TiAl Phase Research Articles

Microstructure and properties of mechanically milled AZ61 powders dispersed with submicron/nanometer Ti particulates

Nanocrystalline AZ61 powders, dispersed with submicron/nanometer Ti particulates, were mechanically milled and a comparative study on thermal stability and mechanical properties was performed. After mechanical milling, Ti dispersions were cracked to submicron/nanometer particulates and Mg grains were refined to nanocrystalline. As illustrated by high-resolution transmission electron microscopy, coherent lattice and semi-coherent lattice interfaces between magnesium and titanium were achieved. Based on high angle annular dark field and corresponding energy dispersive spectrometer maps, Al dissolved into Ti particulates after mechanical milling. Moreover, the emergence of Ti3Al phase was verified by X-ray diffraction during mechanical milling and heat treatment. According to heat treatment results, this nanocrystalline magnesium composite exhibits a preeminent thermal stability among nano-structured magnesium alloys. After annealed at 673K for 600min, the composite still remained nanocrystalline with average grain size being 68.6nm. Due to such microstructure, the composite revealed prominent mechanical property with the hardness being 146.6HV. Simultaneously, the hardness of annealed composite remained high, with the value being 134.7HV, which could ascribe to the excellent thermal stability.

Interfacial characteristics and fracture behavior of spark-plasma-sintered TiNi fiber-reinforced 2024Al matrix composites

Embedding of shape memory alloy (SMA) fibers into materials to fabricate SMA composites has attracted considerable attention because of the potential applicability of these composites in smart systems and structures. In this study, 2024Al matrix composites reinforced by continuous TiNi SMA fibers were fabricated using spark plasma sintering (SPS). The interface between the fibers and matrix consisted of a bilayer. The layer close to the fiber consisted of a multiple phase mixture, and the other layer exhibited a periodic morphology resulting from the alternating phases of Al3Ti and Al3Ni. In addition, a small quantity of TiO2 phases was also observed in the interface layer. Based on detailed interface studies of the orientation relationships between the Al3Ti, Al3Ni, and TiO2 phases and the atomic correspondence at phase boundaries, the effects of the interface phases on the fracture behavior of the composites were demonstrated.

Relationship between microstructure and mechanical properties of TiAl/Ti2AlNb joint brazed using Ti-27Co eutectic filler metal

Partially substituting TiAl alloys for Ti2AlNb alloys has been identified as an effective approach to realize the weight reduction of aircraft engines. Therefore, joining of TiAl alloy to Ti2AlNb alloy is important and meaningful. In this study, TiAl alloy and Ti2AlNb alloy were successfully brazed at 1080–1140°C for 10min using Ti-27Co (wt%) eutectic filler metal. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM) were employed to characterize the interfacial microstructure of the joints. The results showed that α2-Ti3Al, β-Ti, TiCo, Ti2Co and B2 phases were formed in the brazing seam of the joints. As the brazing temperature increased, the thickness of diffusion layer in the TiAl side became thicker correspondingly. Higher brazing temperature accelerated the diffusion of Co element into the substrates, resulting in the disappearance of band-like Ti2Co phase and the growth of acicular Ti3Al phase. The highest shear strength of the joints was 223MPa when brazed at 1100°C for 10min. The hardness and elastic modulus of the phases in the joint were measured by nano-indentation to show the plastic deformation capacity. In addition, the microstructure evolution mechanism of the joints was put forward based on the experimental analysis.

Au-free ohmic Ti/Al/TiN contacts to UID n-GaN fabricated by sputter deposition

The fabrication and characterization of an Au-free Ti/Al/TiN (20/100/100 nm) contact stack to unintentionally doped n-GaN with TiN serving as the diffusion barrier is presented. Sputter deposition and lift-off in combination with post deposition annealing at 850 °C are used for contact formation. After annealing, contact shows ohmic behavior to n-GaN and a specific contact resistivity of 1.60 × 10−3 Ω cm2. To understand the contact formation on the microscopic scale, the contact was characterized by current–voltage measurements, linear transmission line method, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show the formation of Ti-N bonds at the GaN/Ti interface in the as-deposited stack. Annealing leads to diffusion of Ti, Al, Ga, and N, and the remaining metallic Ti is fully consumed by the formation of the intermetallic tetragonal Al3Ti phase. Native oxide from the GaN surface is trapped during annealing and accumulated in the Al interlayer. The TiN capping layer, however, was chemically stable during annealing. It prevented oxidation of the Ti/Al contact bilayer successfully and thus proved to be a well suitable diffusion barrier with ideal compatibility to the Ti/Al contact metallization.

Solid-liquid cast-rolling bonding (SLCRB) and annealing of Ti/Al cladding strip

By feeding the solid titanium strip together with the molten aluminum into the twin-roll caster simultaneously, Ti/Al cladding strips were successfully fabricated in a solid-liquid cast-rolling bonding (SLCRB) process. Annealing treatment was subsequently applied on the Ti/Al cladding strips to eliminate the residual stress and improve the bonding quality. The microstructure, composition of the interface reaction products and bonding strength were investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), peeling and tension-shear test. It indicates that, at the beginning of the cast-rolling process, diffusion reaction occurs at the interface of the liquid aluminum and the titanium strip. At the exit of caster, the bonding interface presents jagged bite due to the hot rolling, and the thickness of diffusion layer is about 3μm. After annealing treatment at 520°C for more than 2h, a continuous compound layer was generated on the bonding interface and the composition is single TiAl3 phase. The influence of annealing time on the thickness of diffusion layer was discussed and it indicates that the interface growth kinetics obeys linear law from 25h to 200h. Annealing treatment can improve the adhesive strength of the cladding strip. After annealing treatment for 25h, the average peeling strength increases from 21N/mm to 27N/mm. The micro-morphology of the peeled surface was subsequently investigated, and the SEM images show that the Ti side surface is almost completely covered with aluminum, which indicates that the fracture is located at aluminum substrate.

The enhanced de/re-hydrogenation performance of 4MgH2-NaAlH4 composite by doping with TiH2

TiH2 was introduced into the 4MgH2-NaAlH4 composite by ball milling to improve the dehydrogenation and re-dehydrogenation performance of the composite. The non-isothermal dehydrogenation curve shows that the onset temperature of hydrogen release from 4MgH2-NaAlH4-TiH2 is at around 100 °C, which is lower by 50 °C than that of 4MgH2-NaAlH4. The investigation by Kissinger's method indicates that 4MgH2-NaAlH4-TiH2 possesses much lower activation energy (68 kJ mol−1) than 4MgH2-NaAlH4 (159 kJ mol−1), indicating an enhanced dehydrogenation kinetics due to the additional TiH2. TiH2 catalyzes the decomposition of MgH2 and NaAlH4, respectively, and also the reaction between their products during the dehydrogenation process below 375 °C. At temperatures above 375 °C, TiH2 will react with Mg17Al12 to form Al3Ti and liberate contained hydrogen. TiH2 also improved the isothermal hydrogenation kinetics of 4MgH2-NaAlH4 desorbed system. The activation energy of re-hydrogenation for 4MgH2-NaAlH4-TiH2 system was greatly reduced from 161.62 kJ mol−1 H2 to 125.80 kJ mol−1 H2 of 4MgH2-NaAlH4. The kinetics enhancement of hydrogen absorption is probably due to the intermediate phase Al3Ti, which is beneficial to the recombination of MgH2.

Developing high toughness and strength Al/TiC composites using ice-templating and pressure infiltration

We prepared Al/TiC composites with different ceramic volume fractions (15, 25 and 35vol%) using ice-templating and pressure infiltration. The thickness of the lamellar layer and the porosity in the ceramic layer of the TiC scaffolds were controlled by varying the slurry concentration. The Al/15vol%TiC composite had a thick metal layer and a low-density ceramic layer, which effectively dissipated the stress at the crack tip and fractured in a multiple-crack-propagation mode, giving bending strength of 355±3MPa and fracture toughness of 81±2MPam1/2. However, the Al/25vol%TiC and Al/35vol%TiC composites had much higher bending strength (417−500MPa) but lower fracture toughness (46−33MPam1/2) as compared to the Al/15vol%TiC composite, and they fractured in a single-crack-propagation mode. In addition, an increase in the brittle TiAl3 phase with increasing ceramic volume at the fracture surface greatly deteriorated the toughness of the Al/TiC composites. Finally, the relationship between cracking mode and structure features in the laminated composites was discussed to account for the toughening mechanism.

Microstructure evolutions of Ni-Ti-Nb-Al alloys with different Al addition

Microstructure evolutions of Ni-Ti-Nb-Al alloys with different Al addition subjected to casting and solution-treatment have been investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The alloys were prepared by vacuum arc melting, and then solution-treated at 1273 K for 20 h. The alloys with 3 at.% Al addition and Al-free show a microstructure consisting by B2-NiTi matrix phase and soft β-Nb precipitate. With increasing Al addition, Al3Ti phase firstly forms, and Ni3Al only can be found in the 7 at.% Al alloy. The addition of Al promotes the diffusion of Nb during casting and solution-treatment, resulting in the refinement of β-Nb precipitate. For the as-cast alloys, the macro-hardness values increase with the increase in Al content and slightly increase after solution-treatment.

Transformation of Ti/Al multilayers to the Ti3Al phase aimed at bonding of titanium alloys

Transformation of Ti/Al multilayers to the Ti3Al phase aimed at bonding of titanium alloys

Effect of the Deformation Parameters on the Microstructure of TiC-Al<sub>2</sub>O<sub>3P</sub>/Al Composites

An aluminum matrix composite reinforced by TiC and Al2O3 particles was synthesized using TiO2, C, and Al powder by the in-situ reaction. The effects of deformation parameters on the microstructure of the TiC-Al2O3/Al composite were investigated using XRD, SEM and TEM. The result shows that the strain rate and deformation temperature influence the size, styles, and distribution of the second phase. Reinforcements are dispersed in a distribution with increasing deformation temperature and strain rate. However, broken Al3Ti exhibits redissolution phenomenon. The new precipitated Al3Ti phase will precipitate from the strain-induced solid solution when the total strain increases to a certain degree at a high deformation temperature.

Influence of laser power on microstructure and mechanical properties of laser welded-brazed Mg to Ni coated Ti alloys

AZ31B Magnesium (Mg) and Ti-6Al-4V titanium (Ti) alloys with Ni coating were joined by laser welding-brazing process using AZ92 Mg based filler. The influence of laser power on microstructure and mechanical properties were investigated. Ni coating was found to significantly promote good wetting-spreading ability of molten filler on the Ti sheet. Acceptable joints without obvious defects were obtained within a relatively wide processing window. In the process metallurgical bonding was achieved by the formation of Ti3Al phase at direct irradiation zone and Al-Ni phase followed by a layer of Mg-Al-Ni ternary compound adjacent to the fusion zone at the intermediate zone. The thickness of reaction layers increased slowly with the increasing laser power. The tensile-shear test indicated that joints produced at the laser power of 1300W reached 2387N fracture load, representing 88.5% joint efficiency with respect to the Mg base metal. The corresponding failure occurred in the fusion zone of the Mg base metal, while joints fractured at the interface at lower/higher laser power due to the crack or excessive intermetallic compound (IMC) formation along the interface.

Experimental investigation of phase equilibria in the Ti–Al–Mo ternary system

Phase equilibria in Ti–Al–Mo ternary system are of fundamental importance to better acquire the phase-forming regulation of TiAl-based alloys. In this paper, phase relations in this system have been investigated with Ti–TiAl–Mo diffusion triples combined with key equilibrated alloys. Five isothermal sections of the Ti–Al–Mo ternary system at 1473, 1373, 1273, 1173, and 1073 K were experimentally determined by means of electron probe microanalysis, scanning electron microscope, and X-ray diffraction. The TiAl and TiAl3 phases show wide ternary composition ranges, while the solubility of Mo in the Ti3Al and αTi phases is relatively limited. The solubility of Al in the β(Ti, Mo) phase decreases from 49.0 at.% at 1473 K to 39.9 at.% at 1073 K while that of Ti in the AlMo3 phase decreases from 25.4 at.% at 1473 K to 20.8 at.% at 1073 K. Moreover, the ternary compound σ forms at a temperature between 1423 and 1373 K through a peritectoid reaction β(Ti, Mo) + AlMo3 → σ. A peri-eutectoid invariant reaction β(Ti, Mo) + AlMo3 → σ + TiAl3 at 1373–1273 K was further deduced and the existence of another invariant reaction β(Ti, Mo) + TiAl3 → σ + TiAl at a temperature between 1273 and 1173 K was also confirmed. The established phase equilibria of the Ti–Al–Mo ternary system may provide essential data required for the construction of reliable thermodynamic assessment of the Ti–Al–Mo system.

Refining and Reinforcing Effects of In Situ AlN–TiN–TiB2/Al Composite Refiner on Aluminum

Typical Al–Ti–B master alloys are good inoculant for aluminum alloys, which have shown obvious dependence on some alloying elements. In this work, in situ AlN–TiN–TiB2/Al composite refiner was successfully prepared by nitrogen alloying based on an Al–8Ti–1B melt. It is found that the refining, tensile strength, yield strength, and hardness of the pure aluminum modified by in situ AlN–TiN–TiB2/Al composite inoculant were all obviously improved. The enhanced refining and reinforcing effects of the refiner can be attributed to the following aspects: the formation of AlN and TiN particles, the homogeneous distribution of AlN, TiN, and TiB2 particles and the miniaturization of Al3Ti phases. The above three factors are all caused by nitridation and have a combining effect for improving the nucleation rate of α‐Al.

Vacuum brazing high Nb-containing TiAl alloy to Ti60 alloy using Ti-28Ni eutectic brazing alloy

High Nb-containing TiAl alloy and Ti60 alloy was brazed successfully by vacuum brazing using Ti-28Ni eutectic brazing alloy. The effect of brazing temperature on interfacial microstructure and mechanical properties of the joints was investigated. The interfacial microstructure of the joint brazed at low temperature was Ti60 substrate/Lamellar (α+β) microstructure/Ti2Ni phase layer + α-Ti/α-Ti layer + Ti2Ni/Ti3Al layer/B2 phase layer/TAN substrate, and the presence of Ti2Ni phase in the joints deteriorated the joining properties. The increasing brazing temperature accelerated the diffusion of Ni atoms into the substrates, leading to the disappearance of Ti2Ni and Ti3Al phase and the growth of B2 phase. The maximum joining strength of ∼140 MPa was obtained when brazing at 1080 °C for 10 min.

Tailoring ultrafine grained and dispersion-strengthened Ti2AlC/TiAl composite via a new fabrication route

In situ Ti2AlC/TiAl composite was fabricated by hot-pressing method via the reaction system of Ti3AlC2 and Ti-Al pre-alloyed powders at low temperature of 1150°C. The composite mainly consisted of TiAl, Ti3Al and Ti2AlC phases. Fine Ti2AlC particles were homogeneously distributed and dispersed in the matrix. Ti2AlC played the significant role of deflecting and blunting the cracks and contributed to improve the mechanical properties of the composite. The Vickers hardness, flexural strength and fracture toughness were 6.2 GPa, 993.9 ± 50.8 MPa and 6.7 ± 0.3 MPa m 1/2, respectively.

Mechanical and microstructural characteristics of Ti6Al4V/AA2519 and Ti6Al4V/AA1050/AA2519 laminates manufactured by explosive welding

The aim of the work was to produce laminated structures consisting of Ti–6Al–4V alloy and AA2519 plates and to investigate their microstructure and mechanical properties with an emphasis on the role of an additional AA1050 interlayer. Explosive welding was selected as a joining technology. The microstructure and chemical composition of the explosively joined samples were investigated. Mechanical properties were evaluated in the tensile testing and by microhardness analysis.The results demonstrated that explosive welding is an effective way to produce Ti/Al laminates. Both Ti6Al4V/AA2519 and Ti6Al4V/AA1050/AA2519 laminated plates exhibit good quality of bonding without voids and major delamination. The explosive welding produced metallurgical bonding with a nanostructured zone consisting of Al3Ti and Al2Ti phases. This zone is thicker in the joint with additional AA1050 interlayer when compared to direct AA2519/Ti6Al4V bonding. In the latter, SEM and STEM analysis reviled the presence of net-like structure in the collision zone. Advanced EDX analysis shows the enrichment of grain boundaries in copper. The formation of this structure is widely discussed. In addition, the explosive welding introduces large plastic deformation which induces the process of grain refinement in aluminium plates. Tensile testing confirms that joining section is not the weakest element of the cladded plates.

Tough TiB2‐Based Ceramic Composites Using Metallic Glass Powder as the Sintering Aid

In this work, the authors report on the formation of toughened TiB2‐based ceramic composites using Ti60.8Al25Nb10V3Mo1B0.2 (at%) metallic glass powder as sintering aid fabricated by spark plasma sintering. The TiB2 ceramic powder blended with 5 wt% metallic glass sintering aid exhibits an interesting sigmoidal shrinkage behavior which is argued to be attributable to the viscous flow of metallic glass powder. The fabricated TiB2‐based ceramic composites have high relative density of 99.1% and high fracture toughness of 9.9 Mpa m1/2. The fracture toughness values obtained in this study are related to the mechanical response of the TiAl phase, which transformed from the metallic glass sintering aid. The results obtained herein provide a novel pathway for the fabrication of high‐performance ceramic composites.

MECHANICAL PROPERTIES AND HIGH TEMPERATURE OXIDATION BEHAVIOR OF Ti–Al COATING REINFORCED BY NITRIDES ON Ti–6Al–4V ALLOY

Ti–Al alloyed coating reinforced by nitrides was fabricated by laser surface alloying technique to improve mechanical properties and high temperature oxidation resistance of Ti–6Al–4V titanium alloy. Microstructures, mechanical properties and high temperature oxidation behavior of the alloyed coating were analyzed. The results show that the alloyed coating consisted of Ti3Al, TiAl2, TiN and Ti2AlN phases. Nitrides with different morphologies were dispersed in the alloyed coating. The maximum microhardness of the alloyed coating was 906[Formula: see text]HV. The friction coefficients of the alloyed coating at room temperature and high temperature were both one-fourth of the substrate. Mass gain of the alloyed coating oxidized at 800[Formula: see text]C for 1000[Formula: see text]h in static air was [Formula: see text][Formula: see text]mg/mm2, which was 1/35th of the substrate. No obvious spallation was observed for the alloyed coating after oxidation. The alloyed coating exhibited excellent mechanical properties and long-term high temperature oxidation resistance, which improved surface properties of Ti–6Al–4V titanium alloy significantly.

Processing, Microstructure and Mechanical Properties of Ti6Al4V Particles-Reinforced Mg Matrix Composites

Novel Ti6Al4V particles-reinforced AZ91 Mg matrix composites were successfully fabricated by stir casting method. The stirring time in semisolid condition directly affected the particle distribution and the quality of the ingots. Furthermore, the optimal speed of the heating and the liquid stirring could overcome particle settlement caused by the density difference between the matrix and the particles. Ti6Al4V particles distributed uniformly in the composites with different particle contents. The average grain size decreased with the increase in the particle contents. The Ti6Al4V particles bonded pretty well with the alloy matrix. In addition, there were some interfacial reactions in the composites. There were rod-like Al3Ti phases at the interface. The precipitates extended from the particle surface to the matrix, and they might improve the interfacial bonding strength. The ultimate tensile strength, yield strength and elastic modulus were enhanced as the particle contents increased, and the elongation was much better than that of the same matrix material reinforced with SiC particles. Thus, the novel composites exhibit better comprehensive mechanical properties.

Nanoscale origins of the oriented precipitation of Ti3Al in Ti[sbnd]Al systems

Oriented precipitation of Ti3Al in TiAl alloys has been already proposed, however the underlying physical mechanism behind the specific precipitation behavior is still unclear. Here, we report that the origin is preferred formation of stacking faults on prismatic planes. In other words, stacking faults induced by Al alloying act as nucleation sources for the precipitation of Ti3Al phases, and their particularly spatial distributions assist in “orienting” Ti3Al precipitates (oriented precipitation). These findings enrich current understanding of nanoscale precipitation behavior, and provide the inspiration of controlling the precipitation behavior by tailoring the physical type and spatial distribution of defects.