Hydrogen embrittlement in a magnesium grain boundary: a first-principles study

First-principles fully relaxed tensile and shear test simulations were performed on Σ10(1124)/[1100] tilt Mg grain boundary (GB) models, with and without H segregation, to investigate mechanisms of H embrittlement of Mg. Strengthening as a result of covalent-like characteristics of Mg-H bonds prevailed over weakening of Mg-Mg bonds resulting from charge transfer; as a result, an H atom strengthened the GB. In addition, because the strong Mg-H bonds suppressed macroscopic GB fracture, elongation to failure was not reduced by H segregation. However, the resistance to GB shearing was increased by H segregation. It is therefore suggested that H segregation enhances crack growth at the GB, because dislocation emission from the crack tip is suppressed, resulting in H embrittlement of Mg.     

The multiple hydrogen location near a α-Fe vacancy

The interaction between 10 hydrogen atoms and a α-Fe structure having a vacancy (V) has been studied using a cluster model and a semi-empirical theoretical method. The energy of the system was calculated by the atom superposition and electron delocalization molecular orbital method. The electronic structure was studied using the concept of density of states and crystal orbital overlap population curves.For the study of a sequential absorption, the hydrogen atoms were positioned in their energy minima configurations, near to the tetrahedral sites neighboring the vacancy, except the last H atom that was located far from the vacancy. The energy difference for H agglomeration was also computed. The vacancy-H_n complexes become less stable than VH species for more than three hydrogen's.The changes in the electronic structure of Fe atoms near the vacancy were also analyzed. The interactions mainly involve Fe 3d and 4s atomic orbitals. The contribution of Fe p orbitals is much less important. The Fe-Fe bond weakened as new Fe-H and H-H pairs were formed. The effect of H atoms is limited to its first Fe neighbors. The detrimental effect of H atoms on the Fe-Fe bonds can be related to the mechanism for embrittlement in α-Fe.     

Quantum chemical study of sulfur and hydrogen at the Σ=5 BCC Fe grain boundary

We investigate the effect of segregated atoms on metal-metal bonding at a grain boundary (GB). The structure and electronic properties of S and H impurities in Fe Σ=5 (013) [100] symmetrical tilt GB are studied using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. A large cluster containing 196 Fe atoms is used to simulate the local environment of the boundary. The results show that impurities induce large relaxation in the GB and that the GB gives rise to an energetically favorable zone for the H and S accumulation. No S-H association is found and the deleterious effect of H is of much less important as compared with S.     

Direct method for determining the segregation in silver-copper solid solutions not prone to brittle breakage of grain boundaries

A new method is proposed for measuring the chemical composition of grain boundaries (GBs) in copper-based alloys not prone to embrittlement of boundaries. This method is based on embrittlement of copper GBs by bismuth penetrating from the gas phase of bismuth telluride (Bi₂Te₃). Chemical analysis of the GB surface is performed via Auger-electron spectroscopy. The GB composition is measured in a solid solution of silver (its volume concentration is 1.4 at %) and copper. The segregation annealing temperature is 570°C. The silver concentration across the broken surface of the GB (its thickness is five to ten atomic layers) is 4.7 at %. Therefore, the ratio between silver concentrations within the GB and the grain volume (the enrichment coefficient) is approximately 3.5.     

Segregation of H, C and B to Σ = 5 (0 1 3) α-Fe grain boundary: A theoretical study

The ASED-MO theory is used to study the effects of H and the HC and HB pairs in the electronic structure of a Fe grain boundary (GB). The results obtained for H in a GB model are consistent with its behavior as a chemical embrittler. The total energies calculated for FeH, FeC and FeB clusters indicate that all interstitials segregate to the GB. C has the lowest energy, followed by B, and could compete with other impurities for the site location on the GB.The results obtained for FeCH and FeBH are consistent with the observed behavior of C and B as cohesion enhancers. A strong repulsive interaction between C and H and B and H atoms is developed if they occupy the nearest interstitial site on the GB. When C or B are present, the total energies are similar to that obtained for the FeH cluster. This indicates that H is displaced from the capped trigonal prism (CTP). Also, we do not detect any CH or BH interaction.Density of states (DOS) and crystal orbital overlap population (COOP) curves are used to shed more light on the interstitial-Fe GB interaction. The existence of strong metal-metalloid bonds is shown, which are primarily due to Fe 3d, 4s and C (or B) 2s, 2p interactions.     

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.     

Er3(Fe,Co, M)29化合物(M=Cr,V, Ti,Mn, Ga,Nb)的结构与磁性

合成了Er_３Fe_29-x-yCo_xM_y化合物(M=Cr, V, Ti, Mn, Ga, Nb )并用x射线衍射和磁测量等手段研究了它们的结构和磁性. 发现Fe基Er_３(Fe,M)_２９化合物结晶成哑铃对Fe-Fe无序替代的Th₂Ni₁₇型结构(P6₃/mmc空间群)而不能形成Nd₃(Fe,Ti)_２９型结构，因此其化学式也可以用Er_2－n(Fe,M)_17+2n (n=0.2)表示. 当Er_３Fe₂₉化合物中部分Fe原子被M原子所取代时，其居里温度均有一定程度的提高. 所有Er_３(Fe,M)_２９化合物在室温均为易面型各向异性. 当Er_３(Fe,M)_２９ (M=Cr, V)中的部分Fe原子被Co原子取代且Co原子数与Fe原子数达到一定比值时，得到一个单斜结构的新相. 磁测量表明Er_３Fe_19.5Co₆V_3.5在室温可能为单轴各向异性，在162K出现自旋重取向，其各向异性由易轴型变为易面型. 在5K下于难磁化方向磁化时观察到一个一级磁化过程(FOMP). 关键词： 稀土金属间化合物 晶体结构 磁晶各向异性     

Structural and Chemical Widths of General Grain Boundaries: Modification of Local Structure and Bonding by Boron-Doping in β-Silicon Carbide

Using a hot-isostatic-pressed boron-doped silicon carbide (SiC) material as an example, we demonstrate that the structural width and the chemical width of general boundaries may be quite different. The high-resolution electron microscopy (HREM) observation did not detect the existence of 1 nm thick amorphous film at such grain boundaries (GB). There is only a core structure of 1–2 atomic planes at GB. The chemical width of GB, obtained by the spatially-resolved electron energy-loss spectroscopy (EELS) analysis, is visibly wider than the core region. Furthermore, the spatially-resolved energy-loss near-edge structures (ELNES) analysis not only revealed the chemical bonding between boron and carbon, oxygen and silicon, but also distinguished an extended GB region with chemical bonding modified from that of the grain interiors. Such ELNES analysis defines an ELNES width that is even wider than the chemical width. The three GB widths of different scale construct a comprehensive picture of general boundaries that is remarkably different from general boundaries with amorphous film. Instead of a film confined by the two atomically sharp grain surfaces, there is only one interface, the rough GB core having most of the B–C and Si–O bonds, and the extended grain surface layers, to form such general GB in B-doped SiC.     

NiAs、PbO型FeX(X=S,Se,Te)结构的稳定性与电子特征

利用基于密度泛函理论的第一性原理方法,Ni As与Pb O型Fe X(X=S,Se,Te)结构的稳定性与电子特征得到了研究.计算结果显示Fe的内聚能与X-Fe元素之间的相互作用是影响Fe X结构稳定性的重要因素.当X原子半径较小、电负性较大时(X=S),Fe X趋向于形成Fe-X相互作用较强、密度较大的Ni As型结构;当X原子半径较大、电负性较小时(X=Se,Te),Fe X趋向于形成Fe-Fe相互作用较强、密度较小的Pb O型结构.此外,压强使得Pb O型Fe X结构的稳定性降低.当压强分别大于5、9 GPa时,Fe Te、Fe Se趋向于形成Ni As型结构.Pb O型Fe Se中Fe原子周围的电子密度随压强的增大而增大.     

钢中小角度晶界区的电子结构及掺杂效应

根据位错的弹性理论,建立了9Ni钢中53°掺杂小角度晶界的原子模型,利用Recursion方法,计算了杂质在53°小角度晶界典型环境中的能量和电子结构,由此得出,钢中小角度晶界的强度敏感地依赖于隔离杂质的类型,S,P杂质使晶粒间结合减弱,从而导致晶界疏松 ;相反,B,C,N则会使晶界间的结合加强.在所有的杂质中,B显示出独特的性质,在钢中B 不仅能增强晶界的结合,而且由于占位竞争效应会使其他杂质远离晶界,具有净化晶界的作用. 关键词： 小角晶界 杂质 占位竞争 晶界隔离     

R(Fe，Mo)12型稀土永磁材料热膨胀反常现象研究

采用x射线粉末衍射方法测量了不同温度下R(Fe，Mo)₁₂化合物(R=Nd，Y，D y)的晶格常数，对沿不同轴向的热膨胀反常程度进行了计算.分析认为R(Fe，Mo)_{12化合物热膨胀反常主要取决于Fe-Fe相互作用，根据Nd(Fe，Mo)12的结构参 数，不仅较短的Fe-Fe键负相互作用对热膨胀反常有贡献，8j-8j等较强的Fe-Fe正相互作用 也有很大贡献.另外还讨论了第三元素Mo的替代作用影响热膨胀反常的机理. 关键词： x射线衍射 R(Fe 12}化合物')" href="#">Mo)₁₂化合物 热膨胀反常     

GB1与金属离子相互作用的NMR研究

G群链球菌G蛋白的B1结构域——GB1蛋白,常被用作发展体外及体内基于顺磁核磁共振（NMR）的蛋白质结构测定方法的研究模型.为确保赝接触化学位移（PCS）、顺磁弛豫增强（PRE）等顺磁约束数据的准确性,了解GB1和金属离子,尤其是顺磁离子的相互作用非常必要.然而GB1和二价金属离子以及镧系金属离子的相互作用并不十分清楚.本文利用NMR波谱研究了GB1和镧系金属离子以及多种二价金属离子的相互作用.我们发现GB1和镧系金属离子之间存在弱特异性相互作用,和Mn²⁺、Cu²⁺以及Co²⁺等顺磁二价离子弱结合,但不与Ca²⁺、Mg²⁺以及Zn²⁺等抗磁二价离子结合.该研究表明在GB1上链接顺磁探针时,应使用与固有位点结合常数差异明显的顺磁标签以获取正确的PRE数据.     

Assessment of chemical elements in cosmetics' eyeshadows by X‐ray fluorescence and International Nomenclature of Cosmetic Ingredients characterization

Energy dispersive X‐ray fluorescence (EDXRF) technique, as a qualitative and quantitative analysis, was used for inorganic chemical elements determination (K, S, Cl, Al, Si, Ca, Ti, Mn, Fe, Pb, Zn, Rb, and Bi) in eyeshadows for safe human use. International Nomenclature of Cosmetic Ingredients standardized nomenclature was used for labels investigation to obtain data on legal regularity. Data of 23 samples were clustered by similarity, measuring relative concentrations of detected chemical elements. Calculating the correlation among such values, a similarity matrix was used to generate a dendrogram. Pb was found in samples silver color S12E and copper color S22I above permissible limits (20 μg/g). Same composition was reported for the pink (S01A), black (S02A), and brown (S03A) samples, but the same chemical elements were not detected by EDXRF in them. The best correlation was found between samples S08D and S23 J (0.961). The least correlation was 0.0012 between S01A and S12E. The clustering analysis showed 7 groups of similar samples according to EDXRF data. Relations among 6 eyeshadows' colors and chemical compositions were discovered by using decision trees, where the most determinant elements were Mn, S, Cl, Ca, and Fe, in this order. Commercial regularization and International Nomenclature of Cosmetic Ingredients standardization of eyeshadows in Brazil are not fully complied by the manufacturers of the investigated brands.     

Al，Cr共偏析Fe晶界对其结合强度影响的第一性原理研究

在密度泛函理论的框架下,采用广义梯度近似(GGA)研究了合金化元素Al,Cr在Fe (210)晶界共偏析的作用。结果表明Cr提高了Fe晶界结合,为韧性杂质;而Al减弱了晶界的结合,是脆性杂质。Cr不能够彻底地消除Al的脆化作用,反而使其脆性增强。基于偏聚能分析表明Cr能有效地阻止Al偏析到晶界,改变Fe的力学性能。     

Investigations into the mechanism of material removal and surface modification at atomic scale on stainless steel using molecular dynamics simulation

A molecular dynamics simulation (MDS) has been carried out to investigate the material removal phenomenon of chemo-mechanical magnetorheological finishing (CMMRF) process. To understand the role of chemical assisted mechanical abrasion in CMMRF process, material removal phenomenon is subdivided into three different stages. In the first stage, new atomic bonds viz. Fe–O–Si is created on the surface of the workpiece (stainless steel). The second stage deals with the rupture of parent bonds like Fe–Fe on the workpiece. In the final stage, removal of material from the surface in the form of dislodged debris (cluster of atoms) takes place. Effects of process parameters like abrasive particles, depth of penetration and initial surface condition on finishing force, potential energy (towards secondary phenomenon such as chemical instability of the finished surface) and material removal at atomic scale have been investigated. It was observed that the type of abrasive particle is one of the important parameters to produce atomically smooth surface. Experiments were also conducted as per the MDS to generate defect-free and sub-nanometre-level finished surface (Ra value better than 0.2 nm). The experimental results reasonably agree well with the simulation results.     

Effect of solute atoms on fracture toughness in dilute magnesium alloys

The effect of alloying elements on the toughness and the fracture behaviour was investigated on seven kinds of Mg-0.3?at.% X (X?=?Ag, Al, Ca, Pb, Sn, Y and Zn) alloys with a grain size of 3–5?μm. The fracture toughness and fracture behaviour in magnesium alloys were closely related to the segregation energy. The Mg–Al and –Zn alloys that had small segregation energy showed high toughness and ductile fracture in most regions, while the Mg–Ca alloy with large segregation energy exhibited low toughness and intergranular fracture. These different tendencies resulted from solute segregation at grain boundaries (GBs). The change in the lattice parameter ratio was the influential material parameter regardless of whether the GB embrittlement was for enhancement or suppression.     

The microstructure of the base oxide on 304L steel

304L stainless steel specimens were subjected to 168 h of oxidation in dry oxygen at 600°C. The composition and microstructure of the oxide as well as the steel near the steel–oxide interface were investigated using SEM and TEM in conjunction with EDS analysis. It was found that the first oxide layers, referred to as the base oxide, consisted of 50–100 nm-sized corundum-type grains. Each grain contained oxides of chromium, iron plus a small amount of manganese in a solid solution with the chemical formula (Cr,Fe,Mn)₂O₃. In the oxide regions closest to the steel–oxide interface, the grain center compositions varied laterally from grain to grain, with the cation balance ranging between very chromium-rich (Cr:Fe=4:1) and very iron-rich (Cr:Fe=1:8). Within each oxide grain, the chromium level was generally higher in the center than near the boundary. This was due to the much quicker diffusion of iron (compared to chromium) at and through the oxide grain boundaries than in the bulk of the oxide grains, creating iron enrichment near the oxide grain boundaries. Compositional variation with respect to the distance from the steel–oxide interface was also noted. Chromium was richer in the oxide regions closer to the steel–oxide interface than farther out in the oxide and depleted from the steel regions bordering the steel–oxide interface. Manganese was also depleted in those steel regions although hardly enriched in the oxide just outside the steel–oxide interface. Nickel was less reactive than chromium, iron or manganese and hence was virtually absent from the oxide.