| As a new type of high temperature structural material,TiAl alloy has a broad application prospect in the aerospace and automotive industries due to its low density,high strength,good high temperature creep resistance and excellent high temperature oxidation resistance.However,the crystal grains of as-cast TiAl alloys are coarse,inhomogeneous,poor hot-working ability and room temperature ductility restrict the development and application of TiAl based alloy.Therefore,in order to improve the microstructure and the hot-workability of as-cast TiAl alloy,Ti-xAl-M?M is a strong?-stabilizing element?alloy is designed in this paper.The microstructure of the alloy is controlled by changing the content of Al and other alloying elements and the heat treatment conditions.In addition,the TiAl binary alloys Ti-33Al,Ti-46Al,Ti-48Al,Ti-52Al and Ti-43Al-3Mo alloys were designed and the heat treatment processes,room temperature and high temperature compression performance and characteristics of tissue evolution of?2,?phase,lamellar??2/??and the alloy containing beta phase were systematically studied.Based on Ti-xAl-M?M represents?phase stabilizing elements,such as V?Mo?alloys.It was found that the?phase solidification microstructure produced just only by decreasing Al content in TiAl alloys do not contains?phase.The microsegregation can not be removed although adding?phase stabilizing elements when the Al content of alloys is higher.The martensite mode will be observed in the microstructure subjected to water quenching condition,with increasing the content of V?Mo elements or decreasing the content of Al.During two-step heat treatment procedure,Ti-Al-Mo alloys subjected to water quenching from 1320?will be in the supersaturation condition,and then furnace cooling from 1200?,the precipitation of?and?from the microstructure will be observe,leading to mixed microstructure of?+?.Taking Ti-33Al,Ti-46Al,Ti-48Al,Ti-52Al and Ti-43Al-3Mo alloys as the research object,the as-cast microstructures were first annealed at different temperatures.It can be found that 900°C can prevent grain growth,but is lower than the eutectoid transition temperature,and the effect of improving the structure is not ideal.1200°C which is higher than the eutectoid transformation temperature can significantly eliminate the as-cast defects.Therefore,the annealing parameters are generally selected to be kept at a relatively high temperature for a short time and then slowly cooled in the furnace.Next,the annealed alloy was subjected to a room temperature compression test.The flow stress and microstructure evolution process of?????phase and lamellar structure were studied respectively by isothermal compression test at different temperatures.It is found that the DRX was more difficult in?phase while DRX was more sufficient in?phase.By analyzing the high-temperature compressive microstructure of Ti-46Al and Ti-48Al alloys,it can be seen that the lamellar structure will be bent and twisted after compression,the lamellar structure remains unchanged,and dynamic recrystallization occurs when the stress is transmitted to the grain boundary.With the increase of deformations,the lamellae at the grain boundaries begin to decompose,and the?lamellae first undergoes spheroidal recrystallization,while the?lamellae still maintain their original orientation.The flow stress and microstructure evolution of Ti-43Al-3Mo alloy at 1100?/0.05s-1/70%and 1200?/0.05s-1/70%were analyzed.With increasing temperature,the flow stress is significantly reduced.This is because there are more?-phases at high temperatures,and the high-temperature disordered?-phase is a body-centered cubic structure with more slip systems.During deformation,the?phase can coordinate the deformation of the lamellar colonies,promote the grain boundary slip,and act as a lubricant.At the same time,the?phase at the grain boundary is prone to dynamic recrystallization,absorbing the distortion energy generated by the deformation and reducing the stress concentration. |
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