Microstructure Evolution Of Ti-43.5Al-4Nb-1Mo-0.1B Alloy Under Different Annealing Conditions

Ti-43.5Al-4Nb-1Mo-0.1B titanium aluminum alloy has high yield strength,high specific strength,low density,good creep performance at high temperatur e.It is a competitive lightweight material for aerospace.In this paper,the Evolu tion of microstructure,relationship between microstructure and mechanics of Ti-43.5Al-4Nb-1Mo-0.1B alloy were studied systematically by Vickers hardness test,X-ray diffraction analysis,scanning electron microscopy and transmission elect ron microscopy from the aspects of secondary annealing time,secondary anneal ing temperature and annealing frequency.The research results are as follows:?1?Ti-43.5Al-4Nb-1Mo-0.1B alloy is subjected to secondary annealing at 800°C for 500 h to 2000 h.The content and morphology of the alloy phase s change with the extension of the second annealing time,but does not change the type of phases in the alloy.As the secondary annealing time prolongs,the interface of the?₂+?colonies is gradually roughened,and a cellular structure composed of?₂,?,and?_o phase is formed.After 500 h of secondary annealing,?_o particles were formed inside the?₂lamellar,and the formation of?_o phase made the single?₂ lamellar become?₂/?_o/?₂bamboo-like structure.As the annealing time prolongs,the?_o phase formed in the?₂lamellar gradually grows into the vicinity of the adjacent?lamellar,which makes the original flat lamellar interface become curved.The formation of?_o inside the?₂lamellar makes the structure inside the lamellar colonies more refined.The hardness of the alloy is improved to some extent due to the refinement of the lamellar structure.no?_o phase particles were formed in the?_o phase generated inside the?₂ lamellar.?2?Ti-43.5Al-4Nb-1Mo-0.1B alloy is mainly supersaturated?₂ particles after air-cooling heat treatment at 1230°C for 1h,only part of the area has lamellar structure.After secondary annealing at 850?to 950?,the supersaturated?₂ particles in the alloy form substantially all of the lamellar structure.Moreover,as the secondary annealing temperature increases,the average spacing of the lamellar gradually increases.The collapse of the lamellar colony and the increase of the average spacing result in a decrease in the hardness of the alloy.?3?The structure of the Ti-43.5Al-4Nb-1Mo-0.1B alloy after annealed at 1230?for 1 h and cooled by furnace is near?structure.Secondary annealing at 950?for 6 h will cause the lamellar spacing to increase,the spherical?_o particles decrease,and spherical?particles will be formed inside the?₂+?lamellar colonies.Generated,these factors cause the hardness of the alloy to decrease.At the same time,the specific gravity of the Al element content in the?_o equiaxed grain is increased by annealing at 950°C,the specific gravity of the Nb element content in the?equiaxed grain is increased,and the specific gravity of the Mo element content in the?equiaxed grain is decreased.After the second annealing,the alloy is subjected to a third annealing at 900°C for 200 h,the number of?_o phases is significantly increased,and a large amount of?_o phase is distributed at the grain boundaries and inside the lamellar,which increases the hardness of the alloy.In addition to the granular shape,the?_o phase precipitated inside the?₂+?lamellar is also a strip-like?_o.In the site of the strip-shaped?_o the lamellar has disappeared.Annealing at 900°C for a long time causes a decrease in the specific gravity of Ti in the?_o equiaxed grain,a decrease in the specific gravity of the Nb element in the?equiaxed grain,and a decrease in the specific gravity of the Mo element in the ?₂+ ?colony.