In Situ Studies On The Fatigue Properties Of High-Nb TiAl Alloys At High Temperature

The TiAl alloys with high Nb content, due to their superior high-temperature properties and low density, have been considered as the high-temperature structural materials with broad application prospects in aeroplane and automobile engines. After many years of research, preparation and processing technology for high-Nb TiAl alloys, as well as the microstructure and control technology of conventional mechanical properties have been basically mastered, however, the studies on their high temperature fatigue properties and life safety design are still lacking. In this thesis, some crucial issues on tensile properties and fatigue properties of high-Nb TiAl alloys by the SEM in situ observation were studied, including cyclic hardening or softening characteristics and mechanisms, fatigue crack initiation and propagation, effect of microstructures and micro-defects on fatigue crack propagation rate and fatigue life and fatigue life prediction etc. The main results and new contributions were listed as follows:The tensile behevior of a high-Nb TiAl alloy with nearly lamellar microstructure was studied at750℃. The results showed that ductility and strain hardening characteristics were found during the process of tensile testing, and the ductility was attributed to the crack propagation, lamella torsion and dislocation glide; and the hardening was attributed to the dislocation pinning, tangles and pileups. TEM analysis results showed that the main mechanism of the alloy tensile deformation at750℃was dislocation glide.The cyclic deformation tests of a nearly lamellar high-Nb TiAl alloy were carried out at750℃. The deformation process showed an obvious cyclic hardening characteristic and the cyclic stress-strain behavior of the alloy was characterised by an initial hardening followed by a stabilisation of the strain amplitude. The results of TEM observation showed the high-Nb TiAl alloy at high temperature deformed first by deformation twinning and then by dislocation climb. Because of the interaction between deformation twins and dislocations, cyclic hardening occurred in the initial stage of cyclic deformation.The fatigue crack initiation of the high-Nb TiAl alloy with nearly lamellar microstructure at750℃showed a multi-point iniation characteristic. During the fatigue crack propagation three or two propagation stages were found depending the microstructure of crack tip:As the fatigue crack was parallel to the lamellar laths, it exhibited the rapid, steady and accelerated propagation stages successively, while as the fatigue crack was perpendicular to the lamellar laths, it exhibited only the steady and accelerated propagation stages, with no rapid propagation stage was found.The high-Nb TiAl alloy has good resistance to fatigue crack propagation at high temperature. For the high-Nb TiAl alloy, the elevated temperature degraded the interface strength of the lamellar laths and lamellar colonies, causing there were more fatigue cracks initiated at the interfaces of lamellar laths and lamellar colonies, so more deflection, branching, and ligament bridging phenomena occured accordingly at high temperature; meanwhile, just because of these phenomenon, the fatigue crack propagation resistance at high temperature was higher than at room temperature.For the hign-Nb TiAl alloy, the fully lamelar microstructure alloy has a higher fatigue crack initiation resistance and fracture toughness than the nearly lamellar microstructure alloy. The fractures occurred in fully lamellar microstructure high-Nb TiAl alloy were mainly translamellar and interlamellar fracture. The microcracks in nearly lamellar microstructure mainly nucleated in y grains, and more cracks initiated and propagated along the boundaries of y grains. As cracks length increased, the cracks could propagate in a mixed mode of translamellar, interlamellar and intergranular, transgranular.The fatigue thresholds of a high-Nb TiAl alloy were determined by in situ SEM observation. The results indicated that the fatigue crack propagation thresholdΔKth of Ti-45Al-8Nb-0.2W-0.2B-0.1Y alloy at room temperature and750℃was determined as12.89MPa·m1/2and8.69MPa·m1/2, respectively.The ratio of fatigue crack initiation life and fatigue fracture life (Ni/Nf) of the high-Nb TiAl alloy at high temperature was constant. It was95%for notched specimen and43%for unnotched specimen.On the basis of damage mechanic mode developed by Lemaitre, the fatigue crack initiation life and fracture life prediction formula of high-Nb TiAl alloy were proposed, which are and respectively.