镍基单晶超合金中孔洞长大的试验和有限元分析

在高温状态下，镍基单晶超合金的变形、损伤及断裂分析中，孔洞的长大都起着主要的作用。本研究进行了系列的蠕变、疲劳及热机械疲劳（TMF）试验。对试件的断面进行的SEM观察表明，所有的断面都是有许多小断面构成，在断面的中心，至少有一个孔洞。孔洞的尺寸与加载的条件相关。使用晶体塑性有限元程序对单胞模型进行分析，模拟孔洞的长大规律。给出了蠕变和弹塑性两种条件下的模拟结果及不同的晶体取向对孔穴长大的影响结果。对孔洞长大的有限元分析有助于对实验结果的理解。     

晶体取向对镍基单晶高温合金纳米压痕行为的影响

为了研究晶体取向对镍基单晶高温合金纳米压痕行为的影响，采用带原子力显微镜的Berkovich压头对[001], [011]和[111]取向的镍基单晶高温合金开展了纳米压痕试验。试验结果显示晶体取向对压痕载荷-深度曲线、硬度和弹性模量有显著影响。晶体取向会影响γ^"强化相形状和体积分数，进而会影响其力学性能。采用晶体塑性理论对三种典型晶体取向下的纳米压痕响应和分解切应力分布开展有限元模拟，模拟结果与试验结果符合得较好。     

孪生诱发软化与强化效应的Cu晶体塑性行为模拟

基于晶体塑性理论,考虑孪生软化效应建立了描述孪晶形核、增殖和长大的位错密度基晶体塑性有限元模型。应用该模型揭示了不同晶体取向Cu单晶拉伸变形过程中位错滑移、孪生激活及其交互作用下的宏观塑性行为演化规律,进一步分析了Cu多晶拉伸变形过程中晶粒间交互作用对孪生软化、应变硬化等宏观塑性行为的影响。结果表明：孪生具有明显的取向效应,在孪生主导塑性条件下,Cu单晶塑性变形过程中孪晶增殖导致应力-应变曲线存在明显的应力突降现象,其塑性变形分为滑移、孪生及位错与孪晶交互作用3个阶段;此外,随着饱和孪晶体积分数增加,Cu单晶塑性变形过程中第3阶段的应变硬化率也随之提升。进一步模拟Cu多晶拉伸变形的塑性行为可知,在晶粒间交互作用下孪晶形核、增殖和长大过程中不会出现应力突降现象,与Cu单晶相比整个塑性变形过程具有更高的应变硬化率;Cu多晶塑性变形过程中位错密度在晶界处出现集中现象,孪晶也容易在晶界处形成。     

孪生诱发软化与强化效应的Cu晶体塑性行为模拟

基于晶体塑性理论,考虑孪生软化效应建立了描述孪晶形核、增殖和长大的位错密度基晶体塑性有限元模型。应用该模型揭示了不同晶体取向Cu单晶拉伸变形过程中位错滑移、孪生激活及其交互作用下的宏观塑性行为演化规律,进一步分析了Cu多晶拉伸变形过程中晶粒间交互作用对孪生软化、应变硬化等宏观塑性行为的影响。结果表明：孪生具有明显的取向效应,在孪生主导塑性条件下,Cu单晶塑性变形过程中孪晶增殖导致应力-应变曲线存在明显的应力突降现象,其塑性变形分为滑移、孪生及位错与孪晶交互作用3个阶段;此外,随着饱和孪晶体积分数增加,Cu单晶塑性变形过程中第3阶段的应变硬化率也随之提升。进一步模拟Cu多晶拉伸变形的塑性行为可知,在晶粒间交互作用下孪晶形核、增殖和长大过程中不会出现应力突降现象,与Cu单晶相比整个塑性变形过程具有更高的应变硬化率;Cu多晶塑性变形过程中位错密度在晶界处出现集中现象,孪晶也容易在晶界处形成。     

微孔洞对高周循环载荷作用下双相钛合金微塑性行为的影响(Ⅰ):晶体塑性分析

建立二维晶体塑性有限元模型研究微孔洞与高周循环载荷作用下双相钛合金微塑性变形行为的关系.结果表明,几何必需位错(GND)倾向聚集于微孔洞周围,且导致平均GND密度升高.孔洞尖端塑性区域(TPZ)的曲率对GND密度的影响大于孔洞尺寸的影响.随着TPZ曲率和空洞尺寸的增大,初生α、次生α及β基体内的累积剪切应变增大.初生α...     

晶体取向对单晶高温合金一次枝晶间距的影响

在固液界面微单元热量平衡分析的基础上，建立了晶体取向对一次枝晶间距影响的理论模型。该模型表明，一维择优晶体取向与宏观定向凝固方向偏离越远，一次枝晶间距越小，对ＤＤ８单晶高温合金凝固组织尺度的实验研究结果表明，晶体取向对一次枝晶间距的影响趋势和理论模型相吻合，其影响程度和固液界面温度梯度及定向凝固速率有关。模型及实验都表明，提高固液界面温度梯度和定向凝固速率，晶体取向对一次枝晶间距的影响程度减弱。     

脉冲电沉积纳米晶体镍镀层热稳定性的研究

采用脉冲电沉积法制备了纳米晶体镍镀层，平均晶粒尺寸约为20nm。采用热分析法、透射电子显微镜(17EM)和X射线衍射方法(XRD)研究了纳米镍镀层的热稳定性。结果表明，纳米晶体镍镀层晶粒开始明显长大温度约为255℃，晶粒长大过程分为两个阶段：低温晶粒异常长大阶段(200～300％)和晶粒正常长大阶段(300～500％)。原始镍镀层的(111)和(200)面双织构在低温晶粒异常长大阶段仍存在，但在正常长大阶段逐渐消失。100％加热后镍镀层显微硬度略有增高，随后随着加热温度的升高不断降低。     

晶体取向和载荷模式对Ni3Al合金单晶体疲劳行为的影响

在名义Ⅰ型和Ⅰ／Ⅱ复合型载荷下研究了晶体取向和加载模式对Ｎｉ３Ａｌ合金单体疲劳开裂行为的影响。结果表明：疲劳开裂行为强烈地依赖于晶体取向和加载模式。在单晶体中，站劳裂纹总是沿着具有最大切应力的滑移须开裂。根据单晶体的实验结果，解释了用三种主要的复合型断裂准则所预测的多晶体在复合型加载下的疲劳开裂行为。     

润湿角对振动激冷表面晶体形核及剥离的影响

研究了润湿角对LY12铝合金和1Cr18Ni9Ti不锈钢两种材质的激冷振动表面晶体形核及剥离的影响。结果表明,润湿角较大的不锈钢材质激冷振动表面几乎没有形核发生,仅在固-液界面处有晶雨的下落。对于润湿角较小的铝合金材质在固-液界面处也存在晶雨的下落,但是晶体形核行为非常明显,并且长大的晶粒形成游离晶。     

晶体塑性模型分析微圆柱墩粗变形尺寸效应机理(英文)

为了分析单个晶粒变形行为对微成形的影响,将自由表面的晶粒看作单晶体构建晶体塑性模型。基于率相关晶体塑性理论,考虑试样尺寸、初始晶体取向及其分布,分析微圆柱体墩粗变形中尺寸效应机理。结果表明,流动应力随着试样尺寸的减小而明显减小,晶体取向的分布对试样流动应力具有显著影响,并随着塑性变形的进行而减小。由于单晶体的各向异性,在试样表层发生了明显的非均匀变形,这将导致表面粗糙度的增加,小尺寸试样则更加明显。过渡晶粒的存在使得晶粒各向异性对表面形貌的影响减小。模拟结果得到了实验验证,这表明所建立的模型适合于以尺寸依赖性、流动应力分散性和非均匀变形为特点的微成形工艺分析。     

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.     

Void Shape Effects on Void Growth and Slip Systems Activity in Single Crystals

建立了含椭球形微孔的三维体胞,该模型包含了椭球形微孔的一种特例:即球形微孔。采用晶体塑性滑移理论对不同取向下,单晶合金铸造微孔形状对微孔生长和滑移系激活的影响进行了研究。结果表明,材料的晶体坐标系、椭球微孔坐标系和载荷坐标系之间的坐标转换角度以及椭球微孔的形状对于微孔的演化具有非常重要的影响。对于三维应变状态下,椭球微孔的形状、晶体取向与载荷之间的相互关系共同决定了铸造微孔体积的增长、滑移系的激活和微孔旋转。当单胞滑移系的对称性被椭球型微孔破坏,即使载荷与滑移系统具有对称性,铸造微孔也会发生旋转。虽然单晶合金具有强烈的正交各向异性,但是当铸造微孔初始形状不为球形时,材料性能的正交各向异性对铸造微孔体积增长的影响被削弱。     

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.     

Discrete dislocation dynamics modelling of microvoid growth and its intrinsic mechanism in single crystals

In the present paper, an infinite face-centered cubic single crystal containing an isolated cylindrical micron-sized void, which is subjected to proportional and monotonically uniform equal biaxial tension loading, is adopted to study the scale-dependent void growth and its intrinsic mechanism by employing a two-dimensional planar discrete dislocation dynamic framework. First, a typical dislocation distribution near the microvoid is presented and the void growth mechanism is revealed by dislocation shear loop expansion for each of three typical fcc slip systems. The effect of size on void growth is then investigated. The general conclusion that voids at the micron or submicron scale are less susceptible to growth than larger ones is drawn. Another result, which cannot be deduced from the continuum theories, is also achieved: at the micron or submicron scale, larger voids grow smoothly with remote strain, while smaller voids usually grow in a “leapfrog” manner. Specifically, when the void is even smaller, it grows in an approximately linear-elastic manner since only few dislocations are present around the void. Further analyses indicate that these size effects are closely related to the dislocation density on the void surface and the dislocation mobility around the void. Finally, the influences of the dislocation sources/obstacles density and their random distribution in materials on the void growth are studied briefly. Results show that there exists remarkable scatter in the microvoid growth due to random distribution of the dislocation sources or obstacles, especially for voids at the submicron scale. These results are helpful for us in understanding the size-dependent damage mechanism at the micron or submicron scale.     

Effects of Crystal Orientations on the Low-Cycle Fatigue of a Single-Crystal Nickel-Based Superalloy at 980 °C

The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the fatigue life not only depends on the elastic modulus, but also the number of active slip planes and the plasticity of materials determine the LCF life,especially for the [011] and [111] specimens. The [011] and [111] specimens with better plasticity withstand relatively concentrated inelastic deformation caused by fewer active slip planes, compared to the [001] specimens resisting widespread deformation caused by a higher number of active slip planes. Additionally, fatigue fracture is also influenced by cyclic plastic deformation mechanisms of the alloy with crystal orientations, and the [001] specimens are plastically deformed by wave slip mechanism and fracture along the non-crystallographic plane, while the [011] and [111] specimens are plastically deformed by planar slip mechanism and fracture along the crystallographic planes. Moreover, casting pores,eutectics, inclusions and surface oxide layers not only initiate the crack, but also reduce the stress concentration around crack tips. Our results throw light upon the effect of inelastic strain on the LCF life and analyze the cyclic plastic deformation for the alloy with different orientations.     

Orientation dependence of nanoindentation pile-up patterns and of nanoindentation microtextures in copper single crystals

We present a study about the dependence of nanoindentation pile-up patterns and of microtextures on the crystallographic orientation using high purity copper single crystals. Experiments were conducted on a Hysitron nanoindentation setup using a conical indenter in order to avoid symmetries others than those of the crystal structure. Orientation measurements were conducted using a high resolution electron back-scatter diffraction technique for the automated acquisition of texture mappings around the indents. Simulations were carried out by means of a 3D elastic–viscoplastic crystal plasticity finite element method which takes full account of crystallographic slip and orientation changes during indentation. The experiments as well as the simulations show that the pile-up patterns on the surfaces of (0 0 1)-, (0 1 1)- and (1 1 1)-oriented single crystals have four-, two-, and sixfold symmetry, respectively. The different pile-up patterns can be explained in terms of the strong crystallographic anisotropy of the out-of-plane displacements around the indents. Pronounced accumulation of material entailing characteristic pile-up patterns occurs along the intersection vectors between the primary crystallographic slip planes and the indented surface planes.     

镍基单晶合金压痕蠕变研究

为了探索用压痕法确定晶体学蠕变应力指数的可行性,采用晶体塑性理论模型对不同取向镍基单晶合金压痕蠕变行为进行了有限元模拟。研究表明,晶体学对称性会导致压痕表面应力呈现特定的对称性;晶体取向对压痕蠕变深度及其扩展速率具有显著的影响。通过开展不同载荷下的压痕蠕变模拟,获得相应的稳态压痕深度扩展速率,再通过线性回归的方法可有效获得晶体学蠕变应力指数。本文提出的确定蠕变应力指数的方法不受晶体取向的影响,有望应用于各向异性高温合金蠕变性能评估。     

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.     

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。     

Modeling void growth in polycrystalline materials

Most structural materials are polycrystalline aggregates whose constituent crystals are irregular in shape, have anisotropic mechanical properties and contain a variety of defects, resulting in very complicated damage evolution. Failure models of these materials remain empirically calibrated due to the lack of a thorough understanding of the controlling processes at the scale of the materials’ heterogeneity, i.e. the mesoscale. This paper describes a novel formulation for a quantitative, microstructure-sensitive three-dimensional mesoscale prediction of ductile damage of polycrystalline materials, in the important void growth phase of the process. Specifically, we have extended a formulation based on fast Fourier transforms to compute growth of intergranular voids in porous polycrystalline materials. In this way, two widely used micromechanical formulations, i.e. polycrystal plasticity and dilatational plasticity, have been efficiently combined, with crystals and voids represented explicitly, to predict porosity evolution. The proposed void growth algorithm is first validated by comparison with corresponding finite-element unit cell results. Next, in order to isolate the influence of microstructure on void growth, the extended formulation is applied to a face-centered cubic polycrystal with uniform texture and intergranular cavities, and to a porous material with homogenous isotropic matrix and identical initial porosity distribution. These simulations allow us to assess the effect of the matrix’s polycrystallinity on porosity evolution. Microstructural effects, such as the influence of the Taylor factor of the crystalline ligaments linking interacting voids, were predicted and qualitatively confirmed by post-shocked microstrostructural characterization of polycrystalline copper.