The effect of anisotropy on microstructure and high temperature stress rupture property of Ni₃Al base single crystal alloy was investigated. The single crystal specimens were produced by screw selection crystal method. The microstructures were examined by OM, SEM, TEM and X-ray EDS techniques. The stress rupture tests were carried out in air by constant load creep machines under 1 100 ℃ and 130 MPa. The experimental results show that the dendrites preferential orientation deviates certain angles to heat flow orientation, and the secondary arms occur for different crystallographic orientations. The single crystal alloy with different orientations shows obvious anisotropy during tensile stress rupture tests under 1 100 ℃ and 130 MPa. The <111> orientation specimen has the best stress rupture life of 211 h. The high ductility at 1 100 ℃ of the <001> orientation specimen may be attributed to the most multiple equivalent slip systems.

The influence of microstructure stability on the creep properties of single crystal nickel-based superalloys was investigated by means of the measurement of the creep curves and microstructure observation. Results show that the superalloy with 4%(mass fraction)W in Ni-Al-Cr-Ta-Co-5.5%Mo-x%W systems displays a better microstructure stability, but the m phase is precipitated in the superalloy with 6% W during aging. The strip-like m phase is precipitated to be parallel or perpendicular to each other along the <110> orientation, and grown into the slice-like morphology along the {111} planes. The superalloy with 4%W displays a better creep rupture lifetime under the applied stress of 200 MPa at 982 ^oC, but the creep lifetime of alloy is obviously decreased with the increase of the element W content up to 6%. The fact that the m phase is precipitated in the superalloy with 6% W during applied stress and unstress aging results in the appearance of the poor regions for the refractory elements. This is one of the main reasons for reducing the creep rupture lifetime of the superalloy.

The challenge to rapid manufacturing high performance metal components is how to consolidate the uncompressed powder preforms to full or near full density without shape distortion. A new approach developed by ProMetal was proposed, in which a bimodal powder, typically a coarse prealloyed powder blended with a fine metal powder was used to build green preforms by three-dimensional printing and then sintered at a temperature above the solidus temperature of the coarse prealloyed powder and below the melting temperature of the fine powder particles or the solidus temperature if the fine powder is a prealloyed powder as well. This approach was successfully applied to sinter Ni-based superalloy 718 preforms, which were built through three dimensional printing into near full density.

Based on analysis of three alloys with various Re contents, the effects of Re on the interfacial structure characteristic and interfacial dislocations of γ/γ′ in single crystal superalloys were investigated. A high resolution transition electron microscopy (HRTEM) technique was used to detect the structure characteristic of γ/γ′ interface. The interfacial dislocations of γ/γ′ were analyzed with TEM. The results show that as Re content increases, the interfaces of γ/γ′ become orderly, the atomic arrangements at interfaces are more uniform, the number of mismatch dislocations increases and transition areas between γ and γ′ phases become narrow. That Re changes the interfacial structure promotes the formation of dense, regular and homogeneous interfacial dislocation networks in short time during creep at 1 100 ℃ and 140 MPa, which results in strengthened γ/γ′ interface at elevated temperature.

In order to research the microstructure of TiAl alloy and TiAl-mould reaction between TiAl and ceramic mould shells prepared with the low cost binder in investment casting, the ceramic mould shells were prepared with low cost binder and refractory materials. Using two kinds of casting methods (gravity casting and centrifugal casting), the titanium aluminum alloys with rare earth element (Ti-47.5Al-2Cr-2Nb-0.3Y and Ti-45Al-5Nb-0.3Y) were cast into the mould shells. The microstructures of investment casting titanium aluminum alloys were observed by optical microscope (OM). The distributions of elements of topping investment on the surfaces of titanium aluminum alloys castings were analyzed by the means of electron probe micro-analysis (EPMA), and the mechanical properties were studied. The results show that the microstructures of two kinds of titanium aluminum alloys are both lamella shape, and lamella is thin. The thickness of reaction and diffusing layer of Ti-47.5Al-2Cr-2Nb-0.3Y alloy is about 80 μm, and that of Ti-45Al-5Nb-0.3Y is less than 30 μm.

Pulse current heating (PCH) was used to join Ti-6Al-4V alloy at two cases of with die and without die. Hot-pressing (HP) method was used to provide a comparison between the two methods. Microstructures near the contacting surfaces were observed in optical microscope. Temperature distribution was analyzed. After joining, the tensile properties of the samples were evaluated. Experimental results show that grains and phases grow transversely on contacting surfaces, which makes two parts into a whole with a certain of tensile strength. PCH joining is a different temperature joining method. The highest temperature is located at the interface. The comparison of tensile strengths of samples joined by the two methods indicates that joining temperature and holding time needed by PCH are lower and less than those needed for HP.

Al₂O_3(f)/TiAl composites were synthesized by an exothermic reaction method using Ti, Al and TiO₂ powders doped with Nb₂O₅ and La₂O₃. The effect of Nb₂O₅ and La₂O₃ additives on the growth and morphology of the fibers, the phases and microstructure of the composites were investigated by means of XRD and SEM. The result indicates that the in situ alumina fiber can be developed in Ti-Al matrix with the Ti/Al mole ratio of 1:2-1:7, and the addition of rare earth powders can improve the dispersion of the fibers in the matrix and increase the length-to-diameter ratio of the fibers.

Al₂O₃/TiAl composites were fabricated by PAXD (pressure-assisted exothermic dispersion) method. The effects of Nb₂O₅ content on the microstructure and mechanical properties of the composites were investigated. The results show that the ultimate phases of the composite consist of TiAl, Ti₃Al, Al₂O₃ and a small amount of NbAl₃. SEM reveals that a submicron γ+(α₂/γ) dual phases structure can be presented after sintered at 1 200 ℃. Furthermore, with the increase of Nb₂O₅ content, the ratio of TiAl to Ti₃Al phase decreases correspondingly, the grains of the composites are remarkably refined, and the produced Al₂O₃ particles are uniformly dispersed. When 6% Nb₂O₅ is added, the composite has the best comprehensive properties. It exhibits a Vickers hardness of 4.77 GPa and a bending strength of 642 MPa. Grain-refinement and dispersion-strengthening are the main strengthening mechanisms.

The amorphization process during mechanical alloying (MA) was investigated for the Ti-50%/Al (mole fraction) powder mixtures with no special protection conditions. During the milling process, with the milling time prolonging, the metallic powder Ti and Al were finely mixed, gradually, aluminum completely dissolved into titanium to form an Ti(Al) hcp supersaturated solid solution, and finally, transformed to the amorphous phase after milled for about 39 h. As a result of heat treatment in hot press sintering processing for the mechanically alloyed amorphous powders in vacuum, a submicrostructure intermetallics of TiAl/Ti₂AlN composite can be produced by in-situ crystallization. Furthermore, the structure evolution, phase formation and transformation during the heat treatment were also investigated by X-ray diffractometry and differential thermal analysis. The results show that the reaction involves many transitional stages, including formation of TiAl₃ and transformation into TiAl and Ti₃Al. The examination show that the composite materials fabricated by this in-situ crystallization from amorphization have good mechanical properties due to fine grain size and uniform microstructure.

As the important evaluation parameters concerning the spray qualities, the porosity and surface roughness of the coatings obtained by thermal spray forming have great influence on their forming accuracy, mechanical properties and service lifetime. But it is difficult to predict or control the two parameters for such a highly nonlinear process. A two-dimensional simulation of coating porosity and surface roughness of nickel-aluminum alloy (Ni-5%Al) in plasma spray forming was presented, which was based on the multi-dimensional statistical behaviors of the droplets as well as the simplification and digitization of the typical splat cross sections. Further analysis involving the influence of the droplet diameters and the scanning velocities of the spray gun on the two parameters was conducted. The simulation and analysis results indicate that the porosity and surface roughness are more influenced by the droplet diameters, but less influenced by the spray gun velocities. The results will provide basis for the prediction or control of coating mechanical properties by depositing parameters.

Phase composition and microstructures of grain boundary of oversintered yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) ceramics by vacuum sintering at 1 850 ℃ were investigated. For synthesizing YAG, grain boundary is a key factor for YAG ceramics. The morphology of grain boundary was observed by SEM, TEM and its composition was analyzed by EDS. It is identified that the grain boundary is composed of α-Al₂O₃ and yttrium aluminum perovskite (YAP, YAlO₃) eutectics. At the edge of YAG crystal grain, YAG phase is decomposed into perovskite YAP and α-Al₂O₃ during high temperature sintering. Due to refractive indexes of YAP and α-Al₂O₃ phases in wide grain boundary are different from those of YAG, the transmittance of oversintered YAG ceramics is lower than that of YAG ceramics sintered at 1 750 ℃.

Continuing the effort to redesign IN718 alloy in order to provide microstructural and mechanical stability beyond 650 ℃, IN718 alloy was modified by increasing the Al, P and B contents, and the microstructure and mechanical properties of the modified alloy were compared with those of the conventional alloy by SEM and TEM. The precipitation of the grain boundaries of the two alloys is different. The Cr-rich phase, Laves phase and α-Cr phase are easily observed in the modified alloy. The γ″ and γ′ phases in the modified alloy are precipitated in a “compact form”. The tensile strengths of the modified alloy at room temperature and 680 ℃ are obviously higher than those of the conventional one. The impact energy of the modified alloy is only about half of that of the conventional alloy. Ageing at 680 ℃ up to 1 000 h lowers the tensile properties and impact energy of both the conventional and modified 718 alloys, except increasing the ductility at 680 ℃. It is concluded that the modified alloy is more stable than the conventional one.

A series of experimental studies were conducted on the recrystallization of directionally solidified cobalt-base superalloy DZ40M. It is found that the secondary M₂₃C₆ precipitation influences the size and shape of the recrystal grains. When the annealing temperature is below 1 473 K, a large amount of the fine secondary M₂₃C₆ precipitations are distributed around the primary carbides, and such carbides impede the movement of grain boundary because the effect, the size and shape of recrystal grains become irregularly. When the temperature exceeds 1 473 K, the recrystal grains grow rapidly due to the dissolved secondary M₂₃C₆ precipitation.

The tensile and fracture behavior of DZ951 directionally solidified Ni-base superalloy was studied in the temperature range of 20？1 100 ℃. The fracture mode was examined by scanning electron microscopy. The results show the experimental temperature has no significant effect on the tensile strengths, which are greater than 1 000 MPa from room temperature to 800 ℃. The yield strength reaches its maximum (970 MPa) at 800 ℃. When the experimental temperature is higher than 800 ℃, the tensile and yield strengths decrease evidently and the ductility increases remarkably. The fractograph of fracture surface for the tensile specimen at room temperature shows a dimple-ductile fracture mode. The fractograph from 600 to 800 ℃ shows a slide fracture mode. The fractograph from 900 to 1 100 ℃ exhibits a creep rupture mode with uneven deformation.

The effects of electric field intensity and treatment temperature on the microstructures of GH4199 superalloy after long-term aging were investigated. The results show that the number and size of carbides and TCP(σ phase and μ phase) phase in the alloy increase with increasing electric field intensity at the same heat treatment temperature and holding time. While the number and size of carbides and TCP phase are weekly influenced by treatment temperature with lower electric field intensity of 2 kV/cm. When the treat temperature is up to 1 093 K, annealing twins appear in the alloy, and the number of twins increases with increasing holding time. Since the electric field can provide the enough energy for the movement of vacancies and atom, it is considered that the nucleus of the twins formed with formation stack faults due to the mismatch of local atom in crystal caused by the vacancies, and the twins will grow with the increase of holding time. Meanwhile, such promoting effects on atom movement of the electric field increase with the increase of the electric field intensity, meantime the carbides and TCP phase grow fast with the increase of electric field intensity.

The effect of phosphorus on the oxidation resistance of low thermal expansion alloy IN909 was studied. The composition and structure of the oxidation layer were analyzed. It is found that the oxidation initiates at the grain boundaries. During the oxidation, Fe atoms diffuse toward the surface and form the outside oxidation layer as the oxide of iron. The transition oxidation layer lies between the oxidation layer and the matrix which is enriched with Nb, Ti and Si, forming FeTiO₅, Nb₂O₅, Fe₂SiO₄ and TiO₂. Phosphorus hardly influences the thermal expansion coefficient of IN909 alloy. However, it increases the formation of ε phase at the boundary of the transition oxidation layer and matrix. As a result, the oxidation rate is decreased efficiently because the ε phase inhibits the diffusion of the element such as iron from the matrix to the oxidation layer.

The densities of liquid NiCoAlCr quaternary alloys with a fixed molar ratio of Ni to Co to Al (x(Ni)？x(Co)？x(Al)≈73？12？15) which is close to the average value of the commercial Ni-based superalloys TMS75, INCO713, CM247LC and CMSX-4, and the mass fraction of chromium changes from 0 to 9% were measured by a modified sessile drop method. It is found that with increasing temperature and chromium concentration in the alloys, the densities of the liquid NiCoAlCr quaternary alloys decrease, whereas the molar volume of the liquid NiCoAlCr quaternary alloys increases. And the liquid densities of NiCoAlCr quaternary alloys calculated from the partial molar volumes of nickel, cobalt, aluminum and chromium in the corresponding Ni-bases binary alloys are in good agreement with the experimental ones, i.e. within the error tolerance range the densities of the liquid Ni-based multi-component alloys can be predicted from the partial volumes of elements in Ni-based binary alloys in liquid state. The molar volume of liquid NiCoAlCr binary alloy shows a negative deviation from the ideal linear mixing and the deviation changes small with the increase of chromium concentration at the same temperature.

The oxidation behavior of a cast Ni-base superalloy K44 in air at 850？1 000 ℃ for l00 h was studied. The scales on the surface were determined by SEM and EPMA equipped with an EDXS. The results show that oxidation kinetics obey the parabolic law from which the values of activation energy Q_p1=221.1 kJ/mol and Q_p2=247.6 kJ/mol are estimated. The oxidation scales are composed of loose outer layer of TiO₂/TiO-Cr₂O₃ and a small amount of NiCr₂O₄ and NiA1₂O₄, compact intermediate layer Cr₂O₃, and precipitate of internal oxides A1₂O₃.

The effects of carbon on the microstructure and mechanical properties of DD99 single crystal superalloy were investigated. The results show that stress rupture life of DD99 alloy possesses peak value at carbon content of 0.03%(mass fraction). As carbon addition is greater than 0.03%, the stress-rupture life decreases with the increase of carbon content. The tensile strength and yield strength of DD99 alloy reach peak value at 0.08% carbon and 760 ℃. On the contrary, the tensile strength and yield strength have minimal values at 0.08% carbon and 900 ℃. The tensile ductility of DD99 alloy basically decreases with the increase of carbon content at 760 ℃ or 900 ℃. The amount of carbides greatly increases with the addition of carbon content. Dislocation moving is retarded by carbides so that dislocation networks are apt to form, which has an important role on the mechanical properties in DD99 single crystal superalloy.

The effect of cobalt on chemical segregation and solution process in three nickel base single crystal superalloys was investigated. Three alloys containing the mass fraction of cobalt of 12% (named Alloy 1), 3% (named Alloy 2) and 0 (named Alloy 3), respectively were studied, in which the contents of other elements were same. The results show that the segregation extent of W, Re, Ta, Al between dendrite and interdendritic region rises with the increase of cobalt content. The incipient melting points decrease by 10 ℃ and 20 ℃ respectively when the content of cobalt increases from 0 through 3% to 12%. During solid solution at 1 340 ℃, the solid solution of large gamma prime in interdendritic region and the dissolution of eutectic in alloy 1 become easier than in other two alloys. After heat-treatment at 1 340 ℃ for 8 h, the segregation extent of elements in alloy 1 decreases dramatically, while in alloy 2 and alloy 3, the segregation ratios decrease slowly. It suggests that the higher content of cobalt can accelerate the diffusion process at high temperature.

Laser engineered net shaping(LENS) process was investigated using Co-based superalloy powder with a high power continuous wave CO₂ laser. Thin wall part with smooth surface was obtained by LENS of layer-by-layer deposition of the powder materials. This thin wall sample was tested for metallographic examinations, micro-hardness, X-ray diffraction and mechanical property test. Microstructural results show that the layers possess rapid solidification microstructural feature, fine dendritic crystal and M7C3-type carbides (essentially chromium-rich carbide) dispersed in the γ(Co,Cr) phase matrix. Dendrite spacing as well as the solidification mode can be controlled through control process parameters. In addition, this microstructural feature of the as-formed Co-base sample leads to an evident hardening and a superior tensile strength and toughness.

The alternative heating/cooling cycles（thermal fatigue）of K465 superalloy were carried out. The specimens were held at 1 050 ℃ for 300 s, then quenched into 20 ℃ recycling water for 10 s as a cycle. During thermal fatigue, γ′ precipitates changed typically from cubical to irregular shape after 10 cycles, to complex configuration after 20 cycles and raft-like shape after 30 cycles. The very fine γ′ particles precipitated inter the original γ′ particles. The elastic energy dominated morphological evolution of large γ′ precipitates, and the thermal stress induced the directional growth of precipitates that minimized the total energy of the system, and the nucleation theory controlled the formation of fine γ′ precipitate. The results show that the volume fraction of γ′ precipitates is increased with the increase of heating/cooling cycles, which improves the mechanical property of this alloy.

The finite element method was applied to study the mechanics of rafting of γ′ precipitates in a single crystal Ni-based superalloy with the [001] orientation. The results show that the creep and rafting are closely related with the stress and strain energy density distributions in the matrix channels. The application of an external stress leads to differential levels of von Mises stress and strain energy density, and the largest value of the stress appears at the corners of the matrix near the interface. Creep dislocations penetrate preferentially into the most highly stressed matrix channels where the γ′-phase rafting is also enlarged. Meanwhile, the von Mises stress of γ matrix and γ′ precipitate increases with the increase of temperature, thus the rafting becomes easier at a higher temperature. Moreover, according to the analysis of slip systems for the Ni-based superalloy, the critical external load for bowing a dislocation through a matrix channel at 950 ℃ is about 180 MPa, which is consistent with the related experimental results.

Fabrication technology and mechanical properties of the Fe₃Al based alloys were studied by spark plasma sintering from elemental powders (Fe-30Al, volume fraction, %) and mechanically alloying powders. The mechanically alloying powders were processed by the high-energy ball milling the elemental mixture powders with the milling time of 5, 8 and 10 min, respectively. The spark plasma sintering process was performed under the pressure of 50 MPa at 1 050 ℃ for 5 min. The phase identification by X-ray diffraction presents the Fe reacts with Al completely during the processing time. The samples are nearly full density (e.g. the relative density of sinter of raw powder is 99%). The microstructure was observed by TEM. The mechanical properties were tested by three-point bending at room temperature in air. The results show that the mechanical properties are better (e.g. bend strength of 1 500 MPa ) than those of the ordinary Fe₃Al casting.

The NiAl-28Cr-5.85Mo-0.15Hf alloy was prepared by high-pressure die casting (HPDC) and subsequent hot isostatic pressing(HIP), and tested for compressible strength and fracture behavior at 300？1 373 K. The results show that the elevated temperature 0.2% compressible yield strength as well as the room-temperature compressible fracture strain of as-HIP alloy are larger than those of the same alloy prepared by directional solidification (DS). It suggests that the fine structures with a homogeneous distribution of fine Cr（Mo）and Hf-rich phase created by high-pressure die casting lead to these improvements.

The oxidation behavior of TiAl-Cr(mole fraction of Cr？0-20%) was investigated at 1 173 K in air. The microstructure and composition of the oxide scale were studied by X-ray diffractometry(XRD), scanning electron microscopy(SEM) and electro-probe micro-analyses(EPMA). The results show that with the addition of Cr content from 0 to 8%, the oxidation resistance decreases, especially at 3%, which is mainly attributed to the doping effect of Cr³⁺. TiAl-Cr(mole fraction of Cr？15%？20%) has good oxidation resistance, and the protective alumina layer is preferentially formed on the surface of TiAl alloy, which is due to an increase of mole ratio of Al to Ti in TiAl-Cr alloys.

The oxidation behaviors of Nb_67-xW₁₅Si₁₈Hf_x (x=0, 5, 10) alloys were studied at 1 250 ℃ in air. It is found that the Nb₆₇W₁₅Si₁₈ alloy has the best oxidation resistance among the three alloys; and Hf addition is harmful to the oxidation resistance of the Nb₆₇W₁₅Si₁₈ alloy. The oxides formed on the Nb₆₇W₁₅Si₁₈ alloy are mainly Nb₁₂WO₃₃ and NbO₂, and that on the Nb₆₂W₁₅Si₁₈Hf₅ and Nb₅₇W₁₅Si₁₈Hf₁₀ alloys is Nb₂O₅. Effect of Hf on the oxidation behavior of the Nb_67-xW₁₅Si₁₈Hf_x alloys has been discussed based on microstructures and kinetics of oxidation.

The electron structure of γ/α₂ phase boundaries in lamellar colonies in Ti-47Al-2M(M=Nb, Cr, V) (mole fraction, %) alloys was theoretically investigated by Empirical Electron Theory of Solid and Molecules (EET) and the bond-length-difference (BLD) method. Average-Atom-Model was employed to calculate valence electron structure of TiAl intermetallics containing site substitution elements. On this basis, the boundary condition of electron movement was employed in the improved Thomas-Fermi-Dirac (TFD) theory to decide the continuity of the electron density of the lamellar colonies interface and it is found that of γ/a₂ interface is continuous(△ρ<10%). Furthermore, it is found that adding alloying elements (including Nb, Cr, and V) can improve the electron density (ρ) of γ/a₂ interfaces, and decrease the electron density difference(△ρ) of γ/α₂ interfaces. Adding V element decreasing △ρ is more remarkable than other site substitution elements. According to electron structure study of γ/α₂ interfaces in Ti-47Al-2M alloys, the added elements improve mechanical properties of the alloy in the following order: V>Cr>Nb.

The phase separation in Ni₇₅Al_xV_25？x alloys incorporated with the elastic stress was investigated using the microscopic phase-field model. The final morphology of γ′ and θ is similar in spatial alignment, but the volume fraction of γ′ phase increases and that of θ decreases as the Al concentration increases. For the small elastic interactions of early-stage phase separation, the coarsening of γ′ and θ can be approximated by a linear growth law as predicated by Lifshitz and Slyozov and Wangner (LSW) theory. As the elastic interactions increase at late-stage coarsening, the growth rate decreases, and the growth presents quick increase at early-stage and slows down at late-stage.

Nb-15W-18Si-xHf (x = 0, 5, 10 and 15, mole fraction, %) alloys were prepared by arc melting and then homogenized at 1 750 ℃ for 50 h. The microstructure and mechanical behaviors, such as Vickers hardness, fracture toughness, room- and high- temperature strength of the alloys were investigated. The microstructure of the annealed Nb-15W-18Si-xHf alloys is composed of large primary Nb_SS dendrite and fine eutectic mixture of Nb_SS and M₅Si₃ silicide. Both the hardness and the fracture toughness show an increase tendency at room temperature with increase of Hf content. The 0.2% compressive yield strength, σ_0.2 of the alloys with 5 %Hf and 10%Hf are larger than 960 MPa at 1 200 ℃, and about 500 MPa even at 1 500 ℃.

Plasma carburization at two different methane-to-argon gas ratios (5:5 and 6:5) was carried out on the cast TiAl based alloy of Ti-46.5Al-2.5V-1Cr (mole fraction, %) in order to enhance its wear resistance. The results show that after carburization under both carburizing atmospheres, Ti₂AlC and TiC are the main carbides in the carburized layer and the value of surface hardness reaches more than HK 822, but for the carburized TiAl treated at CH₄？Ar of 5？5, the surface carbon concentration is higher and the carburized depth is slightly thicker than that of alloy carburized at CH₄？Ar of 6？5. The result of the ball-on-disk test against hardening-steel counter bodies shows that the wear resistance of the TiAl based alloy carburized under two different carburizing atmospheres is improved compared with non-carburized TiAl. The tribological property is related to the carbon content, and the carburized layer obtained at CH₄:Ar of 5:5 possesses a stable friction coefficient, lower volume loss or wear rate and narrow wear scar. The characteristic of the carburized layer was examined by using optical microscopy, glow discharge spectrum and micro-hardness tester.

In Nb-Si based alloys with a two-phase Nb_SS/ intermetallic Nb₅Si₃ structure, the Nb₅Si₃ provides high-temperature strength, while the Nb solid-solution phase, Nb_SS, contributes to room-temperature ductility and toughness. The results show that in Nb-15W-10Hf-xSi alloys (x = 0.5, 5 and 18, mole fraction, %), the volume fraction of the Nb₅Si₃ is 0 for the 0.5% Si sample, 15% for the 5% Si sample and the 50% for 18% Si sample. With increasing Si content, i.e., the Nb₅Si₃ fraction, the high-temperature strength is improved considerably, but room-temperature ductility and toughness are degraded. For the sample Nb-15W-10Hf-18Si with 50% Nb₅Si₃, the compressive strength at 1 500 ℃ and the room-temperature fracture toughness are 500 MPa and 6.8 MPa？m^1/2, respectively, those for the Nb₅Si₃ free sample, Nb-15W-10Hf-0.5Si, are 190 MPa and 13.6 MPa？m^1/2.

The structural defects of L1₀ FePt are investigated by the molecular dynamics (MD) with a modified analytic embedded-atom method (MAEAM). The L1₀ ordered structure of FePt is relaxed from a trial fcc structure. The defect formation energies are calculated. The vacancy formation energies of Fe and Pt are 1.89 eV and 2.11 eV respectively. The antisite formation energy of Fe in Pt sublattice is 0.35 eV. The antisite formation energy of Pt in Fe sublattice is 0.09 eV. The tendency of the vacancy formation energy is in agreement with other calculation. The point defect structure types are Pt antisite in rich-Pt side and Fe antisite in rich-Fe side.

The oxidation of two two-phase ternary Fe-Cu-Al alloys containing about 5%(mole fraction) aluminium, one Fe-rich and one Cu-rich, has been studied at 900 ℃ in 1×10⁵ Pa pure oxygen. The Fe-rich Fe-15Cu-5Al alloy presents two quasi-parabolic stages, with a large decrease of the parabolic rate constant after about 50 min. The presence of 5% Al greatly reduces the oxidation rate of this alloy with respect to a binary Fe-Cu alloy of similar composition by forming an external protective Al₂O₃ layer, which is present near the scale/alloy interface. Due to the high stress-growth effect, this thin Al₂O₃ layer cannot completely prevent further oxidation of the alloy underneath. On the contrary, the Cu-rich Fe-85Cu-5Al alloy presents a single parabolic stage and forms a thick and porous external scale, coupled to the internal oxidation of Fe and Al. As a result, the oxidation of Cu-rich alloy at 900 ℃ is much faster than that of the Fe-rich alloy.

The effect of 5%Cr(mole fraction) on the scaling behavior of Ni-10Al was investigated at 900 ℃ and 1 000 ℃ with the aim to discuss the third element effect(TEE). The oxidation rate of the ternary alloy containing Cr is much slower than the corresponding binary alloys at both temperatures. A dense external scale of NiO overlying a zone of internal oxide precipitates forms on Ni-10Al, while Ni-5Cr-10Al develops a protective layer. So the addition of Cr promotes the formation of alumina scale. The possible mechanism of Cr on the oxidation of Ni-Al alloy is discussed.

NiAl-0.3Ru and NiAl-30Cr-4Mo alloys were fabricated by arc melting method and then annealed at 1 423 K for 12 h. It has been revealed that NiAl-0.3Ru was in a single-phase with large grain size and NiAl-30Cr-4Mo consisted of multiple eutectic cells. Each cell consisted of alternating plates NiAl and Cr(Mo). The compressive properties including the brittle-ductile transition temperature (BDTT) were tested. NiAl-0.3Ru and NiAl-30Cr-4Mo fractured with a little plastic deformation after yielding below the BDTT, and almost no fracture was found after large deformation up to 60% above the BDTT. Fractograph showed that at the room temperature, the fracture in NiAl-0.3Ru was intergranular and the NiAl-30Cr-4Mo in transgranular along the interface between b-NiAl and Cr (Mo). The compressive properties of NiAl were obviously improved by eutectic alloying with Mo and Cr additions.

The oxidation and hot corrosion behavior of Ti₂AlNb-based alloy with and without enamel coating at 800 ℃ was investigated. The results indicated that Ti-22Al-25Nb alloy exhibited poor oxidation resistance at 800 ℃. The constitution of oxide scale had the effect on its oxidation rate. Because of the S and Cl accelerating the corrosion process, Ti-22Al-25Nb alloy suffered severe hot corrosion and exhibited very poor hot corrosion resistance. Enamel coating could remarkably improve the high temperature oxidation resistance of Ti-22Al-25Nb alloy because it had good chemical stability and matched thermal expansion coefficient with the substrate. In (Na,K)₂SO₄+NaCl molten salts at 800 ℃, chemical reactions between molten salts and enamel coating occurred and complicated products formed on the surface of the enamel coating; Cl^- in the molten salts could penetrate through the coating and induced the substrate corrosion, but enamel coating still had good hot corrosion resistance.

A sub-microstructure titanium aluminide alloy /Al₂O₃ (3A) composite was obtained by crystallization of the amorphous powders, which were prepared by mechanical alloying (MA) in a planetary ball milling system using Ti-Al-TiO₂ as raw materials. The experimental results show that, when the milling time increases up to 30 h, the hcp Ti(Al) supersaturated solid solution disappears, only amorphous phase is left. The compact samples were synthesized by hot-press to 1 200 ℃ with the amorphous as a precursor; the final phases of the matrix and strengthened phase are γ-TiAl and Al₂O₃. The phases come from in situ crystallization and transformation. The samples, fabricated from the amorphous phase by hot press sintering, have high bending strength and fracture toughness.

Effect of the deformed microstructure on mechanical properties of an orthorhombic (Ti₂AlNb) based alloy of Ti-22Al-25Nb (mole fraction, %) has been investigated. It was found that the deformed microstructures in different portions of a free forged rod with diameter of 30 mm were quite different and thus resulted in the different mechanical properties after the same subsequent heat-treatment. One deformed microstructure with less primary α₂/O particles and a larger and equiaxed B2 grains resulted in poor RT ductility, but the other one with a relatively larger amount of the primary α₂/O particles and non-equiaxed B2 grains had good combination of the tensile strength and ductility both at RT and 650 ℃. It was also found that two different deformed microstructures were obtained for the hot rolling plates with thickness of 3 mm even processed under an identical nominal rolling and the same post-deforming heat treatment conditions. One only has 3.5% of RT tensile elongation and the other up to 8%.

Several tensile samples were prepared using laser rapid forming (LRF) with Ti-6Al-4V alloy as powder material, and the samples were annealed. The microstructure and high temperature mechanical properties of laser formed Ti-6Al-4V alloy through annealing treatment were investigated. The short-term and long-term tensile tests at 350 ℃ were performed. The results show that the microstructure of LRF samples consists of the large columnar prior β grains which grow epitaxially from the substrate along the deposition direction. There are Widmanst？tten α laths in prior β grains, but α laths in annealed microstructure are coarser, and their aspect ratio is lower than that in as-deposited microstructure. In addition, the prior β grain boundary is also coarsened and broken off through the annealing treatment. The high temperature mechanical properties of the annealed LRF samples exceed those of casting alloy significantly, especially the stress-rupture lifetime reaches 661.7 h even while the test stress increases from initial value of 490 MPa to the final stress of 800 MPa gradually.