Research On The High-temperature Deformation Behaviors, Microstructure And Mechanical Properites Of Beta γ-TiAl Alloys

γ-TiAl based intermetallics alloys have attracted much attention using for high-temperature structural materials because of their low density, high melting point, high elastic modulus and goog high-temperature mechanical and oxidation properties.And as such are excellent candidate materials for high temperature structural applications at the aerospace industry. However, there limited room-temperature ductility has so far hindered their extensive applications. TiAl alloys with fine microstructures and improved ductility can be achieved through alloying, heat treatment, thermo-mechanical treatment(TMT), including isothermal forging, pack-forging, hot extrusion and rolling. Recently, β phase containing TiAl alloys have been developed widely, due to good hot deformability. Therefore, based on new type Ta-TiAl at% alloy as the research object, the hot deformation behaviors were researched, processing maps, exposed the fracture behavior and its mechanism, and built the hot deformation fracture criterion were established. Through the element simulation analysis technology, the optimized processing parameters for hot forging and hot extruded of large size TiAl alloy could be achieved. Therefore, this paper systematically studied the relationship between the microstructure and microstructure-hardness for the hot extruded Ta-TiAl alloy in different phase regions, heat treating parameters and cyclic heat treatments. Main research contents and results are as follows:Based on stress-strain analysis behavior for the alloy, the relationship between deformation behaviors and temperature via strain state was revealed. The constitutive equations was developed by the peak flow strain. The processing maps for the new type TiAl alloy were constructed at strains of 0.2 and 0.6 through the Dynamic Material Model and the Prasad’s instability criterion. And then the stable and unstable regimes for this alloy were verified. The optimal processing window was obtained and the pancake of this alloy was manufactured.The mechanism and impacts for the dynamic recrystallization of current TiAl alloy were systematically researched by microstructure, which deformed at the high temperature.The results showed that the deformation microstructure of the alloy is closely related to srtain, strain rate and temperature. During the deformation microstructure, the complicated phase transformation and dynamic recrystallization took place simultaneously at different temperature ranges. Two types of DRX(discontinuous and continuous) were observed at the investigated strain rate, which has connected with the space of the lamellar. Considering the processing maps and the microstructure, the optimal processing window of temperature no less than 1200℃,strain rate no more than 0.5s-1 and the strain no more than 50% was determined for this material.Based on observing of hot compression deformation specimens, the fracture behavior and mechanism of as-cast Ta-TiAl alloy were investigated systematically. The fracture modes mainly consist of 45° shear fracture and longitudinal fracture on free-surface during the hot deformation. Besides, the cracking degree increased with the decreasing of the hot deformation temperature, the increasing of height reduction and strain rate for this alloy. At the high strain rate range, deformation twinning dominated the entire deformation process.Morever, the cracks nucleation sites were observed at grain boundary of the colonies and/or interfaces between lamellae of the specimens, and initiate and propagate perpendicular to the compression axis.Some cracks extend with “Z”shape along the the colonies grain boundaries and the deformed twinning boundary.Based on the peak flow stress σp and the grain size d, a criterion of wedge cracking can be established as.The thirty kilogram size tests were used to quantitatively research on the microstructural evolution during hot extrusion and post-deformation heat-treatment. The results showed that the alloy mainly composed of γ and α2 phases, which had fine near lamellar microstructure with colony size of 50-70μm after hot extrusion.The different microstructure after hot extrusion was obtained by different cooling rates, and then the massive phase was found at grain boundaries by oil quenching, thus this found will be used to refine the grains by the continuous heat treat. The near full lamellar(NL) material holds excellent ductility more than the full lamellar(FL) one.The observation of fracture surfaces showed the mixed way with the translamellar and interlamellar fracture. Besides, the tensile crack path of NL microstructure easily can be tortuous because of the existence of the beta phase.The crack of FL microstructure easily propagated at the tensile sample and the crack orientation was connected with the lamellar orientation of lamellar colonies.The microstructure of the thermo-mochanically treated TiAl alloy can be further refined by cyclic heat treatment. To achieve this refinement, several parameters should be well controlled including the heating rate, annealing temperature, holding time and cooling rate. For the three-step heat treatment, the lamellar structure is continuously refined with increasing the cycling number. When the cycling number reaches 5, the colony size can be refined from 50 to 80μm.And with the microstructure refinement and optimization, the tensile strength of the present alloy at room temperature was improved to exceed the strength level for the hot extruded TiAl alloy. By examining the fracture surfaces and the polished side faces of the samples in the scanning electron microscope the fracture mechanisms were evaluated. Cracks prefer to be initiated and propagated along lamellar interfaces, which is the weakest link in the fully lamellar microstructure.It was found that the the fracture modes mainly depend on structure and phase arrangement at the boundaries as the way of intergranular and translamellar. The preferred path of cracks usually initiate in the weakest site of interface of γ/α2 or γ/γ. The main crack stops in front of grains having lamellar interfaces perpendicular to or inclined with a large angle to the direction of propagation, and new cracks are nucleated along lamellar interfaces behind the barrier grain.