Compression creep tests of a Ti-48%Al (mole fraction) alloy were carried out at 1150K with soft-orientated PST crystal. Parallel twinning took place during the creep. Changes in lamellar microstructure caused by the parallel twinning were investigated, and their effects on creep deformation behavior were discussed. The results show that the parallel twinning occurs in an early stage of creep, and makes significant contribution to creep strain in the domains favorably oriented for the twinning. The nucleation of parallel twins finishes at a strain of about 3%. There is a critical resolved shear stress for parallel twinning, and it is about 50MPa in the Ti-48%Al PST crystals at 1150K. The activity of parallel twinning increases with increasing applied stress or in a coarse lamellar material. The addition of parallel twins reduces the average value of lamellar spacing. In general, the refinement of lamellar structure should improve creep resistance. However the strengthening by parallel twinning is not evident in creep of the soft PST crystals because the soft deformation modes are the dominant deformation mode in the crystals.

The hot working behaviour of intermetallic titanium aluminide alloys was described. The microstructural evolution during hot working was systematically studied on a series of binary and technical alloys with aluminium contents ranging between 45% and 54%(mole fraction). Process regimes in terms of temperature and strain rate were identified which allow large strain hot working to be carried out, either by forging or extrusion, with the production of sound forgings. The major areas addressed in the paper are ingot structure and homogenization, factors determining hot working and recrystallization, ingot conversion , and secondary processing.

Superplastic behaviors of TiAl based alloy with initial grain size of about 2μm obtained by multistep forging were investigated at 800～1075℃  with strain rates of 8×10^-5s^-1～2×10^-3s^-1. The results show that the material exhibits excellent low temperature superplasticity. Flow softing resulting from dynamic recrystallization is observed at relatively low temperatures (≤1000℃) or at higher strain rates (≥2×10^-4s^-1). Continuous strain hardening resulting from strain-enhanced grain growth occurs at higher temperatures or at lower strain rates. A maximum elongation of 533% is obtained at 800℃ with strain rate of 2×10^-5s^-1, and at 1050℃, a maximum elongation of 570% is obtained at strain rate of 8×10^-5s^-1. At a fixed strain rate of 2×10^-4s^-1, when the alloy is deformed at 850℃, the microstructure is refined, however at 1050℃, is coarsened. The as-deformed microstructure shows relatively high strain rate sensitivity value and it keeps nearly stable during deformation. The activation energy is calculated to be 290kJ/mol at 950～1075℃, with the grain size exponent, p=2, and 224kJ/mol at 800～900℃ with p=3. Therefore, it is suggested that the dominant mechanism during superplastic deformation at 800～900℃ is grain boundary sliding controlled by grain boundary diffusion; however at 950～1075℃ is grain boundary sliding controlled by lattice diffusion.

The tensile properties and fracture behaviors of Ti-22Al-27Nb and Ti-22Al-20Nb-7Ta alloys were investigated in the temperature range of 25～800℃. Three typical microstructures were obtained by different thermomechanical processing techniques. The results indicate that the duplex microstructure has an optimum combination of tensile yield strength and ductility both at room and elevated temperatures. Adding Ta to Ti₂AlNb alloy can improve the yield strength, especially at high temperature while retain a good ductility. The study on crack initiation and propagation in deformed microstructure of Ti₂AlNb alloys indicates that microstructure has important effect on the tensile fracture mechanism of the alloys. The cracks initiate within primary O/α₂ grains along O/B2 boundaries or O phase laths in B2 matrix, and propagate along primary B2 grain boundaries for the duplex microstructure. The fracture mode is transgranular with ductile dimples for the duplex and the equiaxed microstructures, but intergranular for the lath microstructure.

Based on activity calculation model, the activity coefficients of Ti, Al and Nb components of Ti-25Al-25Nb (mole fraction, %) melt, the vapor pressures of corresponding components and the evaporation loss rates were calculated. Utilizing these activity coefficients and the vapor pressures, the relative evaporation coefficient is used to judge the evaporation tendency of these components. The evaporation tendency among the three components were compared and the result shows that the evaporation tendency is that: AlTi>Nb. Evaporation loss rate increases with the increase of melting temperature and decreases with the increase of chamber pressure. There exists an impeding pressure p_impe of Al element evaporation during induction skull melting process of Ti-25Al-25Nb alloy. The impeding pressure can be written as p_impe=8.1p_e, where p_e represents the equilibrium partial pressure. The calculation of evaporation loss of Al element also showed that when chamber pressure exceeds p_impe, the Al volatilization losses could be ignored. In order to pr event the evaporation loss of components, the pressure in the vacuum chamber should not below p_impe.

The research achievement on wrought TiAl alloys gained recently in Central Iron and Steel Re search Institute, China, was contributed. The progresses mainly include the improved hot deformability and homogenized microstructure after hot deformation due to the significant effects of micro-alloying process. Isothermal compressive test indicated that the TiAl containing minor Ni exhibits better plastic flow behavior and enlarged process window. The effect of Ni on modifying hot deformability of TiAl can be enhanced by incorporated addition of Mg. TEM observations suggested that Ni addition activates dislocations as well as twins at beginning stage of hot deformation and thereafter the higher-density dislocations promote the dynamic recrystallization inside γ-TiAl lamellae. It is also identified that breakdown of α₂-Ti₃Al lamellae produces new dislocation-free γ-TiAl grains. On the other hand, the homogeneity of deformed microstructure can be increased by transforming the microstructure of the Ni-containing TiAl fro m original lamellar structure to equiaxed grains before hot deformation.

By using thermal simulation technique, the conventional canned-forging process of TiAl based alloy was studied. The effect of can parameters on the microstructures of TiAl alloy was analyzed in this process. The results show that, the deformation microstructure of TiAl based alloy without canning is inhomogeneous. In lateral area, crack and shearing lines can be found; while in central area, fine-grained shearing zone can be found. The effect of can is to reduce the secondary tensile stress. However, only when the deformation of the steel can is coincidental with that of TiAl alloy ingot, can this effect be effective. Moreover, a thick can would enhance the microstructural homogeneity in TiAl based alloy. With the H/D ratio of the ingot increasing, the deformation of TiAl alloy would be more unsteady, therefore, a thicker can should be needed.

By using thermal simulation technique and computer simulation, the conventional canned-forging process of TiAl base alloy was studied. The effect of can parameters on the mechanical behavior of TiAl alloys with different H/D ratios was analyzed in this process. The results show that, the pea k stress of TiAl base alloy without canning is far higher than that with canning. Compared with the samples with the same H/D ratio, the peak stress decreases with increasing can thickness; while compared with the samples with the same can thickness, the peak stress decreases with increasing H/D ratio. The decrease of t he true stress of TiAl base alloy with canning were analyzed according to the theory of plastic deformation and results of computer simulation. 

γ-TiAl based alloys are rapidly being developed for elevated temperature applications, due to their high strength, light mass and good oxidation resistance. However, the disadvantages of TiAl based alloys are low ductility and toughness at room temperature, and poor workability. Grain refinement is one of the most effective ways for improving room temperature tensile properties and hot workability of ordered TiAl based alloys. At present, the majority of research works have focused on alloy modifications through compositional controls, alloying additions, thermo-mechanical processing and production techniques. This article discusses the research status of TiAl based alloys in the areas of microstructure, alloying, processing and applications.

Blended elemental powders with the nominal compositions (mole fraction, %) of Ti₅₄Al₄₆, Ti₅₂Al₄₈ and Ti₅₀Al₅₀ were mechanically alloyed in a planetary ball milling system for up to 100h.The structure evolution in these powders was characterized by scanning electron microscope, X-ray diffraction and differential thermal analysis techniques. It was found that elemental powders were progressively trans formed into nanocrystalline Ti(Al) supersaturated solid solution, then into amorphous phase. With increasing Al content, the formation of a fully Ti(Al) supersaturated solid solution and amorphous phase were accelerated, which are attributed to the fine grain size. And the grain size condition for formation of amorphous phase in this system is ≤16nm.

Microstructure evolution during superplastic deformation of a large-grained TiAl alloys with near-γ microstructure was characterized by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). In OIM, significant grain refinement is observed at different strain levels with an increase in the density of low angle grain boundaries and high angle grain boundaries. A direct evidence of dynamic formation of grain boundaries with misorientation of 15 °～30° during deformation is found, which is a result of subboundary evolution. The formation of subboundaries by intersecting dislocations, and the evidence of dislocation glide in the interior of grains are revealed by TEM observations. A continuous recovery and recrystallization process similar to that in FeAl and Fe₃Al is proposed as superplastic deformation mechanism in the large-grained TiAl alloy.

TiAl alloy was prepared by intense plastic deformation and subsequent reaction sintering. The effect of plastic deformation on the microstructure of sintered TiAl alloy was investigated using energy dispersive X-ray spectroscopy ( EDS), optical microscopy and transmission electron microscopy (TEM). The results show that the intense plastic deformation of reacting Ti and Al phases caused by high energy ball milling refines the as-sintered microstructure. The longer the milling time, the finer the grain size of γ and lamellar (α₂+γ) phases. The finer grain size improves the properties of the TiAl alloy. It is also found that the volume fraction of lamellar (α₂+γ) phases increases first, then decreases with increasing milling time. Based on the experimental results theoretical discussion was presented.

A more complex structural component with small size and very thin walls and blades used for advanced aircraft engine was fabricated well by induction skull melting and centrifugal investment casting with a proper ceramic mold. The tensile elongation and ultimate strength of the hotisostatically pressed (HIPped) Ti-46.5Al-2.5V-1Cr (mole fraction, %) ca sting alloy s are up to 2.5% and 645MPa at room temperature, and 31% and 593MPa a t 800℃. The fracture roughness at room temperature is up to 28MPa·m^1/2. The endurance tensile strength at 800℃ for 150h, is higher than 200MPa. The high cycle rotary bending fatigue strengths for 1×10⁷ cycles at room temperature and 800℃ a re 412MPa and 270MPa, respectively.

Gas nitridation of TiAl based alloys in an ammonia atmosphere was carried out. The evaluation of the surface wear resistance was performed to compare with those of the non-nitrided alloys. It is concluded that high temperature nitridation raised wear resistance of TiAl based alloys markedly. The tribological behaviors of the nitrided alloys were also discussed. The oxidation kinetics of the nitrided TiAl based alloys were investigated at 800～1000℃ in ho t air. It is concluded that nitridation is detrimental to the oxidation resistance of TiAl based alloys under the present conditions. The nitrided alloys exhibit increased oxidizing rate with the prolongation of nitridation time at 800℃. However, alloys nitrided at 940℃ for 50h display a sign of better oxidation resistance than the other nitrided alloys at more severe oxidizing conditions. The parabolic rate law is considered as the basis of the data processing and interpretation of the mass gain vs time data. As a comparison with it, attempts were made to fit the data with the power law. The oxidation kinetic parameter k_n, k_p and n were measured and the trends were discussed.

The site occupations of the alloying elements of O phase in Ti₂AlNb-based intermetallics are clarified. The ordering behaviours of the O phase in Ti-yAl-zNb(y≥25%, mole fraction) orthorhombic alloys are also investigated with a Bragg-Williams model. In the temperature range where the O phases exist, the order parameters change with the alloy composition and temperature continuously, and the first-order transition character is very “weak” .

In order to develop feasible hot processing technologies, hot workability of TiAl alloys need to be improved. The hot deformability of tested alloys, Ti-46.5Al-2.5V-1.0Cr and Ti-46.3Al-2.5V-1.0Cr-(0.3～0.5)Ni-(0.01)Mg (mole fraction, %), was evaluated by hot compression tests over the temperature range from 950 to 1050℃ and strain rate range from 0.01 to 1.0^-1. It was found that the hot deform ability of TiAl alloys was obviously improved by minor additions of Ni and Mg.

The reinforced phase of in-situ formed intermetallic compound Al₃Ti with grain size about 0.5μm in the aluminum matrix composite was achieved by the method of liquid-solid reaction under specific condition. The orientations were al so studied. The results show that under T6 heat treatment regime, the tensile strength, hardness, elastic modulus and elongation of Al₃Ti/ZL101 in-situ composite are increased by 32.8%, 14.4%, 19.2% and 14.6% respectively in comparison with those of ZL101. Al₃Ti is uniformly distributed in α(Al) matrix with a clear phase interface, and the orientation relationship between Al₃Ti and α(Al) is (006)_Al3Ti∥(0-22)_Al，［1-22］_Al3Ti［11-0］_Al. The strengthening mechanisms of Al₃Ti/ZL101 in-situ compo site is proved to be caused by fine grain size, spreading distribution and dislocation multiplication.