流体受迫流动下的柱状枝晶非对称生长

本文对Ｃｒ２５－Ｎｉ２０不锈钢在受迫流动条件下的柱状枝晶生长行为作了研究，结果表明，流体的定向受迫流对柱状枝晶的形貌影响很大，柱状枝晶的一次枝晶主干相互平等且明显细化，而柱状枝晶迎流侧的二次分枝较背流一侧发达，两侧的二次分枝呈不对称生长，但二次分枝始终垂直于一次枝晶主干，受迫流动造成柱状枝晶两侧温度分布的不均匀，是产生柱状枝晶不对称生长的主要原因。     

强磁场对Al-7%Si合金凝固过程中初生α-Al枝晶分布的影响

采用超导强磁场装置研究了磁场强度对有、无细化剂颗粒Al-7%Si(质量分数)合金凝固组织的作用效果。研究发现,施加强磁场使无细化剂颗粒合金中的初生α-Al枝晶转变为发达枝晶形貌,二次枝晶生长充分,三次枝晶分支明显,枝晶尖端清晰可见;晶粒细化,且枝晶主轴与磁场方向呈30°规则排列。施加强磁场后初生α-Al枝晶数量和Si在α-Al中的溶解度都有少量增加。强磁场抑制添加细化剂合金熔体中的对流,加剧了Ti的重力偏析,使初生α-Al相出现明显枝晶化趋势,方向性增强,枝晶臂粗化明显。     

重力对镍基单晶高温合金枝晶生长和微观偏析的影响

使用长50 m的落管研究Ni-Cr-Al-W-Ta镍基单晶高温合金在重力(1g)和微重力(μg)条件下的凝固行为。用金相显微镜(OM)观察合金的凝固组织并用图像分析软件测量和统计一次和二次枝晶间距,使用扫描电镜能谱(SEM-EDS)测定不同位置枝晶干与枝晶间的化学成分并计算微观偏析系数。结果表明,在重力和微重力条件下这种合金的枝晶特征和合金元素的微观偏析明显不同。重力样品一次和二次枝晶间距比微重力样品的大,随着凝固距离的增大一次枝晶间距的差异变大,而二次枝晶间距的差距变化不大。随着凝固的进行,微重力样品枝晶间Ta、Cr和Al元素的含量呈现先明显升高后略微降低的趋势,W元素含量呈现逐渐下降的趋势,枝晶间液相的密度呈现略微降低的趋势。重力样品枝晶间Ta、Cr和Al元素含量的分布趋势与微重力样品基本相似,W元素含量的分布则与微重力样品明显不同,大部分凝固阶段呈上升趋势,使枝晶间液相的密度沿逆重力方向提高。上述结果表明,在重力条件下凝固前沿溶质密度差导致的对流作用微弱,不是造成枝晶间距增加的主要原因,主要原因应该与凝固前沿热对流造成的温度梯度的降低有关。     

单晶凝固组织的样品尺寸效应

通过对不同直径的单晶样品的凝固实验，研究了样品尺寸对单晶凝固组织的影响规律，研究发现，随样品尺寸减小，单晶凝固组织被细化，尤其一次枝晶间距λ1和枝晶间γ′相尺寸对样品尺寸的依赖性更为明显；样品尺寸影响的要质在于此表面积的变化，进而对凝固过程温度梯度产生影响，最终作用于凝固组织和力学性能，一次枝晶间距λ1和枝晶间γ′尺寸与样品的比表面积近拟呈线性关系，样品尺寸变化，明显改变了单晶生长过程的温度梯度和合金的高温持久寿命。     

Al-4%Cu凝固过程枝晶生长的数值模拟

改进了模拟枝晶生长常用的二维元胞自动机和有限差分(CA-FD)模型,新模型引入扰动函数来控制二、三次枝晶的生长;在枝晶生长过程中,将溶质浓度明确地分为液相溶质浓度和固相溶质浓度两部分;并在溶质再分配与扩散过程中采用八邻位差分以减少网格形状导致溶质扩散的各向异性。模拟了Al-4%Cu二元合金过冷熔体中,单个和多个等轴晶沿不同择优方向生长及单方向和多方向柱状树枝晶竞争生长过程中的枝晶形貌、液相溶质浓度和固相溶质浓度分布情况。模拟结果表明:扰动的引入能够促使枝晶产生分支,并控制二、三次枝晶的生长速率;液/固相溶质计算模型能够准确地模拟出枝晶生长过程中液/固相溶质分布;此外改进后的模型实现了枝晶沿任意方向的竞争生长。     

凝固方向对单晶高温合金枝晶组织的影响

用籽晶法制备了沿不同晶体取向凝同的镍基单晶高温合金试样,研究了单晶中枝晶形貌和一次枝晶臂距随凝固取向的变化规律.结果表明:凝同方向偏离[001]取向小于15°时,枝晶排列比较规则,一次枝晶臂距随偏离角度增大而减小;偏离角度为25°时,部分二次枝晶臂阻断了相邻一次枝晶干的生长,导致一次枝晶臂距增大.沿[011]和[111...     

强制对流作用下多晶粒生长的相场模拟

采用Tong和Beckermann等提出的耦合流场相场模型对纯镍凝固中多晶粒枝晶的生长过程进行模拟,研究了多晶粒枝晶的生长形貌和温度分布.结果表明,熔体流动显著改变凝固前沿的传热,从而影响枝晶生长.受熔体流动和多晶粒之间相互影响的共同作用,枝晶在4个最优生长方向上的形貌呈现不对称;熔体流动还改变了枝晶的水平最优生长方向,使得水平主枝向上游倾斜;此外,二次枝晶出现径向熔化和轴向熔化等粗化方式.     

枝晶生长的稳态理论新进展

本文从自由生长和强制生长两个方面着重评述了单相合金枝晶生长的稳态理论新进展。发现稳态理论近年来有两个重要的进展，一是比较好地解决了自由生长枝晶尖端的稳态扩散场问题；另一是理论分析和实验研究的结果都表明，强制生长枝晶一次间距选择一个较宽的容许范围，平均一次间距在这个范围内的具体取值与凝固系统所经历的历史明显相关     

Simulation of dendrite growth of Fe–C alloy using phase field method

采用相场法模拟了Fe--0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长, 研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响, 获得了具有二次分枝的枝晶形貌, 再现了枝晶生长过程及枝晶臂之间的竞争生长. 模拟结果表明: 凝固过程中存在溶质富集和枝晶偏析, 枝晶主干溶质浓度最低, 枝晶臂之间的液相浓度最高. 随着过冷度的增大, 枝晶生长加快且分枝发达; 界面厚度直接影响枝晶的生长速度; 各向异性影响枝晶的形态; 晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.     

用相场方法模拟Fe-C合金枝晶生长

采用相场法模拟了Fe-0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长,研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响,获得了具有二次分枝的枝晶形貌,再现了枝晶生长过程及枝晶臂之间的竞争生长.模拟结果表明:凝固过程中存在溶质富集和枝晶偏析,枝晶主干溶质浓度最低,枝晶臂之间的液相浓度最高.随着过冷度的增大,枝晶生长加快且分枝发达;界面厚度直接影响枝晶的生长速度;各向异性影响枝晶的形态;晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.     

Non—uniform Stress Field and Stress Concentration Induced by Grain Boundary and Triple Junction of Tricrystal

The stress characteristics in the anisotropic bicrystal and tricrystal specimens were analyzed using the anisotropic elastic model, orthotropic Hill‘s model and rate-dependent crystallographic model. The finite element analysis results show that non-uniform stresses are induced by the grain boundary. For bicrystal specimens in different crystallographic orientations, there exist stress concentrations and high stress gradients nearby the boundaries. The activation and slipping of the slip systems are dependent on the crystallographic orientations of the grains and also on the relative crystallographic orientations of the two adjoining grains. For the tricrystal specimens, there is not always any stress concentrations in the triple junction, and the concentration degree depends on the relative crystallographic orientations of the three grains. Different from the bicrystal specimens, there may be or no stress concentration in the vicinity of grain boundaries for the tricrystal specimens, which depends on the relative crystallographic orientations of the three grains. The stress concentration near to the grain boundaries and triple junction can be high enough for the local plastic deformation, damage and voiding or cracking even when the whole specimen is still under the elastic state.It can be further concluded that homogeneous assumption for polycrystalline materials is not suitable to study the detailed meso- or micro-mechanisms for damaging and fracturing.     

Molecular dynamics simulation of incipient plasticity of nickel substrates of different surface orientations during nanoindentation

The present study aims to elucidate the anisotropic characteristics in material responses for crystallographic nickel substrates with (001), (011) and (111) surface orientations during nanoindentation. Molecular dynamic simulation is applied to compensate for the experimental limitation of nanoindentation, particularly for pure nickel substrates. Defect nucleation and evolution in Ni single crystal of these three crystal orientations was examined. Hardness and Young’s modulus are also extracted in different orientations. The Young’s modulus of (111) crystallographic orientation is the largest, while that of (001) surface is the smallest. The sensitivity of the yield point for face centred cubic crystals depends on the crystallographic orientation. The (001) crystallographic orientation reaches the yield point first, while the (111) crystallographic orientation is the most difficult in which to achieve yield. Using a visualisation method of centrosymmetry parameter, the homogeneous nucleation and early evolution of dislocations were investigated, deepening understanding of incipient plasticity at the atomic scale. The present results suggest that defect nucleation and evolution are the root of curve jitter. The indentation depth of the elastic–plastic transition point varies in the different crystallographic orientation models, and appears latest in the (111) model. The strain energy of the substrate exerted by the tip is stored by the formation of homogeneous nucleation and is dissipated by the dislocation slide in the {111} glide plane. The three nickel substrates with different crystallographic orientations exhibit different forms of dislocation propagation.     

The effects of slant angle on local stress distribution around cooling hole in nickel‐based single crystal

Based on crystal plasticity theory, plate specimens with a cooling hole were adopted to investigate the stress distribution and crystallographic slip characteristics, the effects of crystallographic orientations taken into consideration. The slant angles are 0°, 15°, 30° and 45°. The results show that severe stress concentrations and high stress gradients present at the cooling holes. Stress distribution changes significantly with different slant angles and crystallographic orientations. Slip bands advancing to specific directions initiate around the holes and the characteristics of the slip band vary with slant angles. Four apparent maximum values occur along the hole in the three orientations and the locations of the maximum values are much dependent on the slant angles. The influences of slant angle on the activating law of the slip systems are remarkable.     

The influence of grains’ crystallographic orientations on advancing short crack

A model of microstructurally short cracks that accounts for random grain geometry and crystallographic orientations is coupled with crystal plasticity constitutive model. A short crack is then inserted in the slip plane in one of the grains at the model top boundary and extended into one of the available slip planes of the neighboring grain at monotonic remote load of 0.96R_p0.2. Crack tip opening (CTOD) and sliding (CTSD) displacements are then calculated for several different crystallographic orientations and crack lengths. As the crack is contained in a single grain the crystallographic orientation of the neighboring grain can change the crack tip displacements by up to 26%, however, the displacements change by up to a factor of 10, once the crack is extended beyond the grain boundary into the next grain. Significant CTSD values were observed in all the analyzed cases pointing to mixed mode loading. Another important observation is that the random crystallographic orientations of grains beyond the first two crack-containing grains affect the CTOD by a factor of up to 4.4. This effect decreases slightly with increased crack length.     

Effect of Crystallographic Anisotropy on Micro EDM Process

In this paper, experiments are conducted by machining from different crystallographic orientations of monocrystalline silicon, and the effects of crystallographic orientation on the micro electrical discharge machining (EDM) process are discussed. The results demonstrate that the machining speed and surface roughness are varied when crystallographic orientation changes. The surface roughness is seen to vary by as much as twofold with crystallographic orientation, while the ratio between the maximum and minimum values of material removal rate is 1.76. The unique material removal mechanism of micro EDM enhances the effects of crystal anisotropy on micro electrical discharge machining process.     

Energy selective neutron imaging in solid state materials science

In neutron radiography and tomography, the image contrast is caused by a variation of the effective macroscopic cross-section over the sample volume. Narrowing the energy band of the polychromatic neutron beam in the cold energy range increases the image contrast significantly and opens an access to the crystallographic structure of the sample. Here, we show that crystallographic microstructures of welded stainless steel samples can be visualized and quantified in two and three dimensions by the energy selective neutron imaging. The energy selective neutron radiography maps preferred crystallite orientations over the sample and provides energy values of the highest image contrast. Furthermore, a high contrast neutron tomography visualizes preferred crystallite orientations over the whole macroscopic sample volume.     

用球形单晶体研究干涉色与晶体位向的关系

通过对工业纯铁的球形单晶体试样化学染色，全方位地显示了各种位向表面的干涉色及各干涉色在球面上的对称分布特征，经Ｘ射线背射芳厄法位向分析标定出各干涉色与晶体位向的对应关系，绘制出相位化染图案的单位赤面投影三角形，结果表明，单晶体金属表面上的每一种干涉色对应于一定范围内的许多不同位向。球面化染图案具有与被染色金属晶体的结构桢的对称性，可用于晶体的定量测定。     

Fracture Behavior of Single-Crystal Alloys Under Thermocyclic Loading

Strength of Materials - Experimental study results for thermal fatigue fracture of sand-glass specimens from a ZhS32 single-crystal alloy with different crystallographic orientations are presented...     

The influence of crystallographic orientation on crack tip displacements of microstructurally small, kinked crack crossing the grain boundary

The paper presents an analysis of the effects of grain orientations on a short, kinked surface crack in a 316L stainless steel. The kinking of the crack is assumed to take place at the boundary between two neighbouring grains. The analysis is based on a plane-strain finite element crystal plasticity model. The model consists of 212 randomly shaped, sized and oriented grains, loaded monotonically in uniaxial tension to a maximum load of 0.96R_p0.2 (240 MPa). The influence that a random grain structure imposes on a Stage I crack is assessed by calculating the crack tip opening (CTOD) displacements for bicrystal as well as for polycrystal models, considering different crystallographic orientations. Since a Stage I crack is assumed, the crack is always placed in a slip plane. Results from a bicrystal case show that the maximal CTODs are directly related to the stiffness of the grain containing the crack extension. Anisotropic elasticity and crystal plasticity both contribute to this grain stiffness, resulting in maximal CTOD when Schmid factors are the highest on two slip planes. Such crystallographic orientation results in a soft elasto-plastic response. Anisotropic elasticity can additionally increase the softness of a grain at certain crystallographic orientations. Minimal anisotropic elasticity at the crystallographic orientations with the highest Schmid factors causes the CTOD to be maximized. Presuming that the crack will preferably follow the slip plane where the crack tip opening displacement is highest, we show that the crystallographic orientation can affect the CTOD values by a factor of up to 7.7. For a given grain orientation the maximum CTOD is attained when the crack extension deflection into a second grain is between −75.141° and 34°. For the polycrystal case we show that grains beyond the first two crack-containing grains change the CTOD by a factor of up to 3.3 and that the largest CTODs are obtained when placing the crack into a slip plane with crack extension that results in a crack extension being more perpendicular to the external load.     

Significance of crystallographic grain orientation for oxide scale formation on FeCrAl ODS alloys studied by AFM and MCs+-SIMS

Abstract

Single crystalline specimens were prepared by spark erosion of large grains in an extremely coarse textured bar of an FeCrAl based ODS alloy. These flat single crystalline specimens were studied in respect to oxidation behaviour at 800°C and 1,000°C in air. Initially the oxide scales appeared to grow by outward diffusion of aluminium but subsequently scale growth proceeded by grain boundary oxygen diffusion. The composition, grain size and growth rate of the transient oxide appeared to be dependent on the crystallographic orientation of the alloy. After the inward diffusion of oxygen had become dominant, the oxides on specimens with different crystallographic orientations showed clear differences in their growth rates. Correlation of SIMS results with quantitative grain size analysis performed by AFM showed that the different oxide growth rates, which pertained during longer exposure times, were related to differences in the oxide grain size and therefore the density of oxygen diffusion paths with different crystallographic orientations.