Study On Effect Law And Mechanism Of Thermohydrogen Processing On The Superplastic Characteristics Of Ti-55 High Temperature Alloy

Ti-55 high temperature titanium alloy is an excellent aerospace structural material with advantages of high specific strength,high creep resistance,good corrosion and oxidation resistance,etc.However,the high forming temperature of titanium alloy raises the requirements of alloy processing,which limits the application of titanium alloy.Thermo-hydrogen processing of titanium alloy can improve its microstructure and enhance its superplasticity,so as to optimize the superplastic deformation condition and improve the hot working property.In this paper,the effect law and mechanism of thermohydrogen processing on the superplastic characteristics of ti-55 high temperature alloy were studied by means of mechanical tests and microstructure analysis.Based on the strain rate,flow stress and deformation temperature in high temperature tensile tests,deformation constitutive equations of hydrogenated Ti-55 alloy were established by linear fitting method.The hyperbolic sine constitutive equation has the best applicability to the alloy.The 0.1wt%hydrogenated alloy has the highest strain rate sensitivity index.Hydrogenation decreases the thermal deformation activity energy of Ti-55alloy.The observation of the microstructure of superplastic deformed alloy with different hydrogen contents shows that 0.1wt%hydrogenation can stabilize the distribution of?phase,inhibit the growth of voids and promote dislocation initiation to reduce dislocation aggregation and entanglement.Furthermore,the occurrence of discontinuous dynamic recrystallization is promoted to eliminate the dislocation in the matrix,which improves the coordination deformation ability of the alloy.The high temperature deformation behavior of coarse grain Ti-55 alloy was investigated.The results shows that coarsening of the grains significantly reduces the superplasticity of the alloy.0.1wt%hydrogenation of the coarse grain alloy increases the elongation by 41.3%and reduced the peak stress by 40.6%at 875?.The grains of 0.1wt%hydrogenated alloy after deformation are refined and remain equiaxed with dual characteristics of dynamic recovery and recrystallization.The thermo-hydrogen processed ultra-fine grain Ti-55 alloy was studied.Thermo-hydrogen processing refines the microstructure significantly.The dehydrogenated 0.4wt%H alloy with evenly distributed grains is refined from 5~10?m to 500~1000nm.It has the highest elongation of 723.8%at 800?,which is increased by 57.3%compared with the as-received alloy.The mechanism of the grain refinement is the continuous transformationof?_H??+?+H?andTiH_x??+?+H?during dehydrogenation.It promotes the continuous nucleation and growth of?grain and?grain.