Effect of P impurity on NiAlΣ5 grain boundary from first-principles study

First-principles calculations based on the density functional theory（DFT） and ultra-soft pseudopotential are employed to study the atomic configuration and charge density of impurity P in Ni Al Σ5 grain boundary（GB）. The negative segregation energy of a P atom proves that a P atom can easily segregate in the Ni Al GB. The atomic configuration and formation energy of the P atom in the Ni Al GB demonstrate that the P atom tends to occupy an interstitial site or substitute a Al atom depending on the Ni/Al atoms ratio. The P atom is preferable to staying in the Ni-rich environment in the Ni Al GB forming P–Ni bonds. Both of the charge density and the deformation charge imply that a P atom is more likely to bond with Ni atoms rather than with Al atoms. The density of states further exhibits the interactions between P atom and Ni atom, and the orbital electrons of P, Ni and Al atoms all contribute to P–Ni bonds in the Ni Al GB. It is worth noting that the P–Ni covalent bonds might embrittle the Ni Al GB and weakens the plasticity of the Ni Al intermetallics.

Role of grain boundary networks in vortex motion in superconducting films

We study the vortex dynamics of the polycrystalline superconductors in the presence of both random point defects and the generated grain boundary(GB) networks with Voronoi diagram. The synergistic effect of adjacent GBs on restricting the vortex motion in intragranular region is proposed and the corresponding intensity factor of the synergistic effect which characterizes the strength of the synergistic restriction of adjacent grain boundaries is also determined in the present work.The interconnected GBs offer easy-flow channels for vortices in addition to pinning effects on the vortices. The combined channels and the vortex flow patterns in the superconducting film are analyzed in detail from molecular dynamics simulations. Furthermore, it is discovered that the critical current increases with the decrease of magnetic field intensity,temperature, and the average grain size. The large number of vortices results in the enhanced repulsive interaction forcing the vortices to move out from the GBs. The thermal depinning from GBs leads to the lower Lorentz force range. The increase of the grain size causes the number of GBs to decrease. In summary, these effects leads the critical current to become a decreasing function of magnetic field, temperature, and grain size.

Segregation of alloying atoms at a tilt symmetric grain boundary in tungsten and their strengthening and embrittling effects

We investigate the segregation behavior of alloying atoms （Sr, Th, In, Cd, Ag, Sc, Au, Zn, Cu, Mn, Cr, and Ti） near Z3 （ 111 ） [1]-0] tilt symmetric grain boundary （GB） in tungsten and their effects on the intergranular embrittlement by performing first-principles calculations. The calculated segregation energies suggest that Ag, Au, Cd, In, Sc, Sr, Th, and Ti prefer to occupy the site in the mirror plane of the GB, while Cu, Cr, Mn, and Zn intend to locate at the first layer nearby the GB core. The calculated strengthening energies predict Sr, Th, In, Cd, Ag, Sc, Au, Ti, and Zn act as embrittlers while Cu, Cr, and Mn act as cohesion enhancers. The correlation of the alloying atom＇s metal radius with strengthening energy is strong enough to predict the strengthening and embrittling behavior of alloying atoms; that is, the alloying atom with larger metal radius than W acts as an embrittler and the one with smaller metal radius acts as a cohesion enhancer.

Effect of nickel segregation on CuΣ9 grain boundary undergone shear deformations

Impurity segregation at grain boundary（GB） can significantly affect the mechanical behaviors of polycrystalline metal. The effect of nickel impurity segregated at Cu GB on the deformation mechanism relating to loading direction is comprehensively studied by atomic simulation. The atomic structures and shear responses of Cu Σ9（114） 110 and Σ9（221） 110 symmetrical tilt grain boundary with different quantities of nickel segregation are analyzed. The results show that multiple accommodative evolutions involving GB gliding, GB shear-coupling migration, and dislocation gliding can be at play, where for the 2ˉ21ˉ shear of Σ9（114） 110 the segregated GBs tend to maintain their initial configurations and a segregated GB with a higher impurity concentration is more inclined to be a dislocation emission source while maintaining the high mechanical strength undergone plastic deformation for the 11ˉ4ˉ shear of Σ9（221） 110. It is found that the nickel segregated GB exerts a cohesion enhancement effect on Cu under deformation： strong nickel segregation increases the work of separation of GB, which is proved by the first-principles calculations.

Effect of Effective Grain Size and Grain Boundary of Large Misorientation on Upper Shelf Energy in Pipeline Steels

X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very important for exploring even higher grade pipeline steels. We established the relationship between toughness and crystallographic parameters of high grade pipeline steels by studying the crystallographic parameters of X65, X70, and X80 using EBSD and analyzing Charpy CVN of X65, X70 and X80. The results show that the effective grain size, the frequency distribution of grain boundary misorientation and the ratio of high angle grain boundary to small angle grain boundary are important parameters. The finer the effective grain size, and the higher the frequency distribution of grain boundaries （〉 50~）, the more excellent toughness of high grade pipeline steels will be.

Barrier or easy-flow channel: The role of grain boundary acting on vortex motion in type-Ⅱ superconductors

Grain boundaries(GBs),as extremely anisotropic pinning defects,have a strong impact on vortex motion in type-Ⅱsuperconductors,and further on the macro level dominates the superconductivity for example the critical current density.Many previous studies indicated that mostly GB plays the role of a strong barrier for vortex motion,while an easy-flow channel just under some certain conditions.In order to thoroughly make clear of the questions of what is exactly the role of GB on vortex motion and how it works,in this article we developed a large scale molecular dynamic model and revealed the action of GB on vortex motion in type-Ⅱsuperconductors.The most significant finding is that the role of GB on vortex motion can be changeable from a barrier to an easy-flow channel,and which is intrinsically determined by the competition effect correlated with its action on vortex between in the GB and no-GB regions.Such the competition effect essentially depends on the attributes of both the GB(described by the GB strength and angleθ)and no-GB pining regions(by the relative disorder strengthα_(p)/α_(v)).Specifically,for a YBa_(2)Cu_(3)O_(7-x)(YBCO)sample,to obtain a clear knowledge of vortex motion in GB region,we visualized the three typical trajectories of vortices during the three vortex movement stages.Further,in order to understand how GB results in the macro current-carrying property,corresponding to the current-voltage relation of the YBCO conductor,we obtained the average velocity v_(y) of vortices varying with their driving force,which is nearly identical with the previous observations.

Influence of Grain Boundary on Fatigue Behavior of Ni-base Bicrystals

The influence of the grain boundary on the fatigue behavior was studied by two three-point-bending (TPB) specimens. One TPB specimen was named Bicrystal 1, whose pre-crack was along the grain boundary and the applied load paralleled to pre-crack direction, while the other TPB specimen was named Bicrystal 2, whose the pre-crack was perpendicular to the grain boundary and the applied load paralleled also to the pre-crack. It was found that the rate of the fatigue crack growth of Bicrystal 1 was about a tenfold higher than that of Bicrystal 2. The fatigue behavior of Bicrystal 2 specimens was dependent on the distance between the crack tip and grain boundary. The crack growth rate was highest when the crack tip was at a critical distance to the grain boundary, while the rate was the lowest when the crack tip reached grain boundary. After the crack was over the grain boundary, the crack growth rate increased. The crystallographic finite element method was applied to analyze the stress and strain structure ahead of the crack, in order to reveal the above characteristics of the fatigue behavior. It is the grain boundary-induced redistribution of stresses near the crack tip that induces the difference of fatigue behavior.

Study on the Continuous Transition From PTC Effect to GBBL Capacitor of Semiconducting BaTiO_3 Ceramics——Grain Boundary Barrier Model

The Daniels’ barium vacancy layer model of semiconducting BaTiO3 ceramics is analyzed.A grain boundary barrier model with both acceptor states of grain boundary and barium vacancy diffusion layers for the transition from PTC effect to grain boundary barrier layer (GBBL) capacitor is proposed.The proposed model solves the problem occurring in Daniels’ model and makes it possible to calculate physical properties of semiconducting BaTiO3 ceramics and relevant devices.

Effect of grain boundary on the mechanical behaviors of irradiated metals: a review

The design of high irradiation-resistant materials is very important for the development of next-generation nuclear reactors. Grain boundaries acting as effective defect sinks are thought to be able to moderate the deterioration of mechanical behaviors of irradiated materials, and have drawn increasing attention in recent years. The study of the effect of grain boundaries on the mechanical behaviors of irradiated materials is a multi-scale problem. At the atomic level, grain boundaries can effectively affect the production and formation of irradiation-induced point defects in grain interiors, which leads to the change of density, size distribution and evolution of defect clusters at grain level. The change of microstructure would influence the macroscopic mechanical properties of the irradiated polycrystal. Here we give a brief review about the effect of grain boundaries on the mechanical behaviors of irradiated metals from three scales: microscopic scale, mesoscopic scale and macroscopic scale.

Non-Equilibrium Grain-Boundary Segregation of Phosphorus And the Effective Time in Step Cooling Process

The non-equilibrium grain-boundary segregation of phosphorus in step cooling process in an industrial steel, 12CrlMoV, is studied based on the effective-time-method and compared with that in isothermal holding process. The non-equilibrium grain-boundary segregation concentration of phosphorus was measured with Auger Electron Spectroscopy (AES) and calculated based on the kinetic equations of non-equilibrium grain-boundary segregation. Results show that the calculated result is in good accordance with the experimental observation.

Effect of indentation size and grain/sub-grain size on microhardness of high purity tungsten

Hardness of materials depends significantly on the indentation size and grain/sub-grain size via microindentation and nanoindentation tests of high-purity tungsten with different structures.The grain boundary effect and indentation size effect were explored.The indentation hardness was fitted using the Nix-Gao model by considering the scaling factor.The results show that the scaling factor is barely correlated with the grain/sub-grain size.The interaction between the plastically deformed zone（PDZ） boundary and the grain/sub-grain boundary is believed to be the reason that leads to an increase of the measured hardness at the specific depths.Results also indicate that the area of the PDZ is barely correlated with the grain/sub-grain size,and the indentation hardness starts to stabilize once the PDZ expands to the dimension of an individual grain/sub-grain.

Strengthening-softening transition and maximum strength in Schwarz nanocrystals

Recently,a Schwarz crystal structure with curved grain boundaries(GBs)constrained by twin-boundary(TB)networks was discovered in nanocrystalline Cu through experiments and atomistic simulations.Nanocrystalline Cu with nanosized Schwarz crystals exhibited high strength and excellent thermal stability.However,the grainsize effect and associated deformation mechanisms of Schwarz nanocrystals remain unknown.Here,we performed large-scale atomistic simulations to investigate the deformation behaviors and grain-size effect of nanocrystalline Cu with Schwarz crystals.Our simulations showed that similar to regular nanocrystals,Schwarz nanocrystals exhibit a strengthening-softening transition with decreasing grain size.The critical grain size in Schwarz nanocrystals is smaller than that in regular nanocrystals,leading to a maximum strength higher than that of regular nanocrystals.Our simulations revealed that the softening in Schwarz nanocrystals mainly originates from TB migration(or detwinning)and annihilation of GBs,rather than GB-mediated processes(including GB migration,sliding and diffusion)dominating the softening in regular nanocrystals.Quantitative analyses of simulation data further showed that compared with those in regular nanocrystals,the GB-mediated processes in Schwarz nanocrystals are suppressed,which is related to the low volume fraction of amorphous-like GBs and constraints of TB networks.The smaller critical grain size arises from the suppression of GB-mediated processes.

晶界效应下的焊接热裂倾向判据适用性研究

材料焊接热裂倾向评判在产品制造中具有重要作用,以往热裂倾向判据并未考虑晶界效应,故其适用性一直是讨论的热点问题。文中以Al-Cu合金为例,通过多晶凝固定量相场模型获取熔池凝固过程枝晶形貌,提取凝固路径。基于凝固路径开展了目前常用热裂倾向判据的适用性研究,包含RDG判据、Clyne-Davies判据和Sindo Kou判据。研究发现,RDG判据同实际焊接热裂倾向一致,且裂纹产生主要受孤立晶界段枝晶间压力降主导;Clyne-Davies判据及Sindo Kou判据评判由于计算过程忽略部分孤立晶界区间段,导致评判结果出现失真。通过对三个判据的研究,为材料热裂倾向评判提供了参考。

V型特性PTC材料的复阻抗解析

采用复阻抗解析法研究了Ｖ型特性ＰＴＣ材料（Ｓｒ０．４５Ｐｂ０．５５）ＴｉＯ３的晶粒和晶界的电学性能．结果表明，Ｖ型ＰＴＣ效应来源于材料的晶界，Ｖ型ＰＴＣ效应是一种晶界效应．

BaTiO_3基PTCR中晶界氧元素的作用分析

采用柠檬酸盐溶胶-凝胶法制备出复合掺杂的BaTiO3粉体,然后成型烧结制备了PTCR试片。首先通过氩气气氛热处理试验,验证了氧能够有效地提高PTC效应,进而利用透射电镜对典型试片的晶界区和晶粒内部的元素成分进行能谱定量分析,表明晶界区的氧含量明显高于晶粒内部,甚至超过了施、受主含量的总和。分析了氧对晶界势垒的影响,用晶界层状氧吸附模型进行了解释。

氢影响下纳米晶体晶界滑移和晶界三叉点裂纹形核模型

在远场均匀拉伸载荷下,裂纹尖端会产生应力集中,与裂纹尖端相邻的晶界会承受较大的切应力,此切应力会导致晶界滑移.该文研究了氢和纳米晶界滑移对晶界裂纹形核、临界应力强度因子以及屏蔽效应的影响.应用连续分布位错方法给出了模型的理论解.结果表明:由于位错在晶界三叉点和滑移带尖端处的塞积,楔形裂纹会优先沿着晶界三叉点DC方向和晶界BD向上萌生,而且氢会使得裂纹萌生的总能量降低,氢浓度每增加1%,萌生最稳定裂纹的总能量大约降低1.86%.虽然晶界滑移会使得裂纹尖端的临界应力强度因子和屏蔽效应增大,但是氢使得临界应力强度因子降低.最后根据弱键理论,研究了氢对表面能的影响,氢浓度每增加1%,表面能降低5%.这一理论工作提供了氢和晶界滑移对材料微观断裂力学的新信息,有助于解释金属断裂的微观机理.

BaTiO_3半导瓷PTC现象的机理研究进展

综述了近年来PTC机理研究的进展。着重讨论以下几点:Heywang 及Jonker模型;单个晶界电性测定;用阻抗分析区分晶界及晶粒电阻;应力与PTC效应;晶界结构与PTC效应;晶界第二相、晶界层与PTC效应;化学吸附气体作为电子陷阱。最后讨论了今后还需进一步研究的方面。

不同受主掺杂过程对 BaTiO3半导瓷界面势垒和 PTC 效应的影响

本文根据理论上提出的BaTiO_3半导瓷晶界受主态密度N_s 与晶界势垒高度的关系,以及影响 PTC 效应的机理,探讨掺入不同量受主杂质Mn 和不同掺杂工艺过程对晶界势垒和PTC效应的改善。

La掺杂对(In_(0.5)Nb_(0.5))_(0.10)Ti_(0.90)O_(2)陶瓷微观结构和介电性能的影响

In和Nb共掺杂TiO_(2)(INTO)陶瓷在100 kHz以下的巨大介电常数源于内部势垒层电容效应。根据这种电容效应,通过增加晶界对介电性能的贡献来降低其低频介电损耗。因此,将La掺杂到INTO陶瓷中,通过细化晶粒并增加晶界数量来降低陶瓷制备过程中的能耗。结果表明,La掺杂减小了INTO陶瓷的晶粒尺寸,增加了陶瓷中晶界的数量,并且陶瓷中析出LaNbTiO_(6)二次相,在陶瓷晶粒之间形成高阻态的相界,降低陶瓷的低频介电损耗,优化了陶瓷介电常数的频率稳定性。