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An experimental study on high cycle fatigue behavior of selective laser melting (SLM) TC4 alloy was carried out. The fatigue properties of the alloy under two sampling directions (horizontal and vertical) and two temperatures (room temperature and 400 ℃) were compared and analyzed. Also the feasibility of improving the fatigue properties of the alloy by hot isostatic pressing (HIP) was explored. The results show that the fatigue properties of the alloy after annealing are significantly anisotropic, and the fatigue properties of the vertical samples are higher than that of the horizontal ones. Compared with room temperature, the fatigue life of the alloy at 400 ℃ is reduced, but the anisotropy still exists. After hot isostatic pressing, the fatigue life of the alloy presents a certain degree of improvement, and the anisotropy of fatigue properties decreases. The fracture analysis shows that the cracks of SLM TC4 alloy mainly originate from surface and subsurface defects which are mainly pores. The statistical analysis shows that the source defect size of vertical samples is lower than that of horizontal samples, which is the main reason for the decrease of fatigue properties of horizontal samples. After hot isostatic pressing, cracks in both horizontal and vertical samples of the alloy are generated at the surface slip, and the porosity of the alloy is significantly reduced without obvious defects, and the decrease in the number of defects is the main reason for the improvement of fatigue properties of the alloy.

The three-dimensional small crack growth behavior of selective laser melting (SLM) TC4 alloy was investigated by in-situ observation fatigue test method, and the long crack growth curve was measured by load reduction method under the same experimental conditions. The results show that at the early stage of small crack growth, the fatigue crack growth(FCG) rate fluctuates obviously under the influence of microstructure, and the FCG path is zigzag. With the increase of crack length, the influence of microstructure decreases, and the FCG path is straight, and the FCG rate increases steadily with the crack length. Internal defects can still reduce alloy fatigue life. Considering the small and long crack growth data, it is demonstrated that the small crack can still propagate under threshold value of long crack, and under the same stress intensity factor amplitude ΔK, small crack growth rate is higher than that of long crack. There is a typical "small crack effect", thus the small crack behavior should be considered when fatigue life prediction is carried out.

The thermo-mechanical fatigue（ TMF） behavior of casting alloy Ti-48 Al-2 Cr-2 Nb was researched. The tests were performed thermo-mechanically in-phase（ IP） at 400℃ ～ 800℃ . During TMF,tests strain was controlled. The result of this IP TMF was compared with 400℃ and 800℃ isothermal low cycle fatigue（ LCF）. The result shows that the fatigue life of Ti-48 Al-2 Cr-2 Nb alloy in 400℃ ～ 800℃ TMF condition is longer than 400℃ and 800℃ LCF within high strain region. Within low strain region,it is slightly shorter than 400℃ LCF and within the dispersion zone of 800℃ LCF. In this TMF condition,no evidence shows hardening or softening phenomenon. SEM analysis shows that inter-lamellar and inner-lamellar cracking and trans-lamellar tearing can be observed on the fracture surface.

Uniaxial tensile, low cycle fatigue, fatigue-creep interaction and creep experiments of a novel high-Nb TiAl alloy (i.e. Ti-45Al-8Nb-0.2W-0.2B-0.02Y (atom fraction/%)) were conducted at 750℃ to obtain its tested data and curves. Based on Chaboche visco-plasticity unified constitutive model, Ohno-Wang modified non-linear kinematic hardening was introduced in Chaboche constitutive model to describe the cyclic hardening/softening, and Kachanov damage was coupled in Chaboche constitutive model to characterize the accelerated creep stage. The differential equations of the constitutive model discretized by explicit Euler method were compiled in to ABAQUS/UMAT to simulate the mechanical behavior of high-Nb TiAl alloy at different test conditions. The results show that Chaboche visco-plasticity unified constitutive model considering both Ohno-Wang modified non-linear kinematic hardening and Kachanov damage is able to simulate the uniaxial tensile, low cycle fatigue, fatigue-creep interaction and creep behavior of high-Nb TiAl alloy and has high accuracy.

Total strain range controlled low cycle fatigue (LCF) experiments were conducted at 750 ℃ to investigate the effects of the duplex and fully lamellar microstructure on fatigue behavior and life of a TiAl alloy, and the total strain range-life equation was employed to predict LCF life of the alloy. The results show that the mean stress produced at hysteresis loop of TiAl alloy with DP is less that of TiAl alloy with FL at the same temperature and applied strain. The total strain range-life equation is able to predict the fatigue life of the alloy, and the predicted life is located between ±2 scatter band of the experimental life. In addition, the fatigue source of TiAl alloy with DP is located near center of the specimen while that of TiAl alloy with FL is located on subsurface of the specimen, and the failure mechanisms are obviously different between the two types of the TiAl alloy.

The conventional fatigue test method was used to obtain the very high cycle fatigue (VHCF) limits of 3×107 and 1×108 cycles for TA11 titanium alloy in different temperatures and stress ratios. Three parameter power function method was used to obtain the VHCF median S-N curves and equations. The results show that the VHCF strength of 3×107 and 1×108 cycles presented a continue reducing trend compared with the traditional 1 x 107 fatigue limit. This trend is not obvious in negative stress ratio (R=-1), but significant in normal stress ratio (R=0.1 and 0.5), and the reduction amplitude of room temperature tests was greater than that of elevated temperature tests. The fracture morphologies showed that the VHCF cracks initiat at the specimen surface of TA11 alloy in room temperature tests, and the VHCF cracks initiation ways in elevated temperature tests relate to the stress ratio. The cracks initiate at the specimen surface when R=0.1 and 0.5 but in the internal when R=0.5; The surface state of TA11 alloy specimens is the main cause of its fatigue life dispersion.