Study on Microstructure and Key High Temperature Mechanical Properties of Ti65 Alloy

Author:Yue Ke

Supervisor:yang rui wang qing jiang liu jian rong


Degree Year:2019





Ti65 alloy is a near α high-temperature titanium alloy with a nominal composition of Ti-5.9Al-4.0Sn-3.5Zr-0.3Mo-0.4Si-0.3Nb-2.0Ta-1.0W-0.05C based on Ti60 alloy.The microstructure of Ti65 is typically duplex,containing 5~25%of the primary α phase and α2 phase dispersed in the matrix and the silicide precipitated at the α/β phase boundaries,after the hot-working process and heat treatment in the two-phase region.This alloy has good performance matching between strength/plasticity,creep durability and thermal stability,and excellent comprehensive mechanical properties.As an alternative material for high-temperature components of aerospace engines,the study of high-temperature mechanical properties of Ti65 alloy in the range of 600-650℃ is particularly important.The microstructures of the alloys in different heat treatment conditions were observed.The morphology and composition of the phases and precipitates(silicide and α2(Ti3Al)phase)after solution and aging treatment were characterized by TEM and APT methods.The experimental results show that the micropores observed on the surface of the sample after heat treatment are formed by the corrosion-induced shedding of silicides.The silicides in the alloy are all distributed along the α/β phase boundaries,and the stoichiometric ratio is(Ti,Zr)6(Si,Sn)3,α2 phase size is about 1.4 nm and Si element is enriched in α2 phase;Zr exhibits the effect of β-phase stabilization in the near a titanium alloy,and the enrichment degree of alloying elemenst in β phase increases in the order of Zr<Nb<Ta<Mo<W.The effect of cooling rate after solution treatment on the tensile behavior of Ti65 alloy from room temperature(RT)to 650 ℃ and the microscopic deformation mechanisms were studied.The experimental results show that the strength and ductility of the water-quenched(WQ)microstructure are significantly higher than that of the air-cooled(AC1)microstructure as the test temperature increases.Under the tensile condition of 650℃,the WQ microstructure elongation and the area reduction are 55%and 89%higher than that of the AC1 microstructure,respectively.The yielding/ultimate tensile strength of the WQ microstructure is 613 MPa and 743 MPa,respectively,which is 8%higher than AC1 microstructure.Under RT tension conditions,the plasticity of AC1 microstructure is higher than that of WQ microstructure,T1 deformation twinning was found in the microstructure after tensile test.The difference in the tensile behavior is mainly attributed to the distinction of microstructures induced by the cooling rate.The creep deformation behavior and microscopic deformation mechanisms of Ti65 alloy in the range of 600-650℃ and 120-160 MPa were studied.The experimental results show that the primary creep deformation mechanism is dominated by the process of climbing-controlled dislocations crossing the α2 phases;the creep mechanism in the steady-state creep stage is dominated by the process of diffusion-controlled dislocation climbing at the α/β interfaces,and the stress index of steady-state creep stage varies from 5 to 7.The hindering of dislocation motions by α2 phases is the dominating process to strengthen the high-temperature creep resistance of Ti65 alloy during the primary creep stage.The silicides,cooperating with the α/βphase boundaries,impede the dislocation motions and restrict the grain boundary slip(GBS),which is the dominating strengthening mechanism during the steady-state creep stage.Stacking faults occur under the creep conditions of 630-650℃/160 MPa.The existence of stacking faults does not affect the creep deformation mechanism but may interact with dislocations.The creep rupture behavior and microscopic deformation mechanisms of Ti65 alloy in the range of 600-650℃ and 250-370 MPa were studied.The results of Monkman-Grant relationship parameter fittings are α=0.936,C0=0.0939.The creep fracture mode is characterized by the ductile intergranular fracture mode.The creep fracture cavities are preferentially distributed at the colony/primary a grain interfaces,the interfaces of adjacent colonies and the neighboring interfaces of primary a grains.And the cavity distribution positions have no direct relationship with the adjacent grain orientations.Under the creep rupture conditions,the interaction of dislocations with the α2 phase is in the manner of cutting through α2 phase,and the duration of primary creep stage is greatly shortened.The cavity nucleation mechanism of creep rupture is controlled by the pile-up and vacancy-diffusion of dislocations at the grain/phase boundaries.Continuous cavity nucleation occurs during the whole creep deformation process and the growth and coalescence of cavity process is controlled by the joint effect of vacancy-diffusion and plastic deformation.