Microstructure And Mechanical Properties Of Directionally Solidified TiAl-based Alloy Using Yttria Moulds

As a typical ordered intermetallic,TiAl-based alloys have become a new generation of aerospace structural material due to their low density,high specific strength and excellent heat resistance.However,the large-scale practical application of these alloys still requires improvement in the properties,especially room ductility and plasticity.The directional solidification process with ceramic moulds is one of the effective methods which can not only control microstructure and improve performance,but also satisfy the requirements for product with complex components.However,there are some problems in the application of the process,such as serious mould contamination on alloys and single shape in samples.In order to decrease mould contamination,a novel directional solidification process is developed for TiAl-based alloys,where an Y₂O₃ mould was incorporated into electromagnetic cold crucible?EMCC?heating technology.In this paper,two kinds of heating modes,including the most frequently used method:inductive-graphite indirect heating,and EMCC induction heating,were used to prepare directionally solidified?DS?samples with different shapes under varied process parameters.The DS samples prepared by inductive-graphite indirect heating were used as the control group,and the microstructure characteristics of samples prepared by the novel process were analyzed and determined in view of electromagnetic and temperature characteristics.The effects of the process parameters and the shape change in cross-section of mould on the microstructure and properties were systematically studied,which is of great significance to the optimization of the subsequent process and the practical application of TiAl-based alloys.Through the analysis about the directional solidification processes and the microstructure evolution of samples prepared by two heating modes,it was found that similar to the sample prepared by inductive-graphite indirect heating,the typical microstructure of DS Ti-45Al-2Cr-2Nb alloy prepared by EMCC induction heating consisted of?₂/?lamellae,Y₂O₃ particles and a small amount of B2 phase.This novel process could induce bigger electromagnetic force in the tangential direction,generate a more rapid heating and higher temperature gradient,and decrease the interaction between the mould and melt,compared with the process with inductive-graphite indirect heating.The DS sample prepared by this method was achieved finer columnar crystals and?₂/?lamellae,and lower levels of contamination with regard to Y₂O₃ particles and oxygen.Through the analysis about structures of samples prepared by two heating modes under varied process parameters,it was found that the effect of process parameters on the structures of samples prepared by two heating modes could be concluded as follows:When the heating power was certain,in the drawing rate in range of 0.6¹.2 mm/min,the effects of increase in drawing rate on the structures of samples prepared by two heating modes were basically same.The increase of drawing rate could refine columnar grains and lamellae,reduce Y₂O₃ particles,but have little effect on B2 phase and lamellar orientation.When the drawing rate was certain?1.0 mm/min?,with the heating power in the range of 19²5 kW,only when the heating power increased to a certain value,the stable growth region in samples prepared by inductive-graphite indirect heating could change into columnar crystal.While a larger temperature gradient would be generated in the process with EMCC induction heating even at low heating power,and then continuous columnar in sample could be obtained.The increase of heating power would refine lamellae and increase the proportion of lamellae with small angle.However,when the heating power increasing to a high value,a high melt temperature was generated,and then the interaction between mould and melt would be aggravated,which will affect the solidification transformation process,and eventually lead the appearance of lamellae perpendicular to the growth direction.Through the test and analysis about mechanical properties of the DS samples,it was found that when the heating power was certain,the increase of drawing rate would improve the tensile properties and fracture toughness of DS samples prepared by two heating modes.However,the optimum tensile properties and fracture toughness could be obtained at a suitable heating power when the drawing rate was kept certain.Comparatively,the mechanical properties of the samples prepared by EMCC induction heating were greatly improved.In detail,the tensile strength and fracture toughness had been increased by 30%and 60%,respectively,which could be attributed to thinner lamellae and less of Y₂O₃ particles.Through the analysis of the structures of samples prepared by the direction solidification process with two heating modes and varied shapes moulds with the same cross-section area,it was found that the shape change in cross-section of the mould could affect the flow strength in the directional solidification process,and then would influence the microstructure and properties of sample.On the condition of inductive-graphite indirect heating,compared with cylindrical sample,due to the lower strength of flow generated by the shape factor of the mould,the plate sample had coarser lamellae and more B2 phase,but fewer Y₂O₃ particles.On the condition of EMCC induction heating,the strength and distribution of the applied flow field strongly depended on the cross-section shape of the mould.The calculation results showed that the strength of the flow field in a circular mold was slightly larger than that in the elliptical mould,and larger than that in the plate mould.Therefore,the deflection angle of columnar crystal growth and volume fraction of Y₂O₃ particles decreased gradually,while the width of columnar crystal,lamellar spacing,lamellar angle and volume fraction of B2 phase increased as the mould shape changed from cylindrical to plate.The volume fraction of Y₂O₃ particles in the DS samples prepared by inductive-graphite indirect heating was much,and the Y₂O₃ particles content in the plate sample was much less than that in the cylindrical sample.In this case,the room temperature properties of the samples with different shapes depended on the content of Y₂O₃ particles.Therefore,mechanical properties of the plate sample were better.While the Y₂O₃ particles content of samples prepared by EMCC induction heating and difference in the Y₂O₃ particles content between the samples with different cross-section was much lower.In this case,the mechanical properties of DS samples with varied shapes cross-sections depended on all the microstructure characteristics.Therefore,the elliptical sample had the better mechanical properties combination.