Study On The Low Cycle Fatigue Behavior And Microstructure Evolution Of High Nb Containing TiAl Alloy

High Nb containing TiAl alloys have become the most promising lightweight high-temperature structural materials because of their low densities,superior high temperature strength,oxidation resistance and creep properties.Based on the investigation for decades,a series of advances has been achieved on the materials'design,microstructure tailoring,processing technology,and mechanical properties for TiAl alloys.However,the studies on the high temperature fatigue behavior,especially the micro-mechanical behavior,and the cyclic deformation mechanisms are still limited.Besides,exploring the methods to enhance mechanical properties of TiAl alloy is still lacking that the investigations to improve the properties of TiAl alloys were limited by tailoring the composition and microstructure,and/or acquire well-aligned lamellar orientation PST single crystals.According to the above background,the high temperature cyclic behavior,the deformation mechanisms,and the fracture behavior of high Nb containing TiAl alloys with fully lamellar microstructure and duplex microstructure were investigated in this work,using a combination of SEM,TEM,and HE-XRD techniques.Moreover,the high temperature properties of a duplex high Nb containing TiAl alloy were enhanced by the pre-torsional deformation induced gradient microstructure.Based on this result,the effects of pre-torsional deformation on micro-mechanical behavior and the mechanism of properties enhanced by the gradient microstructure were investigated.The investigation of cyclic stress-strain behavior of high Nb containing TiAl alloy suggests that cyclic stress-strain behavior is based on the microstructure of TiAl alloy.The fully lamellar TiAl alloy has three types of cyclic behavior,that is,cyclic saturation,cyclic softening and cyclic hardening.The duplex TiAl alloy has two types of cyclic behavior,that is,cyclic saturation and cyclic softening.The lattice strains/microstresses in the y phase were compressive at the early deformation stage but tension at the cyclic saturation and cyclic softening stages.In contrast,the strains/microstresses were tensile in the ?2 phase at first and then compressive,due to the load partitioning between ?/?2 lamellar structures during the cyclic deformation.There always tensile lattice strains/microstresses in the ?o phase.The evolution of microstructure is related to phase transformations and ?recrystallization.The ?o phase produced by the ?2lamellae occurred at the initial stage of deformation with two different orientation relationships<1120>?2//<0001>?o;{0002}u2//{1120}?o and<0001>?2//<1120>?o;{1120}?2//{0002}?o,and the orientation relationship of the ?o phase deviated from the ?2 lath during the following cyclic deformation.The effect of pre-torsional deformation on mechanical properties of TiAl alloy suggests that high Nb-containing TiAl alloy was successfully enhanced via gradient twin structure.The strength and ductility at 850? of the as-forged TiAl alloy were 494 MPa and 4.6%,respectively,which were enhanced to 557 MPa and 30.9%,respectively,by a pre-torsion of 180°,and to 592 MPa and 47.0%,respectively,by a pre-torsion of 360°.The tensile strength and the ductility are simultaneously enhanced by pre-torsional deformation.High strength is mainly attributed to the twin structure formed during torsion,while high ductility correlates to the recrystallization of y phase at twin-twin sections and the load partitioning regulated by a hierarchical microstructure.Besides,the larger the compressive lattice strains in the y phase of the pre-torsional sample,the higher properties can be improved of the TiAl alloy.The investigation of fracture behavior of TiAl alloy after cyclic deformation suggests that the cracks preffered to nucleate at the surface of sample.The propagation of fully lamellar TiAl alloy is mainly translamellar at relatively low strain amplitude,while a mixed fracture of translamellar and interlamellar at medium strain amplitude,and interlamellar at high strain amplitude.The fracture mode of duplex TiAl alloy is a qusi-cleavage fracture.Secondary cracks are widely found at the ?2/? lamellar interface,due to the large differences of the lattice strains,not only the values but also the directions,in the ?2 phase and the y phase.Based on the above findings,the relationship of micro-mechanical behavior-microstructure-property of high Nb containing TiAl alloy cyclic deformed at 850 ? is established,and the strategy of introducing gradient microstructure by pre-deformation can provide experimental basis and useful insights on polycrystalline intermetallic materials design to enhance the balanced properties.