Study On The Creep-fatigue Interaction Of High-Nb TiAl Alloys

High-Nb TiAl alloys are considered as the potential replacements to Ni-based superalloys in aerospace and automotive industries due to their superior high temperature properties and low density. Currently, the high-Nb TiAl alloys have been listed as one of the key developing materials for aero-engine applications in our country, and have been funded by the national "973" and the military "863" projects. It has made a series of advances in component design, microstructure controlling, preparation molding and processing technology of the high-Nb TiAl alloys. However, the studies on their mechanical properties and reliability assessment are still not adequate, especially the damage caused by the interaction between fatigue and creep at high temperature are even short of systemic research, and it has seriously affected their further application and development.Based on the above background, the creep-fatigue property and other related high temperature properties of a nearly lamellar high-Nb TiAl alloy have been investigated in this paper, including the properties of tensile, fracture toughness, creep, fatigue and creep-fatigue interaction. The main results are listed as follows.The tensile and fracture toughness properties of the high-Nb TiAl alloy are influenced by the microstructure and crack initiation and propagation behavior at high temperature. The SEM in situ observations and fracture surface observations indicate that cracks mainly initiate and propagate along colony boundaries in the nearly lamellar alloy. On the contrary, cracks mainly initiate and propagate along lamellar interfaces in the fully lamellar alloy. The intergranular cracking reduces local stress concentration, as a result, the tensile properties of the nearly lamellar alloy are better than that of the fully lamellar alloy. As intergranular cracks is much easier to propagate than interlamellar and translamellar cracks, the fracture toughness of the nearly lamellar alloy is lower than that of the fully lamellar alloy.The results of the creep property indicate that with the increase of temperature and stress the minimum creep rate decrease. The creep life predication are proposed as follow, log Tr(h)+0.94× logεmin(%/h)= 0.07. The SEM in situ observations indicate that the three different creep stages are correspond to crack initiation, propagation and interconnection. The steady-state stage mainly consists of crack initiation and propagation process, while the accelerating stage mainly consists of crack interconnection. The deformation microstructure analyses show that the creep deformation mechanism changes with the increase of creep stress. Lattice diffusion is the main deformation mechanism at low stress range, dislocation slip dominates the deformation at intermediate stress range, while deformation twinning prevails at high stress ranging.The results of the fatigue property indicate that the stress ratio (R) significantly influences the fatigue life and deformation mechanism. At R ranging from 0.1 to 0.4. creep-fatigue interaction dominates the fatigue life which shows the minimum value. The corresponding fatigue life predication is proposed as follow. σ0=246.01(MPa)×exp{σ/0.31σ}(?)|σ/(?)σ|.Where, σa is the cyclic stress amplitude and σm is the mean stress. At R ranging from 0.4 to 1, creep dominates the fatigue life which decreases with the increase of R The corresponding fatigue life predication is proposed as follow. Nf=1.17×1020σm-546. The SEM in situ observations indicate that with the increase of R fatigue fracture mode transforms from transgranular cracking at R=0.1, to transgranular and intergranular cracking at R=0.2 and 0.3, and to intergranular over R=0.4. Accordingly, the deformation microstructure analyses show that the deformation mechanism changes from dislocation slip and climb, to dislocation slip and twinning, and to twinning.The loading frequence (f) also affects the fatigue property. With the increase of f, the fatigue fracture mode transforms from transgranular cracking at f=10 and 1 Hz to intergranular cracking at f=0.05 and 0.025 Hz. Accordingly, the deformation mechanism changes from dislocation slip and climb to deformation twinning and dislocation slip. The fatigue life prediction under different f is proposed as follow. Nf= 118887.96(f)1.01The study on the creep-fatigue interaction indicates that with the increase of effective dwell time (Δt/tp), the life decreases linearly and the corresponding life prediction is proposed as follow.Nr=Nfo-Ktp/ΔT. The SEM in situ observations indicate that, with the increase of Δ t/tp, the intergranular cracks increase and the alloy shows a mixed cracking mode which is different from that of the pure fatigue and creep. This kind of cracking mode accelerates the crack growth rate and then results in shortening the life. The deformation microstructure analyses show that dislocation slip and deformation twinning are the main deformation characteristic under creep-fatigue interaction.