Initial Mushy Zone Evolution And Microstructure Controlling During Directional Solidification In TiAl-based Alloys

TiAl-based alloys are potentially used as high-temperature structural materials with high specific strength,lower density and good creep resistance in the range of 700?-950?.However,the poor ductility and oxidation resistance above 800? restrict TiAlbased alloy widely used in engineering.The microstructures of TiAl-based alloy can be modified by directional solidification(DS)technology,which can improve the mechanical properties such as room temperature ductility and high temperature creep resistance.The propose of this research is to modify microstructures by DS in TiAlbased alloys with different solifification path.Influence of thermal stabilization(TS),growth rate,seeding and solidification conditions on microstructure evolution,microsegregation,solidification path and microstructure characteristics are investigated in this dissertation,and the room temperature and high temperature mechanical properties of DS alloys are evaluated.TiAl-based alloys with different solidification path have different microstructures.The ?-solidified Ti-43Al-3Si alloy is composed of primary Ti5Si3,eutectic Ti5Si3 and ?2+? lamellar structure.The ?-peritectic solidified Ti-47Al(W,Si),the additions of W and Si induce the formation of the B2 phase and Ti5Si3 particles phase.The addition of alloying elements refine the lamellar structure in TiAl alloys.The TS experiments were performed in TiAl-based alloys with different solidification paths.In the ?-solidified Ti-43Al-3Si alloy,the mushy zone is composed of liquid phase,? grain and Ti5Si3 phase.With the increase of the TS time,the temperature gradient region melting(TGZM)occurs,the volume fraction of the liquid phase gradually decreases,and the Ti5Si3 phase is also growing continuously.In the ?-peritectic solidified Ti-47Al-1.0W-0.5Si alloy,the occurrence of the peritectic reaction can be seen in mushy zone.With the increase of the TS time,the liquid phase gradually decreases,and the peritectic transformation is gradually restricted.In Ti-44Al-4Nb-1.5Cr-1.0W-0.5Si alloy,the solid/liquid interface tends to be flat with the increase of the TS time,the distribution of solute elements in the front solid/liquid interface gradually stabilized,and the aggregated silicides gradually decreased.The hexagonal ? phase is primary phase of Ti-47Al-1.0W-0.5Si alloy during directional solidification.The actual solidification path of Ti-47Al-1.0W-0.5Si alloy deviates the equilibrium solidification path.The main reason can be attributed to nucleation undercooling,solute segregation and the contamination of melt during solidification.phase transformation path of Ti-47Al-1.0W-0.5Si alloy determined in this experiment could be described as follows: L ?L + ? ?? + Ti5Si3 ??2 + ?+ Ti5Si3.Both primary dendritic arm spacing(?)and interlamellar spacing(?s)decreased with increasing of the growth rate(V)according to the relationship of ? (1-0.356 and ? (1-0.49,respectively.The S-segregation was occurred since the enrichment of the solute element W in primary ? phase during solidification.The degree of the segregation increased with the increasing of the growth rate(V).lamellar orientation was not always perpendicular to the growth direction because of the growth direction of primary ? dendritic deviated the preferred <0001> direction.Finally,the parallel columnar crystals with the parallel lamellar structure are obtained at selected growth rates(5-20m?m/s)by controlling the continued growth of the non-preferred crystals.A new seeding directional solidification technique of TiAl alloys,which is based on the seeding material with primary ? phase,is tried to put forward.To form ? phase between seeding and directionally solidified TiAl alloys,pure Ti or Nb had tried to use as seeding material to assure the lower concentration of Al element in transitional interface,which is helpful to the form of ? phase.Compared with the traditional seeding technique,the ? seeding technique has advantages of reducing composition transition zone and simplifying process procedure.After DS process,the strip B2/? phase parallel to the growth direction is obtained in Ti-44Al-5Nb-1.5Cr-1.0W-0.5Si alloy,and lamellar orientation is also controlled in the range of 45 ° with growth direction.With the increase of the growth rate,the strip B2 / ? phase thinning,and the continuity increased,the lamellar spacing also decrease gradually.When the high melting point pure Ti and Nb are used as the seeding material,the DS microstructure is similar to the conventional DS microstructure.However,the DS with high-melting seed material eliminates the influence of the microstructure of the as-casting zone in the initial stage,and the angle between the lamellar orientation and the growth direction can be controlled to a smaller extent.The yield strength of Ti-47Al-1W-0.5Si alloy increases in the order: TD < LD < DS.Compared to the as-cast alloy,both yield strength and ultimate compressive strength increase after directional solidification.The compression strain exhibits an opposite trend and decreases in the order: TD > LD > DS,while the magnitude of the decrease is not obvious.(LD: compressive specimens with lamellar orientation be parallel to compressing direction from as-cast alloy.TD: compressive specimens with lamellar orientation be perpendicular to compressing direction from as-cast alloy.DS: compressive specimens with lamellar orientation be parallel to compressing direction from directionally solidified alloy.)Directional solidification significantly improves the mechanical properties of Ti-47Al-1.0W-0.5Si alloy by aligning the lamellar orientation parallel to growth direction and eliminating transverse grain boundaries.The interlamellar spacing decreases and the morphologies of Ti5Si3 phases transform from roundness to strip along the growth direction after directional solidification process,which are also helpful to improve mechanical properties.The fracture mode transforms from inter-granular fracture to inter-lamellar fracture after directional solidification.In DS Ti-44Al-5Nb-1.5Cr-1.0W-0.5Si alloy,the nano-hardness value of B2 / ? composite phase is between 7.5 and 8.5GPa,the nano-hardness of lamellar structure is between 5 and 7.5GPa,with the growth rate increases,the nano-hardness increases slightly.Compared with the as-cast alloy,the dynamic recrystallization of Ti-44Al-5Nb-1.5Cr-1.0W-0.5Si alloy requires higher deformation temperature and lower strain rate.The directional solidification alloy has stronger resistance to high temperature deformation.