Microstructure Evolution And Mechanical Properties Of β-Solidifying TiAl Alloy Processed By Hot Extrusion

β-solidifying TiAl alloy has the potential of maintaining high specific strength and modulus,excellent creep resistance and fracture toughness,good flame retardant at the temperature of 600-850℃.It’s expected to be widely used.Nowadays,the LPT blades made by forging TNM alloy have been applied in the GTF engine.Canned extrusion is more suitable for the deformation of the TNM alloy than the forging,which will further improve the combination properties.Although some progress has been made in the field of thermal-deformation of TNM in foreign countries,due to the strict confidentiality of deformation parameters,the domestic research must be further explored.The traditional extrusion will cause the anisotropy of extrusion products,high processing cost and low material utilization rate.Therefore,this article employs canned hot-extrusion process to get TNM alloy square bar,its microstructure and tensile properties is analyzed,and studied annealing temperature,time and cooling rate on microstructure and tensile properties of square bar,analyzing creep rupture properties at 750℃ and the characteristics of grain boundary,get the following main conclusions:TNM alloy square bar was prepared by canned hot-extrusion deformation,deformed microstructure is near-lamellar structure,mainly composed of lamellar,γ andβ0 phase.The lamellar orientation tends to parallel extrusion direction.The γ and β0 phase are distributed parallel to the extrusion direction along the grain boundary.There is no obvious difference on microstructure between the center and the edge partition.After extrusion,y phase on the grain boundary has 3 morphology:granular,massive and long strip.During aging,a large number of lens-shape γ phases were precipitated in the β0 phase.During extrusion,ω0 phase precipitation from β0 phase,following relationship:[111]β0//[0001]ω0 {110}β0//{2110}ω0.This is associated with high temperature stress induced phase transformation and segregation of Nb elements.It can’t obtain equilibrium microstructure without ω0 phase by heat treatment.After extrusion,the tensile strength of TNM alloy at room temperature was increased to over 1000 MPa.The fracture at room temperature were result from the small plane.The yield strength is 400～500 MPa at 800℃,showing obvious plasticity.The failure was caused by crack initiation on grain boundary at 800℃.After aging,the tensile strength at room temperature decreased slightly,while the tensile strength at 800℃increased,because the aging treatment effectively released the grain boundary stress,and lens-shape y phase hinder the dislocation moving.There was no significant difference in the tensile properties of different positions,achieving good microstructure-performance consistency.The grain size increases with the increase of annealing temperature and time,as improving cooling rate decreases.When rapid cooling,heat treatment with different annealing temperature and time,it will appear cellular reaction:(α2+γ)lamellar→(α2+β0+γ)cellular within the grain or grain boundary.The discontinuous microstructure is in equilibrium with the total interface area.With the high annealing temperature,the discontinuous microstructure will no longer occur when the interface is greatly increased due to the precipitation of lens-shape γ phase.Neither reaction occurs at a slower cooling rate.The residual lamellar decreases with the increase of annealing temperature,and can be eliminated in the(β+α)phase region.The mean lamellar spacing increases sharply with the decrease of cooling rate.With annealing temperature increasing,the volume fraction of β0 phase in the(α+β/β0+γ)phase region decreases,which is related to precipitation loss,and in the(α+β)phase region increased which is related to the high proportion of equilibrium phase.The decrease of annealing time and cooling rate is related to precipitation loss.The heat treatment conditions with the best comprehensive tensile properties are as follows:the annealing temperature is near Tγsolv,the annealing time is more than 1h,the cooling rate is between FC and AC,and the two-step heat treatment method is adopted.The creep rupture properties at 750℃ of extruded bar is poor.heat-treatment can obviously improve creep rupture properties at 750℃ of extruded bar due to the reduce the content.Different stabilization temperatures have an effect on the strain resistance of the extruded bar,which is mainly related to the morphology of ω0 phase.