Effect Of Thermo-Mechanical Processing On Microstructures And Tensile Properties Of Ti55531 Alloy

Owing to their low density,high strength and toughness,near-β titanium alloys have attracted considerable attention in the fabrication of load-bearing components,such as landing gears and wing connectors.In the present paper,the microstructure evolution and mechanical properties of Ti55531,a typical near-β titanium alloy,under the thermo-mechanical effect have been investigated.Conventional rapid forging and isothermal compression experiments have been conducted under different conditions.Microstructural dependence on deformation parameters has been analyzed and the effect on mechanical properties has been corresponded for Ti55531 alloy,which could provide the theoretical and practical reference for the high-temperature deformation and heat treatment of Ti55531 alloy.Typical recovery texture,deflated β grains,has been observed after rapid forging of Ti55531 alloy.However,equiaxed grains have been constructed at the junctions as the consequence of sub-recrystallization during air-cooling after forging.Rapid plastic flow from deformation has fragmentized β grain boundaries and has significantly affected the distribution and morphology of grain-boundary(GB)α phase during subsequent solution and aging treatment.Complete β grain boundaries have been reserved after isothermal compression of Ti55531 alloy,which were characterized by continuous GB α lamellae.In addition,the width of GB α lamellae significantly depended on deformation temperature and strain-rate.With temperature elevated,obvious decrease of primary α content occurred and more complete the grain boundaries and GB α phase became.Recrystallized microstructures have been reserved under high rates and with deformation degree raised,deflated characteristic could be identified clearly in βmatrix and GB α lamellae became thinner.The strength of near-β titanium alloys has been significantly determined by β phase in near-β titanium alloys.The stress contribution of β boundaries existence has been calculated less than 15% and this fraction would decrease with temperature elevated.The matrix strength could also been promoted by increasing α phase content and refining α precipitation.Hard and brittle ω phase could been precipitated under low-temperature aging treatment,which could seriously weaken the plasticity of Ti55531 alloy.Low-temperature deformation has been identified with inhibitory effect on ωprecipitation,which could improve matrix plasticity.The strength could decrease and plasticity could be promoted with solution temperature decreased and aging temperature elevated.With deformation temperature elevated from 805 to 820℃,the strength of Ti55531 alloy decreased and plasticity increased.The elongation and area reduction could be maintained at 6% and 17 at the deformation temperature ranging from 820 to 845 ℃ and the improve effect on plasticity became remarkable with raising temperature.During deformation with strain rate of 0.01s-1,the strength maximum and plasticity minimum has been reached.Notable plasticity could be obtained at higher rate of 0.1s-1 with lower strength.Moderate strength and plasticity could be realized under lower rate of 0.001s-1.There existed a strength increase with deformation raised from 20% to 60%,however,obvious stress decrease occurred under 80% deformation.In addition,the plasticity has been improved with larger deformation degree from 40% to 80%.