Microstructural Adjustment And Mechanical Properties Of Hot Extruded High Nb-containing γ-TiAl Alloys

High quality hot extrusion bar of y-TiAl alloy is the significant parent materials for hot die forging of aerospace engine blades.High Nb-containing γ-TiAl alloys are the highly promising for lightweight,high-temperature materials,mainly due to their low density,more excellent strength,creep resistance and anti-oxidation properties at elevated temperature.However,high Nb-containing y-TiAl alloys is limited primarily by less organization uniformity for serious segregation,low ambient-temperature ductility and poor hot-workability.In this study,we focused on the following two high Nb-containing y-TiAl alloys,the solidification path through peritectic reaction alloy Ti-49Al-5Nb and the without peritectic reaction alloy Ti-44A16Nb(at.%);their microstructure evolution,mechanical properties and the thermal stabilities of microstructure in fully lamellar were investigated systematically.A high-quality hot extrusion bar of high Nb-containing y-TiAl alloys were successfully produced through the annealing pretreatment and thermophysical simulation of as-cast microstructure.The as-extruded microstructure of Ti-48Al-6Nb alloy was constituted of coarse y grains belt and residual α2/γ lamellar structures.Ti-44Al-6Nb has good uniformity in extrusion,consisting of finely recrystallized fine equiaxed γ grains and elongated residual sheets(α2/γ).The two types of sorghum y-TiAl alloy extruded rods are fully annealed in the α+γ dual phase region to further decompose and recover the residual lamellar structure,and static recrystallization of γ phase grains and γ→α occur.Transformations have improved organizational uniformity.It is found that the thermal deformation and annealed structure of the peritectic reaction of Ti-48Al-6Nb alloy have histological inheritance,and the interdendritic region of the original A1 solidification segregation evolves into a streamlined γ phase coarse grain region.In the two-phase zone hot extrusion and forging composite processing,the uniform double-axis crystal deformation structure of the sorghum γ-TiAl alloy can be obtained.The forged microstructure consists entirely of dynamically recrystallized γ grains and decomposed spheroidized α2 grains.After annealing treatment in single α phase field,a fully lamellar(FL)micro structure of high Nb-containing y-TiAl alloys were obtained,respectively.The extruded and forged Ti48Al-6Nb alloy was annealed at 1460℃/10min/AC,and the Ti-44Al-6Nb alloy was heat treated by a single α phase region at 1380℃/5min/FC.Full sheet organization of the shaft.The fullsheet structure of the two types of sorghum y-TiAl alloys has excellent tensile strength.The tensile strength of Ti-44Al-6Nb alloy is up to 703 MPa and the elongation at break of 23.80%at 800℃..Microstructure stabilities of fully lamellar high Nb-containing γ-TiAl alloys were investigated.The microstructure of Ti-48Al-6Nb alloy under 800℃/200h static heat exposure is seriously destabilized,which reduces the strength of the alloy.The instability mode of the structure is the edge of the lamellar group and the γ phase crystal grains are generated in the crystal.The slice width is obvious.The Ti-44Al-6Nb alloy exhibits excellent stability under static thermal exposure conditions of 800℃C/850℃-200h.Even under the long-term conditions of 800℃/200MPa/50h high temperature,the structure of the whole sheet is basically stable,and only a small amount of γ phase grains are decomposed and precipitated in the edge region of the lamellar crystal group at 850℃/200MPa.Under the persistent condition of 850℃/200MPa,the instability of the structure is obvious.The inner part of the sheet structure and the periphery of the crystal group have significant lamellar decomposition and a large amount of precipitation of y phase.At the junction of the crystal lattice,cracks appear perpendicular to the direction of the force.The sample was broken for a long time of 46 h.