不同冷速对金属铜Cu凝固过程中微观团簇结构演变影响的模拟研究

采用分子动力学方法和Quantum Sutton-Chen(Q-SC)多体势对由5万个液态金属铜(Cu)原子构成的系统在三个不同冷却速率下的凝固过程中微观团簇结构转变的影响进行了模拟研究.运用双体分布函数、Honeycutt-Andersen(HA)键型指数法、原子团类型指数法(CTIM-2)和可视化分析等方法,对凝固过程中微观团簇结构的演变特性进行了分析研究.结果发现:由非晶体向晶体转变的临界速度约为1.0×1013 K/s,在此冷速下系统形成非晶体和晶体以一定比例并存的混合结构;在冷速为1.0×1014 K/s冷却时系统形成以1551、1541、1532、1431键型为主的非晶体结构,非晶转化温度约为673K;在以4.0×1012 K/s速度冷却时,系统从673K就开始结晶,并形成以1421和1422二种键型或由这二种键型构成的面心立方(FCC)(12 0 0 0 12 0)和六角立方(HCP)(12 0 0 0 6 6)基本原子团为主的晶体结构,尤其是由1421键型构成的面心立方(12 0 0 0 12 0)基本原子团在晶体生长和微观团簇结构形成过程中占主导地位.同时发现,冷速对金属Cu系统中的FCC结构和HCP结构的相对比例有显著的影响,冷速越低,FCC基本原子团以及由其构成的团簇结构越多.     

晶体相场方法模拟纳米孪晶结构

应用双模晶体相场模型,模拟共格纳米孪晶结构.结果表明:球状晶粒生长成的共格孪晶片层,在共格面上的原子排列有变形,容易出现位错;条状晶粒凝固生长成的共格孪晶界,比用球状晶粒长大生成的共格孪晶界的原子排列整齐.应用晶体相场模型,可以精确计算纳米孪晶带的厚度,设计和控制带内的原子层数,实现人工操纵纳米共格孪晶片层结构,指导实验研究纳米孪晶结构及其与性能的关系.     

不同衬底条件下石墨烯结构形核过程的晶体相场法研究

利用可描述气-固转变的三模晶体相场模型,在原子尺度上研究了不同衬底条件下石墨烯结构的形核过程.结果表明:无论衬底存在与否,气态原子均是先聚集为无定形过渡态团簇,随着气态原子的不断堆积和固相团簇中原子位置的不断调整,过渡态团簇逐渐转变为有序的石墨烯晶核,在此过程中,五元环结构具有重要的过渡作用;石墨烯在结构匹配较好的衬底(如面心立方(face-centered cubic,FCC)结构(111)和(110))上生长时,可形成几乎没有结构缺陷单晶石墨烯岛;在无衬底或结构匹配性较差的衬底(如FCC结构(100)面)上生长时,形成的石墨烯岛结构缺陷和晶界较多,不利于高质量石墨烯的制备.     

金属铜Cu凝固过程中晶体生长与团簇微观结构演变特性的模拟研究

采用分子动力学方法和Quantum Sutton-Chen（QS-C）多体势,对液态金属铜（Cu）凝固过程中的晶体生长规律及纳米团簇微观结构转变特性进行了模拟跟踪研究．运用Honeycutt-Andersen（HA）键型指数法和新的原子团类型指数法（CTIM-2）分析了金属Cu原子的成键类型和原子团簇结构演变特性.结果发现：当以1.0×1013K/s速率凝固时,系统最终形成晶体和非晶体混合共存结构;在以4.0×1012K/s速度冷却时,系统从673K就开始结晶,并形成以1421和1422二种键型为主的晶体结构;面心立方（FCC）和六角密集（HCP）结构在形成晶体铜时起着非常重要的作用,尤其是由1421键型构成的面心立方（12 0 0 0 12 0）基本原子团在晶体生长和纳米团簇结构形成过程中占主导地位.     

金属铜Cu凝固过程中晶体生长与团簇微观结构演变特性的模拟研究

采用分子动力学方法和Quantum Sutton-Chen（QS-C）多体势,对液态金属铜（Cu）凝固过程中的晶体生长规律及纳米团簇微观结构转变特性进行了模拟跟踪研究．运用Honeycutt-Andersen（HA）键型指数法和新的原子团类型指数法（CTIM-2）分析了金属Cu原子的成键类型和原子团簇结构演变特性.结果发现：当以1.0×1013K/s速率凝固时,系统最终形成晶体和非晶体混合共存结构;在以4.0×1012K/s速度冷却时,系统从673K就开始结晶,并形成以1421和1422二种键型为主的晶体结构;面心立方（FCC）和六角密集（HCP）结构在形成晶体铜时起着非常重要的作用,尤其是由1421键型构成的面心立方（12 0 0 0 12 0）基本原子团在晶体生长和纳米团簇结构形成过程中占主导地位.     

不同冷速对金属铜Cu凝固过程中微观团簇结构演变影响的模拟研究

采用分子动力学方法和Quantum Sutton-Chen（Q-SC）多体势对由5万个液态金属铜（Cu）原子构成的系统在三个不同冷却速率下的凝固过程中微观团簇结构转变的影响进行了模拟研究．运用双体分布函数、Honeycutt-Andersen（HA）键型指数法、原子团类型指数法（CTIM-2）和可视化分析等方法,对凝固过程中微观团簇结构的演变特性进行了分析研究.结果发现：由非晶体向晶体转变的临界速度约为1.0×1013K/s,在此冷速下系统形成非晶体和晶体以一定比例并存的混合结构;在冷速为1.0×1014K/s冷却时系统形成以1551、1541、1532、1431键型为主的非晶体结构,非晶转化温度约为673K;在以4.0×1012K/s速度冷却时,系统从673K就开始结晶,并形成以1421和1422二种键型或由这二种键型构成的面心立方（FCC）（12 0 0 0 12 0）和六角立方（HCP）（12 0 0 0 6 6）基本原子团为主的晶体结构,尤其是由1421键型构成的面心立方（12 0 0 0 12 0）基本原子团在晶体生长和微观团簇结构形成过程中占主导地位. 同时发现,冷速对金属Cu系统中的FCC结构和HCP结构的相对比例有显著的影响,冷速越低,FCC基本原子团以及由其构成的团簇结构越多。     

NiZr,AlZr和BCr相局域原子短程序特征

晶体和非晶合金具有以近邻多面体团簇为特征的微结构,这些团簇包含有形成其母相的空间局域原子短程序结构信息.本文正是从局域原子团簇特征出发,详细分析了传统晶体相描述中具有相同结构的NiZr,AlZr和BCr相,结果表明它们却具有不同的局域原子特征.这种不同可能是由单胞大小及原子占位略有不同造成的.由于这种不同于传统的晶体学描述方法给出了微观结构特点,对晶体相的重新认知及材料设计会大有帮助.     

Ti3Al合金凝固过程晶核形成及演变过程的模拟研究

采用分子动力学方法对Ti₃Al合金的形核机理进行了模拟研究,采用团簇类型指数法(CTIM),对凝固过程不同尺度的原子团簇结构进行了识别和表征,深入研究了临界晶核的形成和长大过程.结果表明,凝固过程体系包含了数万种不同类型的原子团簇结构,但其中22种团簇结构类型对结晶形核过程起关键性作用.在晶核的形成和长大过程,类二十面体(ICO)原子团簇、类BCC原子团簇和缺陷FCC及缺陷HCP原子团簇在3个特征温度点T₁ (1110 K), T₂ (1085 K)和T₃ (1010 K)时达到数量上的饱和,并根据数量和空间分布随温度的变化,得到了它们在形核和长大过程相互竞争的关系.跟踪平行孪生晶粒形成和长大的过程发现,临界晶核是由FCC原子构成的单相结构,并未观察到亚稳BCC相优先形核的过程;平行孪生结构是由FCC单相晶核在沿密排面逐层生长过程中形成的.结果还表明, CTIM相比于其他微观结构表示方法,能更为准确地揭示凝固过程微观结构的转变特征.     

TiAl/Ti3Al体系剪切变形的分子动力学研究

利用分子动力学方法研究了正化学比的TiAl/Ti3Al双相体系中剪切变形诱发位错形核以及相关结构转变的动态过程以及切变力场对最终结构的影响.研究发现,在TiAl/Ti3Al双相体系中剪切变形诱发黏滞-滑移式的滑移行为;界面在其中起到了传递能量、均衡协变的作用,界面两侧的异相结构保留了单相形变特征.六角密堆积(HCP)-Ti3Al部分各原子层较长时间内呈整体剪切协变,其后形变分化为应力集中诱发层错区和初始完整结构回复区;而面心立方(FCC)-TiAl部分因刚性较大仅存在微协变,其后局部受力区直接诱发相邻原子层间相对滑移,发生FCC向HCP结构转变.变形结构方面,HCP-Ti3Al部分在剪切力较大区域形成连续且稳定的FCC堆垛,近界面区FCC薄层与HCP相交替并存;而FCC-TiAl部分内禀层错和孪晶共存,当力场增大时形成亚稳HCP结构.     

Cu_(87)团簇在升温与急冷过程中结构变化的原子尺度模拟

本文采用基于嵌入原子法的正则系综分子动力学方法在原子尺度上计算了包含87个原子的Cu87金属团簇在连续升温和急冷降温时的结构演化过程.根据原子平均势能、对分布函数、原子堆积结构和主要原子键对数目随温度的变化表明,温度的不同极大地影响团簇内的原子堆积结构.在升温过程中,随着温度的升高,团簇内原子堆积结构出现由密排六方、二十面体直到无序堆积的变化.在急冷降温过程中,随着急冷温度的降低,团簇内出现一定数量的二十面体和面心立方局域结构、数量不一的HCP,FCC和二十面体局域结构,直到急冷温度较低时的一定数量的二十面体局域结构.     

The effect of HCP on the formation of twin boundaries and dislocations in Ni–Co alloys

In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni₄₇Co₅₃ and Ni₄₈Co₅₂ alloys at a cooling rate of 10¹² K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni₄₇Co₅₃ alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni₄₈Co₅₂ alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters.     

Chemical synthesis and magnetic properties of HCP and FCC nickel nanoparticles

In this study, we successfully synthesized single-phase hexagonal closed packed (HCP) and face-centered cubic (FCC) nickel nanoparticles via reduction of nickel nitrate hexahydrate and nickel acetate tetrahydrate, respectively, in polyethylene glycol-200. Structural information of the as-synthesized nickel nanoparticles are studied by X-ray diffraction (XRD) as a function of the molar concentration of the nickel precursor. XRD results reveal that low concentrations of nickel precursor (0.005?M and below) favor the HCP, while high concentrations favor the mixture of HCP and FCC crystal structures. Particle size of HCP structure is found in the range of ～15?nm via transmission electron microscope analysis. Vibratory sample magnetometer is employed to study its magnetic behavior and the results reveal that FCC crystalline phase shows ferromagnetic nature with high saturation magnetization (M _s?～?39.6?emu?gm^?1) as compared to metastable HCP crystalline structure (M _s?～?2?emu?gm^?1). The surfactants bonding on the surface of nickel nanoparticles are studied.     

Generalized equilibrium concentration of polyvacancy: case study for trivacancy in hard-sphere crystals

The equilibrium concentration of polyvacancy in crystal is formulated by modifying the grand canonical ensemble. The generalized equation can be applied to obtain the concentration of vacancy in any order and shape in crystal. In this work, a Widom-like particle insertion method is adopted implemented with molecular dynamics simulation to obtain individual formation free energies for constituent vacancies in polyvacancy. As a case study, equilibrium concentrations and formation free energies were obtained of four and nine trivacancies identified in face-centred-cubic (FCC) and hexagonal-close-packed (HCP) hard-sphere crystals, respectively. The result is in excellent agreement with literature data available for the FCC. Further, stabilities of equilateral triangular trivacancies are elucidated and their relative stabilities in FCC and HCP hard-sphere crystals are reported. Overall trivacancy concentration is higher in the HCP HS crystal.     

Structure, properties, and formation models of optical ceramics

The structure, properties, and formation mechanisms of Y₃Al₅O₁₂, Y₂O₃, and Lu₂O₃ laser ceramics are investigated. Their microhardness and fracture toughness are determined. It is shown that the change in mechanical properties is related both to the grain size and grain boundary structure. Processes of plastic deformation of crystals by mechanical twinning are considered. Mechanisms of formation and motion of twins in crystals with FCC structure are determined. It is shown that the realization of similar mechanisms in crystals with HCP structure results in the phase transformations. Models of the formation and motion of twin boundaries are proposed which result in pore healing when preparing monolithic samples of highly transparent ceramics.     

单晶Nd3Co的低温压力效应研究

本文测量了沿Nd3Co单晶b轴在不同压力下电阻率随温度的变化,并对Nd3Co的居里温度和磁性转变场随压力的变化规律进行了研究.结果表明:随着压力的增大,样品剩余电阻率逐渐减小,居里温度平均每GPa 升高2.1 K,磁性转变场平均每GPa增大0.9 T.通过对结果的分析,可以认为压力增大使样品中原子间距变小,晶粒间的连接更加紧密,导致电阻率减小;原子间距变小,4f电子和传导电子间的关联增强,导致样品中Nd离子磁矩的转向变得困难,从而磁性转变场增大.     

熔融Cu55团簇在铜块体中凝固过程的分子动力学模拟

应用基于嵌入原子势函数的分子动力学方法,模拟了嵌入在具有面心立方结构同质块体中的熔融Cu₅₅团簇在不同急冷温度下微观结构的演变情况.通过计算熔融Cu₅₅团簇的均方位移和原子平均能量随时间步的变化,并应用键对分析技术,分析了急冷温度对熔融Cu₅₅团簇结构变化的影响.研究结果表明,由于受到块体结构的影响,在所研究的急冷温度范围内,熔融Cu₅₅团簇在凝固过程中形成了以面心立方结构为主的微观结构.结晶过程是原子不断交换其位置的过程,团簇原子位置的重排敏感于温度的变化.随着急冷温度的升高,原子的扩散范围增大.在100,300和500 K三个较低的温度下有利于形成稳定的面心立方结构,但当急冷到100 K时,团簇中的原子在没有找到其最佳位置之前就已经完成晶化.在急冷到500 K时,团簇中的原子在块体中扩散充分,与块体中的原子形成理想的面心立方结构.在700,900和1100 K三个较高的温度上,局域结构表现为随时间步波动性变化. 关键词： 团簇 分子动力学 计算机模拟 凝固     

面心立方晶体中生成孪晶或六角密堆相的电子衍射分析

用指数变换矩阵将面心立方晶体的晶带轴[uvw]变换成孪晶的[u^tv^tw^t]及六角密堆结构的[u′v′w′],在略去这些指数的公共系数后,如u^(t2)+v^(t2)+w^(t2)>u²+v²+w²或u′,v′≠3n而u′+v′=3n,则孪晶或六角密堆结构与面心立方晶体有完全相重的电子衍射谱。这种标定的不唯一性是广泛存在的。 关键词：     

Microscopic theory of the second- and third-order elastic constants of cobalt in a thermodynamic module

The total potential energy of a crystal is presented as an expansion by irreducible interactions in clusters containing pairs of atomic triplets and quadruplets. The potential energy of the clusters in the adiabatic approximation is a function of vectors \(\vec r_{ik}\) connecting the centers of atoms in the clusters. Arguments (basic functions) affecting the potential energy of clusters are found using the model with allowance for the exchange symmetry of atoms and the irreducibility of the considered energies of doublet, triplet, and quadruplet atoms and interactions. This allows us to present arguments of the potential energy in the form of summed integral numbers (latticed sums) multiplied by a fixed value of the crystal unit cell parameter, and to set the numerical values of the potential energy arguments as a function of vectors \(\vec r_{ik}\). A potential corresponding to the thermodynamic additivity concept of crystal energy is selected as the model potential of pairwise interaction. Secondand third-order moduli of the rigidity of Co crystals with A1, A2, and HCP structures are calculated using this model of multiatom interactions.     

Stacking faults in close-packed clusters

Ground state geometries of small hard sphere clusters were studied using two different type of contact interaction, a pair-potential and a many-atom interaction. Monte Carlo method in an FCC lattice with all possible (111) stacking faults was used to obtain the minimum energy geometries for clusters up to 59 atoms. Due to the surface energy, FCC packing is generally favoured as opposite to the HCP structure. However, in most cluster sizes the ground state obtained with the many-atom interaction has one or more stacking faults. The most symmetric geometry is usually not the ground state. Clusters with 59 and 100 atoms were studied due the possibility of a high symmetry cluster with stacking faults in all four directions. The size dependence of the total energy has similarities with that of the average moment of inertia. Received 6 February 2002 / Received in final form 11 April 2002 Published online 19 July 2002