【作者】 林斌；

【导师】 王帅；

【作者基本信息】 哈尔滨工业大学 ， 材料工程（专业学位）， 2020， 硕士

【摘要】 航空工业对高温结构件的服役要求不断提高,不仅要求高温钛合金具有更高的使用温度,还对材料的室温力学性能、高温力学性能、高温持久性能提出更高的要求。BTi6431S作为一种新型的近α型高温钛合金,其工作温度可以达到700℃,并具有优良的力学性能。但是新型高温钛合金具有短时有效的缺点,可能与其服役过程有关。为了研究这一失效形式,本文研究BTi6431S在室温和高温下,拉伸过程中位错组织的演化规律,以及研究退火后钛合金宏观力学性能的变化与微观位错组织改变,揭示材料退火失效的机理。主要有的研究结果如下:高温钛合金BTi6431S在室温下拉伸具有很高的抗拉强度和较好的延展性,其断裂类型以微孔聚集型为主的韧性断裂,并伴随着断口表面出现少量解理刻面和大量浅而小的韧窝;BTi6431S的初始位错组态为高位错密度的位错缠结和位错墙,均匀分布在α晶粒中;室温下发生塑性变形时,位错运动以平面型滑移带的滑移为主,位错在α晶内迁移,且可以跨过复杂的位错塞积结构继续滑移。BTi6431S在高温下拉伸时位错组态仍以位错墙和位错缠结为主,这使得材料在高温下仍具有较好的强度;随着时间的延长和应变的增加,平面型位错越过亚晶界迁移到相邻晶内,由于动态回复的作用,平面滑移带最终消失,造成应力持续下降,由亚晶界形成过程中位错运动共同协助提供塑性变形。高温钛合金BTi6431S短时有效的原因可能是:在钛合金服役过程中,钛合金原来网状位错缠结结构遭到破坏而消失,形成亚晶界,产生位错塞积,位错运动困难,钛合金无法承担更多变形提前断裂。而且退火钛合金在高温拉伸时脆化效应消失,退火过程中没有新相生成和析出物,因此高温钛合金的位错组织行为具有十分重要的作用。更多还原

【Abstract】 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.更多还原