Cracking Behavior Of High Nb-containing TiAl Alloys And Its Correlation With Microstructure Characteristics

Due to the excellent elevated temperature properties and low density,Ti Al alloys have the potential to meet the service requirenments and reduce the component weight simultaneously.However,the intrinsic brittleness of intermetallic compounds leads to a low ductility of Ti Al alloys at room temperature,which aggravates the cracking during processing and seriously impedes the progress of manufactue and appilication.The development of aero-space engines needs components with higher service temperature,which draws our attention onto high Nb-containing Ti Al alloys.The addition of Nb reduces the ductility of Ti Al alloys while it improves the strength at high temperatures.Moreover,the addition of Nb introduces orderd B2 phase,which further reduce the dutility and increaces the cracking tendency of Ti Al alloys.Based on the above,the cracking of high Nb-containing TiAl alloys and its correlation with microstructure characteristics was investigated in this thesis.The microcrack nucleation,crack propagation and cavity evolution of Ti Al alloys during brittle and ductile fracture,as well as the effects of constituent phases and lamellar structure were analyzed through a seris of mechanical experiments and microstructure characterization.Additionally,we studied the influence of the thermal stresses on lamellar transformation and thermal-induced deformation during the cooling process of high Nb-containing Ti Al alloy.The main contents and conclusions were as follows:Three alloys with different microstructures,including near fully lamellar?NFL?,fully lamellar?FL?and?B2+??,were designed and fabricated.A series of bending tests was conducted to investigate the nucleation of microcracks under tensile stress and the role of microstructural features.Results show that the microcrack nucleation plays an important role in the fracture of multi-phase high Nb-containing Ti Al alloys.The plastic anisotropy of lamellar structure and the brittleness of B2 phase results in strain incompatibility at colony boundaries in lamellar structure alloys and B2/?phase boundaries in?B2+??alloy,which facilitates microcrack nucleation at these locations.Fracture toughness tests were conducted on specimens of different multi-phase high Nb-containing Ti Al alloys.The fracture path profiles and microscopic toughening mechanisms were analyzed to investigate the effect of microstructure characteristics on fracture toughness.The results show that a combination of microscopic toughening mechanisms including plasticity,crack deflection and shear ligaments contributes to the toughness of high Nb-containing Ti Al alloys.The crack tends to deflect at colony boundaries due to the misorientation of lamellar interfaces and habit planes for cleavage fracture.As temperature increases,the size of plastic zone near the crack tip increases and the interface strength decreases at the same time,which promote the crack defleation and shear ligament,and thus improves the fracture toughness of Ti Al alloys.To understand the ductile fracture and cavitation of high Nb-containing Ti Al alloys,tensile tests were conducted between 700? and 1000?.Then,the fracture morphology and cavitation behavior were investigated.The results show that the ductile fracture of Ti Al alloys is related with cavitation behavior.The fracture strain is determined by cavitation and plastic deformation of the alloy.The cavity nucleation occurs at an early stage during softening process of the tensile deformation,which is dominated by plastic deformation and diffusion.The nucleation and growth of cavities in NFL alloys are correlated with strain incompatibility between hard and soft orientated lamellar colonies.?/B2 phase may facilitate the cavity nucleation due to its high diffusivity,while it can also reduce the cavity growth because it can coordinate heterogeneous strain through plastic deformation.Continuous cooling tests were performed on TiAl alloys using a Gleeble3500machine to investigate the effect of thermal stresses on the microstructure.The results show that macroscopic thermal stresses promote correlated nucleation of?lamellae in?/?₂ phase at relatively low temperatures.The trend of the dominance of one twin-group?variants in local regions is weakened,and the?/?interfaces tend to be true twin and pseudotwin boundaries rather than 120orotational faults.Meanwhile,thermal-induced deformation generated under the effect of both microscopic and macroscopic thermal stresses results in numerous low angle grain boundaries?LAGBs?and dislocations.The LAGBs and dislocations distribute heterogeneously among lamellar colonies and phases,which reduces the tensile strength and fracture toughness at room temperature,and may increase the cracking tendency of high Nb-containing TiAl alloys.