【作者】 杨光；

【导师】 叶恒强； 祁阳； 杜奎；

【作者基本信息】 东北大学 ， 材料学， 2018， 博士

【摘要】 γ-TiAl金属间化合物有高弹性模量、高强度、高硬度等特点,即使在高温使役环境下依然如此。由于这些出色的机械性能,基于γ-TiAl金属间化合物的钛铝合金成为了一种前景广阔的高温结构材料,一个典型的应用就是飞机发动机的低压涡轮叶片。由于钛铝合金全片层组织的PST(Polysynthetic twinned)晶体在高温下具有优良的力学性能,PST晶体表现出了提升TiAl低压涡轮叶片高温性能的潜力。低压涡轮叶片的开发需要材料冲击响应的有关知识,因此需要研究PST晶体的冲击(或者动态)变形行为。本文主要使用了透射电子显微术(Transmission electron microscopy,TEM)和球差校正扫描透射电子显微术等先进的材料学表征手段,研究了名义成分为Ti-49.5Al的PST晶体的动态变形行为。(1)发现了γ-TiAl金属间化合物动态变形的主要变形形式是变形孪晶、孪晶交截、剪切带和位错滑移。剪切带这种非晶体学的变形形式表明了 PST晶体的动态变形行为和准静态变形存在显著区别。在动态变形过程中发生了沿(111)主孪生面和(111)次孪生面的孪生过程,并观察到大量主次孪生面孪晶片层交截的现象。在变形的PST晶体中主孪生面孪晶片层可能是生长孪晶也有可能是变形孪晶,而次孪生面孪晶片层肯定是变形孪晶。主孪生面孪晶界上存在柏氏矢量为1/3<111>和1/6<112>的位错。1/6<112]真孪生不全位错可以独立地存在于孪晶界,然而1/6<211]伪孪生不全位错会与1/2<101]超不全位错共存于孪晶界。在实验中观察到两种类型的剪切带。类型Ⅰ剪切带主要是由孪晶片层组成的,其传播方向倾斜于剪切带内部的孪晶片层。类型Ⅰ剪切带是由相应的{111}晶面剪切而形成的,并且新的孪生过程发生在这些被剪切的{111}晶面上。由主孪生面剪切而形成的类型Ⅰ剪切带最常见,其数量远高于由其它{111}晶面剪切而形成的类型Ⅰ剪切带。类型Ⅰ剪切带带间的交互作用的主要形式是形成新的孪生过程,该孪生过程可以释放带间局部应力。类型Ⅱ剪切带是由严重畸变的次孪生面孪晶片层组成的,其传播方向近似平行于次孪生面。类型Ⅱ剪切带应该是由预先产生的次孪生面孪晶片层演化而来的。柏氏矢量为1/2<101]的超不全位错在滑移的过程中会产生反相畴界并发生交滑移。折线状的柏氏矢量为<011]的超晶格全位错贯穿了主孪生面孪晶片层。另外,弧形位错贯穿了主次孪生面孪生片层。(2)研究了原子尺度下位错和孪晶界的交互作用。在孪晶界上观察到了三类台阶位错,这些台阶位错揭示了位错和孪晶界间的交互作用。柏氏矢量为1/2<110>的台阶位错来源于孪晶界和相应可动位错的交互作用。在共格孪晶界残余的1/3<111]和1/6<211]位错揭示了可动的1/2<101>位错与晶界的交互作用。在实验中观察到紧邻共格孪晶界的反常堆垛层错。当沿[110]晶向观察时,反常堆垛层错的两个紧邻的(111)晶面原子层有相同的原子堆垛顺序但不同的原子种类,然而事实上沿[110]方向存在1/4[110]的位移。该反常堆垛应该是由1/12[112]位错的滑移而产生的。几何相位分析(Geometric phase analysis,GPA)的结果显示反常堆垛区域没有明显的应变。当交截前后入射孪晶的数目不一致时,动态变形可以促进在孪晶交截区域处非共格孪晶界的形成,这可以释放交截区域处的复杂应力。当交截前后入射孪晶的数目一致时,入射孪晶的传播常常是通过孪生位错在障碍孪晶片层的{111}晶面上滑移并贯穿障碍孪晶的方式完成的。另外,二次孪晶可以在孪晶交截区域形成。样品在冲击变形的过程中形成了透镜状的孪晶,这种孪晶应该是通过位错的极轴机制形成的。限于冲击应力状态的影响,这种孪晶的尺寸较小。(3)揭示了动态变形过程中孪晶片层特殊的变形形式。观察到了扭折的孪晶片层。当扭折的孪晶片层只有垂直于[101]晶向(即观察方向)的应变时,这些片层在高分辨TEM图像下的两个{111}晶面所成的夹角接近于70.5°。孪晶片层的扭折可以导致孪晶界上1/3<111>或1/6<112>的台阶位错、片层内部的层错和1/2<011>位错的形成。当扭折的孪晶片层沿[101]晶向(即观察方向)有扭折位移时,扭折孪晶片层在高分辨TEM图像下两个{111}晶面所成的夹角偏离70.5°,并且该角度随着片层位置的不同而发生变化。事实上这些扭折片层并没有发生晶体结构上的改变,角度变化只是TEM产生的假象。线衬度近似平行于[011]超位错的晶界贯穿了主次孪生面孪晶片层。一系列距离较近且近似平行的这种晶界可以造成主次孪生面孪晶片层呈现出S型起伏的形貌。在实验中观察到了由迹线近似平行的[011]超位错、晶界和严重扭曲的次孪生面孪晶界组成的变形带。晶界、孪晶片层的S型起伏和变形带这三种变形形式体现了冲击变形的不均匀性。观察到孪晶片层的三种非晶体学塑性变形形式:第一种是相同孪生面孪晶片层的小角度取向差异的交叠;第二种是孪晶片层的蜈蚣状弯曲的形貌;第三种是主孪生面孪晶界在应力作用下发生了迁移,由平直形貌转变成凸起状形貌。更多还原

【Abstract】 Intermetallic compound y-TiAl has advantages of high elastic modulus,strength and hardness,especially at elevated temperatures.Owing to these superior mechanical properties,TiAl alloys mostly composed of the intermetallic compound γ-TiAl can be used as promising high temperature structural materials,for examples,low pressure turbine blades.TiAl polysynthetic twinned(PST)crystal in full lamellar structure has shown a potential of further improving the performance of TiAl turbine blades,due to its excellent properties.The development of low pressure turbine blades needs the knowledge of the impact response of materials.Thus it is of urgent need to study the deformation bahaviours of PST crystal.In this study,advanced technics in materials science characterization including transmission electron microscopy(TEM)and aberration-corrected scanning transmission electron microscopy(STEM)were used to study the impact or dynamic deformation bahaviours of PST crystal with Ti-49.5A1 composition.(1)The main deformation behaviours(i.e.deformation twining,twin intersection,shear banding and dislocation slip)were revealed.The non-crystallographic deformation of shear banding indicates that significant difference exists between the dynamic deformation and quasi-static deformation.Twinning along primary(111)and secondary(111)planes took place,and twin intersection of primary and secondary twin lamellae in large number were observed.The primary twin lamellae are as-grown twins or deformation twins,while the secondary twin lamellae are surely deformation twins.The dislocations with the Burgers vectors of 1/3<111>and 1/6<112>were observed on the coherent primary twin boundaries(TBs).Among them,1/6<211]pseudo-twin dislocations were observed on the coherent TBs co-existing with 1/2<101]super-partial dislocations,while 1/6<112]true-twin partial dislocations can exist independently on the coherent TBs.Two types of shear bands were identified.Type Ⅰ shear bands are composed of twin lamellae,and their propagation directions are inclined to the internal twin lamellae.Type Ⅰ shear bands were formed by shearing {111} planes,and new twinning took place along the sheared {111} planes.The number of type I shear bands formed by the shearing of primary(111)planes is significantly large than others.The interactions of type Ⅰ shear bands are in form of new twinning between shear bands,which release the local stress between shear bands.Type Ⅱ shear bands are composed of curved(111)twin lamellae and their propagation is nearly parallel to the secondary(111)twin plane.Type Ⅱ shear bands were evolved from the pre-existing secondary twin lamellae.The slip of the super-partial dislocations with the Burgers vector of 1/2<101]can induce antiphase domain boundaries,and cross-slip of these dislocations happened.The superlattice dislocations with the Burgers vector of<101]in zigzag shape were observed running through the primary twin lamellae.In addition,dislocation lines in arc shape were also observed running through primary and secondary twin lamellae.(2)The interactions between dislocation and TB in atom scale were investigated in the impacted deformed samples.The step dislocations with he Burgers vector of 1/2<101>resulted from the interactions between corresponding glissile dislocations and coherent TBs.The step dislocations of 1/3<111]and 1/6<211]on coherent TBs reveal the interactions between glissile 1/2<101>dislocations with the TBs.An abnormal stacking fault was found adjacent to the coherent twin boundary.It has the same stacking sequence but different atom species in the[110]direction with an additional displacement of 1/4[110]in two neighboring{111} layers,and is likely induced by the slip of a 1/12[112](i.e.1/4[110]+1/6[211])dislocation.The strain map obtained with geometric phase analysis shows no considerable strain in the abnormal stacking region.When the number of incident twins before and after twin intersection varied,the incoherent twin boundary was formed to release the complicated stress in the twin intersection region.When the number of incident twins before and after twin intersection is constant,the propagation of incident twins is usually fulfilled by the slip of twinning dislocations along {111} planes inside the obstacle twins and then running through the obstacle twins.In addition,secondary twins took place in the twin intersection region.The twins in lenticular shape can be formed by the pole mechanism of dislocations in the impact deformed samples.The size of these twins is small,due to the effect of impact stress.(3)The special deformation behaviours of twin lamellae in the impact deformed samples were revealed.The kink of twin lamellae was observed.When the strain perpendicular to[101]crystallographic direction(i.e.viewing direction)existed,the angles between two {111} planes in the kinked twin lamellae are close to 70.5° in the high-resolution TEM images.The kink of twin lamellae can induce the step dislocations with the Burgers vector of 1/3<111>and 1/6<112>on the coherent TBs,stacking faults and 1/2<011>dislocations inside the lamellae.When the kink displacements of twin lamellae along[101]crystallographic direction(i.e.viewing direction)happened,the angles between two {111} planes in the kinked twin lamellae deviated from 70.5° and varied at different positions in a lamella.In fact,the crystal structure of these kinked twin lamellae did not happen,and the variation of angle between {111} planes in TEM images is a fake.Line contrast of grain boundary(GB)nearly parallel to[01(?)]superlattice dislocations was observed running though primary and secondary twin lamellae.Adjacent GBs can induce the primary and secondary twin lamellae to present S-shape morphology.The deformation band is composed of GBs,S-shape of twin lamellae and secondary TBs distorted severely which are parallel with[011]superlattice dislocations in line contrast.These deformation behaviours above present heterogeneity of dynamic deformation.Three types of non-crystallographic plastic deformation behaviours of twin lamellae were observed.The first one is the overlapping of twin lamellae in same twin plane with a low-angle difference.The second one is distorted twin lamellae in centipede morphology.The third one is migration of primary TBs under stress,whose morphology was changed from straightness to bulge.更多还原