TiAl Intermetallic Compound Research Articles

Experimental and theoretical investigation of the dislocation mechanisms of microcrack nucleation in Ti3Al

The dislocation structure and microcracks in Ti3Al samples subjected to deformation with an indenter at room temperature are analyzed by electron microscopy. The reactions of interaction between superdislocations that carry out deformation in the basal, prismatic, and pyramidal planes (of types I and II) are considered. The types of interactions that result in the formation of dislocation barriers, i.e., microcrack nuclei, are found. The Rice-Thompson model is used to study the relation between the fracture sensitivity and the plastic-relaxation ability of the Ti3Al intermetallic compound during microcrack propagation.

Mg-Al系合金とTiの高温濡れ現象

Magnesium alloy is expected as one of the environmentally benign materials for significant weight reduction of the components due to its high specific strength. It is, however, necessary to improve its Young's modulus when Mg alloys are applied to structural components. Ceramics hard particles are generally used as reinforcements to improve the stiffness of Mg matrix composite. However, they also cause the decrease of ductility of the composite materials. In the present study, pure Ti particles were used as reinforcement because they had enough ductility and high Young's modulus compared to Mg alloys. For the materials design of Mg matrix composites reinforced with Ti particles, the wetting behavior of Ti by the molten Mg alloy should be investigated to fabricate a good coherence at the interface between the matrix and dispersoids. The sessile drop test results showed an extremely good wettability between pure Mg and pure Ti due to a thermite reaction at the interface between Mg and TiO2 surface films of the Ti substrate. The wettability between Mg-Al alloy (AZ80) and Ti was investigated in high-purity argon gas atmosphere at 1073K. In particular, the effect of Al element contained in AZ80 on the wetting behavior was examined. When AZ80 was dropped on the pure Ti plate (t=0s), the contact angle between the droplet and the Ti plate was 10°. The contact angle and the droplet height were gradually reduced with increasing time. Ti plate was completely covered with molten AZ80 in 60s. The result indicated the wettability of pure Ti by molten Mg-Al alloy was extremely good compared to the combination of pure Mg and pure Ti. When the specimen was kept at 1073K for 180s, Al was enriched on interface between AZ80 and Ti, but Al-Ti intermetallic compounds were not formed. It was concluded the wetting phenomenon of Ti by Mg alloy droplet reached termination before formation of Al-Ti intermetallics and Al element of AZ80 had no harmful effect on the wetting behavior between Mg alloy and Ti.

306 燃焼合成法によるV, Nb, Ta添加TiAl金属間化合物の創製と特性(GS9-2 機械材料)

306 燃焼合成法によるV, Nb, Ta添加TiAl金属間化合物の創製と特性(GS9-2 機械材料)

Effect of calcium on intermetallic compound layer at interface of calcium added magnesium–aluminum alloy and titanium joint by friction stir welding

Abstract Commercial AMCa602 alloy (Mg–6% Al–2% Ca) and AM60 alloy (Mg–6% Al) were joined to titanium plates by friction stir welding to evaluate the effect of a calcium on the reaction layer at the dissimilar joint interface and the joint tensile strength. At the titanium and AM60 joint interface, a TiAl3 intermetallic compound layer was formed. The thickness of this layer was about 2 μm. A layer containing calcium and a layer containing aluminum and titanium were observed at the titanium and AMCa602 joint interface. The aluminum and titanium layer in the titanium and AMCa602 joint interface was very thin. The content of aluminum in solid-solution in the matrix of AMCa602 was lower than that of AM60 due to the formation of a Ca–Al compound, and this suppressed the formation of TiAl3 at the titanium and AMCa602 joint interface.

Effects of mechanical alloying on the characteristics of a nanocrystalline Ti–50at.%Al during hot pressing consolidation

Two different powder metallurgy processes, i.e., reactive and non-reactive sintering, were investigated for the production of titanium aluminides. Ti–Al intermetallics have been successfully produced by mechanical alloying and hot pressing of powders with a nominal composition of Ti–50 at.%Al. A Ti(Al) solid solution was formed at the early stage of milling that transformed to an amorphous phase at longer milling times. On further milling, the amorphous structure transformed to a supersaturated hcp-Ti(Al) solid solution with trace amounts of TiAl after 80 h of milling, which was completely transformed to TiAl, Ti3Al, and TiAl3 intermetallic compounds after additional milling up to 100 h milling with particles of about 200 nm. Blended elemental powders and 100 h MA-ed powders were used for the reactive and non-reactive sintering processes, respectively. The results showed that the HP-ed pre-alloyed powder had better properties in terms of density, hardness, homogeneity of microstructure, yield stress, and ductility than that produced by reactive sintering. The major contribution to the yield stress of the sintered pre-alloyed powder comes from the nanometer-sized grains of intermetallics in accordance with the Hall–Petch relation.

A new approach to grain refinement of an Mg–Li–Al cast alloy

Crystallographic calculation based on the edge-to-edge matching model predicted that both TiB2 and Al3Ti intermetallic compounds have strong potential to be effective grain refiners for beta phase in the Mg-14Li-1Al alloy due to the small atomic matching misfit across the interface between the compounds and beta phase. Experimental results showed that addition of 1.25 wt%Al-5Ti-1B master alloy reduced grain size of beta phase in the alloy from 1750 to 500 mu m. The possible grain refining mechanisms were also discussed

Cyclic deformation and fatigue in TiAl intermetallic compound under plastic strain control

For Mn-containing Ti–48 at.%Al intermetallic compound with a lamellar structure and an equiaxed near-γ structure obtained by heat treatment, a tension–compression fatigue test under plastic strain control was conducted to clarify the fatigue behavior of TiAl. Cyclic hardening occurs significantly in the equiaxed near-γ structure but not in the lamellar structure. The lamellar structure shows the pointed hysteresis loop, whereas the equiaxed near-γ structure exhibits the relatively rounded loop. The variation of the hysteresis loops during cycling was investigated by using the modified energy parameter β E′, and it has been clarified that the strain localization process depends greatly on the microstructure.

Mechanochemical synthesis of (Fe,Ti) 3Al–Al 2O 3 nanocomposite

Formation mechanism of (Fe,Ti) 3Al–Al 2O 3 nanocomposite prepared by mechanical alloying of Fe, Al and TiO 2 with molar ratio of 6:7:3 was studied. The structural changes of powder particles during mechanical alloying were investigated by X-ray diffractometery. Morphology and microstructure of powder particles were characterized by scanning electron microscopy. It was found that during mechanical alloying Al first reacts with TiO 2 leading to the gradual formation of crystalline Ti and amorphous Al 2O 3 phases. In the second stage Ti and remaining Al diffuse into the Fe lattice and as a result a Fe(Al,Ti) solid solution develops. This structure transformed to (Fe,Ti) 3Al intermetallic compound at longer milling times. Heat treatment of this structure led to the crystallization of Al 2O 3 and ordering of (Fe,Ti) 3Al phases.

Phase transformations and thermal stability of the structure of Ti3Al intermetallic compound at deuteration and plastic deformation

The structural transformations in Ti3Al intermetallic compound at deuteration with concentrations x = 1.2 and 1.7, heating at 100–400°C, and shear deformation under pressure have been studied. It is established that at a given deuterium concentration deuterides with fcc and orthorhombic lattices are formed; under severe shear deformation, nanocrystalline and amorphous (or close to amorphous) deuterides arise. The reasons for the structure amorphization at deuteration and subsequent plastic deformation are discussed.

Fabrication of (TiB<sub>2</sub>, TiC<sub>x</sub>)/B<sub>4</sub>C Composites via Ti<sub>3</sub>Al IMC Added in B<sub>4</sub>C Ceramics

Ti3Al inter-metallic compounds were successfully prepared via high energy ball milling and the subsequently heat treatment. Near full dense (TiB2, TiCx)/B4C matrix composites were fabricated by the addition of Ti3Al. The maximum values of hardness, flexural strength and fracture toughness are HRA 94.1, 580.34MPa and 8.07MPa•m1/2 respectively for 20wt% Ti3Al content. Furthermore, the microstructures of the composites were analyzed by XRD, SEM and energy spectrum methods.

Microstructure and Properties of Ti-Al Based Composite Coating Prepared by Electrospark Deposition Process

Ti-Al based composite coating was fabricated on LY12 aluminum alloy surface by electrospark deposition (ESD) process, with γ-TiAl as electrode. The microstructural characteristics, element distribution, microhardness and wear resistance of the coating were investigated. The results show that the coating forms metallurgical bonding to the substrate, which consists of strengthened layer, transition layer and heat affected zone. Besides the Al phase, the coating is also composed of Ti-Al intermetallic compounds and a small mount of Ti/Al oxides and nitrides. The microhardness of composite coating decreases gradually along the depth direction. The sliding wear test results suggest that the worn volume loss of the coating is less than one third of that of the substrate. The abradability of LY12 aluminum alloy is significantly improved. The phases with higher hardness dispersed in the coating increase not only composite coating hardness but also the resistance to micro-cutting and plowing during the sliding wear test. In addition, the excellent sliding wear resistance of composite coating is also related to the metallurgical bonding between the coating and the substrate.

Study on graphite fiber and Ti particle reinforced Al composite

Graphite fiber and Ti particle-reinforced aluminum matrix composite were produced by squeeze casting technology. A small amount of needle aluminum carbide at graphite fiber and Al interface was observed, and TiAl3 intermetallic compound at Ti particle and Al interface was detected. Tensile strength and bending strength of the composite have been measured. The fracture surface of the composite after tensile and bending tests was observed; graphite fiber-reinforced Al was brittle fracture, whereas Ti particle-reinforced Al was ductile fracture. The corresponding fracture mechanism was discussed.

Structural transformations in the deuterium-containing intermetallic compound (Ti3Al)D1.2 induced by high-pressure torsion

The intermetallic compound Ti3Al (which is a very brittle material) with a high deuterium concentration (x = 1.2) is fabricated in a monolithic state by high-pressure torsion at room temperature. The structure of the intermetallic samples is studied by X-ray diffraction and transmission electron microscopy after deformation at various degrees. Under certain conditions, the main volume of the material is found to transform into an amorphous state, and areas 1–2 nm in size with an atomic arrangement close to the initial arrangement are also present in the material. The possible causes of the deformation-induced amorphization of the material alloyed with interstitial atoms are discussed.

Environmental corrosion resistance of porous TiAl intermetallic compounds

Porous TiAl intermetallic compound, as a novel substitute for current inorganic porous material, offsets the shortages of both ceramics and metals. The environmental corrosion resistance of porous TiAl intermetallic compound was investigated. The kinetic equation for the cyclic oxidation of porous TiAl alloy at 600 °C is determined to be Δ m 2=1.08×10 −5 t. After total oxidation of 140 h, porous TiAl intermetallic compound shows more stability of pore structure and the mass gain of TiAl alloy is 0.042 g/m 2, which is only 10.6% that of porous 316L stainless steel. The kinetic equation for the cyclic corrosion behavior of porous TiAl alloy in hydrochloric acid with pH=2 at 90 °C is determined to be Δ m 2=5.41×10 −5 t−2.08×10 −4. After 50 h exposure, the mass loss of TiAl alloy is 0.049 g/m 2, which is only 14.8% and 5.57% that of porous Ti and stainless steel, respectively. The kinetic equation in hydrochloric acid with pH=3 is determined to be Δ m 2=2.63×10 −6 t−3.72×10 −6.

In situ production of Al–TiB2 nanocomposite by double-step mechanical alloying

An in situ Al–TiB2 nanocomposite was synthesized by mechanical alloying (MA) of pure Ti, B and Al powder mixture in a planetary ball mill. A double-step process was used to prevent the formation of undesirable phases like Al3Ti intermetallic compound. In the first step, a powder mixture was tailored to obtain nominal Al–90 wt% TiB2 composition and the second step involved the addition of Al to the mixture in order to achieve Al–20 wt% TiB2. The structural and thermal characteristics of powder particles were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), and transmission electron microscopy (TEM). The results showed that the MA process leads to the in situ formation of nanosized TiB2 particles in an Al matrix with a uniform distribution. It was also found that the double stage addition of aluminum can prevent the formation of undesirable compounds even after annealing at high temperatures.

First-principles study of deformation-induced phase transformations in Ti–Al intermetallics

The structural phase stability and electronic properties of the Ti–Al intermetallic compounds were investigated by means of density-functional theory (DFT) calculations in a generalized gradient approximation. Through comparison of the calculated formation energies of the parent and product phases, an in-depth theoretical understanding of the deformation-induced γ ↔ α2 phase transitions observed previously in TiAl alloys was achieved. The formation energy plays an important role in evaluating the feasibility of these phase transformations during plastic deformation of TiAl alloys. In addition, the density of states (DOS) was also calculated and used to analyze the stability of Ti–Al intermetallic compounds. The reasons for the absence of the deformation-induced (DI)-α2 and DI-γ (L12) phases in underformed TiAl alloys were analyzed.

Formation of Metastable Phases and their Stability in Mechanically Alloyed Ti-48Al-12Nb-1Cr (at %) System

The reported work is on Ti-48Al-12Nb-1Cr (at %) system synthesized in a high-energy planetary ball mill with optimized milling parameters. The synthesized powders are characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-Ray Diffraction (XRD) and Differential Thermal Analysis (DTA) in order to understand the structural and phase transformation. The results obtained from the above said studies show the particles refinement, formation of non-equilibrium structures, etc. High-energy ball milling results super saturation of Al and Nb in titanium matrix at 15 hrs of milling, nanocrystalline nature at 20 hrs and amorphous phase formation at 50 hrs of milling. Differential Thermal Analysis (DTA) and XRD scans of MA powder thermally treated at 750°C for 1 hr show the formation of TiAl and TiAl3 intermetallic compounds.

The kinetics of two-stage formation of TiAl 3 in multilayered Ti/Al foils prepared by accumulative roll bonding

The solid-state reactions of Ti/Al multilayered samples produced by Accumulative Roll Bonding (ARB) have been investigated using differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. The kinetics of the formation of the intermetallic compound TiAl 3 was highlighted. Experimental evidence and analysis of the data shows that, there was a two-stage process in the formation of TiAl 3 in the ARB Ti/Al reactive multilayered samples. Calorimetric and microstructural analyses also suggest that the interdiffusion of Al and Ti which led to solid solutions preceded the formation of intermetallic compounds. Despite the apparent chaos in the thickness of the ARB multilayered samples, the distribution of layer spacing did not become broad enough to lose the main features of the double exothermal behaviour. Isothermal DSC shows a larger Avarami constant in ARB Ti/Al multilayered structures than was found in Ti/Al thin films. A modified model based on thin films was set up to describe the kinetic characteristics of the formation of the intermetallic compound TiAl 3 in ARB samples.

First-principles characterization of the anisotropy of theoretical strength and thestress–strain relation for a TiAl intermetallic compound

We perform first-principles computational tensile and compressive tests (FPCTTand FPCCT) to investigate the intrinsic bonding and mechanical properties of aγ-TiAl intermetalliccompound (L 10 structure) using a first-principles total energy method. We found that the stress–strainrelations and the corresponding theoretical tensile strengths exhibit strong anisotropy inthe [001], [100] and [110] crystalline directions, originating from the structural anisotropy ofγ-TiAl.Thus, γ-TiAl is a representative intermetallic compound that includes three totally differentstress–strain modes. We demonstrate that all the structure transitions in the FPCTTand FPCCT result from the breakage or formation of bonds, and this can begeneralized to all the structural transitions. Furthermore, based on the calculations wequalitatively show that the Ti–Al bond should be stronger than the Ti–Ti bond inγ-TiAl.Our results provide a useful reference for understanding the intrinsic bonding and mechanical propertiesof γ-TiAl as a high-temperature structural material.

Effect of alloying elements on interface microstructure of Mg–Al–Zn magnesium alloys and titanium joint by friction stir welding

An effect of alloying element on interfacial microstructure of Mg–Al–Zn alloys and Ti dissimilar butt joints by friction stir welding (FSW) was studied. Mg–Al–Zn alloy and Ti plates of 2 mm in thickness were successfully butt joined by inserting a probe into the Mg alloy plate and slightly offsetting it by 0.5 mm into the Ti plate side. Al-rich thin layers and Ti–Al intermetallic compound layers formed at the interface of these joints. The thickness of the intermetallic compound layer increased with increasing aluminum content of the Mg–Al–Zn alloy. The tensile strength of the joint decreased with increasing aluminum content. A fracture occurred near the joint interface in a tensile test. This suggests that the thickness of the Ti–Al intermetallic compound layer affected the tensile strength of the joints.