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Friction and wear properties of a high Nb-containing TiAl alloy against WC-8Co, Si3N4, and GCr15 in an unlubricated contact were investigated under various friction and wear conditions. The counterface material plays a key role in the friction and wear properties of the high Nb-containing TiAl alloy. The Si3N4/TiAlNb pair had the highest average friction coefficient caused by the abrasive wear and adhesive wear. The applied temperature had a strong effect on the friction coefficient of the Si3N4/TiAlNb and GCr15/TiAlNb pair. The WC-8Co/TiAlNb pair has the stable average friction coefficient under various friction and wear conditions due to the abrasive wear.

A CuTiZrNi amorphous filler metal was fabricated and utilized for brazing high Nb–TiAl alloys and GH4169 superalloys. The microstructure and mechanical properties of the high Nb–TiAl/GH4169 joints brazed from 900 °C to 1020 °C temperature range were studied. Results showed that the high Nb–TiAl/GH4169 joints consisted of three zones. The interfacial microstructure of the joints was high Nb–TiAl alloy/Ti3Al + Al3(Ni, Cu)Ti2 + Al(Ni, Cu)2Ti/AlCu2(Ti, Zr) + (Ti, Zr)(Ni, Cu) + Ti2(Ni, Cu)/Cr-rich (Cr, Ni, Fe)ss + Ni-rich (Ni, Cr, Fe)ss/GH4169 alloy. The element diffusion between the substrates and the filler alloy was intensified as the brazing temperature increased, and the interfacial reaction layer thickened. At 960 °C, the joint exhibited a maximum shear strength which was approximately 241.9 MPa. At higher brazing temperatures, the excessive AlCu2(Ti, Zr) phase and coarse Ti2(Ni, Cu) deteriorated the joint properties due to their inherent brittleness.

Microstructural instability induced by shot peening was investigated in a Ti-45Al-8.5Nb-(W, B, Y) alloy following high temperature exposure. After shot peening and thermal exposure at 1000 °C for 300 h, fine grains are formed in the outermost (FG layer). Underneath, coarse grains (CG layer) are formed. The FG layer is composed of completely recrystallized γ grains. The CG layer is composed of incompletely recrystallized γ grains, where the critical strain for recrystallization is not reached. During long term thermal exposure, α 2 lamellae can undergo dissolution, precipitation and growth. After thermal exposure for 300 h, large α 2 grains precipitated at the γ/γ interface or inside the γ grains in both FG and CG layers. The precipitated α 2 particles almost have the same orientation with primary α 2 lamellae, indicating that nearly no recrystallization phenomena occur for α 2 phase. So the γ lamella is easier to recrystallize than that of α 2 lamella at the same temperature and residual strain.

Three alloys with composition range of Ti-(25–35)Al (atomic %) were in-situ investigated using neutron diffraction to clarify the phase stability and equilibria in the Ti-rich part of the Ti–Al binary system. The phase transformation sequences of these three alloys were identified based on the neutron diffraction results and differential scanning calorimetry. No superstructure diffraction of β0 has been found in the neutron diffraction patterns. It is concluded that the β0 phase in the examined composition range is thermodynamically unstable. Based on the phase transformation sequences of these three alloys, combining the characters of congruent type relationship and peritectoid type relationship, the phase equilibria among β, α and α2 phases were confirmed as a peritectoid type, with two peritectoid reactions, i.e., β+α→α2 and β +α2→α.

Phase equilibria in the Ti–Al–V system at 1300 °C were studied experimentally by examining 50 samples with compositions up to 75 at % Al. An isothermal section of Ti–Al–V at 1300 °C was constructed by means of X-ray diffraction (XRD), electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The isothermal section is characterized by five single-phase regions (i.e., α phase with hcp_A3 structure, β phase with bcc_A2 structure, γ phase with L10 structure, δ phase with D82 structure, and e phase with D022 structure) and three three-phase regions (α+γ+e, α+δ+e, and α+β+δ). Compared with the tentative results at 1200 °C, the single α phase at 1300 °C is highly extended, while the other single-phase regions remain almost unchanged, resulting in the emergence of new three-phase regions.

The ordered ω phase in as-cast Ti-45Al-8.5Nb-0.2B alloy and its phase transformation during heat treatment are investigated. Ordered ω variants are observed to uniformly precipitate within the βo area in as-cast Ti-45Al-8.5Nb-0.2B alloy. After annealing at 850 °C for 500 h, the βo areas are replaced by large B82-ωo grains. Small γ precipitates are observed at the grain boundaries of the ωo phase and are thought to be transformed from the βo phase. Moreover, the ωo precipitates directly transformed from the parent α2 laths are found within the lamellar colonies. The orientation relationship between the ωo phase and the lamellar structure is 2 ¯ 0]α2; (111)γ//(11 2 ¯ 0)ωo//(0001)α2. The interfaces between the ωo and γ are semi-coherent. The ωo phase is an equilibrium phase at 850 °C in high Nb-containing TiAl alloys. When annealed at 1250 °C, the ordered ω is eliminated in a short time, and the βo phase is substituted by the coarsened α2 laths in the lamellar colonies after 12 h annealing.

Directional solidification (DS) experiments with thermal stabilization (TS) treatments were performed on Ti–46Al–5Nb (at.%) alloys in a Bridgman-type furnace using a quenching technology. Influence of the TS treatment on mushy zone and directional growth afterwards were investigated. The results show that the length of the mushy zone decreases but the β dendrite spacing in directional growth significantly increases with increasing TS time. During the DS process, β dendrite spacing is more homogeneous and its growth direction is more inclined to parallel to the axial direction with increase of the TS time. Al solute concentration in the mushy zone in a steady-state is always lower than that in original as-cast alloys. The mushy zone with the columnar β and α grains is easily produced after TS treatment on the alloys with microstructures of the directional dendrite segregation morphology before DS starting. TS treatment results in the redistribute of solute Al thus changes the phase constituent in the mushy zone. An appropriate TS is necessary to produce the L + β + α region in the mushy zone, which is of great benefit to control DS microstructures of TiAl peritectic alloys.

Compressive tests were conducted on metallic glass matrix composites at a series loading rates. It was found that mechanical properties of the composite, e.g. yielding stress and plasticity, have a week dependence on strain rates of 4.0 × 10 −4 s −1 –4.0 × 10 −1 s −1 . Four composites were tested at a constant strain rate of 2.3 × 10 s −1 to uncover the dynamic deformation behaviors. Compared with the quasi-static case, the yielding strength increased under dynamic loading rate, but the plasticity decreased significantly. On the other hand, the dynamic compressive has closely relation with the dendrite size and volume fraction. The decreasing of the dendrite size and volume fraction leaded to the dynamic yielding strength increased but the plasticity decreased. For a same composite, e.g. T1 alloy, the yielding strengths increased slightly but fracture strain decreased with increasing of dynamic strain rates.

The ordered intermetallic Mo5SiB2 displays a ceramic-like brittleness at the ambient temperature. The state density, charge distribution and elastic parameters were calculated by first-principles, based on the density functional method. The results indicated that the two different kinds of covalent bonds were intricately woven into the refractory phase. The improved Peierls–Nabarro stress which is caused by this kind of distribution mode makes dislocations move difficultly, resulting in intrinsic brittleness. The effects of substitutional alloying on the ductility of Mo5SiB2 were also assessed by the calculations on the elastic properties and dislocation line energy. It was shown that the metal (Nb, Tc) alloying was not to enhance effectively its toughness, but to improve the brittle-to-ductile transition temperature.

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α 2 -TiAl 3 /γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.

It is shown that a uniform and refined fully lamellar microstructure can be generated by tempering the quenched samples in the single α region of as-forged Ti–43Al–9V–Y alloy. The refining effect was mainly connected with the quenched microstructures especially for their substructure, grain size and the residual β particles after the quenching. A series of β-decomposed products: martensite plate, massive α and coarsened α lath with increasing grain size were produced due to the decreasing cooling rate during the wedge-shaped sample quenched into iced brine. In addition, the decomposition of β directly to γ has also been observed during the quenching with different cooling rate. The lamellar colony size can be controlled in the range of 25–50 μm after tempering.

High Nb containing TiAl alloys have prospect to be used for turbo wheel and exhaust valve for vehicle engine. Friction welding is a suitable process for fabricating these components. For investigating the weldability of these alloys, cylindrical samples of Ti–45Al-8.5Nb-0.2W-0.2B-0.02Y (in at%) alloy with a diameter of 10 mm were welded by continuous drive friction welding process, and the microstructures and the mechanical properties of the joints were analyzed in this paper. The results showed that the weldability of the present alloy was very good. At parameters of rotational speed of 2400 r/min, friction pressure of 320 MPa, burn-off distance of 3 mm and upset pressure of 360 MPa, completely metallurgical bonding interfaces were obtained. The joint had a severely plastic deformation zone (SPDZ) with a biconcave lens geometry. The thicknesses of the center and periphery of SPDZ were about 40 μm and 800 μm, respectively. There existed a transition zone (TZ) between parent metal (PM) and SPDZ. SPDZ exhibited the deformation flow lines and had a randomly-oriented recrystallized duplex (DP) microstructure with the average grain size of 2.5 μm, in contrast, PM presented a nearly lamellar (NL) microstructure with a colony size of 150 μm. TZ showed a mixture microstructure combining deformed lamellar colonies and partially recrystallized grains. At room temperature, 750 °C and 800 °C, welding joints had better tensile properties than PM.

The fracture behavior of the polycrystalline Mo5SiB2 alloy was investigated by three-point bending tests, in vacuum, in the range of 25–1400 °C at a crosshead speed of 10−3 mm/s. At room temperature (RT), the alloy exhibited a higher fracture toughness of 3.34 MPa m1/2 compared to single crystalline Mo5SiB2 with a value of ∼1.8 MPa m1/2. The improved toughness is attributed to crack overcoming the barrier effects of grain boundaries. With increasing temperature, the fracture mode underwent a conversion from transgranular cleavage at RT to a mix of transgranular cleavage and intergranular failure at elevated temperatures. At 1000 °C, the maximum increment (∼3.4 MPa m1/2) in toughness was found to be related to the brittle-to-ductile transition (BDT). At 1200 °C, the effects of extrinsic toughening mechanisms (microcracking, etc.) are limited, i.e. its toughness only increasing by ∼1.1 MPa m1/2. However, the cavities formed at triple junctions degraded toughness at 1400 °C.

High Nb-containing TiAl porous and gradient porous alloys were synthesized by elemental powder metallurgy (EPM). The effects of powder size on pore structure and gas permeability were investigated. It is found that pores are interconnected and skeletons consist of typical γ-TiAl/α 2 -Ti 3 Al fully lamellar microstructure for Ti–48Al–6Nb porous alloys. With the fining of particle size of Ti, Al and Nb powders, the pore size and porosity of Ti–48Al–6Nb porous alloys decrease. Otherwise, the gas permeability increases almost linearly when the porosity increases from 17% to 52% and the pore size increases from 3.18 μm to 26.69 μm. A good agreement is shown between the simulations by capillary permeability model and the results of the actual gas permeability. In addition, gradient porous alloys were fabricated using different powder sizes by constrained sintering process, and a prediction of the relationship between permeability and coating thickness was discussed. Results from this study indicate the potential applications in microfiltration development by tailoring porosity of the coating.

Grain boundary and texture evolution of as-forged Ti–45Al–9Nb–Y alloy have been investigated during tensile test between room temperature (RT) and 900 °C. The volume of high angle grain boundaries (HAGBs) increases a certain extent during RT tensile strain, but the volume of low angle grain boundaries (LAGBs) increases with the temperature rising in the range of 800 °C–900 °C. Up to 850 °C, The texture, presented in inverse pole figures, contains deformation textures, i.e. , , and . While the temperature rises to 900 °C, dynamic recrystallization and plastic deformation occur simultaneously instead of single plastic deformation progress at the temperature below 900 °C and a new recrystallization texture of replaces the deformation textures.

In the present paper, high temperature tensile and creep behaviors of Tie45Ale9(Nb,W,B,Y) alloy with duplex (DP) microstructure were investigated. In addition to tensile tests at 815 � C and a strain rate range of 1 � 10 � 4 s � 1 � 1 � 10 � 3 s � 1 and tensile, creep tests at 760 � C and 815 � C under the stress of 180 MPa, the microstructure evolutions during tensile and creep tests were studied. The results show that high temperature high Nb containing TiAl alloy with DP microstructure has a good balance between ductility and strength and intermediate creep resistance. The tensile properties have the strain rate dependence, and ultimate tensile strength (UTS) and yield strength (YS) vs. strain rate obey a single-logarithm linear relationship. Minimum creep rate is affected by the test temperature and stress. Using loading change experiment a stress exponent of 4.3 is determined. DP microstructure is unstable after long-term exposure at high temperatures, and the spheroidization of lamella and recrystallization along grain boundaries occur during the high temperature deformation. It is assumed that the diffusionassisted climb of dislocations might be the controlling mechanism at the minimum creep rate stage. 2006 Elsevier Ltd. All rights reserved.

Microsegregation in big ingots of Ti–45Al–(8–9)Nb–(W, B, Y) alloy had been studied. The composition and microstructural morphology of the large ingot exhibited significant microinhomogeneity. Three types of microsegregation were observed in as-cast microstructure of the large ingot. First is the solidification segregation (S-segregation) at interdendritic area, in which the composition is characterized by higher Al, B (boride), and Y (oxide) contents and lower Nb and W contents. Second is the β-segregation at the boundary and triple junctions among α grain due to the phase transformation of β → α. The composition at the segregation area is characterized by higher Nb and W additions that lead to the formation of β particles and γ phase. Third is the α-segregation that forms local lamellar structure composed of β, γ and α plates due to phase transformation of α → α 2 + β + γ. The microsegregation for the PAM ingot is lower than that for SM ingot in terms of the volume fraction of β phase. The reason is that the PAM melting can offer better control of pouring temperature and rather fast cooling rate by water-cooled copper crucible.