TiAl Phase Research Articles

Microstructures of Three In-Situ Reinforcements and the Effect on the Tensile Strengths of an Al-Si-Cu-Mg-Ni Alloy

In the present paper, the microstructures of three kinds of in-situ reinforcements Al-Ti-C, Al-Ti-B, and Al-Ti-B-C-Ce were deeply investigated using a combination of scanning electron microscopy, X-ray diffraction spectroscopy, and transmission electron microscopy. The effect of in-situ reinforcements on the room temperature and elevated temperature (350 °C) tensile strengths of Al-13Si-4Cu-1Mg-2Ni alloy were analyzed. It is found that doping with trace amounts of B and Ce, the size of the Al3Ti phase in the in-situ reinforced alloy changed from 80 µm (un-reinforced) to about 10 µm, with the simultaneous formation of the AlTiCe phase. The Al-Ti-B-C-Ce reinforcement which is rapid solidified, was more effective and superior to enhance the tensile strengths of the Al-13Si-4Cu-1Mg-2Ni alloy, both at room and high temperatures than those of addition other reinforcements. The room temperature (RT) strength increased by 19.0%, and the 350 °C-strength increased by 18.4%.

Investigation of the Mechanism of Precipitation of the α2-phase in a Two-Phase Titanium – Aluminum Alloy

We study the processes of formation of an ordered Ti3Al phase (α2-phase) in Ti – Al alloys depending on the modes of heat treatment. It is shown that the high-temperature treatment performed in the single-phase α-region followed by supercooling at different temperatures leads to a more active precipitation and the growth of dispersed particles than the preliminary treatment in the β-region.

Formation of Ti-Al Phases during SHS Process

Formation of Ti-Al Phases during SHS Process

Microstructure and wear resistance of compositionally graded Ti[sbnd]Al intermetallic coating on Ti6Al4V alloy fabricated by laser powder deposition

In this study, a compositionally graded TiAl intermetallic coating has been successfully fabricated by laser powder deposition (LPD) with Ti6Al4V and AlSi10Mg powder. The microstructure, composition distribution and phase structure of the graded TiAl intermetallic coating were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), electron probe microanalysis (EPMA) and micro-area X-ray diffraction analysis (XRD). In order to evaluate the wear resistance of the graded TiAl intermetallic coating, the sliding wear test was performed. Besides, the morphologies of the worn surface were analyzed by SEM and confocal microscope. The results show that the coating had a crack-free and well-graded compositional microstructure. Intermetallic Ti3Al, TiAl, TiAl3 and Ti5Si3 hard phase were the dominant constructive phases of the graded TiAl intermetallic coating. These phases enhanced the micro-hardness of the graded coating and consequently improved its wear resistance. The average friction coefficient and the wear rate of the graded TiAl intermetallic coating were 0.3704 and 1.989 × 10−4 mm3/Nm, respectively, which were significantly lower than those of the substrate (0.5667 and 8.384 × 10−4 mm3/Nm). The wear mechanism of the coating was mainly abrasive wear. Based on our finding, the graded TiAl intermetallic coating is a promising wear resistant coating for Ti6Al4V alloy.

A Comparative Study on Titanium-Reinforced Aluminium Matrix Composites Produced by Melt Infiltration Casting and Squeeze Infiltration

The unresolved production problems including wettability and undesired intermetallic phase formation at the interface of aluminium matrix composites (AMCs) with ceramic reinforcements require a different approach to the subject. Titanium as a reinforcement is a strong candidate to overcome the current problems, without sacrificing beneficial features of conventional AMCs. This study aims to compare microstructural, mechanical and wear properties of bimetal composites manufactured by two different techniques: melt infiltration casting (MIC) and squeeze infiltration (SI). The temperature was set to 730 °C for both production methods. MIC was carried out in an open die with vacuum assistance, while SI was performed in a closed die in an atmospheric environment. Optical microscope, SEM, XRD, EDS, nanoindentation test device and ball-on-disc type tribometer with 3-mm-diameter Al2O3 ball were used for composite characterization. Al, Ti, Si and TiAl3 phases were formed in composite structure for both techniques. Homogeneous and continuous TiAl3 layer was obtained at Al/Ti interfaces. The volume fraction of TiAl3 was 20 times higher in SI than that in MIC due to long interaction time between Al and Ti. The thickness and the hardness of TiAl3 obtained were higher in SI. Abrasive and adhesive wear mechanisms were observed in worn surface examinations. The composite produced by SI showed slightly better performance against wear under the same test conditions. It was exposed to less plastic deformation and abrasive wear than composite produced by MIC. Although initial investment cost was higher, SI appeared to be more advantageous than MIC, considering the applicability in the industry.

Evolution of microstructure and mechanical properties of Al-B4C composite after recycling

A remelting method was used to recycle Al-15wt.% B4C composite and the microstructure and mechanical properties of the composite were investigated before and after recycling. Scanning electronic microscope (SEM) showed that some of the particle clusters present in the original composite were broken down and the distribution of B4C in the matrix was more uniform after recycling. The interface reaction was further intensified and the protection layer coated on the surface of B4C was thicker after recycling. In addition, there were more TiB2 particles which peeled off from the surface of B4C and distributed around B4C. Al3Ti phase, which was present in the original composite, disappeared after recycling, while Al3BC phase increased markedly. Tensile test and hardness test revealed that the mechanical properties were preserved well after recycling. Therefore, it is proposed that remelting method is a feasible method for Al-B4C composite recycling.

Some aspects of grain refining of Al-Si cast alloys

ABSTRACTA study on the effect of different grain refiners and addition technique in Al-7%Si alloys was carried out. The results show that the Al3Ti phase reacts with silicon (Si) in the molten alloy forming a new compound (Al,Si)3Ti. This reaction is independent of the grain refiner addition technique. The temperature of the liquid metal causes a significant change in the Al3Ti phase morphology, which precipitates in the form of platelets at 750°C and in a dendritic form at 950°C. It has also been observed that while (Al,Si)3Ti phase platelets precipitate within the α-aluminum dendrites, the TiB2 or AlB2 particles are rejected into the surrounding interdendritic regions. The results also reveal that addition of 100ppm B will reduce the initial grain size by ~ 85%, which is more than the reduction obtained with the addition of 0.2%Ti in the form of Al-10%Ti (about 65%).

Influence of Ni-coating thickness on laser lap welding-brazing of Mg/Ti

Dissimilar lap joining of magnesium alloy to titanium alloy was performed using Ni coating by laser welding-brazing process. The influence of different Ni-coating thicknesses on microstructure and mechanical properties were investigated. In laser irradiation region, Ti3Al phase formed at fusion zone/Ti interface. In middle region, Ti3Al phase still existed at fusion zone/Ti interface, while Mg-Al-Ni ternary IMC generated at fusion zone. In weld toe region, Ti-Ni binary IMC were observed at the interface of fusion zone/Ti. The particles of Mg-Al-Ni ternary compound grew bigger and thickness of Ti-Ni compound layer increased with the increase of Ni-coating thickness. TiNi IMC transformed into (TiNi + Ti2Ni) mixtures when the Ni-coating thickness exceeded 5.8 μm. Interfacial microstructure evolution was clarified according to thermodynamic calculation results of formation enthalpy and chemical potential. The fracture load first increased and then decreased slightly with increasing coating thickness, the maximum fracture load (2430 N/cm) represented 90% joint efficiency in relation to the Mg base metal. The fracture mode changed from interfacial failure into fusion zone fracture when the Ni-coating thickness was greater than 4.0 μm. The interfacial bonding were relatively low, while the performance of fusion zone fracture was relatively high. Further increase of coating thickness has little effect on the performance deterioration. The bonding mechanism of Mg to Ni-coated Ti were illuminated.

An ab initio study on stacking and stability of TiAl3 phases

TiAl3 persists in many Al alloys and plays a detrimental role in solidification of related melts. Knowledge about TiAl3 phases and phase relations is of importance to get some insight into the solidification processes, the microstructures and the properties of Al alloys. In this manuscript, we present a systematic study of the basic structures of TiAl3 with aid from ab initio density functional theory (DFT) calculations. The study confirms that the ground state of TiAl3 has the D023 structure, whereas the observed D022 type is a metastable phase. The calculations have identified the stability of a series of stacking composed of both D022- and D023-TiAl3 units. At elevated temperature, the equilibrium configuration contains neither pure D023 nor pure D022 but will consist of a combination of the TiAl3 cubes arranged to minimise its Gibbs energy. Stacking default investigation reveals a large energy barrier for the D022 ↔ D023 transition. The obtained information sheds some light on the rich variety of the experimental observations in Al alloys, and further to understand the complex titanium aluminides and their thermo-structural properties.

Amorphous Phase Formation and Heat Treating Evolution in Mechanically Alloyed Al-Ti Powders

This paper focuses on the microstructural characterization of Al25Ti75, Al37Ti63, Al50Ti50, Al63Ti37and Al75Ti25powders mixtures prepared by mechanical alloying (MA). The high-energy ball-milling, up to 75 h, of aluminium and titanium powders leads to a nanocrystalline or an amorphous structure. It is showed that a stable amorphous Al–Ti phase with uniform elemental distribution forms after 50 h of milling in Al50Ti50alloy. Heat treatment of the different alloys leads to the crystallization of AlTi3, AlTi, Al2Ti and Al3Ti intermetallic compounds. A comprehensive study by laser granulometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) was carried out on the structure, surface morphology and thermal behaviour of the MA Al-Ti mixtures, both of milled and heat treated powders.

Development of a new filler for discrete reinforcement of cast aluminum-matrix composites by titanium carbide

In order to improve the method for obtaining cast aluminum-matrix composites dispersion-filled by titanium carbide and to increase characteristics of these materials were investigated the structure and properties of the composites reinforced by in-situ and ex-situ methods. For this aim were used the synthesis of the TiC under the melt layer by the introduction of compacts of pressed mixture of powders of titanium and graphite, titanium and aluminum carbide and mechanical mixing into the aluminum melt of the pre-synthesized powders of TiC and TiC in the Al3Ti matrix. It was found that the in-situ method of reinforcing aluminum and aluminum alloys with discrete particles of titanium carbide has low efficiency, because of intensive wetting of titanium powders by melt in the volume of compacts, when they are introduced. As a result, the actively formed Al3Ti phase prevents the flowing self-propagating high-temperature synthesis (SHS) between titanium and carbon or aluminum carbide. It was established, that this method is not suitable for reinforcing aluminum alloys, doped with silicon and magnesium, in view of the almost complete cessation of the synthesis of titanium carbide and the active formation of titanium alumino-silicides and aluminides, which is accompanied by spattering of the melt. The SHS-reaction between Al4C3 and Ti under heating conditions at a rate up to 6000 °C per hour leads to the formation of dispersed TiC and Ti3AlC2 of globular shape in an Al3Ti matrix. With rise of the heating rate and the heat dissipation, the number of non-equilibrium phases formed during the reaction increases. The formation of metallic aluminum found under such conditions allows suggest that the reaction between titanium and aluminum carbide is going by stages. The introduction of briquettes into the aluminum melt after initiation of the SHS-reaction in them does not ensure the dissolution of the matrix and the distribution of titanium carbide in the volume of the melt. This requires their preliminary grinding. Ex-situ aluminum reinforcement with TiC-Al3Ti powders is high efficiency due to good wetting of their surface by aluminum melts and subsequent active dissolution of the Al3Ti matrix. In this case, the Al3Ti phase is recrystallized, and the released titanium carbides are distributed in the form of clusters in the melt volume. The resulting materials are superior to the characteristics of the composites reinforced with ex-situ TiC powders of same dispersity. This allows recommend the usage of TiC-Al3Ti powders as fillers to obtain discretely reinforced aluminum-matrix composite materials.

Influence of Ni coating on interfacial reactions and mechanical properties in laser welding-brazing of Mg/Ti butt joint

Laser welding-brazing characteristics of AZ31 Mg to Ti-6Al-4V in butt configuration without and with Ni coating was comparatively investigated with varied laser power. The interfacial reaction and mechanical properties were studied and the microstructure evolution mechanism was clarified based on thermodynamic calculation. The interfacial microstructure evolved from an ultra-thin Ti3Al layer, to a layer of Ti2Ni mingled with Ti3Al, to Ti3Al layer with increasing laser power. In addition, Ti2Ni phase formed on Ti substrate at high laser power. The driving force of Al diffusing to Ti substrate was higher than that of Ni at low laser power while the diffusion time of Ni was longer, resulting in formation of Ti3Al phase or mingled structure of Ti3Al and Ti2Ni at the interface. The driving force of Ni to Ti was larger than that of Al to Ti at high laser power, producing Ti2Ni phase on Ti substrate. Mg-Al-Ni intermetallics formed in the fusion zone near the interface and became scattered with increasing laser power. The fracture load of Mg/Ni-coated Ti joint was approximately three times that of Mg/bare Ti joint. The Mg/Ni-coated Ti joint reached the maximum value of 3900 N at laser power of 1500 W. In this case, the joints failed at Mg base metal while others cracked from the fusion zone-Ti interface.

Study of Ti/Al/Ni Ohmic Contacts to p-Type Implanted 4H-SiC

This work reports on the electrical and microstructural properties of Ti/Al/Ni contacts to p-type implanted 4H-SiC obtained by rapid thermal annealing of a metal stack of Ti (70 nm)/Al (200 nm)/Ni (50 nm). The contact characteristics were monitored at increasing value of the annealing temperature. The Ohmic behavior of the contact, with a specific contact resistance value of 2.3×10-4 Ω·cm2, is obtained for an annealing at 950 °C. The structural analyses of the contact, carried out by XRD and TEM, reveal the occurrence of reactions, with the detection of the Al3Ni2 and AlTi phases in the upper part of the contact and of an epitaxially oriented TiC layer at the interface. These reactions are considered the key factors in the formation of an Ohmic contact in our annealed Ti/Al/Ni system. The temperature-dependence study of the electrical characteristics reveals a predominant thermionic field emission (TFE) mechanism for the current conduction through the contact, with a barrier height of 0.56 eV.

Microstructures and Mechanical Properties of Al3Ti/Al Composites Produced In Situ by High Shearing Technology

Due to its low density, high strength, and stiffness the intermetallic phase Al3Ti is a good candidate as reinforcement for Al alloys. In this work, in situ Al3Ti particle reinforced Al composites are fabricated from Ti particles and Al melt via melt stirring with a high shearing mixer. Microstructure and mechanical properties are investigated. The results indicate that, owing to the high shearing effect and intensive macroscopic flow of the melt, reinforced particles are distributed homogeneously on the microscopic and macroscopic scale. Furthermore, Al3Ti particles are proved to be effective nuclei for heterogeneous nucleation of α‐Al, thus the grain size of the Al matrix is significantly decreased. As a result of the fine grains and the uniform distribution of Al3Ti particles, E‐modulus, yield, and tensile strength of the composites are enhanced.

The Effect of Aluminium Amount on the Combustion Temperature and Microstructure of Ti-Al alloy After Reactive Sintering

Titanium aluminides with various amounts of aluminium were prepared by Self-propagating High-temperature Synthesis (SHS). Ti-20 wt. % Al, Ti-38 wt. % Al and Ti-63 wt. % Al were chosen according to Ti-Al phase diagram, because these chemical compositions represent Ti3Al, TiAl and TiAl3 phase, respectively. The effect of the amount of aluminium on the combustion temperatures, microstructure and phase composition was studied. Heating of compressed samples was observed by optical pyrometer to determine exothermic reaction which is associated with SHS reaction. It was found that reaction temperatures increased with increasing addition of aluminium as well as reaction time and the start of ignition. The expected dominant phases were determined in all systems after SHS reaction. However, other phases accompanied their formation. The largest variety of phases formed in Ti-38 wt. % Al system.

TiAl/RGO (reduced graphene oxide) bulk composites with refined microstructure and enhanced nanohardness fabricated by selective laser melting (SLM)

This work for the first time investigated the effect of laser scan line spacing on the microstructure, phase evolution and nanohardness of Ti-48Al-2Cr-2Nb/RGO (reduced graphene oxide) metal matrix composites (MMCs) fabricated by selective laser melting (SLM). The results show that with increasing the laser scan line spacing from 80 to 140 μm, the average grain size generally decreases from 10.13 to 8.12 μm. The SLM-processed Ti-48Al-2Cr-2Nb/RGO parts are dominated by high-angle (>15°) grain boundaries (HAGBs) and α2 (Ti3Al) phase. With the increase in laser scan line spacing, the contents of HAGBs and α2 phase both decrease. Due to instantaneous high temperature during the SLM process, some RGO sheets transform to amorphous carbon. The nanohardness of SLM-processed Ti-48Al-2Cr-2Nb/RGO parts increase from 8.13 ± 0.39 GPa to 9.85 ± 0.46 GPa when increasing the laser scan line spacing from 80 to 140 μm, which is much higher than that of the traditional casting TiAl counterparts (4.98 ± 0.10 GPa).

Effects of Nb doping on the mechanical properties and interfacial reactions of Ti/Al2O3 composites

New type of structural functional materials, Ti/Al2O3 composites, were successfully fabricated by vacuum hot-pressing sintering at 1350 °C, 1400 °C, 1450 °C, and 1500 °C under a pressure of 30 MPa with a holding time of 90 min. The brittle TiAl and Ti3Al phases, which were regarded as detrimental to the mechanical performance, were present in the reaction products, as observed in X-ray diffraction patterns. Therefore, metallic Nb was introduced into the system to explore its inhibitory effect between Ti and Al2O3. The EDS and EPMA results illustrated that Nb played an active role in inhibiting interfacial reactions. Moreover, the mechanical properties and microstructure of the composites were improved owing to Nb doping.

Hydrogen-Induced Phase Transformation and Microstructure Evolution for Ti-6Al-4V Parts Produced by Electron Beam Melting

In this paper, phase transitions and microstructure evolution in titanium Ti-6Al-4V alloy parts produced by electron beam melting (EBM) under hydrogenation was investigated. Hydrogenation was carried out at the temperature of 650 °C to the absolute hydrogen concentrations in the samples of 0.29, 0.58, and 0.90 wt. %. Comparative analysis of microstructure changes in Ti-6Al-4V alloy parts was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Furthermore, in-situ XRD was used to investigate the phase transitions in the samples during hydrogenation. The structure of Ti-6Al-4V parts produced by EBM is represented by the α phase plates with the transverse length of 0.2 μm, the β phase both in the form of plates and globular grains, and metastable α″ and ω phases. Hydrogenation to the concentration of 0.29 wt. % leads to the formation of intermetallic Ti3Al phase. The dimensions of intermetallic Ti3Al plates and their volume fraction increase significantly with hydrogen concentration up to 0.58 wt. % along with precipitation of nano-sized crystals of titanium δ hydrides. Individual Ti3Al plates decay into nanocrystals with increasing hydrogen concentration up to 0.9 wt. % accompanied by the increase of proportion and size of hydride plates. Hardness of EBM Ti-6Al-4V alloy decreases with hydrogen content.

A Study on the Characterization of MASHS Processed Ti-Si-Al System

Many promising titanium aluminides and silicides could be synthesized in Ti-Si-Al system via mechanically activated self-propagating high-temperature synthesis (MASHS). The influence of mechanical activation and initial composition on phase formation, microstructure and synthesis behavior of mechanically activated Ti-rich mixtures in Ti-Si-Al system is investigated. Results indicate that Al can participate in the reaction and forms Ti3Al and TiAl3 phases beside Ti5Si3 phase by using mechanically activated reactants. Si and Al could be substituted each other in titanium aluminide and silicide to form solid solution of Ti3(Si,Al) and Ti5(Si,Al)3. In Ti-Si-Al system, combustion temperature is increased by increasing Si amount in the system while, ignition temperature is decreased profoundly from 1100 to 800 ∘C by applying mechanical activation prior to reaction and increasing Al in the system. In addition, Mechanical activation prior to synthesis gives rise to narrow grain size distribution and more uniform microstructure in comparison with SHS products.

Laser conduction welding characteristics of dissimilar metals Mg/Ti with Al interlayer

The immiscible and nonreactive characteristics of Mg/Ti dissimilar metals restrict the metallurgical bonding and reliable joining of them. To solve this problem, the Al interlayer between Mg/Ti was added. The Mg/Ti joint without addition of Al interlayer could not be welded, while relatively acceptable appearance was obtained when adding Al interlayer. The interfacial reaction was improved with the assistance of Al interlayer resulting in metallurgical bonding at the interface. The TiAl3 phase formed at the direct laser irradiation zone. Note that the interfacial reaction layer increased little with the increasing thickness of Al interlayer. The interfacial reaction layer was mainly restricted by dissolution of Ti element. Temperature field and thermodynamic calculation assisted to prove the interfacial reaction at the fusion zone/Ti interface and the formation of TiAl3. The tensile-shear testing indicated that the maximum tensile-shear force could reach 2230 N/cm (about 81% of that of AZ31B Mg alloy base metal) when the laser power was 2300 W and the thickness of added Al interlayer was 0.05 mm. The tensile-shear force decreased when network structure of Mg17Al12 precipitated in the weld zone. The fracture path changed from fusion zone/Ti to Mg fusion line when the thickness of Al interlayer exceeded 0.12 mm.