Influence of grain sizes on the ionic conductivity and the chemical diffusion coefficient in copper selenide

Ionic conductivity and chemical diffusion coefficient have been studied for superionic polycrystalline Cu_1.75Se copper selenide within the temperature interval 300–500 K. An increase in ionic conductivity with an grain size increase is observed. In our opinion, this fact is caused by lower activation energy for the bulk diffusion than that for the grain boundary diffusion.     

Effect of manganese on grain boundary segregation of sulfur in iron

The ASED-MO theory was used to study the electronic effects of S and the S-Mn couple upon the chemical embrittlement of Fe grain boundaries. The results obtained for S alone in a model of grain boundary (GB) are consistent with its observed behavior as a chemical embrittling agent. It was found that the total energy of the cluster decreases when the S atom is located at the GB. When S segregate at the Fe GB containing Mn, the embrittlement process was modified. The crystal orbital overlap population (COOP) curves gives a measure of Fe-Fe bond weakening due to the segregated atoms at the GB. Our calculations show that Mn behaves as a weak embrittler on the Fe GB. The Fe-Mn bonds were strengthened, while Fe-Fe bonds of the capped trigonal prism of the GB (CTP) were weakened. On the other hand, when S segregate at the Mn/Fe cluster, some metallic bonds were resistant to chemical embrittlement.     

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol–gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of ≈1 T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of magnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.     

Contributions of atom probe tomography to the understanding of nickel-based superalloys

Atom probe tomography enables atomic level microstructural characterization to be performed on complex engineering materials such as superalloys. The technique provides information on the size, morphology and compositions of coexisting phases, the solute partitioning of the elements between the phases, and solute segregation to interfaces and grain boundaries. This information leads to a more complete understanding of nickel-based superalloys. The types of atomic level information that may be obtained with atom probe tomography are illustrated with examples of the formation of fine γ precipitates within the central region of the γ′ phase in PW 1480, the evolution of the dual γ′/γ″ nature of secondary precipitates in alloy 718, the interphase precipitation of the γ′ phase at the primary γ″–γ interface in alloy 718, and the quantification of the level and spatial extent of the boron segregation at grain boundaries in a nickel–molybdenum superalloy.     

Gradient nanoscale polycrystalline elasticity: Intergrain interactions and triple-junction conditions

Using the principle of virtual power, we develop general balance equations, interface conditions, triple-junction conditions, and boundary conditions for second-grade nanocrystalline elastic materials undergoing infinitesimal deformations. We further develop thermodynamically consistent constitutive equations and provide a weak formulation of resulting boundary-value problems that automatically yields internal conditions such those that hold across interfaces and at triple junctions.     

镍钨硼合金沉积机理及镀层微晶尺寸

应用循环伏安、恒电位阶跃和Ｘ射线衍射（ＸＲＤ）等研究了Ｎｉ－Ｗ－Ｂ合金电沉积特点和镀层微晶尺寸。结果表明,在以柠檬酸铵为络合剂的溶液中,Ｎｉ－Ｗ－Ｂ合金沉积层较Ｎｉ－Ｗ合金有较低的电化学活性。电位阶跃ｉ￣ｔ曲线分析表明,在玻碳电极上Ｎｉ－Ｗ－Ｂ合金电结晶过程遵循扩散控制瞬时成核三维成长模式,且随过电位的增加,电极表面晶核数增多。ＸＲＤ测试结果表明,随沉积电流密度提高,合金镀层微晶尺寸逐渐增大,说明     

Thermodynamic behaviour of solid–liquid grain boundary grooves

ABSTRACT

Grain boundary grooves (GBGs) are local features that develop along polycrystalline solid–liquid interfaces. Interest in GBGs lies in their ability to form interface defects during crystallization that promote, in fact, dominate, morphological instability and affect microstructure formation in cast alloys. Recently, we reported on unobserved subtle aspects of the thermodynamic behaviour of GBG microstructures by combining sharp-interface field theory with diffuse-interface phase-field simulations. A surprising feature revealed about steady-state GBGs is that despite their stationarity they nonetheless support persistent capillary-mediated energy fluxes with divergences that continuously cool their interfaces and increase local curvatures. We now analyse the energetic behaviour of GBGs as ‘open’ thermodynamic systems, and report further details of their formation free energy that show how geometric constraints and capillary-mediated thermodynamic fields self-interact and influence the steady-state shapes of GBGs.     

The influence of tilt grain boundaries on the mechanical properties of bicrystalline graphene nanoribbons

The mechanical properties of bicrystalline graphene nanoribbons with various tilt grain boundaries (GBs) which typically consist of repeating pentagon–heptagon ring defects are investigated based on the method of molecular structural mechanics. The GB models are constructed via the theory of disclinations in crystals, and the elastic properties and ultimate strength of bicrystalline graphene nanoribbons are calculated under uniaxial tensile loads in perpendicular and parallel directions to grain boundaries. The dependence of mechanical properties is analyzed on the chirality and misorientation angles of graphene nanoribbons, and the experimental phenomena that Young's modulus and ultimate strength of bicrystalline graphene nanoribbons can either increase or decrease with the grain boundary angles are further verified and discussed. In addition, the influence of GB on the size effects of graphene Young's modulus is also analyzed.     

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.     

Influence of the grain size on the quality of standardless WDXRF analysis of river Nile sediments

In multichannel wavelength-dispersive X-ray fluorescence spectrometry (WDXRF) the fluorescence intensity might depend on grain size and heterogeneity of the sample. Six river Nile sediment samples were collected two meters below the water surface from different locations covering the greater Cairo, Egypt. Each sample was dried at 65 °C for 48 h and divided into four grain size fractions: < 32 μm, 32–63 μm, 63–125 μm and 125–200 μm using different sieves. The dry sediment samples were mixed with low contamination binder (Wax, C₆H₈O₃N₂) in a mass ratio of wax:sample = 4:0.9 g. Sample pellets were made using a hydraulic press at a pelletizing pressure of 120 KN cm^{− 2}. The results show that the XRF intensities of the K_α radiation might increase or decrease with decreasing grain size, depending on the atomic number of the analyte. In the present thick pelletized samples, the penetration depth of the characteristic radiation increases at low grain sizes, and consequently the probability of the grain-size effect on the characteristic radiation decreases. Depending on the experimental data, a general theoretical equation, relating fluorescence intensity, grain size and atomic number, was derived by using cubic spline interpolation. The fractions were identified by WDXRF using standardless quantitative analysis, depending on the fundamental parameter approach. According to the present statistical analysis and the Certified Reference Material (CRM) results, the quantitative analysis results were found acceptable when the grain size of the river Nile sediments less than 32 μm. At a grain size > 63 μm, standardless analysis using fundamental parameter approach was found to be useful for qualitative and semi-quantitative analysis only whereas there are a strong positive correlations.