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.     

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.     

Defects in rutile and anatase polymorphs of TiO2: kinetics and thermodynamics near grain boundaries

The direct consequence of irradiation on a material is the creation of point defects-typically interstitials and vacancies, and their aggregates-but it is the ultimate fate of these defects that determines the material's radiation tolerance. Thus, understanding how defects migrate and interact with sinks, such as grain boundaries, is crucial for predicting the evolution of the material. We examine defect properties in two polymorphs of TiO(2)-rutile and anatase-to determine how these materials might respond differently to irradiation. Using molecular statics and temperature accelerated dynamics, we focus on two issues: how point defects interact with a representative grain boundary and how they migrate in the bulk phase. We find that grain boundaries in both polymorphs are strong sinks for all point defects, though somewhat stronger in rutile than anatase. Further, the defect kinetics are very different in the two polymorphs, with interstitial species diffusing quickly in rutile while oxygen defects-both interstitials and vacancies-are fast diffusers in anatase. These results allow us to speculate on how grain boundaries will modify the radiation tolerance of these materials. In particular, grain boundaries in rutile will lead to a space charge layer at the boundary and a vacancy-rich damage structure, while in anatase the damage structure would likely be more stoichiometric, but with larger defects consisting primarily of Ti ions.     

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.     

Microstructure evolution of T91 steel after heavy ion irradiation at 550 ℃

Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr⁺ ions at 550 ℃. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a₀/2<111> type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a₀<100> type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.     

Study of the effects of grain size on the mechanical properties of nanocrystalline copper using molecular dynamics simulation with initial realistic samples

Abstract

Molecular dynamics simulations have been performed to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 9.0 and 24 nm. A melting–cooling method has been used to generate the initial samples: this method produces realistic samples that contain defects inside the grains such as dislocations and vacancies. The results of uniaxial tensile tests applied to these samples reveal the presence of a critical mean grain size between 16 and 20 nm, for which there is an inversion of the conventional Hall–Petch relation. The principal mechanisms of deformation present in the samples correspond to a combination of dislocations and grain boundary sliding. In addition, this analysis shows the presence of sliding planes generated by the motion of perfect edge dislocations that are absorbed by grain boundaries. It is the initial defects present inside the grains that lead to this mechanism of deformation. An analysis of the atomic configurations further shows that nucleation and propagation of cracks are localised on the grain boundaries especially on the triple grains junctions.     

Finite temperature effect on mechanical properties of graphene sheets with various grain boundaries

The mechanical properties of graphene sheets with various grain boundaries are studied by molecular dynamics method at finite temperatures.The finite temperature reduces the ultimate strengths of the graphenes with different types of grain boundaries.More interestingly,at high temperatures,the ultimate strengths of the graphene with the zigzagorientation grain boundaries at low tilt angles exhibit different behaviors from those at lower temperatures,which is determined by inner initial stress in grain boundaries.The results indicate that the finite temperature,especially the high one,has a significant effect on the ultimate strength of graphene with grain boundaries,which gives a more in-depth understanding of their mechanical properties and could be useful for potential graphene applications.     

Estimating the surface tension of grain boundaries in pure metals

The surface tension of grain boundaries with liquid-like structure in a pure metal is presented as a function of the melting temperature and the molar volume of the metal in the solid state at the temperature of melting. Several empirical expressions for estimating the average surface tension of high-angle GBs at high homologous temperatures are proposed on this basis.     

Partial melting at interfaces and graln boundaries for high-strain-rate superplastic materials

The present paper describes partial melting at matrix/reinforcement interfaces and grain boundaries for high-strain-rate superplastic metallic materials. It is suggested from the mechanical testing results that partial melting is associated with the deformation mechanisms of the high-strain-rate superplasticity. DSC measurements and TEM observations reveal that solute additions are segregated at the interfaces and grain boundaries, so that partial melting occurs at elevated temperatures. This supports the concept of the accommodation helper mechanisms such as the accommodation by a liquid phase. However, when a liquid phase is continuous and thick, intergranular decohesion is caused at liquid interfaces and grain boundaries. Therefore a discontinuous and thin liquid phase is required both to play a vital roll as an accommodation helper and to limit intergranular decohesion.     

Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.     

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.     

Connection between surface properties and the parameters of contact melting for solid solutions with metals

Reference data that show the connection between the parameters of contact melting (CM) (temperature and rate of CM) for metals with solid solutions (SSes) and surface properties of components of SSes are presented. It is noted that this connection is due to the segregation mechanism of CM: impurities with lower surface energies, and thus lower melting temperatures, are adsorbed at grain boundaries and the outer surface of samples because of surface and grain boundary segregation affecting the CM parameters.     

Plasma-stimulated penetrability of hydrogen as a tool for studying phase transitions at grain boundaries

It is well known that the diffusion of hydrogen atoms through the intrinsic defects of a crystal lattice has characteristics different from those of bulk diffusion and, at certain parameters for some polycrystalline metals, ensures the determining contribution to the transfer of hydrogen atoms through the material. Grain boundaries (and dislocations) are the most important and shortest paths, the diffusion through which is much faster than bulk diffusion through a crystal lattice. It is particularly important to take into account this diffusion in materials with grains having sizes of about several nanometers. The possibility of using the method of the plasma-stimulated penetrability of hydrogen to analyze phase transitions at the grain boundaries is demonstrated on the example of polycrystalline niobium foils. In contrast to the existing methods, this method proposed for studying grain-boundary diffusion and phase transitions is simple and ensures control over the surface. The temperature characteristics of the diffusion of hydrogen atoms through niobium grain boundaries have been measured.     

双晶铝晶界弛豫的转变温度

测量了夹角分别为60°和153.4°的[110]对称倾侧铝双晶晶界的扭转滞弹性蠕变曲线,指出了晶界的弛豫强度在低于温度T₀时变为零,此温度(T₀)约等于该金属的熔点温度(T_m)的二分之一,这说明晶界的内部结构在这个温度的附近发生了显著的变化。 关键词：     

Behaviour of vacancies in dilute Fe–Re alloys: a positron annihilation study

Positron annihilation lifetime spectroscopy was used in a room temperature study of the influence of heat treatment on behaviour of vacancies in Fe_0.97Re_0.03 and Fe_0.94Re_0.06 alloys. In this experiment, the vacancies were created during the formation and further mechanical processing of the iron systems under consideration so the lifetime spectra of positrons were collected at least twice. The first samples were taken just after the melting process in an arc furnace, and the second ones were taken for the specimens annealed at 1,270 K and then cold-rolled at room temperature. After that, the spectra were measured for all studied samples after annealing at some temperatures gradually increasing from 300 to 1,270 K. It was found that vacancy-Re pairs are the dominant type of structural defects in alloys just after the melting process. In the case of alloys after a cold rolling process, the dominant type of structural defects is vacancies associated with edge dislocations. Moreover, for cold-rolled samples annealed at 473–573 K, the growth of the vacancy clusters associated with edge dislocations is observed by an increase in the mean positron lifetime. Finally, at temperatures above 573 K, vacancy clusters associated with edge dislocations as well as vacancy-Re pairs become unstable, and freely migrating vacancies sink at grain boundaries.     

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)     

Scanning infrared microscopy investigation of copper precipitation in cast multicrystalline silicon

The behavior of copper precipitation in cast multicrystalline silicon (mc-Si) annealed at different temperatures under air cooling (30 K/s) or slow cooling (0.3 K/s) was investigated by scanning infrared microscopy (SIRM). Comparing to Czochralski-grown silicon (Cz-Si), copper precipitated more easily in mc-Si, and the lowest temperature of copper precipitation in mc-Si was about 700 °C, lower than that in Cz-Si. It was also observed that copper preferably precipitated on grain boundaries so that near the grain boundaries the denuded zone formed. The results indicate that the defects including dislocations, grain boundaries and microdefects, as the heteronucleation sites, enhanced copper precipitation. Moreover, cooling rates had a great influence on the copper precipitation, especially at lower annealing temperatures. Generally air cooling led to the formation of high density of copper-precipitate colonies.     

Diffusion coefficients of vacancies and interstitials along tilt grain boundaries in molybdenum

The diffusion coefficients of vacancies and interstitials along symmetrical tilt grain boundaries in molybdenum have been calculated using the molecular dynamics method. The migration energies of defects have been obtained. The activation energy and coefficients of grain boundary self-diffusion have been deter-mined. A comparison of the obtained results with the studies of other authors indicates that boundaries formed between particles in the powder in sintering experiments have a higher diffusion activity as compared to stable grain boundaries in polycrystals.     

Structure and morphology of nanosized lead inclusions in aluminum grain boundaries

Grain boundary lead inclusions formed by ion implantation of mazed bicrystal aluminum films have been investigated by transmission electron microscopy. The vapor-grown bicrystal films contained mainly 90°(110) tilt boundaries with fixed misorientation but variable inclination, as well as some growth twins with 70.5°(110) symmetrical tilt boundaries and a few small-angle boundaries. It was found that the shape, size and orientation of the inclusions in the grain boundaries depend on the orientation of the aluminum grain boundary plane. Inclusions at 90°(110) tilt boundaries were invariably sharply faceted toward one aluminum grain and more rounded toward the other grain. The faceted side was a section of the cuboctahedral equilibrium shape of bulk lead inclusions in parallel topotaxy with the aluminum matrix. The rounded side, where the aluminum grain was rotated by 90° with respect to the lead lattice, approximated a spherical cap. At specific low-energy segments of the grain boundary where a (100) plane in grain 1 meets an (011) plane in grain 2, only two of several possible shapes were observed. One of these was preferred in as-implanted samples while both types were found after melting and re-solidification of the lend inclusions. The observations are discussed in terms of a modified Wulff construction.