Bi，Sb及稀土元素对AZ91镁合金高温性能影响机理研究

利用大角重位点阵概念建立了AZ91镁合金基体(α相)和镁［0001］对称倾斜晶界原子结构模型,应用实空间的连分数方法计算了体系的结构能,环境敏感镶嵌能以及相互作用能.结果发现,在镁合金基体中,Al和稀土形成团簇时比较稳定,Al,Bi或Sb与稀土形成团簇时不稳定.Bi或Sb和稀土元素同时存在于AZ91镁合金中时,一方面Bi或Sb将可与RE结合形成RE₂Bi(RE₂Sb)或RE-Bi(RE-Sb)化合物弥散分布于晶界,另一方面镁合金基体中会形成Al_{11关键词： 电子理论 晶界偏聚 合金元素 高温性能}     

稀土对镁合金应力腐蚀影响电子理论研究

建立了镁合金纯净晶界及其析出Mg₁₇Al₁₂相的晶界原子集团，应用实空间的递归方法计算了铝、稀土元素在晶界的偏聚能，晶界处铝、稀土原子间相互作用能和不同体系的费米能级.讨论了铝、稀土在晶界的偏聚行为，铝、稀土原子间的相互作用与有序化的关系及稀土对镁合金晶间应力腐蚀影响的物理本质.研究发现：铝、稀土原子偏聚于晶界；铝原子间相互排斥，在晶界区形成有序相Mg₁₇Al₁₂，稀土原子间互相吸引，形成原子团簇；稀土原子团吸引铝原子，使铝原子渗入稀土团簇中，形成稀土化合物.因此，稀土具有抑制铝在晶界形成导致应力腐蚀的阴极相Mg₁₇Al₁₂的作用，提高镁合金的晶间应力腐蚀抗力. 关键词： 电子理论 镁合金 应力腐蚀 稀土     

纳米SiO2复合对Mg3Sb2基材料热电性能的影响

Mg₃(Sb,Bi)2基热电材料由于其优异的热电性能和较低的成本近来受到广泛的关注.本研究通过将纳米SiO₂复合进成分为Mg_3.275Mn_0.025Sb_1.49Bi_0.5Te_0.01的基体相中,考察其热电输运性能的变化及机制.结果表明,当SiO₂复合进Mg₃Sb₂基材料中时,由于引进大量的微小晶界,能有效地散射声子,促使晶格热导率降低,优化热输运性能,如SiO₂体积含量为0.54%时,室温时热导率由复合前的1.24 W/(m·K)降至1.04 W/(m·K),降幅达到15%;同时其对电子也产生强烈的散射作用,导致迁移率和电导率大幅下滑,结果表现为近室温区功率因子剧烈衰减,恶化了电输运性能.电性能相对于热性能较大降低幅度使得材料在整个测试温区的热电优值没有得到改善.纳米SiO₂作为Mg₃Sb₂     

Ce在纯铁中存在状态和对Fe-Sb合金的作用

采用低O,S含量的纯铁和高真空无坩埚悬浮熔炼技术制备合金。研究了微量Ce(0.1wt%)在Fe中存在形态及对Fe-Sb合金的作用。结果表明:(1)Ce加入Fe中,发现明显提高α-铁素体晶界抗腐蚀性。其晶界上没有发现Ce的存在。但在高温δ相晶界上有网状析出物,并证实该析出物由O,Ce的化合物组成,从萃取粒子的电子衍射分析发现,许多粒子是由CeAlO₃相构成。(2)Ce加入Fe-Sb合金中能起净化晶界的作用,使原先沿晶界分布的Fe₃Sb₂相不再存在,而在晶内发现有CeSb相存在。 关键词：     

退火诱导机械合金化Fe42.5Al42.5Ti5B10合金的结构演变与晶粒生长动力学

利用XRD和TEM方法研究Fe_42.5Al_42.5Ti₅B₁₀合金在机械合金化及等温热处理过程中的结构演变及晶粒生长动力学,讨论了机械合金化合成机理和热处理过程中的晶粒生长机理.结果表明,球磨过程中Al,Ti,B原子向Fe晶格中扩散,形成Fe(Al,Ti,B)固溶体.机械合金化合成Fe(Al,Ti,B)遵循连续扩散混合机理.球磨50h后,金属Fe,Al,Ti,B已完全合金化,球磨终产物为纳米晶Fe(Al,Ti,B).球 关键词： XRD TEM 42.5Al_42.5Ti₅B₁₀合金')" href="#">Fe_42.5Al_42.5Ti₅B₁₀合金 机械合金化     

Fe-C—Sb合金中液态急冷获得的ε相的分解

本工作用示差热分析法,淬火样品X射线衍射法和Guinier-Lenn透射式高温单色聚焦粉末照相法研究了Fe-C-Sb合金中液态急冷获得的ε相的高温分解过程,实验结果指出,ε相在125℃开始分解,首先分解成ε-Fe₂C,而后在450℃分解成α-Fe,并析出Sb和Fe₃Sb₂,还保留有ε-Fe₂C;在550℃以后ε-Fe₂C转变成Fe₃C,并在770℃时发生Fe的α→γ转变,在800℃时主要组成相为:γFe,Sb,Fe₃C(Fe₃Sb分解);在冷却过程中750℃发生Fe的γ→α转变,冷至室温时的主要组成相是:αFe,Fe₃C,Sb,Fe₃Sb₂。 关键词：     

Mg-Sn-Si合金中Mg2(Si,Sn)复合相的结构与性能研究

研究了铸态Mg-Sn-Si合金中Mg₂(Si,Sn)复合相的结构、 特性以及该相对Mg-Sn-Si合金变质作用的影响. 结果表明: Sn原子能取代Mg₂Si中的部分Si生成Mg₂(Si,Sn)复合相, 该三元相与Mg₂Si, Mg₂Sn相的结构相同, 属于面心立方结构, Mg₂(Si,Sn)相的元素含量并不固定, 在Si富集区形成的Mg₂(Si,Sn)相中, Si元素含量高, 在Si贫乏区形成的Mg₂(Si,Sn)相中, Si元素含量低. Si含量较多的Mg₂(Si,Sn)相性能与Mg₂Si相接近, Sn含量较多的Mg₂(Si,Sn)相性能与Mg₂Sn相接近, 实验中发现Mg₂(Si,Sn)复合相的纳米硬度、 弹性模量与维氏硬度等物理性能介于Mg₂Si与Mg₂Sn之间, Mg₂(Si,Sn)相对汉字状Mg₂Si相的变质处理起到桥梁作用. 关键词： Mg-Sn-Si合金 2Si')" href="#">Mg₂Si 2Sn')" href="#">Mg₂Sn 2(Si,Sn)复合相')" href="#">Mg₂(Si,Sn)复合相     

钢铁材料中形变诱导相变超细化机理研究

通过计算机编程建立奥氏体相中12[1 1 0]刃位错、奥氏体相中非形变区和形变区奥氏体/铁 素体相界模型.用实空间的连分数方法计算了非形变区和形变区奥氏体/铁素体相界界面能， 计算了碳、氮及微合金元素在完整晶体及位错区引起的环境敏感镶嵌能，进而讨论形变过程 中铁素体形核的难易程度，碳、氮及合金元素在位错区的偏聚及析出与铁素体细化的关系. 计算结果表明：α-Fe易于在高密度位错区（形变带、亚晶界、晶界）形核，在奥氏体形变 过程中，就会大大提高α-Fe形核率，细化铁素体晶粒；碳、氮和微合金元素易于单独或共 同 关键词： 奥氏体/铁素体相界 刃位错 形变 晶粒细化     

Mg-Sn-Si系合金的热力学基础及合金相演变过程分析

研究了Mg-Sn-Si系合金的热力学基础及合金相的演变过程. 结果表明: 对于Mg-Sn-Si系合金, 合金相的比热容随着温度增加而增加, 在低温下变化迅速, 而在高温下变化平缓, 其热膨胀系数在低温范围内随温度升高呈指数形式增加, 而在高温范围内呈线性增大. 在Mg₂ (Si_x, Sn_1-x)、Mg₂ (Sn_x, Si_1-x)相结构中, Sn(Si)原子的取代位置不固定, 可以是面心, 也可以是顶点. 常规凝固过程中, 由于处于非平衡状态, x的取值范围有所波动, 对于Mg₂ (Si_x, Sn_1-x)和Mg₂ (Sn_x, Si_1-x) 两种结构, x的取值范围在0.25或0.75附近. Mg₂ (Si, Sn)的生成温度较高, 可从液相中直接析出, 也可由Mg₂Si转化而来, 而Mg₂ (Sn, Si)的生成温度较低, 只能从基体中析出, 随着Sn含量的增加, 开始析出Mg₂ (Sn, Si)相的温度升高.     

压痕塑性变形诱导非晶合金的晶化

利用透射电镜研究了Zr₆₅Al_7.5Ni₁₀Cu_12.5Ag₅非晶合金的Vickers压痕内微观结构的变化. 结果发现，压痕塑性变形诱导非晶合金发生了晶化，在压头棱角下面的区域内有尺寸大于1 μm的晶体析出. 选区电子衍射分析表明，该析出相是稳定的CuZr₂或NiZr₂四方晶体，而没有析出该非晶合金在加热过程中的初生相二十面体准晶相,说明非晶合金的机械稳定性与热稳定性是有区别的. 打压痕过程中的温度升高是可以忽略的，本工作进一步证实了塑性变形诱导非晶合金晶化的主要动力是粘性流动而非局部热效应. 关键词： 非晶合金 塑性变形 粘性流动 局部热效应     

Interfaces in Iron-Rich Ordered Fe-Al Alloys: An Atomic-Scale Simulation Study

The aim of this paper is to investigate the low-temperature structure of interfaces (the (3 1 0) [0 0 1] symmetrical tilt grain boundary (GB) and the (3 1 0) surface) in stoichiometric ordered Fe-Al alloys with B2 and DO₃ structures. In both alloys, (i) the GBs cannot be realistically described by geometrical models, (ii) GBs and surfaces show strong segregation effects. A simple independent-defect model cannot be applied: the interactions between point defects sometimes lead to results opposite to those predicted from the formation energies of isolated point defects. The excess energies and configurations of the most stable interface variants are determined. All interfaces show a tendency to Al segregation except the B2 GB for which the most stable structure is an Fe-rich one. The interface structures are more complex in the DO₃ than in the B2 alloy, with a high multiplicity of DO₃ configurations with close energies. Finally, values of the GB and surface energies are introduced into a Griffith model of brittle fracture, in order to assess the trends of both alloys to intergranular fracture. Comparisons are also drawn with the similar Ni-Al ordered alloys.     

Improved thermoelectric performance of n-type Mg3Sb2–Mg3Bi2 alloy with Co element doping

n-type Mg₃Sb₂–Mg₃Bi₂ alloy shows as a potential new thermoelectric material (TE) and has been widely researched recently. The pure phase n-type Mg_3·20(Sb_0·3Bi_0.7)_1.99Te_0.01 were prepared by adjusting Mg content with the Bi impurity phase being effectively suppressed. Then, Co element was doped into the pure phase and the electrical conductivity of samples were improved. With a high power factor of 20.3 μW cm⁻¹K⁻² for Mg_3·185Co_0·015(Sb_0·3Bi_0.7)_1.99Te_0.01 at 525 K. Additionally, it was found that the phonon scattering is enhanced due to the larger atomic mass of Co comparing to Mg and the lattice thermal conductivity is reduced. As a result, a high ZT value of ~ 1.03 at 525 K is achieved for the Mg_3·185Co_0·015(Sb_0·3Bi_0.7)_1.99Te_0.01.     

Growth of Bi-Sb alloy thin films and their characterization by TEM, PIXE and RBS

Polycrystalline bulk materials of Bi₉₃Sb₇ Bi₈₈Sb₁₂, Bi₈₅Sb₁₅ and Bi₈₀Sb₂₀ were synthesized by melt-quench technique starting from the stoichiometric mixture of constituent elements. The phase purity and compositional uniformity of bulk materials were investigated using powder X-ray diffraction (XRD) and proton induced X-ray emission (PIXE) experiments. The single phase formation and the compositional analysis of thin films were confirmed by transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS). X-ray diffraction studies confirmed the phase homogeneity of the materials. Atomic concentration ratio of constituent elements (Bi and Sb) determined by PIXE and RBS revealed that near-stoichiometric composition is nearly the same in the bulk as well as in thin film forms.     

Effect of heat treatment on the structure and the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> thin films and composites based on them

This work considers the effect of vacuum annealing on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 thin film and Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb_0.9Bi_1.1Te_2.9Se_0.1–C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution, which determine the type of conductivity of Sb_0.9Bi_1.1Te_2.9Se_0.1 granules. The power factors are estimated for films of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution and films of Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)₂(Te,Se)₃ solid solutions.     

Plasma electrolytic oxidation coating on AZ91 magnesium alloy modified by neodymium and its corrosion resistance

Ceramic coatings on the surfaces of Mg-9Al-1Zn (AZ91) magnesium alloy and Mg-9Al-1Zn-1Nd magnesium alloy (AZ91 magnesium alloy modified by neodymium, named as AZ91Nd in this paper) are synthesized in aluminate electrolyte by plasma electrolytic oxidation (PEO) process, respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses show the PEO coating on the Mg-9Al-1Zn-1Nd alloy comprises not only MgO and Al₂O₃, which are found in the coating on the AZ91 alloy, but also a trace amount of Nd₂O₃. Microstructure observations indicate the addition of Nd can decrease the sizes of β phases and form Al₂Nd intermetallics in the AZ91 alloy. The fine β phases can effectively restrain the formation of unclosed-holes and greatly decrease the sizes of pores in the coating during the PEO process. In addition, the Al₂Nd intermetallics can be completely covered due to the lateral growth of the PEO coatings formed on the α and β phases. As a result, the coating on the AZ91Nd alloy possesses a dense microstructure compared with that on the AZ91 alloy. The following corrosion tests indicate the corrosion resistance of the PEO coating on the AZ91Nd alloy is evidently higher than that of the PEO coating on the AZ91 alloy.     

Effect of Sb substitution on the superconductivity of Bi2(Ca,Sr) n+1Cu n O4n+2 (n=1, 2 and 3) systems

Effect of incorporation of Sb in place of Bi in the bismuth cuprate superconductors has been examined. The nominal compositions studied are MCa_1·5Sr_1·5Cu₂O_8+δ and MCa₂Sr₂Cu₃O_10+δ, whereM=Bi_2−x Sb _x or Bi_1·5−x Sb _x Pb_0·5. Different preparative routes such as the ceramic method, the matrix route as well as the melt route were employed to prepare the materials. No indication of either Sb entering the lattice or enhancement ofT _c is noted from resistivity, magnetic susceptibility and microwave absorption measurements. Communication No. 162 from Materials Research Centre.     

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg₂Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg₂Sn and La-doped Mg₂(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg₂Sn.     

Experimental determination of the mean scattering length of low energy electrons in bismuth

The mean scattering length of electrons emitted in the Auger decay of Sb4d core holes in Sb₂Te₃ and propagating through thin Bi films has been experimentally determined for energies ～5 to 12 eV above the Fermi level.