morphologically large regions of the TiAl phase. Incompatibility of slip systems across γ/α₂ and the inherent resistance to slip in hyperstoichiometric Ti₃Al are suggested as possible explanations for the observed phenomena. Formerly with Martin Marietta Laboratories     

Shock densification/hot isostatic pressing of titanium aluminide

Consolidation of rapidly solidified titanium aluminide (Ti₃Al) powders employing explosive shock pressure followed by hot isostatic pressing (“hipping”) was carried out successfully. Shock densification was achieved by using a double tube design in which the flyer tube was explosively accelerated, impacting the powder container. Elemental mixtures of Ti (15 wt pet) and Al (15 wt pct) powders were added to intermetallic compound powders (Ti₃Al). Hipping was used to chemically induce bonding between Ti₃Al particles. The highly exothermic reactions were activated by hipping at 1000 ‡C and enhanced the bonding between the inert intermetallic powders. Compression tests indicated strong bonding between Ti₃Al particles. Well-bonded Ti₃Al compacts having an average ultimate compressive strength of 2 GPa and compressive fracture strain of 20 pct were produced by this technique. The ultimate tensile strengths, due to the presence of flaws in the microstructure (microcracks and voids) and intergranular fracture observed in the reacted regions, were much lower (~250 MPa).