【作者】 孙斌；

【导师】 孟祥龙；

【作者基本信息】 哈尔滨工业大学 ， 材料物理与化学， 2019， 博士

【摘要】 Ti-Nb合金与其他高温记忆合金相比,具有优异的冷热加工性能,马氏体相变温度可达300^℃以上,在高温场合下的驱动和连接等方面具有广阔应用前景。然而,Ti-Nb合金的形状记忆效应差,完全可恢复应变不到2%,这已阻碍了它的进一步推广应用。本文采用透射电镜、高分辨电镜、差示扫描量热分析以及拉伸试验等系统研究了热机械处理对Ti-16Nb高温记忆合金组织和界面结构、马氏体相变、形状记忆效应的影响,阐明了合金在室温拉伸变形时的组织结构演化规律和变形微观机制,揭示了热机械处理提高Ti-16Nb合金形状记忆效应的微观机制。透射电镜观察表明,固溶态Ti-16Nb合金中α″马氏体主要呈“V”字型自协作形态。构成“V”字型变体组的马氏体具有{1 1 1}I型或〈2 1 1〉II型孪晶关系。其孪晶界面共格性良好。但当“V”字型马氏体的侧边和其他变体的端部接触时,除孪晶面以外,还可观察到以（（?）2 0）_CV（i）^{1 3 （?）}_CV（j）或（（?）2 0）_CV（i）^{（?）0 2}_CV（j）晶面作为连接面,形成半共格界面。Ti-16Nb合金经适当的热机械处理后,冷加工引入的位错在退火时被部分消除并发生重排,形成局部内应力场,使马氏体变体在相变过程中发生择优取向,从而形成单一取向的马氏体板条。升高退火温度或延长退火时间,择优取向的马氏体板条数量减少,呈“V”字型组态的马氏体数量增多。热机械处理对Ti-16Nb合金的马氏体相变具有显著影响。当冷轧/冷拔变形量为60%,随着退火温度的升高或退火时间的延长,马氏体逆转变峰值温度A_p升高。升高退火温度,α相含量降低,基体中Nb含量也随之降低,导致A_p升高。随着退火时间的延长,α相含量保持不变,而残余位错密度逐渐降低,其对马氏体形核的促进作用减弱,导致A_p升高。当退火工艺参数不变时,A_p随冷轧/冷拔变形量的增加而降低。适量的残余位错不明显阻碍马氏体相变,又有利于马氏体的形核,因此相变温度降低。固溶态Ti-16Nb合金室温拉伸变形时,在粗大马氏体变体内部形成大量细小的{1 1 1}I型和〈2 1 1〉II型孪晶。当变形量超过2%时,位错滑移与马氏体的孪生同时发生。当应变量增至10%时,还观察到有少量{0 1 1}复合孪晶形成。热机械处理改变了Ti-16Nb合金的变形微观机制。当拉伸变形量小于5%时,经适当热机械处理后的Ti-16 Nb合金中主要发生马氏体的合并及再取向。当拉伸应变量大于5%时,位错滑移开始出现,导致形状不能完全恢复。固溶态Ti-16Nb合金变形时,{1 1 1}堆垛层错可作为{1 1 1}I型和〈2 1 1〉II型孪晶的形核位置。层错存在时{1 1 1}面上的原子通过微小切变就能移动到孪晶的点阵位置。柏氏矢量为〈0.3044 0.1465（?）〉的不全位错在{1 1 1}晶面连续滑移5个原子层,便可形成{1 1 1}I型孪晶。此外,当某一区域内同时存在多个{1 1 1}堆垛层错时,与之对应的Shockley不全位错将使其附近的（0 0 2）_m晶面间距增大,接近与之平行的〈0 （?）0〉_t晶面,经微小调整后,便可形成〈2 1 （?）〉II型孪晶。变形时{1 1 1}I型孪晶界面运动由孪生位错在孪晶面滑移实现。当拉伸应变量为5%,孪晶界面上出现高度不等的台阶,在部分孪晶界面附近观察到少量{1 1 1}晶面堆垛层错。当应变量增至10%时,孪晶界面附近产生畸变层。变形过程中〈2 1 （?）〉II型孪晶通过多个（1 （?）1）晶面协同的切变实现孪晶面〈（?）5 （?）〉的运动。热机械处理显著提高了Ti-16Nb合金的形状记忆效应。当冷轧/冷拔变形量为60%,退火时间为0.5h时,Ti-16Nb合金的可恢复应变随退火温度的升高而增加,当退火温度为700^℃时达极大值;而当冷轧/冷拔变形量为60%,退火温度为700^℃时,可恢复应变随着退火时间的延长而减小。Ti-16Nb合金获得的最佳的热机械处理工艺参数为冷轧/冷拔变形量60%,退火温度700^℃,退火时间0.5h,此时可获得5%的完全可恢复应变。Ti-16Nb合金经热机械处理后形成择优取向的板条状马氏体再取向临界应力低,界面可动性好是形状记忆效应提高的主要原因。更多还原

【Abstract】 Compared with other high temperature shape memory alloys,Ti-Nb alloys have excellent hot and cold workability,martensitic transformation temperature of up to300^℃,and has broad application prospects in driving and connection in high temperature applications.However,the shape memory effect of the Ti-Nb alloys is poor and the fully recoverable strain is less than 2%,which has hindered their further popularization and application.In this paper,the effects of thermomechanical treatment on microstructure and interface structure,martensitic transformation and shape memory effect of Ti-16Nb high temperature shape memory alloy were studied by means of transmission electron microscopy（TEM）,high resolution transmission electron microscopy（HRTEM）,differential scanning calorimetry（DSC）and tensile test.The microstructure evolution and deformation mechanism of Ti-16Nb alloy during tension at room temperature were revealed and the mechanism of thermomechanical treatment to improve the shape memory effect of Ti-16Nb alloy was proposed.TEM observation shows that theα′′martensite in the solution treated Ti-16Nb alloy is in the V-shaped self-accommdated form mainly.The martensite variants forming the V-shaped cluster have the{1 1 1}type I or the〈2 1 1〉type II twin relationship.The twin interface has good coherence.Except for the twin interfaces,it is observed that（（?）2 0）_CV（i）^{1 3 1（?）}_CV（j）or（（?）2 0）_CV（i）^{（?）0 2}_CV（j）lattice plane acts as a junction plane to form a semi-coherent interface,when the side of the V-shaped martensite is in contact with the ends of other variants.After proper thermomechanical treatment of the Ti-16Nb alloy,the dislocations introduced by cold working are partially eliminated and rearranged during annealing to form a local internal stress field,which makes the martensite variants occurs the preferred orientation forms a single-oriented martensite lath.Increasing the annealing temperature or prolonging the annealing time,the number of preferred orientation martensite laths decreases,and the amount of martensites with the V-shape configuration increases.Thermomechanical treatment has a significant effect on the martensitic transformation of Ti-16Nb alloy.When the cold rolling/cold drawing deformation is60%,the reverse martensitic transformation peak temperature,A_p,increases as the annealing temperature increases or the annealing time increases.When the annealing temperature is raised,the volumn fraction ofαphase is decreased and the Nb content of theα″martensite is decreased accordingly,resulting in an increase in A_p.As the annealing time prolongs,the volumn fraction ofαchanges little and the residual dislocation density gradually decreases.And its promotion of martensite nucleation is weakened,resulting in an increase in A_p.When the annealing parameters are constant,A_p decreases as the degree of cold rolling/cold drawing deformation increases.Appropriate residual dislocations do not significantly hinder the martensitic transformation,but also favor the nucleation of martensite,so the phase transformation temperature decreases.When the solution treated Ti-16Nb alloy is deformed at room temperature,a lot of fine{1 1 1}I type and〈2 1 1〉II type twin bands form inside the coarse martensite variant.When the deformation variable exceeds 2%,the dislocation slip occurs simultaneously with the martensite twin.When the strain was increased to10%,a small amount of{0 1 1}compound twin formation was also observed.Thermomechanical treatment changes the microscopic mechanism of deformation of Ti-16Nb alloy.When the tensile deformation is less than 5%,the martensite is mainly combined and reoriented in the Ti-16 Nb alloy after appropriate thermomechanical treatment.When the tensile strain is greater than 5%,dislocation slip begins to appear,resulting in a shape that cannot be fully recovered.When the solution treated Ti-16Nb alloy is deformed under tension,the{1 1 1}stacking faults can act as the nucleation sites of the{1 1 1}type I and the〈2 1 1〉type II twin.Stacking faults themselves are possible nucleation sites for the{1 1 1}type I twins.If the Shockley partial dislocation with a Burgers vector of〈0.3053 0.1463（?）〉glides continuously on the{1 1 1}lattice plane for 5 atomic layers,the{1 1 1}type I twin forms.While for the〈2 1 1〉type II twins,they may form at the end of a series of{1 1 1}stacking faults,where Shockley partial dislocations exit.The（0 0 2）_m plane is approximately parallel to the〈0 （?）0〉_t plane for the two crystals in〈2 1 1〉type II twin related orientation.The d-spacing of（0 0 2）_m lattice planes increase,resulting from the Shockley partial dislocations nearby and the〈2 1 1〉type II twin forms after tiny modification of the（0 0 2）_m lattice planes.When deforming is occuring,the movement of the{1 1 1}type I twin interface is achieved by the slip of the twinning dislocation on the twin plane.When the tensile strain is 5%,there are unequal steps on the twin interface,and a small number of{1 1 1}stacking faults are observed near the partial twin interface.When the strain is increased to 10%,a distortion layer is generated near the twin interface.During the deformation process,the〈2 1 1〉type II twins realize the movement of the twin plane（（?）5 （?））by the shearing of multiple（1 （?）1）crystal planes.Thermomechanical treatment significantly improved the shape memory effect of Ti-16Nb alloy.When the cold rolling/cold drawing deformation is 60%and the annealing time is 0.5h,the recoverable strain of Ti-16Nb alloy increases with the increase of annealing temperature,and reaches the maximum when the annealing temperature is 700 ^℃.When the rolling/cold drawing deformation is 60%and the annealing temperature is 700 ^℃,the recoverable strain decreases as the annealing time increases.The optimal thermomechanical treatment parameters for Ti-16Nb alloy are 60%cold rolling/cold drawing,annealing at 700 ^℃ for 0.5 h.In such cases,5%fully recoverable strain can be obtained.The thermal stress of Ti-16Nb alloy after forming a preferred orientation of lath martensite has low reorientation critical stress,and good interface mobility is the main reason for the improvement of shape memory effect.更多还原