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 theα₂+γcolonies is gradually roughened,and a cellular structure composed ofα₂,γ,andβ_o phase is formed.After 500 h of secondary annealing,β_o particles were formed inside theα₂lamellar,and the formation ofβ_o phase made the singleα₂ lamellar becomeα₂/β_o/α₂bamboo-like structure.As the annealing time prolongs,theβ_o phase formed in theα₂lamellar gradually grows into the vicinity of the adjacentγlamellar,which makes the original flat lamellar interface become curved.The formation ofβ_o inside theα₂lamellar 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.noω_o phase particles were formed in theβ_o phase generated inside theα₂ lamellar.（2）Ti-43.5Al-4Nb-1Mo-0.1B alloy is mainly supersaturatedα₂ 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 supersaturatedα₂ 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 nearγstructure.Secondary annealing at 950℃for 6 h will cause the lamellar spacing to increase,the sphericalβ_o particles decrease,and sphericalγparticles will be formed inside theα₂+γlamellar 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 theβ_o equiaxed grain is increased by annealing at 950°C,the specific gravity of the Nb element content in theγequiaxed grain is increased,and the specific gravity of the Mo element content in theγequiaxed grain is decreased.After the second annealing,the alloy is subjected to a third annealing at 900°C for 200 h,the number ofβ_o phases is significantly increased,and a large amount ofβ_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,theβ_o phase precipitated inside theα₂+γlamellar is also a strip-likeβ_o.In the site of the strip-shapedβ_o the lamellar has disappeared.Annealing at 900°C for a long time causes a decrease in the specific gravity of Ti in theβ_o equiaxed grain,a decrease in the specific gravity of the Nb element in theγequiaxed grain,and a decrease in the specific gravity of the Mo element in the α₂+ γcolony.