TiAl Intermetallic Compound Research Articles

Cryomilling and Characterization of Ti/Al<sub>2</sub>O<sub>3</sub> Powders

In order to prevent the oxidation of Ti, which ultimately leads to the generation of intermetallic compound Ti3Al, a new method of cryomill in liquid nitrogen was used to deal with the Ti/Al2O3 powders. The size distribution, phase composite and microstructure of the powders were analyzed using laser particle size analyzer, XRD, and TEM, respectively. Then, the performances of Ti/Al2O3 cermet sintered using cryomilled powders and room temperature milled powders were compared. The results show that, with the increase of cryomilling time, the grain size decreases shapely and high reactivitive nanoscale powders are finally obtained. With the cryomilling in liquid nitrogen, the Ti-N bonds are formed, which successfully prevent the oxidation of Ti. Ti/Al2O3 cermet sintered using cryomilled powders shows higher density, better mechanical properties than that using RT milled powders.

Fabrication of reinforced α+β titanium alloys by infiltration of Al into porous Ti-V compacts

We present an efficient process to fabricate reinforced titanium α + β alloys. Porous Ti compacts containing vanadium were prepared by a metal injection molding process. Infiltration of molten Al and subsequent heat treatments were performed to synthesize a variety of Ti-V-Al alloys. The V-added specimens showed better mechanical properties than those of V-free Ti-Al intermetallic compounds. Microstructural observation of the fracture surface revealed that the lamellar structure in the Ti-V-Al alloys plays an important role in improving their mechanical properties by postponing the propagation of cracks. Notably, the lamellar structures in the Ti-V-Al alloys disappeared and the mechanical properties deteriorated as the Al infiltration duration time increased. Therefore, Al infiltration time is a crucial factor for successful growth of well-defined lamellar structured α + β phases in Ti-V-Al alloys.

Influence of Nb and Cu Elements on the Intermetallic Particles Morphology in Ti/Al Multilayer Composite Processed Through Hot Pressing and Rolling

Ti–Al–Nb composites were produced by solid state diffusion bonding through hot pressing and rolling followed by annealing at 700 °C for 0.5, 1, 1.5 and 2 h. The morphologies of TiAl3 intermetallics were investigated by Scanning Electron Microscopy combined with Energy-dispersive X-ray spectroscopy. Titanium tri-aluminide (TiAl3) particles with blocky morphology were dispersed into Aluminum matrix. In the presence of niobium and copper, TiAl3 particles were produced in different sizes and morphologies. The presence of Nb in the composite led to the formation of irregular angular morphology, while the copper resulted in cubic morphology of the intermetallic particles. The EDS results indicated that TiAl3, (Ti, Nb)Al3 and (Ti, Nb, Cu)Al3 intermetallic compounds appeared near Ti zone, Nb Zone and in the presence of Cu, respectively.

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.

Grain Refinement of Al-Si-Fe-Cu-Zn-Mn Based Alloy by Al-Ti-B Alloy and Its Effect on Mechanical Properties.

We investigated the effects of Al-5.0wt%Ti-1.0wt%B addition on the microstructure and mechanical properties of the as-extruded Al-0.15wt%Si-0.2wt%Fe-0.3wt%Cu-0.15wt%Zn-0.9wt%Mn based alloys. The Aluminum alloy melt was held at 800 °C and then poured into a mould at 200 °C. Aluminum alloys were hot-extruded into a rod that was 12 mm in thickness with a reduction ratio of 38:1. AlTiB addition to Al-0.15Si-0.2Fe-0.3Cu-0.15Zn-0.9Mn based alloys resulted in the formation of Al3Ti and TiB2 intermetallic compounds and grain refinement. With increasing of addition AlTiB, ultimate tensile strength increased from 93.38 to 99.02 to 100.01 MPa. The tensile strength of the as-extruded alloys was improved due to the formation of intermetallic compounds and grain refinement.

Friction push plug welding in airframe structures using Ti-6Al-4V plug

Friction push plug welding, a recent variation of friction welding process has tremendous applications in joining and repairing operations in aerospace and automotive engineering. The main objective of the present work demonstrates joining feasibility of high performance Ti-6Al-4V to low density AA2024 employed in passenger seat track, vertical stabilizers and wing spars of aircraft structures of aerospace applications. Mechanical testing is carried out to analyse the joint integrity and strength of Ti-6Al-4V/AA2024 joints. The microstructural aspects are investigated to study the bonding mechanism to obtain defect-free welds. SEM micrograph reveals the formation of different zones with significant reduction in grain size. EDX analysis confirms the formation of Ti3Al intermetallic compound which is less brittle. Appreciable impact strength of about 286.354 kJ/m2 indicates friction push plug welding as a promising candidate for joining of Ti-6Al-4V and AA2024 in airframe structures.

TiO2 ceramic particles-reinforced aluminum matrix composite prepared by conventional, microwave, and spark plasma sintering

Aluminum-10 wt% TiO2 metal matrix composites were fabricated with conventional, microwave, and spark plasma sintering processes. Aluminum and nano-sized TiO2 powders were mixed using a high-energy mixer, and the sintering process was done at 450℃ by spark plasma sintering and 600℃ under both microwave and conventional heating. The results showed microwave sintering led to form Al3Ti intermetallic compounds with flaky shape, while in the conventional heating at the same sintering temperature, Al3Ti was formed and confirmed by X-ray diffraction and scanning electron microscope investigations. Moreover, the nano-sized TiO2 particles as reinforcement with no additional phase were obtained by spark plasma sintering at the lowest sintering temperature. The maximum bending strength of 254 ± 12 MPa and Vickers hardness of 235 ± 13 were measured for samples sintered in microwave as a consequence of Al3Ti formation. The SEM and energy-dispersive X-ray spectroscopy analyses showed uniform distribution of Al3Ti particles in the microstructure of microwave sintered samples and nonuniform distribution of agglomerated Al3Ti particles and porosities in samples sintered by spark plasma sintering and conventional heating.

The Influence of the Synthesis Temperature on Phase Composition and Structure of Tenary Compounds Obtained from the Powder Mixture of the TiH2-Al-C System

This paper presents the results of an investigation of the features of phase and structure formation of a ternary compound during thermal sintering use of compacted TiH2-Al-C powder blends. The thermal sintering was carried out in a vacuum furnace at temperature 1150, 1300, and 1400 0С. The x-ray diffraction pattern and structural analysis show that the main phase after synthesis at 1150 0С is titanium carbide. The ternary Ti2AlC and intermetallic Ti3Al compound were also identified in the phase composition of the alloy. Increasing the sintering temperature to 1300 °C leads to significant increases in the content of Ti2AlC ternary compounds and accordingly decreases the content of titanium carbide TiC. Is propose a modified model thermal synthesis of ternary compounds of the Ti-Al-C system, which includes the melting of aluminum and its interaction with titanium at low-temperature stages of the process, the formation of the Ti3Al intermetallic compound, formation titanium carbide grains as a result of the interaction of the Al4C3 intermediate metastable phase with titanium or Ti3Al intermetallic compound and the synthesis of ternary Ti2AlC and Ti3AlC2 compounds as a result of the interaction of the Ti3Al intermetallic compound with carbon and Ti2AlC with titanium carbide TiC.

Microscopic plastic response in a bulk nano-structured TiAl intermetallic compound processed by high-pressure torsion

Intermetallic compounds of titanium aluminides (Ti-Al) have attracted much attention for their excellent properties including light-weight, high specific strength and high-temperature creep strength. Nevertheless, bulk Ti-Al require suitable processing routes for improving the mechanical properties, especially plasticity and ductility. Accordingly, the present study applies a strategy of grain refinement through high-pressure torsion (HPT) processing which is an effective processing technique for difficult-to-work alloys due to the large compressive hydrostatic stresses that are developed during processing. A γ-based Ti-45Al-7.5Nb intermetallic compound was processed by HPT for 5 and 10 turns under 6.0GPa at room temperature. The duplex microstructure was successfully refined to have ultrafine laths with thicknesses of 40–100nm after 10 HPT turns. Improved hardness was recorded by both Vickers microhardness and nanoindentation measurements. The X-ray diffraction profiles confirmed the occurrence of phase transformation from γ-TiAl to α2-Ti3Al after HPT. The macroscopic plastic flow was estimated through the nanoindentation technique by applying both Berkovich and spherical indenters. The results predict an improved plastic behavior by increasing the strain rate sensitivity and a significant increase in yield strength whereas a slight decrease is measured in the strain hardening capability after HPT for 10 turns. Furthermore, this nanoindentation study demonstrates an alternative and promising technique of small-scale testing for predicting the macroscopic plastic flow behavior of ultrafine-grained materials.

Study the Effects of Dielectric Type on the Machining Characteristics of γ-Ti Al in Electrical Discharge Machining

The current study surveys the results of using deionized water and kerosene as dielectrics in the machining outputs of γ-TiAl intermetallic compound obtained in electric discharge machining. Influences of these different dielectrics properties on machining speed, tool wear, surface cracks and roughness were compared. Scanning electron microscopy micrographs were prepared to investigate influences of dielectrics on the surface characteristics of electrically discharged samples. Results indicate which by kerosene dielectric; the material removal rate (MRR) is further compared to another one. But deionized water as dielectric causes higher tool wear ratio than kerosene dielectric. Electrical discharged samples in deionized water have higher surface roughness, in addition it contains surface cracks, whereas kerosene dielectric results better surface finish in low pulse current. According to XRD spectra electric discharge machining in deionized water and kerosene dielectrics produces Ti3 Al intermetallic compound on the produced surface.

Interface-correlated bonding properties for a roll-bonded Ti/Al 2-ply sheet

We investigated the influence of annealing conditions on the interface-correlated microstructural evolution and subsequent bonding properties of a warm roll-bonded Ti/Al clad sheet. A TiAl3 intermetallic compound layer with a thickness of 160nm was initially generated at the joint interface between the Ti and Al alloys by warm rolling. When the annealing time and temperature were increased to a maximum of 6h and 650°C, respectively, the thickness of the TiAl3 layer increased to 320nm. The feasible annealing conditions for the optimum bonding strength were within 550°C-6h and 600°C-3h. An improvement in the bonding strength between Ti and Al is strongly correlated with the generation of considerable metallurgical bonding. This is manifested in the zipper-like failure mode that occurs at the TiAl3 joint interface for a layer thickness of less than 300nm.

The Kirkendall void formation in Al/Ti interface during solid-state reactive diffusion between Al and Ti

In the current study, the formation of Kirkendall voids in Al/Ti interface during solid-state reactive diffusion between Al and Ti was investigated. In this regards, pure Al wire (grade 1100) was deposited on commercially pure Ti surface by means of GTAW processing and the prepared samples were annealed at 625 °C for different periods of time. The achieved results indicated that, during cladding process, a continuous layer of Al3Ti intermetallic compound with average thickness of about 2 µm, can be formed in Al/Ti interface. During annealing process, both Ti and Al diffused into each other and the growth of the Al3Ti layer occurred mainly towards the Ti side. At this condition, Kirkendall voids are formed and accumulated near to Al/Al3Ti interface. These voids started to connect to one another, coalesce into large gaps and resembling a line crack at the interface. The formed crack acts as a barrier in Al/Ti diffusing couple and inhibits the growth of Al3Ti phase.

Design of co-existence parallel periodic surface structure induced by picosecond laser pulses on the Al/Ti multilayers

Formation of periodic nanostructures on the Ti/5x(Al/Ti)/Si multilayers induced by picosecond laser pulses is studied in order to better understand the formation of a laser-induced periodic surface structure (LIPSS). At fluence slightly below the ablation threshold, the formation of low spatial frequency-LIPSS (LSFL) oriented perpendicular to the direction of the laser polarization is observed on the irradiated area. Prolonged irradiation while scanning results in the formation of a high spatial frequency-LIPSS (HSFL), on top of the LSFLs, creating a co-existence parallel periodic structure. HSFL was oriented parallel to the incident laser polarization. Intermixing between the Al and Ti layers with the formation of Al-Ti intermetallic compounds was achieved during the irradiation. The intermetallic region was formed mostly within the heat affected zone of the sample. Surface segregation of aluminium with partial ablation of the top layer of titanium was followed by the formation of an ultra-thin Al2O3 film on the surface of the multi-layered structure.

Numerical simulation of titanium dissolution in the aluminum melt and synthesis of an intermetallic compound

Titanium dissolution in the aluminum melt and synthesis of an intermetallic compound at constant temperature and pressure are numerically simulated by the molecular dynamics method. Owing to titanium dissolution, the TiAl3 intermetallic compound is formed near the interface between the titanium crystal and aluminum melt. Based on the theory of weak solutions, a mathematical model of titanium dissolution in the aluminum melt is constructed. Dependences of the diffusion coefficient, equilibrium concentration of titanium, and dissolution rate on temperature are obtained.

Self-Propagating High-Temperature Synthesis of Boron-Containing MAX-Phase

An attempt was made to obtain boron-containing MAX-phase by the process of self-propagating high-temperature synthesis (SHS) of Ti3AlC2, replacing some carbon atoms by boron atoms. This was conducted by burning powder mixtures (charges) of the composition 3Ti+2Al+2((1-x)C+xB), where x is the fraction of boron atoms (0.10, 0.15, 0.25, 0.50, 0.75, 0.90), replacing the carbon atoms. X-ray diffraction analysis of the products of combustion have shown that the replacement of carbon with boron to half of the content of carbon atoms in the charge (x=0.10-0.50), does not change the phase composition of the products, including Ti3AlC2 and TiC, but leads to a shift of the peaks of these phases in the diffraction pattern in the direction of smaller angles. When replacing more than half of the carbon atoms with the boron (x=0.75 and 0.90), the peaks of titanium carbide and MAX-phase are not observed, and the XRD peaks appear of the titanium borides TiB and TiB2, and intermetallic compound Al3Ti. Photomicrographs obtained with an electron microscope show that the SHS products synthesized from the charge with replacing up to half of the carbon atoms with the boron represent plates with a thickness of about 1 μm typical for MAX-phases, but rounded particles of borides and intermetallic compound of titanium appear at a higher boron content. Based on these results, it is concluded that replacement of a part (up to 50%) of the carbon atoms with boron atoms in the SHS charge 3Ti+2Al+2C leads to the synthesis of boron-containing MAX-phase based on the crystal lattice of Ti3AlC2.

Tensile property improvement in Ti/Al clad sheets fabricated by twin-roll casting and annealing

A new fabrication method of Ti/Al clad sheet by bonding a thin Ti sheet on to Al alloy melt during twin-roll casting was presented to broaden industrial applications of Ti alloys. In the twin-roll-cast Ti/Al clad sheet, a homogeneously solidified microstructure existed in the cast Al side, while a Ti/Al interface did not contain pores, cracks, or lateral delamination, which indicated a successful fabrication. When this cast sheet was annealed at 400 °C, metallurgical bonding was expanded by interfacial diffusion, thereby leading to improvement in tensile properties. After the 600 °C-annealing, a TiAl3 intermetallic compound was formed at the Ti/Al interface, which deteriorated tensile properties because of its brittle characteristics. The yield and tensile strengths of the 400 °C-30-min-annealed sheet (132 MPa and 218 MPa, respectively) were higher than those (103 MPa and 203 MPa, respectively) calculated by a rule of mixtures using tensile properties of a 5052-O Al alloy and a pure Ti. Its ductility was also higher than that of 5052-O Al alloy (25%) or pure Ti (25%) because the strain at the interfacial delamination point reached 41%. The 400 °C-30-min-annealed sheet showed the overall homogenous deformation behavior by complimenting drawbacks of mono-layer of Al or Ti.

Crack Resistance Characterization in TiAl Intermetallics with Enhanced Toughness

The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable α phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of α+g phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.

Fracture Toughness of Massively Transformed and Subsequently Heat Treated TiAl Intermetallic Compound

The effects of massive transformation and subsequent heat treatments on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studied. Massive transformation occurs at the center region of the specimen by cooling from α single phase state. At the surface side of the specimen, α phase has remained. Fine convoluted microstructure with α2, γ phases and lamellar structure has formed by heating at (α+γ) two phase state after massive transformation. Colony size or grain size is about 25 μm. Fine fully lamellar structure is obtained after heat treatment of convoluted microstructure at α phase for 60 s. Fracture toughness seems to be increasing with the increase in lamellar colony size. However, some massively transformed specimens show lower toughness due to the formation of microdamage present in samples before the test.

AIH-FPP処理によるチタン合金表面のTi-Al金属間化合物化プロセスの検討

AIH-FPP処理によるチタン合金表面のTi-Al金属間化合物化プロセスの検討

Effect of Cr and Nb on the phase transformation and pore formation of Ti-Al base alloys

Abstract The phase transformation and its relation to the porosity of Ti-Al base alloys are investigated in order to understand the effect of composition on the pore formation during sintering at low and high temperatures. Ti-Al base alloys with nominal compositions of Ti52Al48, Ti50Al48Cr2, Ti50Al48Nb2 and Ti48Al48Cr2Nb2 are prepared by the conventional elemental powder metallurgy route. The sintering of mixed powders was carried out at 600° C (low-temperature sintering below the Al melting point) and 1200° C (high-temperature sintering). Al-rich intermetallic compound TiAl3 was primarily formed during low-temperature sintering while Ti-rich intermetallic compound Ti3Al was formed during high-temperature sintering. The phase transformations increase the overall porosity and pore size of alloys to a similar extent regardless of the alloy composition during the low-temperature sintering. However, large differences in the pore size and pore volume distribution occur depending on the composition of Ti-Al alloys during high-temperature sintering. Chromium and niobium do not affect the formation of Kirkendall voids and thus show a similar void formation behavior at 600° C, but affect the void agglomeration behavior to a different extent due to the different diffusion activities at 1200° C.