TiAl3 Layer Research Articles

Annealing effect on microstructure and mechanical properties of Al/Ti/Al laminate sheets

Trimodal composites, consisting of ultrafine grains, coarse grains, and particles, are known to possess desirable combinations of physical and mechanical properties. In this paper, we report the fabrication of a sheet of trimodal material using roll bonding and annealing of an Al/Ti/Al laminate at 873K for durations ranging from 6h to 168h. The Al/Ti/Al laminate was roll bonded from 625µm (Al sheet thickness 300µm, Ti foil thickness 25µm) to 130µm. The Ti layer was seen to break up and disperse in the aluminium matrix after rolling. It was found that when the annealing time was less than 12h, there were residual voids at the interface between the TiAl3 and Al layers, resulting in reduced ductility and strength of the composite sheet. When the annealing time was increased to 24h, there were no residual voids and the laminate became a kind of trimodal material, consisting of a combination of coarse-grained Al, ultrafine-grained Ti and TiAl3 particles. This kind of laminate shows the highest yield strength and good ductility. With a further increase in the annealing time to 168h, no residual pure Ti was seen in the laminate and the TiAl3 particles were found to be distributed in the Al matrix close to the laminate surface, leading to lower strength and ductility. We also discuss the microstructure evolution and the deformation mechanism during tensile testing.

Interface formation and properties of friction spot welded joints of AA5754 and Ti6Al4V alloys

This study aims to understand the influence of dwell time on the microstructure of the interface and lap shear strength of friction spot welded joints of AA5754 and Ti6Al4V alloys. The interface reaction layer consists mainly of TiAl3 intermetallic compound. It was noticed that dwell time significantly influences the diffusion process during the friction spot welding, thereby modifying the thickness of the interface and thus affecting the mechanical performance of the joints. Minimizing or optimizing the brittle TiAl3 phase was demonstrated to be the key issue to achieve high strength Ti/Al dissimilar joints. The intermetallic compound growth kinetics was also examined showing that an incubation time of approximately 2.7s is necessary before it nucleates at the Ti/Al interface and grows laterally to form a continuous layer. Afterwards, the TiAl3 layer grows toward the Al during thickening with a corresponding growth rate of k=2.92×10−7m/s. These results are critical to understand the microstructure–properties relationship and contribute with additional improvements on the joint performance.

Reaction behaviors occurring in Ti/Al foil metallurgy

Reaction behaviors occurring in Ti/Al foil metallurgy were systematically investigated. Particular emphasis was focused on the reaction between solid Al and Ti as well as subsequent reaction between TiAl3 and Ti layer. In the solid reaction between Al and Ti, the presence of residual Al is mainly caused by inhomogeneous growth of TiAl3 layer and micro-voids existing at the interface. However, through reaction between molten Al and Ti, TiAl3/Ti multilayer can be achieved with complete consumption of Al. During subsequent high-temperature heat treatment, TiAl3/Ti multilayer will eventually turn into Ti3Al/TiAl multilayer accompanying with simultaneous formation and successive disappearance of intermediate phases, such as TiAl2 and Ti2Al5. Moreover, it is found that the growth direction of TiAl layer changes as a function of annealing time between different couples in multi-intermetallics system.

Reactive synthesis and characterization of titanium aluminides produced from elemental powder mixtures

The formation of titanium aluminides in Ti–Al elemental powder mixtures containing 25, 50 and 75 at.% Al, has been studied using differential scanning calorimetry (DSC). Phase evolution in the mixture was followed by heating the compacted samples up to 1273 K at 7.5 and 15 K min−1. The cooled samples were characterized using X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. The results showed that the primary combustion product in all the samples was TiAl3, and the combustion reaction occurred below the melting point of aluminum only in Ti-rich samples. In Al-rich samples (75 at.% Al), TiAl3 was obtained as a porous, single-phase product after combustion. In samples containing 25 and 50 at.% Al, the combustion reaction was incomplete and the unreacted titanium particles were covered by a layer of TiAl3. In these samples, other intermetallic compounds such as TiAl2, TiAl and Ti3Al were observed to form upon heating beyond the combustion peak and are attributed to the solid-state reaction between unreacted titanium and TiAl3. Heating the samples with 25 at.% Al to 1273 K for an hour led to the formation of a homogenous Ti3Al product, while a multiphase product with a dominant TiAl phase was observed in samples containing 50 at.% Al. Calculations based on DSC data show that the formation of TiAl3 through the reaction between solid titanium and molten aluminum is associated with an apparent activation energy of 195 ± 20 kJ mol−1 and an enthalpy of −114 ± 5 kJ mol−1.

Brazing high Nb containing TiAl alloy using Ti–28Ni eutectic brazing alloy: Interfacial microstructure and joining properties

High Nb containing TiAl alloy (Ti–45Al–8.5Nb–(W, B, Y) (at%)) was successfully brazed by using Ti–28Ni (wt%) eutectic brazing alloy. The interfacial microstructure was characterized by SEM, EDS and XRD. Especially the effect of brazing temperature on interfacial microstructure and joining properties was investigated in details. Microstructure analyses results showed that the brazed joints presented a symmetrical characteristic and mainly consisted of Ti3Al, Ti2Ni and B2 phases. As the brazing temperature increased, the amount of Ti2Ni phase gradually decreased, meanwhile the amount of B2 phase increased significantly, while the continuous Ti3Al layer was insensitive to the brazing temperature. The maximum shear strength reached 204MPa when the joints were brazed at 1100°C for 10min. Brittle fractures were observed in all of the brazed joints after shear test although the fracture location changed obviously with the increase of brazing temperature.

Formation of Diffusion Aluminide Coatings on γ-TiAl Alloy with In-Pack and Out-Pack Processes

In this investigation, Ti–48Al–2Nb–2Cr (at.%) alloy was aluminized by two kinds of pack cementation processes, in-pack and out-pack processes. In both methods, pack powder mixture consisting of 10 wt% Al, 5 wt% AlCl3, and balance Al2O3 was used to aluminize γ-TiAl at 1000 °C for 6 h. The coated samples were characterized by XRD and SEM. The results indicated that a single-layered TiAl3 coating by thickness of approximately 100 μm was formed by the use of in-pack method, but a two-layered aluminide coating including an outer thin layer of TiAl3 with thickness of approximately 2 μm and also an inner layer of TiAl2 with thickness of approximately 10 μm was formed by out-pack process. In both methods, aluminide coatings with uniform structure free from any microcracks were formed on the surface of the substrates. The results showed that the flux of active aluminum atoms on the substrate surface in out-pack method is to be expected to be much slower than in-pack method.

Effects of Joining Conditions on Microstructure and Mechanical Properties of Cf/Al Composites and TiAl Alloy Combustion Synthesis Joints

Cf/Al composites and TiAl alloy were joined by combustion synthesis in different joining conditions. Effects of additive Cu, joining temperature and holding time on joint microstructure and shear strength were characterized by employing DTA, SEM, EDS, XRD and shear test. Results show that the additive Cu in the Ti–Al–C interlayer could significantly decrease the reaction temperature owing to the emergence of Al–Cu eutectic liquid. Reaction degree of the interlayer was influenced by joining temperature and holding time. Due to the barrier action of formed TiAl3 layer, reaction rate of Ti and Al was determined by the atoms diffusion. The reaction between Ti and Al was more sensitive to the joining temperature rather the holding time. The joints shear strength was influenced by joining condition directly. The maximum shear strength of CS joints was 25.89 MPa at 600 °C for 30 min under 5 MPa. Interface evolution mechanism of the CS joint was analyzed based on the experimental results and phase diagram.

Formation kinetics of multi-layered interfacial zone between γ-TiAl and glass–ceramic coatings via interfacial reactions at 1000 °C

Glass–ceramic coatings are investigated for high temperature protection of γ-TiAl. This investigation focuses on the interfacial reactions between γ-TiAl and a glass–ceramic coating at 1000°C in air by means of transmission/scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. A thin interfacial zone of multilayered glass/Ti5Si3/α-Al2O3/Ti3Al at the interface was observed after oxidation for several hours. Both the growth of Ti5Si3 and α-Al2O3 layers obeyed parabolic rules, while the thickness of the Ti3Al layer increased first and then decreased. The mechanism for formation kinetics of the interfacial zone was discussed.

Study on Microstructure Formation of TiAl Based Micro-Laminates Deposited by EB-PVD

TiAl based micro-laminated sheet was deposited by electron beam physical vapor deposition (EB-PVD) with dual-target in this work, and then the microstructure and phase analysis of as-deposited samples were studied by SEM, XRD and TEM. The results show that the difference of melting points between Ti and Al led to the distinctive structure: the Ti-Al layers were mainly constituted of equiaxed grains and Ti layers were constituted of column grains. Because of the deviation of saturated vapor pressure between Ti and Al element, the component showed a gradient change periodically along the normal direction of Ti-Al layers and results in several sub-layers. The Ti layers, Ti-Al layers and interfacial layers were constituted of α phase, γ+α2 phase and α2 phase respectively.

Formation of Alumina Layer on Ti Alloy for Artificial Hip Joint

The purpose of this research is to form a layer of alumina on Ti-6Al-4V alloy for hip joint by deposition of Al layer on the Ti alloy and its subsequent oxidation. In this work, a thick layer of Al was deposited onto the Ti alloy by cold spraying. The reaction layer of Al3Ti was formed by heat treatment of cold sprayed Al at 640°C in air/Ar atmosphere to ensure a good adhesion between cold sprayed Al layer and the Ti alloy. A thick Al3Ti layer formed by heat treatment of Al layer at 640°C for 12 h in air, was subjected to heat treatment at 850°C for 96 h in air to form a-alumina and Al2Ti. Thus, alumina scales can be formed on the top surface of the Ti alloy and can be densified by increasing the time duration of heat treatment.

Improving of interfacial microstructure of Ti/Al joint during GTA welding by adopting pulsed current

Butt welding of titanium alloy TA15 to aluminum alloy Al2024 dissimilar lightweight metals was conducted using gas tungsten arc welding. Pulsed current was adopted in the welding process. Influence of pulsed current on morphologies and microstructure of Ti-Al intermetallics near the Ti/Al interface was investigated. Microstructure characteristics and phase constitution of weld zone near the Ti/Al interface were analyzed. In top surface and upper region of the joint, Ti base metal was partially melted, and continuous intermetallic layers with Ti3Al, TiAl, and TiAl3 were formed in the fusion zone. In middle and bottom regions of the joint, Ti base metal was not melted and a thin TiAl3 layer was formed near the Ti/Al-brazed interface. Most of the Ti-Al intermetallics formed into discrete TiAl3 precipitations in the weld metal in upper and middle regions of the joint. No precipitation was observed in bottom region of the joint. Thickness of continuous Ti-Al intermetallic layers in the fusion zone was controlled at a low degree by adopting pulsed current in the welding. Crack sensitivity of weld zone near the Ti/Al interface was decreased.

Microstructure and Wear Resistance of in situ Formed Duplex Coating Fabricated by Plasma Nitriding Ti Coated 2024 Al Alloy

In this study, plasma nitriding was carried out on pure titanium film coated 2024 Al alloy to improve its surface mechanical property. Ti film with the thickness of 3.0 μm was firstly fabricated by means of magnetron sputtering method. Then, the Ti coated specimen was subjected to plasma atmosphere comprising 40% N2–60% H2 at 430 °C for 8 h. The microstructures of the nitrided specimens were characterized by X-ray diffraction and scanning electron microscopy. Microhardness tester and pin-on-disc tribometer were used to test the mechanical properties of the untreated and nitrided specimen. The results showed that the surface of the nitrided specimen was composed of three layers (i.e. the outside nitride TiN0.3 layer, the middle Al3Ti layer and the inside Al18Ti2Mg3 layer). The surface hardness and wear resistance of 2024 Al alloy were increased simultaneously by duplex treatment. The untreated specimen exhibited severe adhesive wear while the nitrided one behaved in middle abrasive wear.

Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying

The band structure of pure and Ti-alloyed anodic aluminum oxide has been examined as a function of Ti concentration varying from 2 to 20 at. %. The band gap energy of Ti-alloyed anodic Al oxide decreases with increasing Ti concentration. X-ray absorption spectroscopy reveals that Ti atoms are not located in a TiO2 unit in the oxide layer, but rather in a mixed Ti-Al oxide layer. The optical band gap energy of the anodic oxide layers was determined by vacuum ultraviolet spectroscopy in the energy range from 4.1 to 9.2 eV (300–135 nm). The results indicate that amorphous anodic Al2O3 has a direct band gap of 7.3 eV, which is about ∼1.4 eV lower than its crystalline counterpart (single-crystal Al2O3). Upon Ti-alloying, extra bands appear within the band gap of amorphous Al2O3, mainly caused by Ti 3d orbitals localized at the Ti site.

Microstructure and Tensile Behavior of Laser Arc Hybrid Welded Dissimilar Al and Ti Alloys.

Fiber laser-cold metal transfer arc hybrid welding was developed to welding-braze dissimilar Al and Ti alloys in butt configuration. Microstructure, interface properties, tensile behavior, and their relationships were investigated in detail. The results show the cross-weld tensile strength of the joints is up to 213 MPa, 95.5% of same Al weld. The optimal range of heat input for accepted joints was obtained as 83–98 J·mm−1. Within this range, the joint is stronger than 200 MPa and fractures in weld metal, or else, it becomes weaker and fractures at the intermetallic compounds (IMCs) layer. The IMCs layer of an accepted joint is usually thin and continuous, which is about 1μm-thick and only consists of TiAl2 due to fast solidification rate. However, the IMCs layer at the top corner of fusion zone/Ti substrate is easily thickened with increasing heat input. This thickened IMCs layer consists of a wide TiAl3 layer close to FZ and a thin TiAl2 layer close to Ti substrate. Furthermore, both bead shape formation and interface growth were discussed by laser-arc interaction and melt flow. Tensile behavior was summarized by interface properties.

Microstructure and mechanical properties of multiphase layer formed during depositing Ti film followed by plasma nitriding on 2024 aluminum alloy

In this study, a novel method was develop to fabricate an in situ multiphase layer on 2024 Al alloy to improve its surface mechanical properties. The method was divided into two steps, namely depositing pure Ti film on 2024 Al substrate by using magnetron sputtering, and plasma nitriding of Ti coated 2024 Al in a gas mixture comprising of 40% N2–60% H2. The microstructure and mechanical properties of the multiphase layer prepared at different nitriding time were investigated by using X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), microhardness tester and pin-on-disc tribometer. Results showed that multiphase layer with three sub-layers (i.e. the outmost TiN0.3 layer, the intermediate Al3Ti layer and the inside Al18Ti2Mg3 layer) can be obtained. The thickness of the Al18Ti2Mg3 layer increased faster than TiN0.3 and Al3Ti layer with increasing nitriding time. The hardness of the layer has reached about 593 HV, which is much higher than that of 2024 Al substrate. The wear rate of the coated samples decreased 53% for 4h nitriding and 86% for 12h nitriding, respectively, compared with that of the uncoated one. The analysis of worn surface indicated that the coated 2024 Al exhibited predominant abrasive wear, whereas the uncoated one showed severe adhesive wear.

Fabrication of TiAl intermetallic phases by heat treatment of warm sprayed metal precursors

Warm Spraying is an atmospheric coating process based on high-velocity impact bonding of powder particles. By decreasing the temperature of combustion gas via mixing with nitrogen the oxidation of feedstock powder can be effectively controlled. This is particularly important for Ti-based coating materials, which rapidly oxidize at elevated temperatures.In this study, Ti–Al composite coatings were fabricated by the Warm Spray process using a mixture of titanium and aluminum powders as a feedstock and applying a two-stage heat treatment at 600 and 1000 °C to obtain intermetallic phases. The microstructure, chemical and phase composition of the deposited and heat-treated coatings were investigated using SEM, EDS and XRD. The experimental results show that TiAl3 was the first intermetallic phase formed during the first-stage heat treatment. The growth of TiAl3 layer occurred mainly by diffusion of Al into Ti particles. Significant porosity that developed during the heat treatment was caused mainly by Kirkendall effect. After the second-stage heat treatment, a coating layer with TiAl as the dominant phase was obtained with about 20 vol % porosity.

Fabrication of in situ Ti2AlN/TiAl composites by reaction hot pressing and their properties

Ti2AlN/TiAl composites with different volume fractions of reinforcement were successfully fabricated by hot-pressing sintering method (reaction hot pressing) using Ti, Al and TiN powders as starting materials. The synthesis process includes four stages: first, the reactions between Al and Ti powers and between Al and TiN powders respectively occur and result in TiAl3 phase; secondly, Al powders in the sample are exhausted; the remaining Ti cores react with TiAl3 layer to form Ti-Al intermetallics; moreover, a few Ti2AlN particles precipitate from the TiAl3 phase; thirdly, Ti-Al intermetallics react with the remaining Ti cores to form Ti3Al and TiAl phases. TiAl phase and original TiN powers are in direct contact each other; finally, the residual TiN powers react with TiAl phase and result in a plenty of Ti2AlN phase. Compared with TiAl matrix, the hardness, elastic modulus and high-temperature compressive strength of Ti2AlN/TiAl composite are improved obviously and they are all enhanced with increasing the volume fraction of Ti2AlN phase.

Combined Functional Biocoatings on the VT-6 Alloy

Electrochemical oxidation of spark-deposited coatings from intermetallic TiAl3 and TiN–3AlN nitride ceramics in 3% NaCl solution leads to a nanosized surface layer composed of titanium oxides of various composition: from higher to lower oxides toward the substrate. The highest corrosion resistance is shown by the intermetallic coating, which is confirmed by anodic oxidation polarization curves and layerwise Auger spectroscopy of protective titanium and aluminum oxide films. A combined biocoating is produced as follows: electrospark deposition of TiAl3 layer onto VT-6 alloy—electrochemical oxidation in 3% NaCl solution—deposition of hydroxyapatite layer—laser fusion. Energy-dispersive X-ray spectroscopy has revealed a wide adhesion area at the interface between the spark-deposited coating and hydroxyapatite, which agrees with smooth variation in microhardness in this area. The coating shows strong adhesion to the substrate and biocompatibility with the bone tissue.

Relations between Microstructure and Mechanical Properties in Laminated Ti-Intermetallic Composites Synthesized Using Ti and Al Foils

Ti-(Al3Ti+Al) and Ti-Al3Ti laminated composites have been fabricated in vacuum using 50, 100, 150, 200, 400 and 600 μm thick titanium and 50 μm thick aluminum foils. The composites were synthesized with controlled temperature and treating time. Microstructural examinations showed that Al3Ti was the only phase formed during the reaction between Ti and Al. The initial foil thicknesses only affected the volume fraction of the resultant Ti, Al and Al3Ti layers. Treating time at 650 °C was a main factor determining microstructures and properties of the composites. After 20 minutes not all aluminum was consumed and therefore the composites consisted of alternating layers of Ti, Al and Al3Ti. After 60 minutes aluminum layers were completely consumed resulting in microstructures with Ti residual layers alternating with the Al3Ti layers. Mechanical tests were performed on the materials with different microstructures to establish their properties and fracture behavior. The results of investigations indicated that mechanical properties of the composites strongly depended on the thickness of individual Ti layers and the presence of residual Al layers at the intermetallic centerlines.

Microstructure and joining mechanism of Ti/Al dissimilar joint by pulsed gas metal arc welding

Butt joining of titanium alloy Ti–2Al–Mn to aluminum 1060 using AlSi5 filler wire was conducted using pulsed gas metal arc welding. Joining mechanism of Ti–2Al–Mn/Al 1060 dissimilar joint with different welding heat input was investigated. Formations of precipitation and Ti/Al interface were discussed in detail. Fusion zone near aluminum is composed of α-Al dendrites and Al–Si hypoeutectic structures. A few TiAl3 precipitations appear in the weld metal owing to metallurgical reactions of Al with dissolved Ti. When the welding heat input was in the range of 1.87–2.10 kJ/cm, titanium alloy Ti–2Al–Mn and Al 1060 were joined together by the formation of a complex Ti/Al interface. With a low welding heat input, a serrate TiAl3 interfacial reaction layer was formed near Ti/Al interface. With the increasing of the welding heat input, α-Ti, Ti7Al5Si12, and TiAl3 layers were formed orderly from Ti–2Al–Mn to weld metal.