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.     

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.     

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.     

Nonclassical rotational inertia in helium crystals

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large (4)He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon.     

Observation of unusual mass transport in solid hcp (4)He

Solid (4)He has been created off the melting curve by growth at a nearly constant mass via the "blocked capillary" technique and growth from the (4)He superfluid at constant temperature. The experimental apparatus allows injection of (4)He atoms from superfluid directly into the solid. Evidence for the superfluidlike transport of mass through a sample cell filled with hcp solid (4)He off the melting curve is found. This mass flux depends on temperature and pressure.     

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.     

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.     

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)     

Elevated temperature directional recrystallization of high-purity nickel

Electron backscattered diffraction has been used to characterise the three different kinds of boundaries that occur in grains that are generated by secondary recrystallization during directional annealing of high-purity nickel. Boundaries between columnar grains (CC boundaries) can be twin boundaries, low-angle boundaries or high-angle grain boundaries. The frequency of low-angle CC boundaries dropped from 25% to 0% while the frequency of the high-angle CC boundaries increased from 19% to 67% when the annealing temperature was increased from 1000°C to 1200°C. The misorientation angles of boundaries between columnar grains and small equiaxed grains ahead of them (CE boundaries) was random at 1200°C but had a 40° rotation relationship about ?111? at 1000°C. It was found out that the character of the CC boundaries is determined by relative mobility of the CE boundaries, which is determined by the processing temperature rather than the energy of the CC boundaries themselves. The character of the island grain boundaries sometimes found with columnar grains was not affected by the annealing temperature or the drawing velocity.     

Superfluid density in a two-dimensional model of supersolid

We study in 2-dimensions the superfluid density of periodically modulated states in the framework of the mean-field Gross-Pitaevskiǐ model of a quantum solid. We obtain a full agreement for the superfluid fraction between a semi-theoretical approach and direct numerical simulations. As in 1-dimension, the superfluid density decreases exponentially with the amplitude of the particle interaction. We discuss the case when defects are present in this modulated structure. In the case of isolated defects (e.g. dislocations) the superfluid density only shows small changes. Finally, we report an increase of the superfluid fraction up to 50% in the case of extended macroscopical defects. We show also that this excess of superfluid fraction depends on the length of the complex network of grain boundaries in the system.     

Local stress and superfluid properties of solid 4He

We provide a semiquantitative tool, derived from first-principles simulations, for answering the question of whether certain types of defects in solid 4He support mass superflow. Although ideal crystals of 4He are not supersolid, the gap for vacancy creation closes when applying a moderate stress. While a homogeneous system becomes unstable at this point, the stressed core of crystalline defects (dislocations and grain boundaries) can turn superfluid.     

Model of local melting of grain boundaries containing impurity atom clusters

A model describing the initial stages of the local melting of grain boundaries containing impurity atom clusters is developed. The local melting process is considered as the formation of liquid zones whose geometry and number depend on the initial concentration and thermodynamic characteristics of the impurity atoms, as well as on the material parameters and temperature. The evolution of the liquid-solid interface structure with increasing temperature is discussed. The dependences of the liquid-solid interface parameters on temperature are obtained at various initial concentrations and thermodynamic characteristics of the impurity atoms.     

Grain boundaries of nanocrystalline materials – their widths, compositions, and internal structures

Nanocrystalline materials contain many atoms at and near grain boundaries. Sufficient numbers of Mössbauer probe atoms can be situated in grain boundary environments to make a clear contribution to the measured Mössbauer spectrum. Three types of measurements on nanocrystalline materials are reported here, all using Mössbauer spectrometry in conjunction with X-ray diffractometry, transmission electron microscopy, or small angle neutron scattering. By measuring the fraction of atoms contributing to the grain boundary component in a Mössbauer spectrum, and by knowing the grain size of the material, it is possible to deduce the average width of grain boundaries in metallic alloys. It is found that these widths are approximately 0.5 nm for fcc alloys and slightly larger than 1.0 nm for bcc alloys. Chemical segregation to grain boundaries can be measured by Mössbauer spectrometry, especially in conjunction with small angle neutron scattering. Such measurements on Fe-Cu and Fe₃Si-Nb were used to study how nanocrystalline materials could be stabilized against grain growth by the segregation of Cu and Nb to grain boundaries. The segregation of Cu to grain boundaries did not stabilize the Fe-Cu alloys against grain growth, since the grain boundaries were found to widen and accept more Cu atoms during annealing. The Nb additions to Fe₃Si did suppress grain growth, perhaps because of the low mobility of Nb atoms, but also perhaps because Nb atoms altered the chemical ordering in the alloy. The internal structure of grain boundaries in nanocrystalline materials prepared by high-energy ball milling is found to be unstable against internal relaxations at low temperatures. The Mössbauer spectra of the nanocrystalline samples showed changes in the hyperfine fields attributable to movements of grain boundary atoms. In conjunction with SANS measurements, the changes in grain boundary structure induced by cryogenic exposure and annealing at low temperature were found to be somewhat different. Both were consistent with a sharper density gradient between the crystalline region and the grain boundary region.     

Superfluidity and quantum melting of p-H2 clusters

Structural and superfluid properties of p-H2 clusters of size up to N=40 molecules, are studied at low temperature (0.5 K相似文献   

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.     

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.     

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.     

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：     

Solid-phase wetting at grain boundaries in the Zr-Nb system

The microstructure of Zr-Nb polycrystalline alloys with niobium concentrations of 1, 2.5, 4, and 8 wt % is investigated in the temperature interval of 620–840°C. It is revealed that the second solid phase β-Nb forms either a chain of separate lens-like precipitates or continuous homogeneous layers at grain boundaries in zirconium, depending on the annealing temperature and the energy of the Zr/Zr grain boundary. It is shown that the greater the quantity of the second solid phase, the lower is the temperature of the termination of grain-boundary wetting. A model is constructed that explains the dependence of the temperature of grain boundary wetting on the amount of wetting phase. It is found that the complete wetting of all grain boundaries in zirconium by the second solid phase does not occur in Zr-Nb alloys.     

Helium trapping at dislocations,precipitates and grain boundaries

Helium bubble formation was observed in austenitic stainless steels by transmission electron microscopy following implantation of 30 to 1000 appm helium at room temperature and annealing at 700 to 800°C. Helium bubble distributions at dislocations and at various grain boundaries and precipitates were studied. It was found that interfacial dislocations play a dominant role in bubble nucleation at grain and interphase boundaries but not at Tic-matrix interfaces. Particularly high trapping of helium was observed at Tic precipitate-matrix interfaces which is attributed to an inhomogeneous ripening mechanism.