Investigation On The Thermal Stabilities Of Microstructure In Fully Lamellar High Nb Containing TiAl Alloys

High Nb containing TiAl alloys are considered as the potential replacements to Ni-based superalloys in aerospace and automobile industries for high temperature applications,mainly due to their excellent high temperature properties and low density.After nearly decades of research,the composition design and microstructure control technology of high Nb containing TiAl alloys,as well as the preparation and processing technology have been made much exciting progress.However,there are little systematic studies about microstructure stabilities of the alloys under service environment,which seriously impedes their further application.In this study,the thermal stabilities of microstructure in fully lamellar high Nb containing TiAl alloys were investigated,and the effects of microstructure degradation on mechanical properties were also studied.The main conclusions are listed as follows:(1)Microstructure stabilities of fully lamellar high Nb containing TiAl alloys during thermal cycling between room temperature and 700?-1000?were investigated.The results show that the coarsening of lamellar colony boundaries was produced after long term thermal cycling.At 700?and 800?,the colony boundary phases were mainly constituted of?and?phases.At 900?and 1000?,the phases became y and B2.These findings suggest that there should be a(?2+?+B2/?)region within this temperature range for the studied alloys.The corsening of colony boundaries,in a way of precipitation of the coarse y grains,prefers to occur at boundaries with high misorientation angle.The precipitation is inhibited at the boundaries with low misorientation.And there is a coincidence-site lattice relationship between the precipitated?grains and the?lamellae at one side of the boundary.(2)The effect of Al-segregation on the microstructure stabilities of high Nb containing TiAl alloys during thermal cycling was revealed.After long term thermal cycling at 900?,the discontinuous coarsening was inclined to occur in the Al-segregation region.After long term thermal cycling at 1000?,the massive y grains were generated in the Al-segregation region and within the lamellar structure or at colony boundaries.And the a2 phase with different orientation,which is coherent with the?matrix by Blackburn orientation,precipitated on the{111}?plane in the?grain interior.There is no precipitates of B2 observed both at900?and 1000?.(3)The effect of Nb content on the microstructure stabilities of TiAl alloys during thermal cycling was studied.After long term thermal cycling,the colony boundaries were also coarsened by precipitation of coarse y grains in low Nb containing TiAl alloy.However,the colony boundary phases were constituted of y and?2 phases,the B2 particles cannot be observed in the alloys.The plate-shaped and particle-shaped?2 could be found,and there is {111}?//{0001}?2.<1120>?2//<110>?orientation relationship between the plate-shaped?2 phase and?matrix.(4)The effects of microstructure stabilities on mechanical properties of high Nb containing TiAl alloys during thermal cycling at 900?and 1000?were stuied.After thermal cycling at 900?,both the tensile strength at room temperature and the compression yield strength were reduced gradually with increasing thermal cycles.After thermal cycling at 1000?,the two kinds of strength decreased firstly and then increased.The strength decreased about 10%.which is much lower than that for conventional TiAl alloys after long term thermal cycling.Regardless of 900?or 1000?,no noticeable deterioration in the fracture toughness was observed throughout the thermal cycling process.The KIC value decreased first and then increased.However,tensile ductility at room temperature showed significant declines after thermal cycling both at 900?and1000?,which is attributed to the "B2 formation induced embrittlement" and"oxygen-release induced embrittlement".High temperature tensile ductility was enhanced with increasing thermal cycles,due to the sufficient slip system of high temperature?phase.(5)The microstructure stabilities induced by shot peening were investigated in high Nb containing TiAl alloys following high temperature treatments.After high temperature treatment at 900?and 1000?,a fine-grained(FG)layer consisted of equiaxed grains and a coarse-grained(CG)layer consisted of elongated grains,were formed in transversal direction of the surface.The FG layer is composed of completely recrystallized y grains,which might transform from the deformed lamellae due to the residual deformation energy.The CG layer is composed of incompletely recrystallized y grains.The growth of CG layer after a certain holding time is caused by the reduction of interfacial free energy rather than residual deformation energy.After high temperature treatment at 900?,the phases in the two layers are constituted of?,?2 and B2 phases.However,almost no B2 particles can be observed in the two layers after high temperature treatment at 1000?.The precipitated?2 particles after high temperature treatment almost have the same orientation with primary?2 lamellae,indicating that nearly no recrystallization phenomena occur for?2 phase.(6)The effects of hot compression deformation,shot peening(compressive residual surface stress)and thermal stress on the thermal stabilities of microstructure in high Nb containing TiAl alloys were compared and studied.Under thermal stress condition,the formation of new?grains both at colony boundaries and inside of the lamellar colonies does not belong to recrystallization.The?2 lamellae undergo only dissolution process.After hot compression deformation and shot peening,the lamellar structure transformed itself into recrystallized?grains when the deformation exceeds the critical value during high temperature treatment process.The?2 lamellae can undergo dissolution,precipitation and growth.And most of?2 particles precipitated on the {111}?plane in the recrystallized y grains.In addition,it is the enhanced diffusion rates of Nb and W atoms that cause the disappearance of B2 particles at 1000?high temperature treatment after hot compression deformation and shot peening process.