Dynamic Responses of Nickel-Based Single Crystal Superalloy DD6 Blade

The aim of this article is to address the dynamic responses properties of the second-generation nickel-based single crystal superalloy DD6 with different orientations, rotor speeds, and temperatures, and this work can lay good foundation for predicting damage behaviors of the DD6 blade and starting with the application of gas turbine in aerospace. The finite element method was applied to investigate the dynamic characteristics of the DD6 blade with [001] and [111] orientations under different rotor speeds at 760, 980, and 1070 °C. On the basis of establishing the elastic-plastic constitutive model of single crystal alloy, the dynamic responses model of a DD6 blade was educed. As a result, the corresponding static and dynamic frequencies were analyzed for different temperatures, rotor speeds, and orientations. Comparison of the computational and experimental results shows the proposed model of the DD6 blade is effective. The analyses indicate that the natural frequencies of [111] orientation is higher than that of [001], and the effect of rotor speed on natural frequencies of [001] orientation is greater than that of [111]. In other words, the values of the natural frequency are increased with the elevated rotor speed, and the values of the natural frequency are decreased obviously with the elevated temperature.     

含铸造微孔镍基单晶合金蠕变损伤和裂纹萌生的模拟计算

基于镍基单晶合金蠕变变形过程中的细、微观组织结构变化及损伤特点,建立了考虑材质劣化和孔洞损伤的双参数蠕变损伤本构方程。利用所建模型对裂纹前缘含铸造缺陷（孔洞）的镍基单晶合金紧凑拉伸（CT）试样蠕变损伤和裂纹萌生进行了模拟计算,并考虑了晶体取向偏差和随机性的影响。计算结果表明：晶体取向和孔洞位置对试样蠕变损伤和裂纹萌生行为有着显著的影响。当孔洞距切口根部距离较近时,裂纹形核于切口附近的孔洞表面,裂纹形核时间较短;孔洞距切口根部距离较远时,裂纹形核位置位于切口表面,具体位置取决于试样的晶体取向,裂纹形核时间较长。随着加载轴晶体取向偏角的增大,裂纹形核时间明显缩短,其分散性加大,最大有34.7%的变化幅度;试样在2个不受控的晶体取向变化时,在偏角为45°和80°出现极值,裂纹形核时间最大偏差达3倍。     

基于粘塑性的单晶镍基合金晶体学本构模型

单晶镍基合金在涡轮叶片上得到越来越广泛地应用,对单晶材料本构模型的研究也在不断地深入。本文运用晶体学理论,结合现象学的统一本构模型的优点,进行了单晶镍基合金的本构建模,模型基于晶体学滑移理论,在单晶八面体12个滑移系和立方体6个滑移系的微观水平上建立粘塑性流动方程和硬化方程,该方程采用粘塑性统一本构方程的一般形式。建模过程表明晶体模型物理意义更明确,计算结果则表明模型是准确的。     

定向结晶叶片内部晶体取向无损测定研究

采用短波长特征X射线衍射方法,无损测定镍基高温合金定向结晶叶片内部晶体取向及其分布,阐述其测定原理。通过单点扫描的方式,实现对定向结晶叶片内部一条直线上晶体取向分布的无损检测;通过分析衍射峰宽,实现扫描方向上晶粒尺寸的测量;结合内部晶体取向及扫描测试结果,确定样品内部不同取向的晶粒至少存在5个;通过晶体取向角的梯度变化特征探讨晶界类型的区分。为短波长特征X射线衍射技术用于无损检测定向结晶以及单晶叶片内部晶体缺陷奠定基础。     

蓝宝石热物性能对SAPMAC法晶体生长影响的模拟分析

利用数值模拟计算方法对材料热物性能对晶体生长的影响进行了计算分析，计算表明：晶体与熔体具有相同的热导率时，较小的热导率能够有效降低晶体内温度梯度和界面凸出率；晶体热导率各向异性时，较大的径向热导率，较小的轴向热导率更有利于保持微凸的生长界面；热熔、潜热对温场几乎没有影响。选a轴为结晶取向，成功生长出了直径达230mm、高质量蓝宝石晶体。     

离子轰击对Be上Al膜的影响

以Be为基体 ,采用磁控溅射离子镀在其上镀制Al膜 ,研究了离子轰击对膜基界面和Al膜微结构的影响。研究表明 ,在薄膜沉积初期 ,施加高能量离子轰击和采取循环轰击镀能增加界面形成的Be、Al共混区宽度。不同能量的离子轰击对Al膜微结构有较大影响 ,不施加离子轰击 ,Al膜在 (111)面择优生长 ;随着轰击能量升高Al膜在(111)面择优生长趋势减弱 ,Al膜在 (2 0 0 )面生长趋势加强 ;当轰击能量超过一定值后 ,Al膜在 (111)面择优生长的趋势又得到加强。晶粒在低能量离子轰击时随轰击能量增加而细化 ,当较高能量离子轰击引起基体温度升高时 ,此时晶粒又变大了。     

Effects of initial orientation,sample geometry and friction on anisotropy and crystallographic orientation changes in single crystal microcompression deformation: A crystal plasticity finite element study

The study presents crystal plasticity finite element simulations of cylindrical Cu single crystal micropillar compression tests. The aim is to study the influence of the stability of the initial crystal orientation, sample geometry (diameter-to-length ratio) and friction on the anisotropy and crystallographic orientation changes during such tests. Initial anisotropy (initial orientation) has a strong influence on the evolution of crystallographic orientation changes and also, to a minor extent, on the sample shape during compression. Pronounced orientation changes occur at an early stage of compression (at engineering strains of 0.2), entailing as a rule a large orientation spread within the initially uniformly oriented sample. A non-zero friction has a stabilizing effect on the course of the compression test even in cases where strong orientation changes occur. The evolution of orientation changes during compression is in part due to rigid body rotations (shape inclination due to buckling) rather than exclusively to crystallographic reorientation. Orientations that are crystallographically unstable and non-symmetric during compression tend to entail shape instability of the pillars at an earlier stage than observed for more stable cases.     

FCC晶体中晶体取向对孔洞长大和聚合的影响

通过编制率相关有限元用户子程序，采用包含一个和两个球形孔洞的单胞探求了FCC晶体中晶体取向对孔洞长大和聚合的影响。计算结果表明：晶体取向对孔洞长大的影响较大，孔洞的形状和长大方向与晶体取向密切相关：由于变形不均匀，孔洞在晶界处产生尖角，易形成裂纹。由于约束较少，孔洞周围和两孔洞间的区域塑性变形较大，晶体的转动和滑移主要集中在孔洞周围以及两孔洞间的区域。     

镍基单晶合金复杂应力状态低周疲劳寿命预测

根据损伤应变能释放率的定义表达式，将各向同性材料应力二三轴性因子拓展到正交异性材料，定义了含有3个弹性常数的镍基单晶应力三轴性因子。用它修正Mises应变范围作为疲劳损伤参量，可以显著消除晶体取向和多轴载荷对疲劳寿命的影响。用损伤应变能释放率作为热力学广义力描述正交异性材料的疲劳损伤过程，引入取向函数和损伤驱动力循环特征参数反映晶体各向异性对疲劳损伤的非线性影响以及循环载荷的交变特性，提出了单晶合金低周疲劳损伤模型。利用CMSX-2镍基单晶合金薄壁圆筒试样的拉一扭循环载荷低周疲劳试验数据和DD3镍基单晶合金缺口试样的低周疲劳试验数据，运用多元线性回归分析方法拟合模型的材料常数，试验所得数据分别落在2．5倍和2．0倍偏差分布带内。     

Symmetric and Asymmetric Rolling Pure Copper Foil: Crystal Plasticity Finite Element Simulation and Experiments

A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientation development during foil rolling. Simulations were successfully carried out by using crystal plasticity finite element method(CPFEM),and a novel computational framework is presented for the representation of virtual polycrystalline grain structures. It has been found that asymmetric rolling(ASR) is more efficient in producing plastic deformation since it develops additional shear strain and activity of slip system compared with symmetric rolling(SR). For ASR, increase in the length of the shear zone, and decrease in the amount of the pressure and roll force would lead to further reduction. The shear strain path in SR and ASR is strictly influenced by the misorientation of neighbor grains, and corresponding {1 1 1} pole figures offer direct evidence of the spread of crystallographic orientation around the normal direction. The activity of slip systems was examined in detail and found that the predicted results are consistent with the surface layer model. The accuracy of the developed CPFEM model is verified by the fact that the simulated results of roll force coincide well with the experimental results.     

Recrystallization Textures of a Cold-Rolled Aluminum Single Crystal

An aluminum single crystal cold-rolled from (110) [112] essentially retains its initial orientation after 99.6 pct reduction in thickness. The orientation of the recrys-tallized grains of this material can be described in terms of crystallographic rotations about principal poles of deformation texture.     

Stray grain formation in the seed region of single-crystal turbine blades

Seed crystals are frequently used to provide an off-axial 〈001〉 crystallographic orientation to investment cast single-crystal, nickel-based superalloy turbine blades. However, stray grain defects can form during the melt-back of the seed crystal, requiring the use of a helical grain selector between the seed and the blade to remove them. Using meso-scale numerical simulations, the formation mechanisms of these stray grain defects have been investigated. Also investigated was the influence of the seed’s crystallographic orientation relative to blade axis. The model is first validated by comparison to experimental observations and then by its application to a range of casting situations. The results show that initiation of these defects is difficult to avoid. Instead, the impact of stray grains should be controlled during their growth. For more information, contact P.D. Lee, Department of Materials, Imperial College London, Exhibition Road, London SW7 2BP, U.K.; e-mail p.d.lee@imperial.ac.uk.     

Simulation of lattice orientation effects on void growth and coalescence by crystal plasticity

A three dimensional rate-dependent crystal plasticity model is applied to study the influence of crystal orientation and grain boundary on the void growth and coalescence. The 3D computational model is a unit cell including one sphere void or two sphere voids. The results of three different orientations for single crystal and bicrystals are compared. It is found that crystallographic orientation has noticeable influences on the void growth direction,void shape, and void coalescence of single crystal. The void growth rate of bicrystals depends on the crystallographic orientations and grain boundary direction.     

Coupled Crystal Plasticity–Phase Field Fracture Simulation Study on Damage Evolution Around a Void: Pore Shape Versus Crystallographic Orientation

Various mechanisms such as anisotropic plastic flow, damage nucleation, and crack propagation govern the overall mechanical response of structural materials. Understanding how these mechanisms interact, i.e. if they amplify mutually or compete with each other, is an essential prerequisite for the design of improved alloys. This study shows—by using the free and open source software DAMASK (the Düsseldorf Advanced Material Simulation Kit)—how the coupling of crystal plasticity and phase field fracture methods can increase the understanding of the complex interplay between crystallographic orientation and the geometry of a void. To this end, crack initiation and propagation around an experimentally obtained pore with complex shape is investigated and compared to the situation of a simplified spherical void. Three different crystallographic orientations of the aluminum matrix hosting the defects are considered. It is shown that crack initiation and propagation depend in a non-trivial way on crystallographic orientation and its associated plastic behavior as well as on the shape of the pore.     

Crystallographic orientation relationships in coalescing sintered tungsten spheres

Earlier model experiments have shown that specific processes of coalescence occur during the sintering of tungsten particles in liquid nickel. In this work, measurements of crystallographic orientation were made on such coalescing sintered tungsten spheres using “selected area channeling patterns” in a scanning electron microscope. There is evidence that the growing neck in the contact region between two neighbouring spheres has the same crystallographic orientation as the sphere where growth begins. Furthermore, it is suggested that the necks often grow in favored crystallographic directions. Theoretical considerations on crystal growth mechanisms confirm this hypothesis.     

镍基单晶高温合金一次枝晶间距的统计研究

采用籽晶法制备了3种[001]取向的镍基单晶高温合金。通过正方形估算法和直接测量法统计了合金不同凝固高度的一次枝晶间距。该方案消除了晶界和晶体学取向对一次枝晶间距的影响。随着凝固高度升高,3种合金的一次枝晶平均间距逐渐增大。两种统计方法的一次枝晶平均间距相近,说明两种方法的精度均较高。此外,3种合金的一次枝晶间距在较大的范围内变化,且呈正态分布。3种合金的一次枝晶间距上限是下限的5.4~8.5倍。     

单晶叶片激光修复研究进展

单晶叶片激光修复技术为航空领域节约了大量成本,该技术以增材制造工艺为基础,以期在单晶基底上通过定向凝固生长出理想的单晶结构。目前,主流的单晶叶片修复技术包括以下两类：定向能量沉积技术（Directed Energy Deposition,DED）、粉末床熔融技术（Powder Bed Fusion,PBF）。本文综述了两种主要修复技术的研究进展,总结了修复过程中工艺参数的影响及单晶生长的机理,阐明了单晶修复技术在航空领域的应用潜力。此外,本文还讨论了单晶叶片修复目前面临的主要挑战,并对其未来发展趋势进行展望。     

Advanced Method for Single Crystal Casting of Turbine Blades for Gas Turbine Engines and Plants

A method for fabricating single crystal blades that combines the techniques of seed crystals and selection is suggested. The method realizes the advantages of both techniques, i.e., the high structural perfection and the possibility of fabricating single crystals with specified spatial orientation. Metallographic and x-ray diffraction analyses are used to study the processes of nucleation of the single crystal structure of blade castings fabricated from high-temperature nickel alloys by the method of selection and seed crystals. A commercial process for fabricating cast single crystal turbine blades by the new method is suggested.     

Variation in Machinability of Single Crystal Materials in Micromachining

For practical application of micromechanical machining, four levels of process realization are required; fundamental understanding of process physics, development of microplanning (processing parameter optimization), macroplanning (tool path planning), and design optimization. This study surveyed the influence of localized variation in the microstructure on final process outcome and machinability of brittle optical material in a ductile regime. A clear correlation between burr height, critical depth of cut and crystallographic orientation was found on single crystal materials (copper and magnesium fluoride), giving insight into optimal orientations and process parameters for acceptable micromachining process outcome.     

Crystal orientation and corrosion resistance of CuNi10Fe1Mn alloy billet under rotating electromagnetic field

CuNi10Fe1Mn alloy billet under rotating electromagnetic field (REF) was characterized in this work. The change of the crystal orientation was first explored by X-ray diffraction (XRD) and scanning election microscope (SEM); the corrosion resistance was done by three electrodes system, and the natural sea water was used as corrosion medium. The results demonstrate that the strongest crystal orientation is transformed from crystal plane (200) to (111); moreover, the crystal plane (111) whose intensity is the strongest on the cross section and vertical section with REF causes the tubes be rolled easily, and the corrosion resistance of the billet is increasing with REF. As a result, properties of CuNi10Fe1Mn alloy tubes can be improved by REF.