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In this study, Cf/Al composites and TiAl alloys were joined by a new method named laser-ignited selfpropagating synthesis（SHS）. Mixed powders of 63.0Ni-31.9Al-5.1Ti（wt%） were used as joining interlayer.Perfect joint was got. The microstructure evolution and formation mechanism of the SHS joint were investigated by scanning electron microscopy（SEM）, energy-dispersive spectroscopy（EDS） and X-ray diffraction（XRD）. Results show that localized melting occurs on both sides. One γ-Ni（0.35）Al（0.30）Ti（0.35） and two Ni-Al reaction layers form,respectively, in the TiAl/interlayer and Cf/Al/interlayer interfaces. The combustion of Ni-Al-Ti interlayer begins with the sharp reaction of Ni and Al. The interlayer product is a eutectic organization of NiAl and Al-rich γ.

This study systematically compared the influences of yttrium（Y）,boron（B）,and carbon（C） on the microstructural refinement and properties of a Ti-43Al-5Nb alloy.The microstructural refinement effect in the TiAl alloy closely depends on the refiner used.The refinement effects of the three elements on colony size and lamellar thickness can be arranged as B 〉 Y 〉 C and Y 〉 C 〉 B,respectively.Moreover,a microstructure with a small grain size and ultra-fine lamellar spacing can be obtained by adding B and Y or B and C.The mechanical properties of TiAl alloy are also influenced by the refiners.TiAl alloys with proper B and Y contents exhibit favorable hot workability,tensile properties,and fracture toughness,whereas the C-containing alloy displays poor tensile properties and low fracture toughness.These results indicate that Y and B are more suitable microstructure refiners than C.This study may serve as a reference for practical alloying design.

The hot deformation behavior of powder met- allurgical （PM） TiAI alloys was investigated on Gleeble- 3500 thermomechanical simulator, at a temperature range of 1050-1200 ℃ with an interval of 50℃ and a strain rate range of 0.001-1.000 s-1. The results show that the flow stress of PM TiAI alloy is sensitive to deformation tem- perature and strain rate, the peak stress decreases with the increase in deformation temperature and decrease in strain rate, and dynamic recrystallization occurs during the hot compression. The deformation active energy was calcu- lated and the flow stress model during high-temperature deformation was established based on the Arrhenius equations and Zener-Hollomon parameter. The deformed microstructure consists of refined homogeneous γ and α2/γ grains.

Creep experiments of the TiAl alloy with high Nb content were conducted to consider both the temperature dependence and stress dependence of its creep behavior. The creep curves were characterized by Theta constitutive model, and Newton–Gauss method was applied to obtain the material parameters. Based on the rupture strain, the rupture life of the TiAl alloy was able to be predicted by Theta constitutive model. The results show that the creep curves of the TiAl alloy contain primary creep, secondary creep and tertiary creep stages, especially for the lower stress. The rupture life of the TiAl alloy decreases with the increase in either applied stress or temperature. Theta constitutive model is able to describe the creep deformation of the TiAl alloy accurately, and the predicted life agrees well with the experimental result.

Cf/Al composites and TiAl alloy were joined by combustion synthesis in different joining conditions. Effects of additive Cu, joining temperature and holding time on joint microstructure and shear strength were characterized by employing DTA, SEM, EDS, XRD and shear test. Results show that the additive Cu in the Ti-Al-C interlayer could significantly decrease the reaction temperature owing to the emergence of Al--Cu eutectic liquid. Reaction degree of the interlayer was influenced by joining temperature and holding time. Due to the barrier action of formed TiAl3 layer, reaction rate of Ti and Al was determined by the atoms diffusion. The reaction between Ti and AI was more sensitive to the joining temperature rather the holding time. The joints shear strength was influenced by joining condition directly. The maximum shear strength of CS joints was 25.89 MPa at 600 ℃ for 30 rain under 5 MPa. Interface evolution mechanism of the CS joint was analyzed based on the experimental results and phase diagram.

Effects of surface condition on fatigue properties of a medium-strength γ-TiAl alloy Ti-45Al-5Nb-lW（at%） were investigated.It is found that the maximum stresses of fatigue samples are lower than the yield stresses of the medium-strength γ-TiAl alloy.Meanwhile,the local plastic deformation is unconspicuous to occur at the crack tip.In this case,the fatigue strength is mainly decided by surface conditions of maximum-stressed surface,but compressive stress and deformation especially resulted from shot peening play an important role in the improvement of the condition fatigue strength.The affecting depth of shot peening is about 250 μm.As a result,the relatively weak microstructures and phases become the preferential initiation sites and propagation routes.They are observed to be equiaxed γ grains,B2 ＋ ω grains,and α_2-γ lamellar interface in soft orientations.The existence of V-notch can significantly reduce the fatigue properties of the samples.

Turning machining induced microstructural instability was investigated in a fully lamellar Ti-45Al-8.5Nb-（W,B,Y） alloy during high temperature exposure.After turning machining followed by thermal exposure at900 or 1000℃ for 100,300 and 500 h,a depth-dependent gradient microstructure with random orientations was produced in the region close to the machining surface.Two typical layers,a fine-grained（FG） layer with equiaxed grains and a coarse-grained（CG） layer with elongated grains,are formed in this region in transversal direction.The thickness of the two layers is up to 120 urn after thermal exposure at 1000℃ for 500 h,which is less than the depth of the hardened region（200 μm） after turning machining.Most of the new grains in FG and CG layers are constituted of γ single phase,while short α_2 segments and few B2 particles are precipitated at the γ/γ interface or inside the γ grains.Recrystallization and phase boundary bulging are found to be the major mechanisms responsible for lamellar degradation in FG layer and CG layer,respectively.The residual deformation energy stored is considered to be the main driving force of this process.

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy （SEM） in-situ observation. The results indicated that the fatigue crack growth threshold △Kth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750℃ was determined as 12.89 MPa.m^1/2 and 8.69 MPa.m^1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.

The high-temperature friction and wear properties of TiAl alloys and Ti2AlN/TiAl composites （TTC） in contact with nickel-based superalloy were studied. The results showed that, at 800 and 1 000 ℃, the coefficient of the friction （COF） decreased with the increase of sliding velocity and the wear loss of the TTC decreased with the increase of volume fraction of Ti2AlN. The wear mechanisms of the pairs are adhesive wear and the wear debris mainly comes from the contacting nickel-based superalloy. The intergranular fracture and the cracking of the phase boundary in the lamellar structure are the wear mode of TiAl alloy. The wear mode of TTC is phase boundary fracture and adhesive spalling. The abrasive resistance of TTC is slightly higher than that of TiAl alloy.

Al2O3/TiAl composites were successfully fabricated by hot-press-assisted exothermic dispersion method with elemental powder mixtures of Ti, Al TiO2 and Nb2O5, and the microstructure and mechanical properties were investigated. The results indicate the fine Al2O3 particles tend to disperse on the grain boundaries. The grain size of TiAl matrix decreases and the hardness increases with increasing Nb2O5 content. The bending strength and fracture toughness reach to a maximum when Nb2O5 content is 6 wt%, under 642 MPa and 6.69 MPa·m^1/2, respectively. Based on the fractography and the observation of crack propagation path, it is concluded that the strengthening and toughening of such composites at room temperature can be attributed to the refinement of the TiAl matrix, the deflection behavior in the crack propagation and the dispersion of Al2O3 particles.