Dislocations And Failure Analysis Of A New Near-α Type Titanium Alloy For Aerospace

The development of aerospace industry requires higher properties of high temperature structural frames which include higher durability of high temperature environment and more excellent mechanical properties working at room temperature and high-temperature.BTi6431 S is a newly developed near-α type high temperature titanium alloy and is designed to work at 700℃ which has excellent room temperature strength and high temperature ductility.However,the relationship between microstructure of BTi6431 S and mechanical properties remains unknown,which hampers the application of high temperature titanium alloy on aerospace industry.Besides,when the titanium alloy exposes to thermal environment for a long time or several cycles,the mechanical properties may be detrimental because the microstructure and dislocation may change.If the titanium alloy was applied without studying the mechanism of deformation,there would cause a severe damage.In this paper,the evolution of dislocation in BTi6431 S during tension at room temperature and high temperature is studied.The relationship between macro mechanical properties and dislocation behavior is established.Also,the detrimental effect of annealing on the high temperature titanium alloy and the microstructure subjected to heat treatment are studied,and the mechanism of annealing failure is revealed.The main results are as follows.The titanium alloy performed excellent strength and good ductility when tensioned at room temperature.There are complex dislocation tangling and dislocation wall distributed in α phase of BTi6431 S.When deformed at room temperature,dislocations are activated from the α-β interface and glide in the α matrix.In this case,planar slip bands cross grain boundaries of α phase and move to adjacent grains,which account for the deformation.However,the planar slip bands could not move across the α-β interface and thus cause dislocation hamper near grain boundaries,which is difficult for the moving dislocations to glide and thus cause a high strength of BTi6431 S.When tensions at high temperature,the titanium alloy performs superior ductility.There are also dislocation walls and complex dislocations tangling in the matrix,which accounts for the fair strength at high temperature of BTi6431 S.With the strain increases,sub-grains are found in the matrix and planar dislocations slip across the boundaries of sub-grains.It causes the decrease of stress with strain increase.With the effect of dynamic recovery,the planar slip bands are annihilated due to the thermal effect.The slip of sub-grain boundaries and dislocation movement endure the increasing strain at high temperature.After annealing at 600℃,the BTi6431 S performs embrittlement when tensioned at room temperature.Annealed sub-grains and uneven dislocation structures are observed in the matrix.In the zone with high dislocation density,dislocations also tangle with a relatively simple topography in the matrix.As for the zone with low dislocation density,dislocation topography tends to be dislocation networks and dislocation arrays.Primary dislocation like discrete dislocation is also formed.Moreover,there is no phase transition or precipitate formed after annealing,which means the dislocation behavior is crucial to the brittle effect of BTi6431 S.