| 其他摘要 | Near-net shape TiAl structural parts by investment casting has a potential application . Fatigue of TiAl alloy such as thermo-mechanical fatigue(TMF), isothermal fatigue(IF), low cycle fatigue (LCF) at room temperature(RM) and fatigue crack propagation (FCP) has been systematically investigated in this paper which can be beneficial technical support for securi -ty design and practical application of casting TiAl.
TMF properties of Ti-46Al-2Cr-2Nb-0.15B alloy were examined by mechanical strain control . During mechanical strain amplitude range of 0.2-0.85%, in-phase (IP) and out-of -phase (OP) TMF lifetime curves were described while linear relationships between fatigue lifetime and mechanical strain amplitude, plastic strain amplitude, stress amplitude , total strain energy were also concerned. Studies show that IP and OP-TMF lifetime of gamma- TiAl were substantial affected by creep, oxidation, peak stress and mean stress.
During TMF testing, in cycling stress response(CSR) curves, IP and OP has a different hardening and softening tendency. And hysteresis loops show two apparent asymmetry, one is zero asymmetry, the other is tensile and compressive asymmetry. The factor of asymmetry of CSR curves during TMF testing are materials constant, such as E, G and μ. There are many factors which affect CSR behavior of gamma-TiAl alloy. Strain amplitude and phase play a important role which cannot been separated from deformation microstructure during TMF testing. The results show that slip modes change with decrease of mechanical strain amplitude. Other factors such as Boron compound, α2 phase dissolution, γ phase nucleation, precipitation of precipitated phase greatly influence CSR behavior of TiAl alloy.
So far as, reports about isothermal fatigue and its deformation mechanisms of TiAl alloy have been reported little to the authors’ knowledge. It is necessary to discuss LCF and CRS behaviors of TiAl alloy. In this paper, effect of total strain amplitude, temperature, thermal exposure, porosity and strain rate on LCF and CRS behaviors of TiAl alloy was also interpreted. LCF at RM was controlled by total strain amplitude and range of total strain amplitude is 0.3-0.6%. Some samples were thermal exposed at 500oC, 650oC and 800oC for 20h and 100h respectively so as to understand influence of different temperature and thermal exposure time on LCF properties. It is shown that fatigue lifetime increase at RM following by decrease of total strain amplitude, thermal exposure temperature and time. Strain amplitude is somewhat higher(Δεt/2≥0.45%), hysteresis loops almost show zero and tensile-compressive symmetry, while strain amplitude is lower(Δεt/2≤0.45%), zero and tensile-compressive is asymmetry.
During IF tests, three temperatures(500oC, 650oC and 800oC)related for peak temperature, valley temperature and mean temperature in the temperature range of TMF tests were selected to make a comparison. IF tests were controlled by mechanical strain and 0.6%, 0.5% , 0.45% for 650oC and 0.45% for 500oC, 800oC. Three strain rates(6.67×10-5, 6.67×10-4,6.67×10-3 s-1)were performed to discuss effect of strain rate on IF behavior of gamma TiAl. It is shown that hysteresis loops during IF tests has a better symmetry than that during TMF tests. Microstructures transformation is tightly related to CSR behavior of IF tests , which is contributed by mechanical strain amplitude, temperature and strain rate.
In this paper, influence of microstructures(FL, NL) and stress ratios(0.1,0.3,0.5) on FCP properties of Ti-46.5Al-2Cr-2Nb-0.15B was investigated. The results show that, FL microstructure has slower fatigue crack growth rate than NL microstructure; fatigue crack propagate faster with prolongation of tests lasting and increase of stress ratio; β phase and lamellar orientation can apparently improve FCP resistance of gamma TiAl.
Key words: γ-TiAl thermal exposure thermo-mechanical fatigue
Isothermal fatigue low cycle fatigue porosity fatigue crack propagation |
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