TiAl3 Layer Research Articles

Calculation and experimentation on the formation sequence of compounds at Al/Ti interface in pure Al antioxidant coatings

To investigate the precipitation order of intermetallic compounds at the Al/Ti interface, the enthalpy, entropy, Gibbs free energy, and heat capacity of Ti–Al compounds (including TiAl3, TiAl2, Ti3Al, and TiAl) were calculated using first-principles. In addition, the enthalpy of formation, entropy of formation and Gibbs free energy of formation of Ti–Al compounds were further calculated. By combining thermodynamics and diffusion dynamics, the effective free energy of formation model is proposed to predict the formation sequence of compounds at the Al/Ti interface. The results showed that the formation sequence of Ti–Al compounds followed TiAl3→TiAl2→TiAl→Ti3Al. Subsequently, the pure Al coating was prepared on the surface of pure titanium plate by arc spraying equipment and the specimens were heat-treated. The experimental results show that there was a transition zone composed of four layers of Ti–Al compounds at the interface. From the Al layer to the matrix direction, the four-layer compounds were TiAl3→TiAl2→TiAl→Ti3Al in turn, which was consistent with the calculated results. The EBSD analysis of the Al/Ti interface shows that the grain size of TiAl3 decreases gradually from the coating to the substrate direction when the heat treatment condition is 800 ℃/5 h, and the grain size and thickness of the Ti3Al layer are both larger than that of TiAl2 and TiAl layer. The Ti–Al phases at the interface presented random grain orientation. Finally, the calculation results of the preferred adsorption position of oxygen atoms on the TiAl3 (110)-Al surface show that the position of Al–Al bridge is the most preferred adsorption position of oxygen atoms and oxygen atoms are more inclined to form aluminum oxide by bonding with Al atoms.

Reinforcing effect of laminate structure on the fracture toughness of Al3Ti intermetallic

Metal/intermetallic laminate composites can improve the mechanical properties of intermetallic materials using metal layers. In recent years, titanium aluminide intermetallics have received increasing attention due to their excellent performance properties, such as high melting point, high specific strength and stiffness, and good corrosion resistance. However, the low fracture toughness of Al3Ti alloys at room temperature has greatly limited their application, and fiber or particle reinforcement has not shown a significant toughening effect. Research into the reinforcing effects of the interface and near-interface zone on the fracture behavior of Al3Ti is lacking. Ti/Al3Ti metal/intermetallic laminate composite was synthesized from titanium and aluminum foils using vacuum hot-pressed sintering technology. The microstructure of the prepared material was analyzed by scanning electron microscope and electron backscattered diffraction. Results illustrate that both Ti and Al3Ti were single-phase and there was a noticeable stress concentration on the interface. To obtain indentation and cracks, loads were applied to different locations of the composite by a microhardness tester. The growth path of the cracks was then observed under microscope, showing that crack propagation was prevented by the interface between the Ti and Al3Ti layers, and the cracks that propagated parallel to the laminate shifted to the interface. Fracture toughness of the different areas, including Al3Ti layers, interface, and near-interface zone, were measured by the indentation fracture method. The fracture toughness at and near the interface was 1.7 and 2 times that of the Al3Ti layers, respectively. Results indicate that crack blunting and crack front convolution by the laminate structure was primarily responsible for increased toughness.

Constructing self-supplying Al2O3-Y2O3 coating for the γ-TiAl alloy with enhanced oxidation protective ability

The failure of the ceramic coating usually occurs at high temperatures due to the appearance of the cracks, which are caused by the accumulation of thermal stress. Here a self-supplying Al2O3-Y2O3/Al-Y coating was prepared by magnetron sputtering on the γ-TiAl alloy. After oxidation, the coating cross-section showed that three layers were formed, and the main phases consisted of Al2O3, Y2O3, and Y3Al5O12 (YAG). The isothermal oxidation test revealed that the coating efficiently prevented γ-TiAl alloy from degeneration for 100 h with a mass gain of 0.34% compared to the uncoated sample with a mass loss of 1.51% for 60 h at 950 °C. The study focused on the microstructure evolution of the coating during the oxidation process. We found that the improvement of oxidation resistance primarily could be attributed to three reasons: 1) the Y2O3 phase was more stable than Al2O3 and TiO2 at elevated temperature. 2) Al-Y layer, as the intermediate layer, can compensate for the element consumption of the ceramic layer caused by oxidation. 3) the Al3Ti layer efficiently suppressed the outward diffusion of Ti.

Comprehensive structural and mechanical characterization of in-situ Al–Al3Ti nanocomposite modified by heat treatment

Aluminum-Al3Ti reinforced MMCs were produced with a large amount (up to ~30 vol%) of Al3Ti phase via mechanical alloying, hot extrusion and heat treatment (as a supplementary process) of the Al–Ti elemental powder mixture, respectively. Samples in each stage were studied precise and comprehensive by field emission electron microscope, energy dispersive spectroscopy, X-ray diffraction, differential scanning calorimetry, densitometry, macro and micssro hardness, nanoindentation and tensile tests. The results indicated that after 4-h vibratory milling, the aluminum crystallite size reached 63 nm and Al3Ti phase formed in powder mixture. After hot extrusion, the volume percentage of Al3Ti increased by the formation of new Al3Ti nanoparticles, and a densed structure was achieved. As the milling time increased and crystallite and particle sizes of aluminum and titanium become smaller, the mechanical behavior of the hot extruded nanocomposites was improved. After heat treatments, the amount of Al3Ti phase significantly increased, and by thickening the Al3Ti layers around the titanium microparticles at high temperatures and long time periods, the voids in the structure proliferated. Increase in Al3Ti content can improve the mechanical properties, provided that volume exchanges originating from its formation do not cause widespread formation of voids in the structure. Mechanical properties varied as a function of aluminum and titanium crystallite and particle sizes and amount of Al3Ti phase and voids in the final samples. Aluminum crystallites growth during heat treatment reduces the hardness and increases the matrix ductility, and reinforcement particles growth brings faster failure for the samples.

Formation of Ti-Aluminides on Commercially Pure Ti via the Hot-Dipping Aluminizing Process

This study examined the applicability of the hot-dipping aluminizing technique carried out on commercially pure titanium (Ti) as a new method for the formation of Ti-aluminides on a Ti surface. The process was carried out using pure aluminum (Al) and Al 7075 alloy in molten Al baths at 900 °C and 1000 °C for 4 h and 6 h, respectively. The microstructure, phase fraction, and composition analysis of the formed layers were examined using field emission scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. Three different areas with different thicknesses were formed on the Ti surface by the hot-dipping aluminizing technique. The top (Al coating) layer consisted of Ti and Al elements having a higher hardness than the base metal. The second layer, formed below the Al coating, was the Ti–Al layer having the highest hardness on the surface. Below this layer the Al was diffused. As a result of the Ti aluminizing process carried out at different temperatures and durations in this study, TiAl, TiAl2, TiAl3, and Ti3Al phases were obtained. These phases positively affected the mechanical and corrosion properties.

Preparation and characterization of novel Ti(Al)–TiB2/Ti3Al metallic-intermetallic laminated composites

In this paper, we designed and prepared Ti(Al)–TiB2/Ti3Al laminated composites consisting of alternating Ti(Al) layers and TiB2/Ti3Al layers, which were realized by using Ti foils and TiB2/Al composite foils as raw materials, stacking, and reaction annealing. Phase transformation and microstructure evolution of laminated composites during reaction annealing were systematically investigated by means of optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). Formation mechanism of laminated structure was elucidated in details. The results showed that Ti(Al)–TiB2/Ti3Al laminated composites exhibited a significant increase in ductility while maintaining or even improving the tensile strength, in comparison with monolithic Ti3Al. It is expected that the design strategy can be extended to other brittle intermetallic compounds, such as Ni–Al and Fe–Al systems.

Diffusion bonding of AlCoCrFeNi2.1 eutectic high entropy alloy to TiAl alloy

High entropy alloys have special microstructure and remarkable properties. To explore their potential engineering application in high temperature structures, the microstructure evolution of bonding interface, the elemental diffusion behavior and mechanical property of the diffusion bonded joint between AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) and TiAl alloy were investigated. Four reaction layers (rodlike B2 phase, Al(Co, Ni)2Ti, τ3-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + Ti3Al) formed in the diffusion zone near FCC phase of EHEA, but three layers (Al(Co, Ni)2Ti, τ3-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + Ti3Al) formed near B2 phase. Al and Ni controlled the reaction diffusion of EHEA and TiAl alloy, coarsened the acicular precipitated B2 phase and turned TiAl phase into Al(Co, Ni)2Ti and τ3-Al3NiTi2 phases. All these reaction layers grew in a parabolic manner as a function of bonding temperature. Rodlike B2 phase has the lowest growth activation energy of 125.2 kJ/mol, and the growth activation energy of τ3-Al3NiTi2 + TiAl layer near B2 phase is much lower than that near FCC phase. The penetration phenomenon and convex structure formed in the diffusion zone, which resulted in interlocking effect and enhanced the strength of resultant joints. The highest shear strength of 449 MPa was achieved at 950 °C. And the brittle fracture generally initiated at the interface between Al(Co, Ni)2Ti and τ3-Al3NiTi2 + TiAl layers.

In-situ synthesis of Ti–Al intermetallic compounds coating on Ti alloy by magnetron sputtering deposition followed by vacuum annealing

In this study, a thin Ti–Al intermetallic compounds (IMCs) coating was in-situ synthesized on a titanium alloy substrate through magnetron sputtering of Al followed by vacuum annealing. The microstructure, composition distribution, nanoindentation and microhardness, scratch and wear behavior of the obtained Ti–Al IMCs coating were investigated. It was found that TiAl3 is formed at the surface, and very thin TiAl and Ti3Al layers exist between the TiAl3 layer and the substrate. The microhardness of the TiAl3 (11.17 GPa) is almost twice as that of the substrate (6.03 GPa), and no cracking occurs during the indentation process. Microstructure observation and composition analysis suggest that metallurgical bonding is formed between the Ti–Al IMCs coating and the substrate, leading to a high bonding strength and almost no peeling during the whole scratch test. The tribological test reveals that the Ti–Al IMCs coating exhibits a lower friction coefficient and a much lower wear rate than those of the substrate, and the major wear mechanism of the Ti–Al IMCs coating is abrasive wear. Based on the findings, the in-situ synthesis of Ti–Al IMCs coating via magnetron sputtering followed by vacuum annealing can be considered a promising method to effectively improve the wear resistance of the Ti alloy.

Effect of Al interlayer on resistance spot welding of MB3/Ti6Al4V

The aim of present study was to evaluate the effects of Al foil on microstructure and mechanical performances of Ti/Mg joint obtained by resistance spot welding. Experiment results reveal that a TiAl3 intermetallic compound layer was formed in the interface region of Ti/Mg joint with the addition of Al foil, which was the key to achieve joint strengthening. For Ti/Mg joint without Al foil, the maximum micro hardness value of 138 HV was measured in the interface region of spot weld. However, under the action of Al foil, the micro hardness of interface region increased to 152HV, attributed to the formation of TiAl3 layer. Besides, with optimal current of 9 kA and the aid of Al foil, the maximum tensile shear strength increased from 3.3 kN to 3.8 kN and the relevant fracture surface was characterized by plastic fracture features.

Brazing of Gr/2024Al composite and TC4 using AgCuTi filler with the assist of Ti-Al-C-Cu interlayer

In order to braze Gr/2024Al composite at low temperature to keep its heat insulation property, a novel thermal explosion assisting brazing method was designed. Through this method, the thermal explosion reaction of Ti-Al-C-Cu interlayer offered additional heat, and AgCuTi filler reacted with Gr/2024Al composite under the melting point of AgCuTi. Based on the theoretical calculation and DSC analysis, the composition of Ti-Al-C-Cu interlayer was designed. The microstructure and mechanical properties of brazed joint was investigated, and the brazing parameters were optimized. The results showed that AgCuTi reacted with the Gr/2024Al composite to form Ti3AlC2 layer, and a continuous TiAl3 layer acted as the reaction layer near the TC4 alloy. The high preheating temperature and long holding time would promote the reaction of AgCuTi filler, Gr/2024Al and Ti-Al-C-Cu interlayer, and Al5CuTi2 phase volume in the brazing zone increased. The Al5CuTi2 distribution affected the joint strength significantly. The optimized joint strength reached 11 MPa when the joint was brazed at 650 °C for 15 min.

Finite element analysis of thermal stresses in Ti-Al3Ti metal-intermetallic laminated composites

For improving the mechanical properties of Ti-Al3Ti metal-intermetallic laminated (MIL), it is of vital importance for reducing tensile thermal stresses of this composites. In this work, finite element analysis was used to study the distribution and various reducing methods of thermal stresses in Ti-Al3Ti MIL composites, such as changing the layer thickness ratios, the layer numbers, remaining the residual Al, etc. The results showed that the tensile thermal stresses in outer Al3Ti layer is the highest, and the outer Al3Ti layer is the most likely fail. When the layer numbers are larger than 47, the tensile thermal stresses in outer Al3Ti layer are almost minimal and keep constant. When the thickness ratios are larger than 5, the tensile thermal stresses in Al3Ti layer decrease only a little. With the introduction of residual Al, the thermal stresses in Al3Ti layers switch from tension to compression state.

Microstructures and properties of TiCp/Al coating synthesized on Ti–6Al–4V alloy substrate using mechanical alloying method

A TiCp/Al coating was synthesized on the Ti–6Al–4V alloy substrate by mechanical alloying method. The microstructure, the mircohardeness, the friction and wear resistance and the high-temperature oxidation resistance of the coating, as well as the effect of annealing treatment on them were investigated. The results showed that the coating had a TiC particles reinforced Al matrix composite structure in which the reinforcing particles were uniformly dispersed in the matrix. The microhardness, the wear resistance and the oxidation resistance were improved because of the as-synthesized coating. After annealing treatment, the coating formed a bi-layer structure, including an outer TiCp/Al composite layer and an inner Al3Ti layer. The outer layer had low microhardness and tended to be flaked in both friction and oxidation processes. The inner layer showed more than 5 times of microhardness values of the outer layer and had favorable wear resistance. But during long-time oxidation process, the inner layer had a stronger tendency of forming microdefects compared with the unannealed coating. In general, the TiCp/Al composite coating could protect the substrate from being severe worn or oxidized. The annealing treatment to the coating was conductive to the further improvement of its microhardness and wear resistance, but had limited effects on its oxidation resistance.

Non-parabolic Al3Ti intermetallic layer growth on aluminum-titanium interface at low annealing temperatures

Abstract 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 Al3Ti layer on the Al-Ti interface at a low annealing temperature (TA, 500 °C) obviously didn’t follow a parabolic law. A very long incubation stage of Al3Ti 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 Al3Ti-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.

Cold Arc MIG Welding of Titanium Ti6Al4V to Aluminum 5A05Al Using Al–Mg5 Filler

Butt welding of titanium Ti6Al4V to aluminum 5A05Al was carried out using cold arc MIG welding with Al–Mg5 filler. Microstructure of the Ti/Al interface was strongly influenced by the welding heat input. Two types of Ti/Al interfaces were observed, i.e. a serrated TiAl3 layer in the regions with low heat input, a uniform Ti3.3Al layer and a cellular TiAl3 layer in the regions with high heat input. The latter interface appeared interesting growth sequence, that is, Ti3.3Al layer and cellular TiAl3 layer were formed orderly from titanium side to weld metal side. Tensile test and fractography study were carried out to examine the reliability and fracture mode of the joint, respectively. Crack-free welds were obtained with full-penetration under operational welding process. All the specimens were fractured in aluminum base metal during the tensile test and the average tensile strength reached up to approximately 242 MPa. Interfacial microstructure and properties of the joint indicated that Al–Mg5 filler can be used as effective filler to fabricate sound Ti/Al joints.

Protective aluminide coating by pack cementation for Beta 21-S titanium alloy

Pack cementation method was used to form high activity aluminide coating on titanium alloy Beta-21S using a cement activated by CrCl3 at 760 °C for a period of 9 h. Thermodynamic computations showed that growth mechanism of TiAl3 occurs in tree steps involving different chemical reactions. The effectiveness of the TiAl3 coating to protect Beta-21S alloy was demonstrated through oxidation tests performed at 750 and 850 °C during 300 h. The elevated resistance against oxidation of the TiAl3 layer is provided by a protective Al2O3 scale having low growth kinetics.

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.

Surface modification of Al by high-intensity low-energy Ti-ion implantation: Microstructure, mechanical and tribological properties

A high-intensity metal ribbon ion beam was generated using plasma immersion extraction and the acceleration of the metal ions with their subsequent ballistic focusing using a cylindrical grid electrode under a repetitively pulsed bias. To generate the dense metal plasma flow, two water-cooled vacuum arc evaporators with Ti cathodes were used. The ion current density reached 43 mA/cm2 at the arc discharge current of 130 A. High-intensity ion implantation (HIII) with a low ion energy ribbon beam was used for the surface modification of the aluminium. The irradiation fluence was changed from 1.5 × 1020 ion/cm2 to 4 × 1020 ion/cm2 with a corresponding increase in the implantation temperature from 623 to 823 K. The structure and composition of the Ti-implanted aluminium were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX). The mechanical properties and wear resistance were measured using nanoindentation and “pin-on-disk” testing, respectively. It was shown that the HIII method can be used to form a deep intermetallic Al3Ti layer. It has been established that a thin (0.4 μm) modified layer with a hcp Ti(Al) structure is only formed on the surface at 623 K, while the formation of the ordered Al3Ti intermetallic phase occurs at the implantation temperatures of 723 and 823 K. Despite the significant ion sputtering of the surface, the thickness of the modified layer increases from ~1 μm to ~6 μm, and the implantation temperature rises from 723 to 823 K. It was found that the homogeneous intermetallic Al3Ti layer with a thickness of up to 5 μm was formed at 823 К. The mechanical and tribological properties of the aluminium were substantially improved after HIII. For the Ti-implanted aluminium, the hardness of the surface layer increases from 0.4 GPa (undoped Al) to 3.5–4 GPa, while the wear resistance increases by more than an order of magnitude.

Effect of laser-arc offset and laser-deviation angle on the control of a Ti-Al interlayer

Butt joining of an AA6061 aluminum alloy and a Ti-6Al-4V titanium alloy with a 3 mm thickness was carried out using laser-metal inert-gas hybrid welding-brazing without a bevel or an air gap. The uniformity of the interfacial intermetallic compound (IMC) layer was improved by changing the laser-arc offset and the laser-deviation angle. The weld appearance, IMC layer microstructure and tensile strength of the welded-brazed joints were investigated. A uniform serrated 1 μm thick TiAl3 layer was formed along the butt plane by using a laser-deviation angle. The tensile strength of the joint was closely related to the IMC layer morphology and thickness. Joints with excellent wetting and spreading on both sides achieved a maximum tensile strength of 230 MPa when the laser-arc offset was 1.2 mm and the laser-deviation angle was 5°. Three different fracture phenomena existed during the tensile test that correspond to different fracture positions at the interface layer, weld seam and heat-affected zone of the aluminum base metal.

Microstructure and Kinetics of Intermetallic Phase Formation during Solid State Diffusion Bonding in Bimetal Ti/Al

In this study, tri-layered Ti/Al composites were synthesized using hot press/roll bonding, and then the annealing treatment was executed in the temperature range of 550–650°C for 2, 4, and 6 h. Interfacial layers were characterized using scanning electron microscopes (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD). The results of microstructural characterization for all annealing temperatures and times indicated that the Al3Ti compound is the only intermetallic compound observed in the diffusion layers. The Al3Ti growth obeyed linear kinetics and was characterized by an activation energy of 128.7 kJ mol–1. The results indicated that the dominant diffusing component was aluminum and the growth of the Al3Ti layer occurred mainly at Ti/Al3Ti interfaces.

Effect of copper-nickel interlayer thickness on laser welding-brazing of Mg/Ti alloy

Dissimilar lap joining of AZ31B Mg alloy to Cu-Ni coated Ti-6Al-4V was carried out by laser welding-brazing method. The effect of Cu and Ni contents on interfacial reaction and joint fracture load were analyzed. For the joint in which the Ni coating (15.36 µm) was thicker than the Cu coating (5.47 µm), thick intermetallic compound (IMC) composed of a mixture of light gray Al-Ni-Ti + Ti3Al + Ti2Ni phases mingled with dark gray Ti3Al was produced at the interface. In comparison, a mixed interfacial reaction layer consisted of Ti2Ni and Ti3Al was formed from the direct irradiation zone to the weld toe zone of the joint with comparable Ni and Cu coating thicknesses (10.78 µm Cu–9.30 µm Ni). In this case, the thickness of the mixed layer was below the critical thickness of 10 µm. For the joint in which the Cu coating is much higher than the Ni coating thickness (17.12 µm Cu–4.23 µm Ni), Ti3Al and Ti2Ni mixed reaction layer was produced at the brazed interface of direct irradiation zone, whereas, only Ti3Al phase was formed at the middle zone. At the weld toe zone, Ti2Cu uneven interfacial reaction layer was evolved. With increasing Cu and decreasing Ni coating thicknesses, the fracture load first increased and then slightly decreased, the maximum tensile-shear fracture load attained 2020 N for joints with comparable Cu and Ni coating thicknesses. This is twofold higher than that of uncoated joint. The tensile-shear investigation showed that the joint would fracture at the fusion zone when the coating thickness of Ni was comparable or higher than Cu. In contrast, interfacial failure was observed when the thickness of Cu was much higher than the Ni. For the joint with interfacial failure mode, tear ridge was observed from the fracture surface, whereas, the fusion zone fracture surfaces was noted to display a typical dimple feature.