以团簇加连接原子模型解析Cr-C共晶成分

Cr-C体系材料是重要硬质防护涂层的代表,具有共晶特征.我们的前期工作指出,共晶合金满足双团簇近程序结构模型,由两种稳定液体亚单元构成,各自满足理想非晶团簇成分式,这里的第一近邻团簇来自相关共晶相.显然共晶成分解析的关键在于获得团簇,而相结构中往往存在多种团簇,进入到非晶/共晶团簇成分式的主团簇定义是关键环节.本文通过应用Friedel振荡理论及原子密堆,以团簇分布的球周期性及孤立度为判据,以Cr-C共晶相为例,进一步细化了共晶相中的主团簇选择流程,再搭配以2,4或6个连接原子,获得了描述共晶成分Cr_(86)C_(14)和Cr_(67.4)C_(32.6)的双团簇成分式:[Cr-Cr_(14)+C-Cr_9]Cr C_3和[C-Cr_9+C-Cr_8]C_6,其中四种团簇分别来自共晶相Cr,Cr_(23)C_6,Cr_7C_3和Cr_3C_2.该工作进一步证实了团簇加连接原子模型在共晶点解析中的普适性,并从理论上支持了相关的材料设计.     

面心立方固溶体合金的团簇加连接原子几何模型及典型工业合金成分解析

工业合金牌号的成分选择体现了固溶体合金的化学短程有序结构,满足由最近邻两层原子组成的"团簇加连接原子"模型,例如对于置换型面心立方固溶体Cu-Zn,其合金牌号成分可以表述为[Zn-Cu_(12)]Zn_(1-6)和[Zn-Cu_(12)](Cu,Zn)_6,其中方括号内为第一近邻配位多面体团簇.基于此,本文赋予团簇式以具体原子结构的含义,对置换型面心立方固溶体结构中团簇的可能存在形式进行了穷尽,得出团簇式所对应的所有"团簇加连接原子"结构单元模型,给出团簇和连接原子之间的比例和空间排列的所有可能,并对Cu-Zn和Cu-Ni合金常用牌号对应的团簇式给出了三维结构模型,进一步验证了前期关于合金团簇式解析的正确性.用这些模型中原子化学序描述合金的成分,赋予团簇式以具体的原子结构意义,为进一步开发新的合金提供了理论依据.     

Ni-Al-Cr合金中团簇加连接原子模型的第一性原理计算

通过团簇加连接原子模型研究了Ni-Al-Cr合金的近程序结构和物理特性.以Al原子为中心,其周围第一近邻的12个Ni原子作为壳层原子,位于次近邻的Al原子和Cr原子作为连接原子,即[Al-Ni₁₂]Al_xCr_3–x,其中x=0, 0.5, 1.0, 1.5, 2.0, 2.5.形成能表明团簇加连接原子模型对应的结构比其他结构更稳定.差分电荷密度显示了Ni, Al, Cr原子间的电荷密度转移主要聚集在Ni-Al和Ni-Cr之间,说明Ni-Al和Ni-Cr之间比AlCr和Ni-Ni更容易成键.能带结构显示了Ni-Al-Cr合金材料均具有导体性质,且Ni-3d, Al-3p和Ni-3d, Cr-3d之间发生了明显杂化效应,验证了Ni-Al和Ni-Cr之间存在较强的相互作用.     

源于“团簇-共振”模型的理想金属玻璃电子化学势均衡

理想金属玻璃是指完全满足电子结构稳定性的金属玻璃. 在我们前期工作中提出的“团簇加连接原子"及理想金属玻璃的“团簇-共振"结构模型的 基础上, 本文指出理想金属玻璃应该满足电子化学势均衡判据, 可定量给出团簇与连接原子的比例, 最终确定了理想金属玻璃成分式[团簇](连接原子)_x. 运用此判据, 解析了Cu-Zr基和Co-B基块体金属玻璃, 实验确定的最佳形成能力成分满足电子化学势均衡.     

基于团簇模型设计的Cu-Cu_(12)-[M_(x/(12+x))Ni_(12/(12+x))]_5(M=Si,Cr,Cr+Fe)合金抗高温氧化研究

文章在稳定固溶体团簇模型的指导下,对白铜合金进行微合金化,将Cu元素在合金中的含量固定为72.22 at.%,改变Ni与M(M为Si,Cr,Cr+Fe)的比例,设计了系列合金成分,并对其抗高温氧化性能及其机制进行了研究.同时加入基体中的Ni-Si元素可以从两方面提高合金的抗氧化性能:以团簇形式加入,形成稳定固溶体结构,可以降低Cu-Ni-Si合金的化学反应活性;Si/Ni比增大后,合金在少量固溶的基础上能析出抗氧化性能优于基体的析出相,且析出越多,抗高温氧化性能越好.所以其抗氧化能力的来源并不是形成致密Si氧化物薄膜.Ni-Cr的同时加入可以明显抑制Cu合金在800?C以下的中温氧化,但其抗高温氧化能力主要与外氧化层中是否形成连续的Cr氧化层有关,因而该系列合金的抗高温氧化能力与Cr/Ni比有密切联系,合理选择团簇内Cr/Ni比例,才能够提高Cu合金的抗高温氧化能力.第四组元Fe和第三组元Cr相比较,不能够起到优先氧化、生成保护性氧化皮的作用,所以Cr,Fe同时添加只能抑制Cu在800?C以下的中温氧化,却不能够提高Cu合金的抗高温氧化能力.     

体心立方固溶体合金中的"团簇+连接原子"结构模型

为体现固溶体合金中的溶质原子产生的化学短程序,文章提出了配位数为14的团簇在体心立方(bcc)点阵中的堆垛模式,并建立了基于bcc结构的"团簇+连接原子"结构模型,用团簇成分式[团簇](连接原子)_x表述. 此模型中,与基体组元具有相对大的负混合焓的溶质原子占据团簇心部,其他原子作为连接或者替代团簇壳层基体原子. 1 ∶1结构模型[团簇](连接原子)₁由于最大程度地保证了团簇与连接原子的近邻,构成了连接原子最有效的合金化方式. 在两个实用bcc固溶体合金体系中, 1 ∶1模型指导设计了低V含量的储氢合金 V₁和低弹性模量高强度的 Nb₁合金. 关键词： 体心立方固溶体成分设计 "团簇+连接原子"结构模型 Ti-Cr-V合金 Ti-Zr-Mo-Nb合金     

以最大原子密度定义合金相中的第一近邻团簇

本文提出利用不同壳层所包含的径向原子密度, 即单位体积内的原子个数随着径向的分布, 来方便而精确地定义团簇, 即具有最大径向原子密度的且表面呈现三角密堆结构的完整壳层为第一近邻团簇. 最后以Al-Ni-Zr合金相为例说明了该方法的合理性与适用性, 及此方法所定义的团簇与非晶形成的关系.     

Mg-Al系工业合金牌号的成分式解析

Mg-Al系牌号是应用最广的镁基工业合金,但其牌号背后的成分根源一直未知,构成研发新合金的主要障碍.本文应用描述固溶体短程序结构特征的团簇共振模型,得到了Mg-Al二元固溶体的最理想化学结构单元[Al-Mg_(12)]Mg_1,然后对《the American Society for Testing Materials》手册中所有Mg-Al系工业合金牌号进行成分解析,得到相应团簇成分式,如AZ63A合金解析后的团簇成分式为[Al_(0.78)Zn_(0.16)-Mg_(12)]Mg_(1.04)Mn_(0.02),AZ81A合金解析后的团簇成分式为[Al_(0.97)Zn_(0.03)-Mg_(12)]Mg_(0.98)Mn_(0.02).再根据成分式与化学结构单元之间的误差,对比该牌号合金的力学性能,验证了该化学结构单元在Mg-Al体系中的准确性,揭示出看似复杂的工业合金牌号后面隐藏的简单成分规律,为发展Mg-Al体系合金指出了一个全新的途径.     

铁铬合金团簇点阵中与成分相关的磁性耦合

本文采用气相团簇束流沉积法制备了不同铁铬比的铁铬合金密集团簇点阵,研究了团簇点阵中复杂的多相结构和各种耦合效应.当合金中铬含量较大时,在团簇中能够观测到一种晶格失配的类四方结构,这种结构的出现导致了不同铁铬比的合金团簇中交换偏置效应的不同.随着铬的含量增加,合金团簇的交换偏置场减小,而团簇间的偶极相互作用增强.在合金团簇的铁磁-超顺磁转变温度以上,能明显观察到在类四方结构团簇中有更大的残存矫顽力与剩磁.在场冷条件下,对矫顽力和偏置场随温度变化的研究表明含有更多类四方结构的合金团簇有更好的热稳定性和更大的各向异性.     

液态Ga-In合金中的团簇分离现象

与以往试验及模拟计算方法不同,文章利用从头计算分子动力学研究了液态Ga, In及Ga-In合金的偏结构. 发现合金偏双体相关函数gGaGa(r), gInIn(r)的第一峰的位置分别与液态纯Ga和纯In的第一峰 位置接近, gGaIn(r)第一峰位置大于纯Ga和纯In第一峰位置的平均值,说明液态Ga-In合金中异类原子 呈现排斥倾向, Ga-Ga, In-In团簇更容易出现.在纯Ga, 纯In中占据最高含量的1311键对在液态合金中占主导 地位,说明Ga-Ga, In-In团簇共存于液态Ga-In合金中. Voronoi多面体分析发现,随着In含量的增加, 在Ga_100-xIn_x(30 ≤x≤qslant 50 at.% ) 区域内, Ga原子周围主配位数出现突变,由12降为10,证明Ga-Ga和In-In团簇倾向于分离.该研究结果不同于 通常的微观不均匀模型,揭示了液态Ga-In合金中团簇分离的机制.     

Investigation of multicomponent chemical short-range order in NiZr2

The local atomic configuration of multicomponent chemical short-range order (MCSRO) in NiZr2 has been investigated by means of molecular dynamics simulation (MD) in a wide temperature range. The potential functions for the system based on the embedded atom method are constructed and the parameters are obtained by fitting the structure and properties of NiZr2 crystal. The static structures such as pair distribution functions and the distribution of coordination number have been calculated. The local atomic configurations of the MCSI~Os in the melt were demonstrated as distorted coordination polyhedron of the compound structure and/or the structure similar to cubooctahedron analogues. It is indicated by the results of MD simulation that above the melting point the atomic packing of long-range order disappears, but the chemical interaction of coordinated atoms still exists, which leads to the formation of various MCSROs with atomic configurations similar to the stable or metastable unit cell of NiZr2 compound. When the system is just melted, many icosahedral polyhedron configurations appear, which decrease as the over-heating temperature increases.     

24 electron cluster formulas as the ‘molecular’ units of ideal metallic glasses

It is known that ideal metallic glasses fully complying with the Hume-Rothery stabilization mechanism can be expressed by a universal cluster formula of the form [cluster](glue atom)_{1 or 3}. In the present work, it is shown, after a re-examination of the cluster-resonance model, that the number of electrons per unit cluster formula, e/u, is universally 24. The cluster formulas are then the atomic as well as the electronic structural units, mimicking the ‘molecular’ formulas for chemical substances. The origin of different electron number per atom ratios e/a is related to the total number of atoms Z in unit cluster formula, e/a?=?24/Z. The 24 electron formulas are well confirmed in typical binary and ternary bulk metallic glasses.     

The characterisation of atomic structure and glass-forming ability of the Zr–Cu–Co metallic glasses studied by molecular dynamics simulations

In the present work, the glass formation process and structural properties of Zr₅₀Cu_50-xCo_x (0 ≤ x ≤ 50) bulk metallic glasses were investigated by a molecular dynamics simulation with the many body tight-binding potentials. The evolution of structure and glass formation process with temperature were discussed using the coordination number, the radial distribution functions, the volume–temperature curve, icosahedral short-range order, glass transition temperature, Voronoi analysis, Honeycutt–Andersen pair analysis technique and the distribution of bond–angles. Results indicate that adding Co causes similar responses on the nature of the Zr₅₀Cu_50-xCo_x (0 ≤ x ≤ 50) alloys except for higher glass transition temperature and ideal icosahedral type ordered local atomic environment. Also, the differences of the atomic radii play the key role in influencing the atomic structure of these alloys. Both Cu and Co atoms play a significant role in deciding the chemical and topological short-range orders of the Zr₅₀Cu_50-xCo_x ternary liquids and amorphous alloys. The glass-forming ability of these alloys is supported by the experimental observations reported in the literature up to now.     

Hardness of Al-based quasicrystals evaluated via cluster-plus-glue-atom model

In this paper, the hardness of ternary Al-based quasicrystals was assessed through an application of the cluster-plus-glue-atom model. In this model, any structure is decomposed into a first-neighbour strongly bonded cluster part and second-neighbour weakly bonded glue-atom part so that the overall structural information is condensed into a local structural unit [cluster](glue atom)_x. For quasicrystals, the averaged local units are formulated as [icosahedron] TM_0,1(Transition Metal) and could be visualized as single icosahedron packing. Then, the hardness of quasicrystals was related to the rupture of weak inter-cluster bonds. Typically, theoretical hardness values of 8–9?GPa were obtained using 19 broken inter-cluster bonds, which accounts for about half of all the surface bonds of an icosahedron in the Mackay-type environment. The unit cluster formulas would act as rigid units during deformation and cracking.     

Composition rule for Al–transition metal binary quasicrystals

Compositions of Al–transition metal (TM) binary quasicrystals are revisited using the cluster-plus-glue-atom model. These compositions all conform to a unified cluster formula [icosahedron](glue)₁, where the icosahedron is taken from a crystalline approximant. In a given binary system, icosahedral and decagonal quasicrystals are expressed by the same icosahedron but glued, respectively, by one Al and one TM, and their e/a ratios fall close to 1.86 and 1.71–1.79, respectively. Their relative stabilities are discussed by referring to compositions satisfying the fully filled TM-3d states according to Häussler's resonance model.     

Influence of alloying elements on the elastic properties of ternary and quaternary nickel-base superalloys

Using new supercells, the influence of the alloying elements Co, Cr, Ta, Re, and Ru on the elastic properties of ternary and quaternary nickel-base superalloys are systematically studied by the first-principles pseudopotential method. The calculated results of the five ternary and three quaternary superalloys suggest that the alloying elements can increase the elastic moduli. Re is found to be most effective in increasing the moduli of ternary alloys. Among the quaternary alloys, the Ni-Al-Re-Ru system has the largest moduli. Our calculated values of the elastic moduli agree very well with the experimental results. The charge density redistribution and partial density of states are calculated for the quaternary system including both Re and Ru. Covalent-like bonding between Re and its nearest neighbor is observed. The hybridization between the Re-d orbital and the host Ni-p, d orbitals are the origin of the modulus strengthening effect.     

预退火时间对Fe_(80.8)B_(10)P_8Cu_(1.2)非晶合金微结构及磁性能的影响

研究了预退火时间对Fe_(80.8)B_(10)P_8Cu_(1.2)非晶合金微结构及磁性能的影响.穆斯堡尔谱研究表明:在660 K的预退火温度下,随着预退火时间的增加,Fe原子不断富集,非晶基体中的类Fe_3B化学短程有序结构向类Fe B结构转变,并且非晶基体中Fe第一近邻壳层中Cu原子的逐渐脱离以及Fe-P配位键数量的明显减少可间接表征CuP团簇的形成过程.同时,本研究通过调节预退火时间来调控非晶基体中CuP团簇和Fe团簇的数量,促进后续退火晶化过程中α-Fe纳米晶相的析出,并细化纳米晶尺寸,从而获得综合磁性能更加优异的非晶/纳米晶软磁合金.