Microstructure And Mechanical Properties Of Directionally Solidified Ti-(43-48)Al-2Cr-2Nb Alloys

With high specific strength and moduli,good oxidation resistance and creep resistance,TiAl-based alloys is one of promising high-temperature structural materials in the aviation and aerospace field.However,the lower room temperature plastic and fracture toughness limit its application.TiAl-based alloy exhibits a significant anisotropy of mechanical properties.Through lamellar orientation control method,a good combination of strength,plastic and toughness can be obtained.Therefore,it is a valuable research subject that how to make use of the texture to give full play to the advantages of TiAl-based alloy performance.However,the melt will be polluted by ceramic crucibles in the directionally solidified process,oxygen content of TiAl-based alloy increased.The technique of cold crucible directional solidification(CCDS)has been developed to prepare directionally solidified(DS)TiAl alloys ingots with low contamination and well-aligned ?2(Ti3Al)/?(TiAl)lamellae.For development with high strength and toughness of TiAl alloy,this technology has important theoretical and practical significance.In this paper,directional solidification experiments were carried out on Ti-(43-48)Al-2Cr-2Nb quaternary alloys using cold crucible directional solidification technique(CCDS).The effects of Al element content and drawing rate on the surface quality,macrostructure and microstructure are investigated systematically.Fracture toughness and crack propagation behavior are studied.To analyze the relationship between mechanical properties and microstructure,hardness,compressive and tensile testing are carried out.It is found that the solidification shell of DS ingot gets thickened,solidification shrinkage of shell takes place much faster,and shell remelting phenomenon is serious with increased drawing rate.It will lead to the uneven distribution of the stress and deformation in the solidification shell,which could produce more surface defects.However,slowly drawing rate could reduce the solidification shell thickness and gradually release stress,which can effectively improve casting surface quality.The research on the relationship of composition,drawing rate and the macrostructure,found that DS ingot with the continuous columnar crystals could be obtained under slowly drawing rate.For DS Ti-43Al-2Cr-2Nb and Ti-45Al-2Cr-2Nb alloys under faster drawing rate,the continuity of coarse columnar crystal is interrupted,and even columnar to equiaxed transition occurs,because of nucleation of ? phase increasing on ? phase.For DS Ti-48Al-2Cr-2Nb with high Al content,and continuous columnar crystal are obtained by t? phase solidification process under faster drawing rate.The microstructure of DS Ti-(43-48)Al-2Cr-2Nb alloys is composed of well-aligned ?2/? lamellar structures,B2 phase and blocky ? phase.Effect of Al content and drawing rate on the microstructure can be concluded as follow:(1)With increase of Al content,volume fraction of ?2 and B2 phase decreases increases,but ? phase volume fraction increasing and ?2/? lamellae thickening.(2)Interlamellar spacing()decreases with the increasing of growth rate according to the relationship = .(1-0.5,but the volume fraction of B2(1)2)exhibits an exponential decay function of the reciprocal of growth rate(1?(1).(3)The solidification path influence the orientation of lamellae.The lamellae of DS Ti-43Al-2Cr-2Nb and Ti-45Al-2Cr-2Nb alloy are aligned at an angle to the growth direction in the range of 0° to 45° at growth rates varying from 0.2 to 1.5 mm/min.For DS Ti-48Al-2Cr-2Nb alloy,the number of 0°-orientation and 45°-orientation lamellae decreased,but that of 90°-orientation lamellae increasing.Results of tensile tests show both tensile strength and elongation of DS Ti-43Al-2Cr-2Nb and Ti-45Al-2Cr-2Nb alloy improved with decreasing volume fraction of the B2 phase,because they are usually employed to act as cracking sources during the tension process.Tensile properties of DS Ti-48Al-2Cr-2Nb alloy mainly depend on the number of 90°-orientation lamellae of specimen.Clearly the presence of 90°-orientation lamellae degrades crack growth resistance under tensile stress.Thus,the specimens without 90°-orientation lamellae have higher both tensile strength and elongation.Conversely,the specimens consisted of 90°-orientation lamellae is easy to fracture failure in elastic deformation stageThe fracture toughness of DS Ti-(43-48)Al-2Cr-2Nb alloys was investigated by single edge notched beam three-point bending test(SENB).The anisotropic fracture behavior of DS alloys is revealed.The fracture toughness is lowest and inter-lamellar crack propagation is very easy,when the crack is parallel to the lamellar interfaces.Conversely,trans-lamellar crack growth through the lamellar interfaces resulted in much higher toughness.B2 phase has greatly impacted on the fracture toughness.When the lamellae are perpendicular to the loading stress,the increasing of B2 phase leads to a lower fracture toughness value.On the contrary,B2 phase becomes barriers for crack propagations and increases fracture toughness value when crack are parallel to the lamellae.Thus,the fracture toughness of DS Ti-43Al-2Cr-2Nb and Ti-45Al-2Cr-2Nb alloy could improve by decreasing the B2 phase volume fraction,and that of DS Ti-485Al-2Cr-2Nb alloy could increases by decreasing 90°-orientation lamellae.