TiAl Phase Research Articles

Microstructure, oxidation and mechanical properties of a diffusion aluminide (Al3Ti) coated lamellar γ-TiAl alloy

The use of γ-TiAl alloys at higher temperatures exceeding 700–800 °C is restricted due to their inadequate oxidation resistance. Their durability and high temperature capability can be enhanced by the application of oxidation resistant coatings. The present study examines the cyclic oxidation behavior of a diffusion Al3Ti coated lamellar Ti–45Al–5Nb-0.2B-0.2C (in at.%) γ-TiAl alloy during exposure at 800 and 950 °C in air for 500 h. The effect of oxidation on the coating microstructure and the compression properties at room temperature, 800 and 950 °C are assessed. The coating forms a protective Al2O3-rich oxide scale, which lowers the overall oxidation rate and induces steady parabolic weight gain in the coated alloy; unlike the extensive weight loss occurring in the bare/uncoated alloy during oxidation at higher temperatures. The parabolic oxidation rate constant (Kp) for the coated alloy at 6–7 x 10−7 mg2 cm−4 s−2 is two orders of magnitude lower than that of the bare alloy. Surface embrittlement during oxidation causes deterioration in the compression yield strength of the bare alloy. On the other hand, the coating prevents oxidation-induced surface degradation of the alloy, which manifests in improved compression strength of the oxidized coated alloy as compared to that of the oxidized bare alloy. Though the coating is constituted of the brittle intermetallic Al3Ti phase, the cracks in the coating do not penetrate the substrate and are innocuous to the compression properties of the underlying intermetallic γ-TiAl alloy. The coating is beneficial in providing oxidation resistance and prevents significant oxidation-induced deterioration in compression strength of the alloy during high temperature exposure. Besides, the Al3Ti coating does not induce significant deterioration in the tensile strength of the γ-TiAl alloy.

Effects of niobium on phase composition and improving mechanical properties in TiAl alloy reinforced by Ti2AlC

The effect of niobium on microstructure and mechanical properties of Ti46Al2.6CxNb (at. %) alloys with Nb content ranging from 0 to 8 at.% have been studied. The alloys have been prepared by vacuum arc melting. The content of γ(TiAl) phase increases and content of α2(Ti3Al) phase decreases with increasing content of Nb. The lattice parameters of γ phase increase and parameters of the α2 phase remain stable when Nb content is higher than 4 at. %. There is no significant segregation of Nb in the studied Ti46Al2.6CxNb alloys. The size of lamellar colonies decreases with increasing content of Nb from 0 to 8 at. %. The increase of the lattice parameters and low segregation of Nb without formation of B2 phase are related to a higher solid solubility of Nb in the γ phase. Compressive strength and compressive strain increase with increasing content of Nb. Tensile strength of Ti46Al2.6C8Nb alloy increases from 330 MPa to 442 MPa and the strain increases from 2.9% to 5.3% with increasing test temperature from 750 °C to 950 °C. Solid solution strengthening by Nb, precipitation strengthening by Ti2AlC particles, increasing content of γ phase, and grain boundary strengthening are beneficial to improvement in mechanical properties of the studied alloys.

The influence of Al content from filler metals on tungsten inert gas welding-brazing of Mg-Ti

Three types of Mg-based fillers with various Al-containing were applied for the tungsten inert gas (TIG) welding-brazing of MB3 alloy and Ti-6Al-4V alloy. Results reveal that coarse α-Mg grains accompanied with black precipitates of Mg17Al12 phase occurred in the fusion zone, whereas thicker TiAl3 phase was formed in the brazing zone with the increasing Al-containing from filler wire. The evident decline of micro hardness was observed in the joint fusion area, owing to the comprehensive effect of grain growth and Orowan hardening mechanism. With optimum welding current of 65A and AZ91 filler, the sound Mg/Ti joint with joining strength of 202 N mm−1 was obtained. Eventually, the formation of interfacial layer was analyzed to further reveal the joining mechanism of Mg/Ti joint.

Preparation, microstructures and mechanical properties of in-situ TiB2/Al composites by mechanical stirring and subsequent ultrasonic treatment

In this work, in situ TiB2/2024Al composites were successfully fabricated by mechanical stirring and subsequent ultrasonic treatment. The effects of reaction temperature and enhanced phase mass fraction on the microstructure and mechanical properties of TiB2/2024Al composites were studied. The results show that the reaction temperature affects the degree of reaction of the Al-Ti phase. When the reaction temperature is too low, a sheet-like Al-Ti intermediate phase still exists inside the melt. However, when the reaction temperature reaches 850 °C, the Al-Ti phase is dissolved sufficiently and evolves into TiB2 particles. The ability of TiB2 particles to refine matrix structure was also discussed. As the mass fraction of TiB2 particles increases, the matrix structure is continuously refined. When compared to the base alloy, the yield strength, tensile strength and elongation are significantly improved.

Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al

In this work, TA15 joints reinforced in situ with TiAl3 IMCs were successfully fabricated using pure Al filler by ultrasonic brazing. TA15 alloys were first wetted by pure Al with and without ultrasonication and then ultrasonically brazed. The wetting of Al to the TA15 alloy was realized by the formation of a TiAl3 phase at the contacting interface. The complete wetting of the Al filler to the TA15 substrate was achieved at 20 min without ultrasonication. Ultrasonication significantly accelerated the wetting process, and the time required to achieve complete wetting was shortened to 10 s. During ultrasonic brazing, the formation of TiAl3 in the joint mainly depended on the holding time. The content of the TiAl3 phase increased from 35.2% to 77.2% as the holding time was increased from 5 min to 30 min. Joint strength increased with the amount of TiAl3 in the joint. Ultrasonication for 10 s and a holding time of 30 min generated a joint filled with homogeneously distributed TiAl3. The maximum shear strength of this joint was 140.6 MPa, which was 75% higher than that of pure Al. Failure crack propagated through the filler at short holding times and through the filler and TiAl3 phase at prolonged holding times.

Phase Equilibria in the Ternary Al–Ti–Pt System. IV. Vertical Sections in the Ternary Al–Ti–Pt System in the Composition Range 0–50 at.% Pt

Physicochemical analysis results for alloys in the Al–Ti–Pt system at 0–50 at.% Pt that were cast and annealed at subsolidus temperatures are used to construct for the first time vertical sections at 7, 25, and 35 ± 1 at. Pt of this system taking into account the constitution of its melting diagram. The congruent crystallization of τ3, τ 4, and τ8 compounds and incongruent crystallization of τ2 and τ5 compounds are confirmed. Changes from incongruent to congruent crystallization of the solid solutions based on binary Al3Pt2, Al2Pt, and TiAl phases in transition to the ternary system are shown.

Influence of additional elements (Si, Ti and B) on the microstructure, mechanical properties and castability of aluminum alloys (A201)

Improving the castability, mechanical properties and modifying the microstructure of the A201 alloy (Al - 4.97wt. % Cu – 0.56 wt. % Ag) were done by adding Si, Ti and B elements. The four alloys of A201 with additions of: (1) Si, (2) Ti and B, (3) Si, Ti and B, (4) non, were investigated in the as-cast, solution treated at 550°C for 20 hr and aged at 170°C up to 32 days conditions. Combination of different microscopes (HRSEM/EDS, TEM) and microhardness tests were used to investigate how the alloy properties influenced by the precipitation. Additions of Si resulted in the grain coarsening and an increase in the amount of near-grain boundary secondary phases. Additions of Ti and B resulted in the grain refinement, prevented hot tears, but caused a large amount of Al3Ti phase which makes the melt more viscose, and less fluid and castable. The mechanisms of the influence were recognized as facilitating the GP zones nucleation: Si atoms provide more probable homogeneous nucleation of the <001>α GP zones transforming to θ″ phase during aging; TiB2 particles serve as the sites for heterogeneous nucleation of <111>α GP zones which then transform to Ω phase. The precipitation sequence during aging was the following: SSSS → GP zones → θ″+ Ω → θ′ + Ω → θ′ → θ. The maximum microhardness was achieved with two kinds of semi-coherent precipitates in the structure: θ′ (Al2Cu-tetragonal) and Ω (Al2Cu-orthorhombic). Therefore, joint additions and corresponding thermal treatment can provide optimal microstructure and mechanical properties of the A201 based alloys.

Microstructure and refinement mechanism of TiB2/TiAl3 in remelted Al-5Ti-1B system

ABSTRACTIn this work, we propose a new method (by remelting Al-5Ti-1B) to investigate the grain refinement mechansim. It is found that the morphology and size of TiAl3 phase had little effect on the grain refinement of pure Al. Therefore, further experimental studies were carried out to understand the potency of TiB2 particles. The high-resolution transmission electron microscopy (HRTEM) observation has confirmed the existence of an atomic layer on the surface of (0001) TiB2, which is possibly a two-dimensional (2D) (1-12) TiAl3. Crystallographic study indicates that it is a more suitable nucleation sites for α-Al than other particles. The TiB2 particle with TiAl3 2D acts as the best effective nucleation sites for α-Al.

The Kinetic of Al3Ti Phase Growth in Explosively Welded Multilayered Al/Ti Clads during Annealing under Load Conditions

The Kinetic of Al3Ti Phase Growth in Explosively Welded Multilayered Al/Ti Clads during Annealing under Load Conditions

Strengthening and toughening of laminated TiAl composite sheets by titanium alloy layers and carbide particles

AbstractLaminated sheets with Ti2AlC/TiAl composite layers synthesized from the Ti–Al–TiC system and TC4 titanium alloy as the toughening layer were fabricated by spark plasma sintering. Their phase composition and microstructure were analyzed and flexural strength and fracture toughness measured. The composite layers mainly comprised Ti2AlC, TiC, TiAl, and Ti3Al phases. The flexural strength and fracture toughness of the sheets perpendicular to the laminated direction were 455.74 MPa and 21.65 MPa · m1/2, respectively. The TC4 layers provided a strengthening–toughening effect by changing the crack propagation model and weakening the fracture energy. Ti2AlC particles provided a strengthening effect. The synergistic effect of the two components enhanced the mechanical properties of these TiAl-based laminated composite sheets.

Al/Ti5Si3-Al3Ti composite prepared via in-situ surface coating of Ti using tungsten inert gas welding

This study tried to enhance simultaneously the surface micro-hardness as well as corrosion resistance of Ti by preparation of in-situ Al/Ti5Si3–Al3Ti composite using TIG process and 4043 Al filler alloy. The alloyed layers were characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). ASTM E384-HV and X-ray diffraction via ψ analysis were employed for determination of micro-hardness of prepared samples, and residual stresses at the interface of prepared coatings. To study the corrosion behavior of the final products used from NaCl 3.5 wt% electrolyte by potentiostat analysis. Surface coating was carried out by consideration of dilution ratio and passes number as operational variables. Results showed that, by increasing of pass number and filler injection rate, the thickness of Ti5Si3 layer at the interface gradually increased. In addition, Al3Ti phase formed far from the interface. In these conditions, the corrosion resistance and micro-hardness of prepared coatings were enhanced through the cladding. Analysis of residual stresses on the prepared coating at optimum condition confirmed the evolution of tensile residual stresses equal to 290 ± 40 MPa at the center line of fusion zone. The prepared sample with the highest surface micro-hardness as well as corrosion resistance was selected as optimum condition for investigating the effect of time (from 1 to 20 h s) and temperature (550–600 °C) through annealing process. Annealing treatment for 10 h s at 600 °C, not only enhanced the amount of Ti5Si3–Al3Ti phases and provided the possibility of the formation of adhesive and uniform Ti5Si3 layer at the interface of the clad/substrate but also raised the micro-hardness and corrosion resistance of the prepared coatings and determined as optimum condition.

Microstructures and Mechanical Properties of In-Situ Al3Ti/2024 Aluminum Matrix Composites Fabricated by Ultrasonic Treatment and Subsequent Squeeze Casting

Squeeze casting is a near net shaping technology which is advantageous to refining the microstructures and improving the mechanical properties. In the present work, in-situ Al3Ti/2024 Aluminum matrix composites with different amount of Al3Ti reinforcements were successfully fabricated by ultrasonic treatment and subsequent squeeze casting. The effects of specific pressure and the amount of reinforcements on microstructures and mechanical properties were studied. The results show that when the specific pressure is increased from 0 to 150 MPa, the average grain sizes of α-Al matrix are decreased by 39.8%, and the yield strength and compressive strength are increased by 16.8% and 22.9%, respectively. However, severe segregations of eutectic structures were generated under an excessive specific pressure of 200 MPa, which also results in deterioration in the mechanical strength. The mass fraction of Al3Ti phases has significant influence on morphology of eutectic structures. When the mass fraction of Al3Ti phases is increased from 4 to 16 wt%, the eutectic structures was changed from continuous network to dispersed structures gradually. The compressive strength was increased from 611.2 to 712.0 MPa (increased by 16.5%), as the Al3Ti content increasing from 0 to 16 wt%. However, as the mass fraction of Al3Ti phases increased, the variation in the yield strength is not monotonically increasing.

Rapid reactive synthesis of TiAl3 intermetallics by thermal explosion and its oxidation resistance at high temperature

Porous TiAl3 intermetallics were synthesized from Ti-75 at.% Al elemental powder mixtures using an energy-saving and rapid reactive method of thermal explosion (TE). The results demonstrated that the actual temperature of the compact climbed rapidly from 673 °C to 1036 °C within 24 s, indicating that an obvious TE reaction occurred during sintering process. The video graphs suggested that the TE in Ti–Al system behaved instant occurrence and overall heating whether from axial or radial direction. The silver wires and NaCl particles that pressed on the surface of the sample disappeared due to the heavy heat released during TE reaction. Only pure TiAl3 phases were synthesized in TE products and the open porosity of 55.4% was easy to obtain. After high-temperature treatment at 1000 °C, large amounts of sintering-neck formed and then improved the compressive strength of porous TiAl3 materials. Moreover, the mass gain curve of porous TiAl3 intermetallics oxidized at 650 °C for 120 h exhibited the parabolic oxidation rate law. XPS analysis confirmed that the strong O 1s peak was 531.4 eV which was the typical binding energy of Al2O3. Therefore, the excellent oxidation resistance of porous TiAl3 foams would be considered as good candidate materials for prolonging the service life at high temperatures.

Effects of Oxidation and Alumina Addition on the Physical and Mechanical Properties of Ti/Al2O3 Composites Prepared by Semi-powder Metallurgy Method

The effects of oxidation and alumina addition on the physical and mechanical properties of Ti/Al2O3 composites were studied. The variation in alumina addition used in this study was 0, 10, 20 and 30 wt%. The mixture of Ti and Al2O3 was prepared by semi-powder metallurgy method and then pressed and followed by sintering in air atmosphere at 1000 °C for 2 h. The present results show that the density of the sintered Ti/Al2O3 decreased with increasing alumina amount and oxidation. XRD and EDX analysis indicates that the sample free of alumina produced the oxides in the form of TiO2 on the surface of the composite. With alumina addition, the AlTiO2 oxide appears besides TiO2. This occurrence confirms that the oxidation of Ti increases with increasing the amounts of alumina. The intermetallic phase Ti3Al has appeared in the Ti/Al2O3 composites, which might be due to reduction in alumina by Ti. The oxidation of Ti/Al2O3 composites decreases the hardness and compressive yield strength and hardness values. The decrease in mechanical properties becomes more obvious with increasing the alumina amount which enhanced the formation of oxidation scales after sintering.

Metal/Semiconductor Barrier Properties of Non-Recessed Ti/Al/Ti and Ta/Al/Ta Ohmic Contacts on AlGaN/GaN Heterostructures

This paper compares the metal/semiconductor barrier height properties of non-recessed Ti/Al/Ti and Ta/Al/Ta contacts on AlGaN/GaN heterostructures. Both contacts exhibited a rectifying behavior after deposition and after annealing at temperatures up to 550 °C. The ohmic behavior was reached after annealing at 600 °C. High-resolution morphological and electrical mapping by conductive atomic force microscopy showed a flat surface for both contacts, with the presence of isolated hillocks, which had no significant impact on the contact resistance. Structural analyses indicated the formation of the Al3Ti and Al3Ta phases upon annealing. Furthermore, a thin interfacial TiN layer was observed in the Ti/Al/Ti samples, which is likely responsible for a lower barrier and a better specific contact resistance (c = 1.6 10−4 Ωcm2) with respect to the Ta/Al/Ta samples (c = 4.0 10−4 Ωcm2). The temperature dependence of the specific contact resistance was described by a thermionic field emission mechanism, determining barrier height values in the range of 0.58–0.63 eV. These results were discussed in terms of the different microstructures of the interfaces in the two systems.

Direct fabrication of bimetallic Ti6Al4V+Al12Si structures via additive manufacturing

Ti6Al4V + Al12Si compositionally graded cylindrical structures were fabricated on a Ti6Al4V substrate using laser engineered net shaping (LENS™) process. LENS™ fabricated materials had two regions of Ti6Al4V + Al12Si compositions, a pure Al12Si, and a pure Ti6Al4V area. Microstructural changes were affected by both laser power and compositional variations. In addition, TiSi2 and Ti3Al phase formations were also identified in low and high laser power processed Ti6Al4V + Al12Si sections, respectively. Moreover, the high laser power processed Ti6Al4V + Al12Si section showed the highest hardness value of 685.6 ± 10.6 HV0.1, which was caused due to the formation of new intermetallic phases. This high hardness section exhibited brittle failure modes during compression tests, while the pure Al12Si sections showed ductile deformation. The maximum compressive strengths of Ti6Al4V + Al12Si compositionally graded material was 507.8 ± 52.0 MPa. Our results show that compositionally gradient bulk structures of Ti6Al4V and Al12Si can be directly manufactured using additive manufacturing, however, performances can vary significantly based on process parameters and compositional variations.

Comparison of oxidation resistance of Al–Ti and Al–Ni intermetallic formed in situ by thermal spraying

A pure Al coating and an Al/Ni composite coating were deposited on the surface of a pure Ti substrate by thermal spraying. To enable the in situ reaction between the coatings and the substrate, the specimens were heated to 800 °C. Furthermore, the high-temperature oxidation resistance and the protection mechanism of the Ti–Al and Ni–Al intermetallic compounds were compared. The results showed that after heating to 800 °C, the diffusion region of the Al/Ti specimen mainly comprised the TiAl3 phase, and the thickness of the diffusion region first increased and then decreased as the reaction time was extended. The diffusion region of the Al/Ni/Ti specimen consisted of the Ni2Al3 and NiAl phases after heating to 800 °C, and the thickness of the Al/Ni diffusion layer showed parabolic growth as the reaction time was extended. After the oxidation treatment (800 °C for 100 h), oxygen diffused not only into the Al/Ti diffusion region, but also into the Ti substrate. For the NiAl intermetallic coating, no elemental Ti was found on the surface of the specimen and no oxygen was found in the Ti substrate after the oxidation treatment. The NiAl intermetallic coating had better oxidation resistance than the TiAl3 intermetallic coating.

Effect of friction stir processing on corrosion of Al-TiB2 based composite in 3.5 wt.% sodium chloride solution

Abstract The microstructure and corrosion behavior of as-cast and friction stir processed in-situ Al-TiB2 based composite in 3.5 wt.% sodium chloride solution were investigated. The microstructure was characterized using X-ray diffractometry, scanning electron microscopy and electron backscattered diffraction technique while corrosion behavior was evaluated using linear/cyclic potentiodynamic, electrochemical impedance spectroscopy and ASTM-G67 tests. The composite contains sub-micron TiB2 particles in an aluminum matrix with both blocky and fine clusters of Al3Ti agglomerated around TiB2 and displays a low uniform corrosion rate. It is also resistant to pitting as substantiated by the absence of a positive loop in cyclic potentiodynamic tests. This is due to the non-conductive nature of TiB2 particles and a controlled amount of blocky Al3Ti phase. However, both friction stir processed and as-cast composites are susceptible to inter-granular corrosion where Al3Ti and TiB2 at grain boundaries provide initiation sites for corrosion. Electrochemical impedance study attributes this to the adverse effect of Al3Ti and TiB2 on the protective oxide surface film, which increases with immersion time.

TiBCN-Ceramic-Reinforced Ti-Based Coating by Laser Cladding: Analysis of Processing Conditions and Coating Properties

In this paper, TiBCN-ceramic-reinforced Ti-based coating was fabricated on a Ti6Al4V substrate surface by laser cladding. The correlations between the main processing parameters and the geometrical characteristics of single clad tracks were predicted by linear regression analysis. On this basis, the microstructure, microhardness, corrosion resistance, and wear resistance of the coating and the substrate were investigated. The results showed that the clad height, clad width, clad depth, and dilution rate depended mainly on the laser power, the powder feeding rate, and the scanning speed. TiBCN-ceramic-reinforced Ti-based coating was mainly composed of directional dendritic TiBCN phases, equiaxed TiN phases, needle-like Al3Ti phases, and Ti phases. The microhardness gradually increased from the bottom to the top of the coating. The highest microhardness of coating was 1025 HV, which was three times higher than that of the Ti6Al4V substrate (350 HV). Furthermore, the coating exhibited excellent corrosion resistance and wear resistance. The corrosion potential (Ecorr) reached −1.258 V, and the corrosion density (Icorr) was 4.035 × 10−5 A/cm2, which was one order lower than that of the Ti6Al4V substrate (1.172 × 10−4 A/cm2). The coating wear mass loss was 4.35 mg, which was about two-third of the wear mass loss of the Ti6Al4V substrate (6.71 mg).

Butt Laser Welding–Brazing of AZ31 Mg to Nickel-Coated Ti-6Al-4V

Mg and Ti alloy are hard to be bonded due to their huge discrepancy in physical and metallurgical characteristics. As an intermediate element, Ni is feasible for joining of Mg and Ti. The interfacial microstructure and mechanical properties of laser-welded–brazed AZ31 Mg/Ti-6Al-4V joints in butt configuration were investigated with different thicknesses of electrodeposited Ni interlayer. The interfacial reaction products evolved from an ultra-thin Ti3Al layer to Ti2Ni layer mingled with Ti3Al, and more Mg3AlNi2 compounds were produced in the fusion zone (FZ) as Ni coating thickness increased. Slag inclusions and cracks were observed in interfacial layer when coating thickness exceeded 2.5 μm. The quantity of Mg3AlNi2 and the thickness of interfacial layer decreased from the top to bottom of groove of the same joint owing to high thermal gradient during laser welding. The chemical potential of Al and Ni indicated that Al-Ti reaction product was produced more easily than Ni-Ti in 0.001Mg-(Ni-Al-Ti) system. The thermodynamic calculation results of Gibbs free energy suggested the preferential formation Ti-Al phase and Ti-Ni phase along the interface while the Mg-Al-Ni ternary phase in the FZ. Fracture loads of joints increased first followed by a decline with the increase in coating thickness. The joint fractured at the Mg base metal, reaching a maximum value of 3900 N with the Ni coating thickness of 1.5 μm. In the case of using thinner or thicker Ni coating, crack propagated along the interface due to the insufficient metallurgical bonding or defects existed in the interfacial layer. The characteristic of fracture surface changed from the tear ridges and terraces to river pattern when interfacial failure occurred.