Non-planar grain boundary structures in fcc metals and their role in nano-scale deformation mechanisms

This work presents the results of a comparative molecular dynamics study showing that relaxed random grain boundary structures can be significantly non-planar at the nano-scale in fcc metals characterized by low stacking fault values. We studied the relaxed structures of random [1?1?0] tilt boundaries in a polycrystal using interatomic potentials describing Cu and Pd. Grain boundaries presenting non-planar features were observed predominantly for the Cu potential but not for the Pd potential, and we relate these differences to the stacking fault values. We also show that these non-planar structures can have a strong influence on dislocation emission from the grain boundaries as well as on grain boundary strain accommodation processes, such as grain boundary sliding. We studied the loading response in polycrystals of 40 nm grain size to a level of 9% strain and found that the non-planar grain boundaries favour dislocation emission as a deformation mechanism and hinder grain boundary sliding. This has strong implications for the mechanical behaviour of nano-crystalline materials, which is determined by the competition between dislocation activity and grain boundary accommodation of the strain. Thus, the two interatomic potentials for Cu and Pd considered in this work resulted in the same overall stress–strain curve, but significantly different fractions of the strain accommodated by the intergranular versus intragranular deformation mechanisms. Strain localization patterns are also influenced by the non-planarity of the grain boundary structures.     

Self-diffusion in liquid interfaces

For studying self-diffusion in liquid interfaces, 59Fe tracer diffusion was measured on ultrafine-grained Nd2Fe14B which undergoes an intergranular melting transition for low Nd excess. The diffusion coefficient in the intergranular liquid layers is found to be lower than in bulk melts indicating a hampered atomic mobility due to confinement. Well above the intergranular melting transition, the diffusivity in the liquid interfaces approaches a value characteristic for bulk melts.     

Quasi-static intergranular brittle fracture: Dynamic embrittlement

A mode of brittle fracture is described which is fundamentally different from the rapid transgranular cleavage or intergranular decohesion that is usually associated with that term. It involves stress-induced diffusion of surface-adsorbed embrittling elements along grain boundaries, and it occurs by slow, step-wise crack growth, the rate of which can, in principle, be calculated from the knowledge of the relevant intergranular diffusion coefficient, the stress profile at the crack tip and the dependence of the stress for grain-boundary decohesion on the concentration of the embrittling element. This mode of fracture is postulated to be possible in any high-strength alloy with a low-melting-point element adsorbed on the surface if the applied stress is high enough. Known examples include the brittle type of stress-relief cracking in steels, tin-induced cracking of Cu-Sn alloys, oxygen-induced cracking of iron-, copper-, and nickel-based alloys, and the group of phenomena known as liquid-metal embrittlement and solid-metal embrittlement.The paper is dedicated to Dr. Frantiek Kroupa in honour of his 70^th birthday.This work is supported by National Science Foundation Grant CMS 95-03980.     

Superfluidity of grain boundaries in solid 4He

By large-scale quantum Monte Carlo simulations we show that grain boundaries in 4He crystals are generically superfluid at low temperature, with a transition temperature of the order of approximately 0.5 K at the melting pressure; nonsuperfluid grain boundaries are found only for special orientations of the grains. We also find that close vicinity to the melting line is not a necessary condition for superfluid grain boundaries, and a grain boundary in direct contact with the superfluid liquid at the melting curve is found to be mechanically stable and the grain-boundary superfluidity observed by Sasaki et al. [Science 313, 1098 (2006)10.1126/science.1130879] is not just a crack filled with superfluid.     

Fundamental understanding of Na-induced high temperature embrittlement in Al–Mg alloys

Sodium is an undesirable impurity in aluminium–magnesium alloys. In trace amounts it leads to high temperature embrittlement (HTE), due to intergranular fracture, which results in edge cracking during hot rolling. In the present work, the results of a thermodynamic investigation to elucidate the mechanism are presented. Correlations between HTE, phase formation, temperature and composition in Al–Mg alloys were determined. It is suggested that: (i) HTE is related to the formation of an intergranular Na-rich liquid phase, which significantly weakens the strength of grain boundaries; (ii) for a given Mg content, there exists a maximum Na content above which HTE cannot be avoided; and (iii) for a given alloy, a proper hot-rolling temperature should be chosen with respect to Na and Mg contents to suppress HTE. The HTE sensitive zone and a hot-rolling safe zone of Al–Mg–Na alloys are defined as functions of processing temperature and alloy composition. The tendency of HTE formation was evaluated based on thermodynamic simulations of phase fraction of the intergranular Na-rich liquid phase.     

High-speed superplasticity of microcrystalline alloys under conditions of local grain boundary melting

A model is proposed for the high-speed superplasticity of materials under conditions of local grain boundary melting at temperatures close to solidus. It is shown that the local melting of grain boundaries containing segregations of impurity atoms, results in the formation of a structure consisting of liquid-phase regions and solid intergranular bridges which provide cohesion of the grains during the deformation process. The equilibrium concentration, dimensions, and activation energy for the formation of solid bridges are determined as a function of the temperature, initial impurity concentration in the boundary, and the boundary thickness. A mechanism is proposed for grain-boundary slip under conditions of local grain boundary at anomalously high strain rates. Zh. Tekh. Fiz. 68, 38–42 (December 1998)     

Effect of annealing on density of metals with small eutectic additions

It is shown that in metals with sparingly soluble eutectic additions pore-type voids can form as a result of high-temperature annealing. The experimental results are explained by the occurrence of liquid layers on certain grain boundaries. In the absence of stresses the pores result from the crystallization of the liquid phase in closed volumes. Thermostructural stresses in cadmium with its hcp lattice cause intergranular slippage, resulting in disclosure of the micropores.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 62–66, July, 1972.     

Mechanisms of corrosion in surface layers at solid-solution interfaces

This review is focussed on solids in the broad categories of oxides, both simple (e.g. binary) and complex (e.g. silicates, aluminosilicates, titanates), and sulfides as ceramics, minerals and surface coatings. Mechanisms of surface reaction, corrosion and leaching associated with protons, hydroxide, water and simple cations and anions are considered. A variety of mechanisms that have some generality in determining the kinetics and reaction products at the solid-solution interfaces is illustrated. The focus of the experimental studies is on the surface layers; their development, distribution of species, depth and control. Mechanisms discussed and illustrated include: diffusion; protonation and ion transfer to solution; lattice reaction; ion exchange (leaching); precipitation; surface oxidation; surface reconstruction; intergranular films and grain boundaries. Theoretical models for the first four mechanisms are presented.     

Heterophase dislocations — An approach towards interpreting high temperature grain boundary behavior

When the conditions of full thermodynamic equilibrium are added to the conventional linear elastic theory of dislocations a hybrid concept of “heterophase” dislocations arises. As developed here, heterophase dislocation theory provides a self-consistent method for calculating the core configuration, energy, and stress-strain field which minimize the thermodynamic potential of a dislocated crystal. Simple grain boundaries in crystals near the melting point are treated as constrained arrays of heterophase misfit dislocations with a liquid-like core phase similar to, but not identical with the properties of ordinary liquid phase. Measurements of the properties and behavior of {011̄} tilt boundaries in bismuth crystals near the melting point show that the new theory accounts accurately for the energetic dependence on tilt misorientation in the range 0° to 6°, and then deviates from the observed dependence. The new theory also permits prediction of a structural transition in grain boundaries which is extremely sensitive to the density of misfit dislocations. This transition, predicted to occur in bismuth at a tilt misorientation of 15° was also confirmed by hot-stage electron microscopy.     

Melting of a polycrystalline material

Calculating the melting temperature of a solid with a known model of interaction between atoms is nowadays a comparatively simple task. However, when one simulates a single crystal by molecular dynamics method, it does not normally melt at the melting temperature. Instead, one has to significantly overheat it. Yet, a real material melts at the melting point. Here we investigate the impact of the defects and the grain boundaries on melting. We demonstrate that defects and grain boundaries have similar impact and make it possible to simulate melting in close vicinity of thermodynamic melting temperature. We also show that the Z method might be non-applicable in discriminating a stable submelting phase.     

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.     

Molecular dynamics simulation of Ga penetration along grain boundaries in Al: a dislocation climb mechanism

Many systems where a liquid metal is in contact with a polycrystalline solid exhibit deep liquid grooves where the grain boundary meets the solid-liquid interface. For example, liquid Ga quickly penetrates deep into grain boundaries in Al, leading to intergranular fracture under very small stresses. We report on a series of molecular dynamics simulations of liquid Ga in contact with an Al bicrystal. We identify the mechanism for liquid metal embrittlement, develop a new model for it, and show that is in excellent agreement with both simulation and experimental data.     

Thermal stability of nanotwinned and nanocrystalline microstructures produced by cryogenic shear deformation

Nanotwinned microstructures are of significant interest due to their high strength and enhanced thermal stability, attributed to the presence of a dense network of coherent twin interfaces. Propensity for twinning during deformation is known to increase at low temperature and/or high-strain-rate. In this study, we use high-strain-rate (~10^3?s^?1) shear deformation in cutting over a range of strains (γ ~1–5) and temperatures (cryogenic to ambient) to engineer a variety of microstructures in three face-centred cubic (FCC) metals – copper, brass and aluminium. The microstructures include nanocrystalline-equiaxed and densely (nano) twinned types of controllable domain size. The effects of low-temperature deformation and stacking fault energy on the resulting microstructure, hardening, stored energy and associated recrystallization kinetics are established. For copper, the nanotwinned microstructures are found to be thermally more stable and stronger than the equiaxed counterparts comprised of random high-angle grain boundaries. This enhanced effect of nanotwins on microstructure stability is, however, not observed in brass, while aluminium did not show any indications of twinning over the investigated range of deformation conditions.     

First observation of a wetting phase transition in low-angle grain boundaries

The wetting phase transition at low-angle intercrystallite grain boundaries has been experimentally observed. In contrast to the high-angle grain boundaries with the misorientation angels θ > 15°, the low-angle grain boundaries (θ < 15°) are not continuous two-dimensional defects, but constitute a discrete wall (network) of lattice dislocations (edge and/or helical). The theory predicts that, depending on θ, either a continuous layer of the liquid phase or a wall (network) of microscopic liquid tubes on wetted dislocation nuclei is formed at completely wetted low-angle grain boundaries. It has been shown that the continuous liquid layers at low-angle grain boundaries in the Cu-Ag alloys appear at the temperature T _wminL = 970°C, which is 180°C higher than the onset temperature T _wmin = 790°C and 50°C lower than the finish temperature of the wetting phase transition at high-angle grain boundaries, T _wmax = 1020°C.     

晶体相场法研究预变形对熔点附近六角相/正方相相变的影响

应用双模晶体相场模型计算二维相图,并模拟了在熔点附近预变形和保温温度对六角相晶界演化以及六角相/正方相相变的影响.研究发现:在相变初期,当预变形为零、保温温度离熔点很近时在晶界发生缺陷诱发预熔;增大预变形,变形与缺陷的交互作用在熔点附近诱发预熔;随着预变形的进一步增大,变形在畸变处同时诱发液相和正方相,且预变形越大、保温温度越接近熔点,液相生长越明显,反之正方相生长明显.持续保温使得畸变能释放,晶粒最终完全转变为平衡正方相.模拟结果表明:预变形六角相在熔点附近保温时,由于晶界固有缺陷和预变形双重作用使得原子无序度增加,从而在晶界或其他缺陷处产生液相,待能量释放后晶粒再转变成平衡正方相,进而延缓了六角相/正方相相变时间.     

Connectivity and percolation in simulated grain-boundary networks

Random percolation theory is a common basis for modelling intergranular phenomena such as cracking, corrosion or diffusion. However, crystallographic constraints in real microstructures dictate that grain boundaries are not assembled at random. In this work a Monte Carlo method is used to construct physically realistic networks composed of high-angle grain boundaries that are susceptible to intergranular attack, as well as twin-variant boundaries that are damage resistant. When crystallographic constraints are enforced, the simulated networks exhibit triple-junction distributions that agree with experiment and reveal the non-random nature of grain-boundary connectivity. The percolation threshold has been determined for several constrained boundary networks and is substantially different from the classical result of percolation theory; compared with a randomly assembled network, about 50-75% more resistant boundaries are required to break up the network of susceptible boundaries. Triple-junction distributions are also shown to capture many details of the correlated percolation problem and to provide a simple means of ranking microstructures.     

Grain boundary wetting phase transition and analysis of the energy characteristics of grain boundaries in the Sns-(Zn/Sn)l system

Regularities of the interaction of tin grain boundaries (special Σ5 and general Σ17 〈001〉) and a Sn-Zn melt of equilibrium composition were studied. The grain boundary wetting phase transition temperature was determined; for Σ5 and Σ17, it is 216°C. More than 90% of the general grain boundaries were completely wetted by the melt over a range of temperatures, from the eutectic melting temperature to the tin melting temperature. It was shown that the anisotropy of interphase energy at the solid tin-Zn-Sn melt interface is 64 ± 10 mJ m^?2 at 216°C. The energies of the Σ5 and Σ17 grain boundaries in the range of 201–216°C were obtained on the basis of the experimental dependence of the dihedral angle on temperature.     

Crack–grain boundary interactions in zinc bicrystals

In polycrystalline materials that fail by transgranular cleavage, it is known that crystallographic misorientation of preferred fracture planes across grain boundaries can provide crack growth resistance; despite this, the micromechanisms associated with crack transmission across grain boundaries and their role in determining the overall fracture resistance are not well understood. Recent studies on diverse structural materials such as steels, aluminum alloys and intermetallics have shown a correlation between fracture resistance and the twist component of grain misorientation. However, the lack of control over the degree and type of misorientation in experimental studies, combined with a dearth of analytical and computational investigations that fully account for the three-dimensional nature of the problem, have precluded a systematic analysis of this phenomenon. In this study, this phenomenon was investigated through in situ crack propagation experiments across grain boundaries of controlled twist misorientation in zinc bicrystals. Extrinsic toughening mechanisms that activate upon crack stagnation at the grain boundary deter further crack propagation. The mechanical response and crack growth behavior were observed to be dependent on the twist angle, and several accommodation mechanisms such as twinning, strain localization and slip band blocking contribute to fracture resistance by competing with crack propagation. Three-dimensional finite element analyses incorporating crystal plasticity were performed on a stagnant crack at the grain boundary that provide insight into crack-tip stress and strain fields in the second grain. These analyses qualitatively capture the overall trends in mechanical response as well as strain localization around stagnant crack-tips.     

晶间界面附近区域中的弹性场——各向异性理论

根据现代关于晶间界面的研究可知,晶界和相界多具有二维周期结构。由此出发,采用Fourier级数展开的表示,在给定了界面结构,在数学上即是形式地给定了边界条件的情况下,求出了两个各向异性物相间相界附近的弹性场。并以Ni中扭转晶界为例,作了应力场的数值计算。本文的方法和方程对于研究各种类型的晶界和相界均普遍可用,为处理这类问题提供了一种有效的简捷手段。 关键词：     

Thermodynamics and structural aspects of grain boundary segregation

Physical and chemical properties of solid materials are strongly. influenced by the chemical composition of internal interfaces, One of the crucial parameters affecting interfacial chemistry is the atomic structure of the interface. Due to its importance. a considerable amount of work was done to elucidate the relationship between structure and chemical composition of interfaces. This article reviews the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation. After a brief outline of grain boundary structure and geometry. thermodynamic approaches to describe grain boundary segregation are summarized and their application to materials is discussed. covering particular sites at a single grain boundary as well as the role of interfaces in polycrystals. Both the experimental evidence of grain boundary segregation anisotropy and the theoretical results of computer simulations of grain boundary segregation are summarized. Useful methods of predicting grain boundary segregation are presented. Finally, segregation behavior of solutes at grain boundaries is compared with that at free surfaces, and examples of chemical composition of intexphase boundaries are given.