Directionally Solidified Microstructure And Mechanical Properties Of Microalloyed High-Nb TiAl Based Alloy

TiAl based alloys have very broad application prospects in the aerospace field,due to their low density,high specific strength and modulus,great high temperature and creep resistance.Alloying and directional solidification are important means to improve the properties of TiAl alloys by adjusting the microstructure.The third-generation TiAl alloy represented by high-Nb TiAl alloy has added a large number ofβ-stabilizing elements,and its high-temperature performance has been significantly improved.But it will introduce a large amount of hard and brittle blocky B2 phase into the TiAl alloy matrix,seriously deteriorating its mechanical properties at room temperature.Large size and pollution-free directionally solidified TiAl based alloy ingot can be prepared by using cold crucible directional solidification technology to achieve microstructure control and performance improvement.In this paper,the directionally solidified microalloyed high-Nb TiAl alloy withα₂/γlamellar structure as the matrix,fine and dispersed B2/γcoupling structure and TiB strengthening phase was prepared by using the technology of cold crucible directional solidification combined with microalloying,thereby greatly improving its mechanical properties at room and high temperature.The microstructure characteristics and evolution mechanism of directional solidification,fatigue and creep properties,crack initiation propagation,deformation and fracture behavior and the coordinated deformation mechanism of directionally solidified high-Nb TiAl alloys were studied.With the addition of Nb element,the dendrite structure of Ti46AlxNb alloy is more developed,and the degree of segregation is intensified.When the Nb content exceeds 8 at.%,the orientations of phase are more uniform,and the tendency of hot cracking is more obvious.The high Nb element（5～9 at.%）addition can significantly improve the room temperature compressive property of Ti46Al alloy.Since the Ti46Al7Nb alloy has the best mechanical properties,this composition was chosen as the master alloy for subsequent studies.The columnar grains of directionally solidified Ti46Al7Nb alloy are arranged in a"herringbone"shape along the heat flow direction,which is a near-lamellar microstructure withβsegregation,and its segregation degree is significantly lower than that of the as-cast state.The angle between the lamellar arrangement direction and the directionally solidified direction is 45°.The room temperature three-point bending fatigue limits of as-cast and directionally solidified alloys are 408 MPa and 468 MPa,respectively.The fracture morphology shows that the fracture mode of the as-cast alloy is brittle cleavage fracture,while the directionally solidified specimen is the trans-lamellar fracture with plastic characteristics.The defects formed on the surface of notched fatigue specimens are mostly cleavage microcracks at the initial stage of fatigue crack formation.The corrugated secondary cracks formed on both sides of the main cracks of smooth specimens are beneficial to the release of stress concentration and increase the fatigue limit.The ultimate tensile strength of directionally solidified Ti46Al7Nb alloy at 800℃is 475MPa,96%higher than that of the Ti46Al alloy.The high temperature fracture is mainly trans-lamellar fracture.There are pinned dislocation structures at the lamellar boundary,and subgrain boundaries are formed during high temperature tensile deformation to further hinder the dislocation movement.The microstructure and room temperature tensile properties of high-Nb TiAl alloys based on Ti46Al7Nb alloy were compared.Three high-Nb TiAl based alloys Ti46Al7Nb0.4W0.6Cr,Ti46Al7Nb0.4W0.6Cr2V and Ti46Al7Nb0.4W0.6Cr0.1B with good room temperature tensile properties were selected for directional solidification experiments.The microstructures of three directionally solidified high-Nb TiAl alloys are all composed ofα₂/γlamellar matrix and strip-like B2/γ coupling phase distributed among them.The increase of the pulling rate will cause the decrease of the average length and width of the columnar grains and the decrease of the spacing between the stripe-like B2/γcoupling phases.In addition,the V element plays a role in refining the columnar crystal size and the stripe-like B2/γphase spacing.The B element can significantly refine the columnar grain width of the directionally solidified high-Nb TiAl alloy,and can form a short rod-shaped TiB phase with a B27 structure and enriched with Nb and W elements.As the pulling rate increases from 3.3μm/s to 16.7μm/s,the directionally solidified Ti46Al7Nb0.4W0.6Cr0.1B alloy undergoes columnar to equiaxed transition（CET）,and the average grain width decreases from 1378±204μm to 69±8μm.The increase of the cooling rates and the satisfaction of heterogeneous nucleation conditions are the main reasons for the occurrence of CET in this alloy.The tensile properties of the micoralloyed Ti46Al7Nb0.4W0.6Cr0.1B alloy at 700～900℃are higher than those of the other two alloys.The ultimate tensile strength at 800℃is 647 MPa,which increased by 23.5%and 19.4%,respectively,compared with Ti46Al7Nb0.4W0.6Cr and Ti46Al7Nb0.4W0.6Cr2V alloy.Therefore,this microalloyed alloy is finally selected for high temperature creep and fatigue performance test.Under the condition of 760℃/275MPa,the Ti46Al7Nb0.4W0.6Cr0.1B alloy has a creep duration of 408 hours without fracture.The creep time is 116%higher than that of Ti46Al7Nb alloy,and the minimum creep rate is one order of magnitude lower.The high temperature creep deformation of Ti46Al7Nb0.4W0.6Cr0.1B alloy is controlled by dislocation movement and twinning deformation.The twin intersections and dispersed TiB phases can restrict the dislocation movement,so as to improve the creep resistance.The directionally solidified microalloyed Ti46Al7Nb0.4W0.6Cr0.1B alloy has excellent rotating bending fatigue performance at room temperature with a fatigue limit of 400 MPa.The fatigue fracture morphology shows that the fatigue fracture of the alloy includes the flat crack origin zone caused by the defect and the brittle fracture zone of the trans-lamellar fracture.The dispersed TiB phases serve to pin the lamellar structure,preventing fatigue crack propagation and improving the fatigue properties of the alloy.The basic mechanism of rotating bending fatigue deformation is the dislocation slipping with the Burgers vector<110>in the B2 phase and the superlattice dislocation with the Burgers vector[01（?）]and twinning in theγphase.The TiB phase in the alloy can hinder the dislocation movement to strengthen the alloy matrix.The nano-hardness of the B2phase andγphase of the alloy are 7.0 GPa and 6.1 GPa,respectively.The addition of trace Cr element can reduce the hardness of the B2 phase without producing a large amount of the B2 phase,and reduce the hardness difference between the B2 phase andγphase to improve the coordinated deformation ability between them.The B2 phase in the alloy has good deformability at room temperature.The deformation twins in theγphase can induce the dislocation formation in the B2 phase by changing the atomic arrangement at the phase interface,so as to release the stress concentration at the interface.The coordinated deformation betweenγphase and B2 phase can promote continuous and uniform deformation of the alloy to prevent premature failure.