Study On The Microstructures And Mechanical Behavior Of Ti-47Al-2Nb-2Cr-(Er,C,Mn) Directionally Solidified By Cold Crucible

Intermetallic titanium aluminide alloys consisting of?-Ti Al?ordered face-centered tetragonal L1₀ structure?and?₂-Ti₃Al(ordered hexagonal D0₁₉structure)are a new type of high-temperature structural materials and considered as candidate materials for replacement of Ni-based alloys at 650oC⁸50oC.However,room temperature brittleness and lower fracture toughness of Ti Al-based alloys remaind to be improved effectively.Directional solidification and alloying are proven to be two effective methods.Cold crucible directional solidification technology,which is applied to prepare the Ti Al-based alloys,can avoid the direct contact between Ti Al melt and the crucible because of electromagnetic confinement effect,thus leading to the decrease of the melt contamination.Moreover,previous studies reveal that alloying can influence solidification path and modify the final microstructure,thus leading to the change in the deformation mechanism and the improvement of performances.The present paper is focused on the Ti-47Al-2Nb-2Cr-?Er,C,Mn?alloys directionally solidified by cold crucible,and studies systemically influcen rules of alloying elements on inter-lamellar spacing,lamellar orientation,primary phase and mechanical properties,and illustrates the relationship among alloying elements,microstructure and mechanical behaviors.Research results on the solidified structure of directionally solidified Ti-47Al-2Nb-2Cr-?Er,C,Mn?alloys by cold crucible reveal that the macrostructures of the alloys containing 0.2 at.%Er,?0.2,0.5?at.%C and?1,3,5?at.%Mn are composed of continuous columnar grains.Addtions of Er and C can refine the size of columnar size,and adding Mn shows no significant influence on columnar grains.However,the alloy containing 0.8 at.%Er is composed of exquiaxed grains because the secondary dendrite can be easily broken off from the primar dendrite,thus causing the columnary grians to exqyuaxed grains transformation.During directional solidification process,alloy elements Er can scavenge the oxygen dissolved in the Ti Al matrix and form Er₂O₃ particles.It reduce the dissolved oxygen from 960 ppm to 400 ppm,which can play a role in purifing the Ti Al melt.Adding C can significantly reduce the volme fraction of B2 phase and Ti₂Al C particles with a hexagonal crystal structure are formed in case of 0.5 at.%C.Mn additions tend to segregate in the dendrite and the volume fraction of B2 phase increase with the increase in the Mn content.Investigations on inter-lamellar spacing show that incorporations of alloying elements Er and C can refirne the lamellar structure and the inter-lamellar spacing decreases with the increasing content.In contrast,the inter-lamellar spacing of the alloy containing alloying element Mn is almost no change although the contern of alloying element increases from 1 at.%to 5 at.%.Increasing the nucleation rate of?phase is the lamellar refinement mechanism of the Er-containing Ti Al alloys.For the C-containing Ti Al alloys,the lamellar refinement mechanism is increasing the nucleation rate and inhibiting the growth of?phase.Moreover,the inter-lamellar spacing of the Er-,C-and Mn-containing Ti Al decreases with the increase of the growth velocity.After linear regression anblysis,the relationship between inter-lamellar spacing and growth velocity can be described as=6)(1,and the reduced amplitude of the inter-lamellar spacing with the increasing growth velocity is TNC-0.2C>TNC-1Mn>TNC-0.2Er.Investigations on the primary phase of the directionally Ti Al alloys show that the final lamellar orientation is distributed mainly in the range of 0o-30oand 30o-60oas?phsse is the sole primary phase.While a certain amount of 60o-90olamellae are involved in the final microstructure as the primary phase changes from sole?phsse to the mixed phase of?phsse and?phase.The primary phase of the alloy containing 0.2 at.%Er,0.2 at.%C and?1,3,5?at.%Mn is?phsse,but the primary phase of the alloy containing 0.5 at.%C changes from sole?phsse to mixed phase of?phsse and?phase.Growth velocity also affects the primary phase.Increasing in the growth velocity can make the primary phases change from sole?phsse to the mixed phase of?phsse and?phase.The critical growth velocity that make the primary phase change depends on alloying element.The critical growth velocity is lower for the alloy containing more alloying element C,while the critical growth velocity is higher for the alloy containing more alloying element Mn.In order to refine the lamellae and remain the lamellar orientation,the directionally solidified Ti Al alloys containing Er and C should be prepared at a lower growth velocity,while the Mn-containing Ti Al alloys should be prepared at a higher growth velocity.Research results on the mechanical properties of directionally solidified Ti-47Al-2Nb-2Cr-?Er,C,Mn?alloys by cold crucible reveal that adding alloying elements Er,C and Mn can significantly improve the strength and ducitly.The effect of strength improvement is C>Er>Mn,and the effect of elongation improvement is Mn>Er>C.The compression yield strength of the alloy containing 0.5 at.%is higher than that of the alloy containing 0.2 at.%C,while the ultimate strength of the former is lower than that of the latter.Both tensile strength and elongation of the alloy containing 0.5 at.%C decreases compared with those of the alloy containg 0.2 at.%C.The compression and tensile performaces,and fracture toughness increases to the maximum and then decreased with the increase of Mn content.The alloy with 1 at.%Mn addiitons shows the best comprehensive performances.Mechanical behaviors analysis of directionally solidified Ti-47Al-2Nb-2Cr-?Er,C,Mn?alloys indicates that the deformation mechanism of the Er-and C-containing is governed by dislocation slip from the intital deforamton stage until fracture.However,dislocation slip dominates the intial deformation stage of Mn-containing alloy and twin mechanism is activated as the deformation proceeds,then the deformation mechanism of the Mn-containing alloy is twinning and slipping conbinated action.Dislocation loop released from the semi-coherent interface is dislocation multiplication mechanism.