Grain boundary segregation,solute drag and abnormal grain growth

By employing a phase-field model, we simulated the grain boundary (GB) kinetics in terms of GB segregation of solute atoms for an isolated grain embedded in a matrix and demonstrated that the phase-field simulation could describe the GB movement under conditions of GB segregation in a quantitatively correct way. We then modeled grain growth in association with GB segregation in two-dimensional polycrystalline systems, and clarified that similarity between the Cottrell effect and the solute drag effect holds even on the macroscopic scale, that is, abnormal grain growth (AGG) can be induced by the solute drag effect. This AGG can take place spontaneously in homogeneous systems without any texture, anisotropy of GB mobility and/or energy, pinning particles and grain size advantage. The basic characteristics of this AGG and the effects of the solute diffusivity and the average grain size were investigated in detail.     

Diffusion controlled creep in nanocrystalline materials under grain growth

We consider diffusion controlled creep in nanostructured materials under the conditions when grain growth occurs concurrently. The Nabarro–Herring and Coble mechanisms are re-visited to calculate the effect of attendant vacancy generation on creep. Strain rate variation that resembles primary and secondary creep behaviour is predicted.     

Unraveling the nature of room temperature grain growth in nanocrystalline materials

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor are there any other systems aware to us that manifest a similar growth behaviour. We detect a transition from an initially self-similar slow growth to abnormal grain growth. Its onset seems to be associated with the simultaneous decrease of microstrain with increasing grain size. Abnormal grain growth is considered as a generic feature of nanocrystallinity but is a transient state since we observed in the late stage of coarsening, using orientational imaging microscopy, a monomodal grain size distribution. We empirically find a nonlinear-response-type of growth law which is in agreement with the observed coarsening kinetics.     

Phase field modeling of grain boundary migration with solute drag

The grain boundary (GB) motion in the presence of GB segregation is investigated by means of phase field simulations. It is found that the solute concentration at the moving GB may increase with increasing velocity and becomes larger than the equilibrium value, which is unexpected according to the solute drag theory proposed by Cahn, but has been observed in some experiments. A non-linear relation between the driving force (curvature) and the GB velocity is found in two cases: (1) the GB motion undergoes a transition from the low-velocity extreme to the high-velocity extreme; (2) the GB migrates slowly in a strongly segregating system. The first case is consistent with the solute drag theory of Cahn. As for the second case, which is unexpected according to solute drag theory, the non-linear relation between the GB velocity and curvature comes from two sources: the non-linear relation of the solute drag force with GB velocity, and the variation in GB energy with curvature. It is also found that, when the diffusivity is spatially inhomogeneous, the kinetics of GB motion is different from that with a constant diffusivity.     

The effect of triple-junction drag on grain growth

Current theories of grain growth presume that grain boundary migration is the rate-limiting step, and either explicitly or implicitly assume that triple junctions can always move with sufficient speed to accommodate the changing positions of the grain boundaries. Following from some recent observations of triple-junction drag effects in tricrystals of zinc and in molecular dynamics models, an analytical theory is developed to explore the effects of triple-junction drag upon grain growth, for a two-dimensional solid. The theory is developed in the framework of the Von Neumann–Mullins formulation, and demonstrates that drag effects operating exclusively at the triple junctions result in a retardation of grain growth. The stability of six-sided grains in the isotropic, drag-free case of the Von Neumann–Mullins analysis is successively extended to grains of 6±N sides, where N increases with the strength of the triple-junction drag.     

纳米ZrO2-8％Y2O3粉末的相转变及晶粒生长动力学

采用共沉淀法制备纳米ZrO2-8％Y2O3(质量分数)粉末,然后将其在大气中于1 100～1 300℃范围内高温煅烧处理2～32 h.利用XRD、SEM、TEM等方法研究纳米ZrO2-8％Y2O3粉末高温煅烧前后的相成分、形貌和晶粒粒径变化,并分析纳米ZrO2-8％Y2O3粉末的晶粒生长动力学及生长机制.结果表明:纳米ZrO2-8％Y2O3经高温煅烧后,单斜相和四方相含量随温度的升高和时间的延长而减少,立方相含量随温度的升高和时间的延长而增加;随温度的升高和时间的延长晶粒粒径逐渐增大;在1 250℃等温煅烧时,其晶粒生长指数为6,晶粒生长速率常数为7.626×1011 nm3/min:等温锻烧温度低于1 200℃时,晶粒生长活化能为64.35 kJ/mol,晶粒生长表现为以表面扩散为主的聚合生长;等温锻烧温度高于1 200℃时,晶粒生长活化能为1 16.40 kJ/mol,晶粒生长表现为以晶格扩散为主的聚合生长;另外,还可见晶粒旋转驱动的聚合生长机制;低的晶粒生长激活能归因于大量氧空位的引入和晶粒旋转驱动的聚合生长机制.     

金属纳米晶粉体材料中的不连续晶粒长大

采用高能球磨法制备了Co纳米晶粉体,平均晶粒尺寸为(17±3)nm.设计了一系列宽温度范围的退火实验,考察纯Co纳米晶粉体的晶粒长大行为.实验发现,低温区和高温区的晶粒长大动力学有明显差异,而在中温区出现不连续晶粒长大特征.高分辨透射电镜观测表明:在低温区,纳米晶中存在较大比例的小角度纳米晶界,而在高温区则基本为典型的大角度晶界.结合纳米晶热力学计算和DSC分析,认为纳米晶粒在中温区的突发迅速长大是由残余储存能作为附加驱动力激发的动力学过程,其主导机制是通过相邻小角度位向差的纳米晶粒的转动而实现晶粒快速粗化.     

Strategies to suppress grain growth of nanocrystalline aluminum

The grain growth behaviors of nanocrystalline aluminum, alloy and composite are compared. First, nanocrystalline aluminum is fabricated by consolidation of ball-milled powder. Second, nanocrystalline aluminum alloy is designed to have elements such as Mn, Zr, and Misch metals, which can form thermally stable second phases at grain boundaries and also drag the movement of grain boundaries. Third, nanocrystalline aluminum-based composites containing multi-walled carbon nanotubes (MWCNTs) are also prepared because MWCNTs are expected to be located at grain boundaries and to suppress the grain growth of nanocrystalline aluminum. These three types of samples are annealed at 550 °C for up to 5 d and the effect of annealing time on Vickers hardness of the samples is compared. As a result, MWCNTs are found to be most effective to impede grain growth of nanocrystalline aluminum.     

第二相颗粒对多晶材料晶粒生长影响的元胞自动机(CA)模拟

采用Moore型邻域定义的元胞自动机模型模拟研究第二相颗粒对多晶材料晶粒生长的影响.结果表明:第二相颗粒的体积分数及尺寸对基体晶粒组织特征的影响很大;第二相颗粒含量增加可以提高晶粒尺寸分布的均匀性,而颗粒尺寸增大则导致晶粒尺寸分布的均匀性降低.通过对模拟数据的回归分析获得极限晶粒尺寸(D)与颗粒尺寸(d)和颗粒含量(f)之间的关系;不同的颗粒尺寸(d)对应不同的拟合指数(n).     

Triple junction drag and grain growth in 2D polycrystals

The process of grain growth in 2D systems is analyzed with respect to the controlling kinetics: from solely boundary kinetics, when grain growth in a polycrystal is determined by the Von Neumann–Mullins relation, to exclusively triple junction kinetics, when grain growth is governed by the mobility of triple junctions. It is shown that in the “intermediate” case, when the driving force for grain boundary motion and the characteristic mobility are grain boundary curvature and grain boundary mobility, respectively, a limited mobility of triple junctions essentially influences grain boundary motion. The Von Neumann–Mullins relation does not hold anymore, and this is the more pronounced the smaller the triple junction mobility. In the case where grain growth is determined by the mobility of grain boundary triple junctions (triple junction kinetics) all grains are transformed into polygons in the course of grain growth. Grain growth would cease if all grains assumed the shape of regular polygons, not only hexagons like in the Von Neumann–Mullins case. The only exceptions are triangles: they collapse without transforming into a polygon. The respective relation for the rate of a change of grain area under triple junction kinetics is obtained and discussed with regard to microstructure evolution.     

Reappraisal of grain boundary diffusion creep equations for nanocrystalline materials

Grain boundary diffusion creep equations developed previously for nanocrystalline materials were reappraised in order to elicit further understanding of plastic deformation of these materials in relation to grain boundary diffusion. From a mechanistic viewpoint, the strain rate is inversely proportional to the second power of the grain size when the grain size is refined to the same order of the grain boundary thickness. The presence of the threshold stress appears to be inherent, as a relatively large volume fraction of the grain boundary region is associated with irregularities.     

Crossover from grain boundary sliding to rotational deformation in nanocrystalline materials

A theoretical model is suggested which describes cooperative action of grain boundary (GB) sliding and rotational deformation in mechanically loaded nanocrystalline materials. Focuses are placed on the crossover from GB sliding to rotational deformation occurring at triple junctions of GBs. In the framework of the model, gliding GB dislocations at triple junctions of GBs split into dislocations that climb along the adjacent boundaries. The splitting processes repeatedly occurring at triple junctions give rise to climb of GB dislocation walls that carry rotational deformation accompanied by crystal lattice rotation in grains of nanocrystalline materials. The role of GB sliding, rotational deformation and conventional dislocation slip in high-strain-rate superplastic flow in nanocrystalline materials is discussed.