Microstructure And Fatigue Properties Of Cold Crucible Directional Solidified Tial Alloy

The lightweighting of aero engines has been the focus of research in various countries.TiAl-based alloys have received extensive attention due to their low density,high strength in high temperature,is the potential applications for engine blades.The engine blade is subjected to high frequency and high cycle cyclic load.In order to improve the reliability of the turbine blade and promote the engineering application of the TiAl alloy blade,it is of great significance to study the fatigue performance of the TiAl alloy.In this paper,Ti-46Al,Ti-44Al-5Nb-3Cr-1.5Zr,Ti-47Al-2Cr-2Nb-0.2Er,high Nb-TiAl titanium alloy ingots were prepared by cold crucible directional solidification furnace.Macroscopic organization and microstructure were analyzed.The three-point bending fatigue performance of the four alloys was tested,and the stress-controlled tensile fatigue and damage bending fatigue tests were selected.Analyze the relationship between fatigue performance and organization.The experimental results show that the surface quality of ingots of Ti-46Al,Ti-47Al-2Cr-2Nb-0.2Er and high Nb-TiAl alloys is good,and the parallelism and continuity of columnar crystals are good.The oriented structure of Ti-44Al-5Nb-3Cr-1.5Zr alloy is equiaxed,the surface quality is poor,and the mechanical properties are not good.The original ingots of the four alloys have improved bending properties after cold crucible directional solidification treatment,and the bending strength,deflection and fatigue limit are significantly improved.The bending fatigue limits of directional solidification microstructures of Ti-46Al,Ti-47Al-2Cr-2Nb-0.2Er and high Nb-TiAl are 390MPa,414MPa and 470MPa,respectively.The results show that high Nb-TiAl has the best resistance to bending fatigue.Ti-46Al alloy is a typical full-lamellar microstructure with good plasticity.The solid solution of Cr and Nb in Ti-47Al-2Cr-2Nb-0.2Er alloy strengthens the matrix of the alloy and improves the mechanical properties of the alloy.The addition of the Er element refines the interlamellar spacing,which improves the mechanical properties of the alloy to a certain extent,but the interface between the Er₂O₃ phase and the matrix is weak,and the phase is generally loose clusters,which often induces the initiation of fatigue cracks.And it is passed as a weak point on the fatigue crack propagation path.The volume fraction of B2 phase of Ti-44Al-5Nb-3Cr-1.5Zr alloy is large,and the dense band structure formed by?single phase separates the continuity of?/?₂ layer,which makes the fatigue fracture appear stepped in macroscopic shape.The hard and brittle B2 phase reduces the ability of the alloy to co-deform.The right angle produces a large stress concentration and induces fatigue cracks.Microcracks first appear in the cyclic loading process at the?single phase around the B2 phase.The Nb element in the high Nb-TiAl alloy has a significant solid solution strengthening effect,and the fatigue fracture is dominated by the fracture of the through-layer,and the number of fractures along the sheet is small.The bending fatigue limit of the"U"shaped notch is about 0.45 times the value of the smooth sample,and the fatigue stress concentration factor K_f=2.22.The tensile fatigue limit of high Nb-TiAl alloy is 215 MPa.The sample often breaks at the right angle of intersection of the wide and thick stress or the transitional fillet.The fatigue crack starts at the interface of the layer or along the layer,and the crack propagates to At critical dimensions,brittle cleavage fracture occurs.