Atomistic Simulation of Curvature Driven Grain Boundary Migration

We present two dimensional molecular dynamics simulations of grain boundary migration using the half-loop bicrystal geometry in the experiments of Shvindlerman et al. We examine the dependence of steady-state grain boundary migration rate on grain boundary curvature by varying the half-loop width at constant temperature. The results confirm the classical result derived by absolute reaction rate theory that grain boundary velocity is proportional to the curvature. We then measure the grain boundary migration rate for fixed half-loop width at varying temperatures. Analysis of this data establishes an Arrhenius relation between the grain boundary mobility and temperature, allowing us to extract the activation energy for grain boundary migration. Since grain boundaries have an excess volume, curvature driven grain boundary migration increases the density of the system during the simulations. In simulations performed at constant pressure, this leads to vacancy generation during the boundary migration, making the whole migration process jerky.     

Interactions between vacancies and prismatic Σ3 grain boundary in α-Al_2O_3:First principles study

Interactions between vacancies and Σ3 prismatic screw-rotation grain boundary in α-Al_2O_3 are investigated by the first principles projector-augmented wave method.It turns out that the vacancy formation energy decreases with reducing the distance between vacancy and grain boundary(GB) plane and reaches the minimum on the GB plane(at the atomic layer next to the GB) for an O(Al) vacancy.The O vacancy located on the GB plane can attract other vacancies nearby to form an O–O di-vacancy while the Al vacancy cannot.Moreover,the O–O di-vacancy can further attract other O vacancies to form a zigzag O vacancy chain on the GB plane,which may have an influence on the diffusion behavior of small atoms such as H and He along the GB plane of α-Al_2O_3.     

Interdiffusion in two-layer Pd/Ag films II. “Cold” homogenization mechanisms

Interdiffusion processes in thin epitaxial polycrystalline Pd/Ag films in the temperature range 20–500°C are studied by transmission electron microscopy, electron diffraction and electrical resistance methods. Homogenization is investigated both during condensation and under conditions of postcondensation annealing.The basic processes of homogenization associated with GB diffusion along migrating boundaries. It is found that in real polycrystal films with wide spectrum of grain sizes few mechanisms can occur simultaneously or subsequently: recrystallization induced diffusion, diffusion induced grain boundary migration, activation of bulk diffusion in fine grain clusters, bulk diffusion through interphase boundary. The conditions for prevailing one of them can be provided by changing condensation and postcondensation annealing temperatures or by choosing certain grain size.     

Correlation among grain boundary character,carbide precipitation and deformation in Alloy 690

The correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715^oC for 15?h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.     

Mobility of low-angle grain boundaries in pure metals

The mobility of low-angle grain boundaries in pure metals is reviewed and several theoretical treatments are provided. The approach that provides the best agreement with the available experimental data is one in which the mobility is controlled by vacancy diffusion through the bulk to (and from) the dislocations that comprise the boundary that are bowing out between pinning points. The pinning points are presumed to be extrinsic dislocations swept into the boundaries or grown in during the prior processing of the material. This approach yields a mobility that is constant with respect to misorientation angle, up to the transition to the high-angle regime. For small misorientations of the order 1°, however, the mobility appears to increase with decreasing misorientation angle.     

多晶材料晶粒生长粗化过程的相场方法模拟

基于采用晶体有序化程度参量ψ和晶体学取向θ来表示多晶粒结构的相场模型,利用自适应有限元方法模拟了多晶材料等温过程中的晶粒粗化现象.模拟结果显示,在曲率作用下,通过晶界迁移弯曲晶界逐渐平直化,小晶粒逐渐被大晶粒吞并,当晶界之间的取向差较小时,满足一定能量和几何条件的两晶粒在界面能作用下会发生转动,合并为单个晶粒.模拟结果与实验结果符合较好.因此,该相场模型可以很好地用来模拟固态相变中多晶材料的生长粗化等现象. 关键词： 相场 晶界迁移 晶粒转动 粗化     

Grain Boundary Migration in Fe-3wt.%Si Alloys

The studies of three different laboratories on grain boundary migration in Fe-3wt.%Si alloys are presented. In all cases bicrystal techniques employing capillarity as driving force were used. [100] tilt boundaries were studied in the temperature range from 1223 K to 1373 K, and [110] tilt boundaries in the range from 1220 K to 1625 K. Proportionality between grain boundary velocity and driving force was confirmed. All data fulfil a linear relation between activation enthalpies and logarithms of the pre-exponential factors, corresponding to a compensation temperature of 1386 K where all boundaries theoretically should possess the same mobility. A considerably lower activation enthalpy was found in one case for an asymmetrical grain boundary compared to the symmetrical boundary of the same misorientation. High values of activation enthalpy of migration were found for special [100] boundaries compared to general ones although an opposite tendency was also observed for [100] boundaries.     

Faceting of a moving grain boundary and its effect on the kinetic properties of grain boundaries

The motion of an individual half-loop grain boundary in zinc is studied experimentally. A correlation is revealed between the half-loop’s mobility and the change in the shape of its structural elements (curved segments, facets, and grain boundary edges). A hysteresis is observed on the temperature dependence curve of grain boundary mobility. The results from investigating grain boundary migration accompanied by faceting inside zinc bicrystals indicate that unsteady grain boundary motion occurs upon high-temperature isothermal annealing. The unsteady motion is due to the considerable difference between the mobility of a facet and that of the curved part of a grain boundary.     

铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为.结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错.在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位.间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚.     

铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为。结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错。在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位。间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚。     

Coupled motion of [10−10] tilt boundaries in magnesium bicrystals

Magnesium bicrystals were grown with symmetric and asymmetric tilt boundaries about the [10–10] axis using the vertical Bridgman technique. Isothermal constant load tensile tests were conducted on these bicrystals in the temperature range 300–500°C and relative displacements of the two grains were measured to obtain an appreciation for grain boundary motion characteristics. Coupled grain boundary motion was noted in almost all cases with the degree of tangential motion versus migration changing with tilt misorientation, temperature and applied stress. Specifically, within the family of symmetric bicrystals evaluated, a minimum in grain boundary displacement in the specimen plane was observed at a tilt misorientation of 20°. In specific stress/temperature regimes, rigid body sliding was observed for the particular case of a 35° asymmetric tilt misorientation. The ease of basal and prism slip in magnesium at the temperatures considered and the consequential impingement of intragranular dislocations on the bicrystal boundary and their decomposition and motion along the boundary are thought to play an important role in the observed coupled motion of these tilt boundaries.     

Thermodynamics of Vacancies and Impurities at Grain Boundaries

The thermodynamics of vacancy and impurity adsorption at interfaces and grain boundaries (GBs) in solids is considered. Theoretical expressions are derived for the GB/interface free energy change caused by various levels of vacancy or impurity adsorption. This information is used to predict the behavior of vacancies at interfaces and GBs in a stress gradient and to forecast the effect of impurities on GB fracture strength. The latter predictions provide an interpretation of intergranular fracture behavior in terms of impurity adsorption and GB structural parameters such as GB width and value.     

Theory of grain boundary motion during high-temperature DIGM

A theory of diffusion induced grain boundary migration (DIGM) is presented for high temperatures where volume diffusion of solute atoms out of the grain boundary is important. It is shown that due to the presence of a gradient term in the expression for the free energy of solid solution, even a relatively small discontinuity in the solute distribution across the gain boundary provides enough driving force for grain boundary migration. From the expression obtained for the grain boundary velocity the coefficient for the Ni diffusion across the grain boundaries in a Cu(Ni) polycrystal has been estimated.     

Effect of grain boundary structures on the behavior of He defects in Ni: An atomistic study

We investigated the effect of grain boundary structures on the trapping strength of He_N(N is the number of helium atoms) defects in the grain boundaries of nickel. The results suggest that the binding energy of an interstitial helium atom to the grain boundary plane is the strongest among all sites around the plane. The He_N defect is much more stable in nickel bulk than in the grain boundary plane. Besides, the binding energy of an interstitial helium atom to a vacancy is stronger than that to a grain boundary plane. The binding strength between the grain boundary and the He _N defect increases with the defect size. Moreover, the binding strength of the He_N defect to the Σ3(12)[110] grain boundary becomes much weaker than that to other grain boundaries as the defect size increases.     

Modelling of grain boundary impurity segregation during high temperature plastic deformation

A modified theoretical model is proposed to predict the grain boundary segregation of impurity atoms during high temperature plastic deformation. The model is based on the supersaturated vacancy-impurity complex created by plastic deformation and involves quasi-thermodynamics and kinetics. Model predictions are made for phosphorus grain boundary segregation during plastic deformation in ferrite steel. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. At a given te...     

Significant decrease in interfacial energy of grain boundary through serrated grain boundary transition

The mechanism of serrated grain boundary formation and its effect on liquation behaviour have been studied in a wrought nickel-based superalloy – Alloy 263. It was newly discovered that grain boundaries are considerably serrated in the absence of γ?′-phase or M₂₃C₆ at the grain boundaries. An electron energy-loss spectroscopy study suggests that serration is triggered by the discontinuous segregation of C and Cr atoms at grain boundaries for the purpose of relieving the excessive elastic strain energy. The grain boundaries serrate to have specific segments approaching one {111} low-index plane at a boundary so that the interfacial free energy of the grain boundary can be decreased, which may be responsible for the driving force of the serration. The serrated grain boundaries effectively suppress grain coarsening and are highly resistant to liquation due to their lower wettability resulting from a lower interfacial energy of the grain boundary.     

The effect of surface contact conditions on grain boundary interdiffusion in a semi-infinite bicrystal

The Kirkendall effect is conditioned by active diffusion as well as by active sources and sinks for vacancies. In the case of grain boundaries under the condition of negligible bulk diffusion, the Kirkendall effect is highly localized and responsible for the formation of an extra material wedge in the grain boundary, which may lead to high stress concentrations. The Kirkendall effect in grain boundaries of a binary system is described by a set of partial differential equations for the mole fraction of one of the diffusing components and for the stress component normal to the grain boundary completed with the respective initial and boundary conditions. The contact conditions of the grain boundary with the surface layer acting as source of one of the diffusing components can be considered as equilibrium ones ensuring the continuity of generalized chemical potentials of both diffusing components. Thus, the boundary conditions are determined by the difference in chemistry (i.e. how the thermodynamic parameters depend on chemical composition) of the grain boundaries and of the surface layer. The simulations based on the present model indicate a drastic influence of the chemistry on the grain boundary interdiffusion and Kirkendall effect.     

Grain Boundary Migration in Metals: Recent Developments

Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.     

氦泡在bcc钨中晶界处成核长大的分子动力学模拟

钨(W)是潜在的聚变堆面向等离子体材料.聚变反应中产生的氦(He)不溶于金属W,并在其中易聚集形成He泡,使W基体发生脆化,从而导致W基体的性能发生退化.在前人工作的基础上,本文采用分子动力学研究了He泡在单晶bcc-W中以及bcc-W中∑3[211](110)和∑9[110](411)晶界处He泡形核长大初期的演化过程.结果发现,晶界处He泡的长大机制和单晶W中有所不同.单晶W中He泡通过挤出位错环促进长大.而He泡在∑3[211](110)晶界处的长大机制为:首先挤出并发射少量自间隙W原子,而后挤出1/2⟨111⟩位错线,随后,该位错线会沿晶界面上[111]方向迁移出去;在∑9[110](411)晶界处,He泡在我们的模拟时间尺度范围内没有观察到W自间隙子的发射和位错的挤出.