CdS晶体的价电子结构分析和结合能、熔点计算

应用固体与分子经验电子理论(EET)分析了半导体化合物硫化镉(CdS)的价电子结构,并计算了化合物的键能、熔点,计算值与实验值一致。CdS的闪锌矿和纤锌矿两种结构的价电子计算结果表明,两种结构的价电子分布非常相似,其键能主要分布在最近的4条Cd-S键上。闪锌矿和纤锌矿结构的CdS中的Cd原子和S原子都应处于第四杂阶。键能分布计算结果表明,常温下,六角结构的CdS要比立方结构的CdS更加稳定,但两种结构的结合能差值相对于键能非常小,晶体结构的各向异性也在减弱,但总体来说其结构稳定性差别不大,很容易受到环境影响而发生相互转化。对熔点的计算结果表明,CdS的固相稳定性与其共价电子结构密切相关,对熔点起主要作用的是最强键的共价电子对数,其对纤锌矿和闪锌矿两种结构熔点的影响率分别为93.4%和99.2%。     

δ（Al3Li）相的价电子结构分析

应用固体与分子经验电子理论对铝,锂及δ（Al3Li)相的电子结构进行了分析,并计算了Al-Al,Li-Li和Al-Li的最强键的键能及铝、锂的结合能,计算结果与实验值比较非常接近。     

Ni-Al系金属间化合物价电子结构与性能分析

NiAl和Ni3Al金属间化合物熔点高、密度低,具有较好的热传导性和良好的抗氧化性,是航空航天领域很有潜力的高温结构材料.运用固体与分子经验电子理论分析了NiAl和Ni3Al金属间化合物的价电子结构,并从电子结构层次初步探讨了NiAl和Ni3Al金属间化合物的强度、稳定性、室温脆性及熔点等问题.计算结果表明,化学计量比的NiAl和Ni3Al的脆性因子均小于0.08,室温下表现为本征脆性,NiAl的脆性比Ni3Al的脆性大;NiAl的熔点和强度均比Ni3Al的高,稳定性比Ni3Al的差.     

Zr元素对纯铝细化机理的电子理论研究

基于固体与分子经验电子理论(EET),计算了Al-Zr合金熔体中相及相界面的价电子结构,获得了表征合金性能关系的价电子结构参数统计值,并利用这些参数讨论了Zr对纯铝晶粒细化的机理。结果表明:Zr对纯铝的细化机理可以追溯到熔体中Al3Zr与α-Al相晶面间的界面共价电子密度差的统计值Δρ′,即Δρ′10%且Δρ′值越小,合金中的异质形核效应越好,晶粒细化效果越好。Δρ′值小(2.789 04%)的Al-Al原子团簇能较容易地被异质形核核心(Al_3Zr晶体)吸附,促进异质形核及晶粒细化作用;而Δρ′值大(15.698 70%)的Al-Zr原子团簇容易从异质晶核上脱离,削弱异质形核及晶粒细化效应。     

Fe－C合金贝氏体在奥氏体贫碳区切变相变机制的价电子理论分析

本文运用固体与分子经验电子理论，计算了Ｆｅ－Ｃ合金发生马氏体转变的临界阻力以及中温等温时的相变阻力，讨论了贝氏体在贫碳区切变形核的可能性。     

硅基外延Mn_4Si_7薄膜电子结构与光学性质研究

基于第一性原理的密度泛函理论赝势平面波方法,对外延关系Mn4Si7(001)//Si(001),取向关系Mn4Si7[001]//Si[001]的Mn4Si7平衡体系下的电子结构和光学性质进行了理论计算,计算结果表明:当Mn4Si7晶格常数选取为a=b=0.5431nm、c=1.747nm时,Mn4Si7为带隙宽度为0.834eV的直接带隙半导体。Mn4Si7费米面附近的价带主要由Mn的3d5态电子构成,导带主要由Mn的3d5态电子及Si的3p态电子构成。静态介电常数ε1(0)=14.48,折射率n0=3.8056。     

Breakup of eutectic carbide network of white cast irons at high temperatures

The fracture toughness of white cast irons is related to the morphology of eutectic carbides,being better when isolated than when network-like. In this paper observations on the breakup of eutectic cementite network during annealing treatment of white cast irons are reported using a high temperature microscope and scanning electron microscopy (SEM). Dissolution-induced breakup and capillarity-induced breakup are identified. The former occurs through growth of holes and pre-existing fissures as well as through fragmentation at narrow necks and narrow roots of branches. The latter is observed through growth of perturbations. Dissolution-induced breakup is closely associated with the morphology of the as-cast eutectic cementite. A combination of solidification processing and heat treatment thus produces a more positive breakup effect.     

The morphology and microtexture of M7C3 carbides in Fe-Cr-C and Fe-Cr-C-Si alloys of near eutectic composition

The microtexture of M₇C₃ carbides in undercooled 40 g samples of hyper- and hypo-eutectic Fe-Cr-C alloys was determined by electron back scatter diffraction. In the hyper-eutectic alloy the carbides were monocrystalline, while those in the hypo-eutectic alloy were polycrystalline. While in the former the preferred growth direction of the M₇C₃ carbides was [0001], in the hypo-eutectic alloy there was a relatively weak texture near [10¯11]. There was no evidence for the presence of growth twins in either the M₇C₃ carbide rods or in the branching mechanism in the joint between the carbide rods of the hypo-eutectic sample. The morphologies of the M₇C₃ carbides resulting from undercooling were used to explain the microstructure of hardfacing Fe-Cr-C weld deposits applied by the manual metal arc process. The effect of silicon additions on the morphology of M₇C₃ carbides in Fe-Cr-C-Si alloys is explained in terms of the effect of silicon on undercooling.     

固溶体的价电子结构与固溶强化

为在价电子结构层次研究代位固溶体与间隙固溶体强化差异的本质,建立了代位固溶体含溶质晶胞点阵常数的计算公式,以余(瑞璜)氏固体与分子经验电子理论为基础,按BLD(键距差分析方法)分析方法计算了代位固溶体含溶质晶胞的价电子结构.结果表明:代位固溶体中含溶质原子晶胞的nA值与不含溶质原子晶胞的nA值相差无几,而间隙固溶体中含溶质原子晶胞的nA值远高于不含溶质原子晶胞的nA值.这-信息从价电子结构的角度深刻揭示了两种固溶体强化程度存有严重差异的本质,从而把固溶体强化的机制追溯到固溶体的价电子结构.     

Stacking faults in chromium,iron and vanadium mixed carbides of the type M7C3

Carbides found in white cast-iron containing 2.88% carbon, 15.22% chromium and 3.08% vanadium have the stoichiometric formula Cr_2.8V_0.7Fe_3.4C₃. They belong to the type M₇C₃ and are isomorphic with the chromium carbide Cr₇C₃. A modification of the approximate structure, given by Westgren, was considered in which carbon atoms were assumed to be situated right at the centre of gravity of perfectly symmetrical right-angled prisms. The great number of crystal defects that the carbides always contain were studied by electron microscopy and electron diffraction. They are stacking faults having as their fault planes one of the three equivalent planes (1 0 ¯1 0), (1 ¯1 0 0) or (0 1 ¯1 0), and as their fault vectorsR=a/2 orb/2 or (a+b)/2. A detailed examination of the diffraction patterns which contain streaks or extra reflections indicates that, in strongly faulted carbides, the stacking faults are ordered. A simple model which views the structure of M₇C₃ as a stacking sequence of right-angled prisms containing carbon atoms is proposed. Using this model, the order of the stacking faults can be easily interpreted. It suggests that the stacking fault energy is very weak, hence the frequent occurrence of the stacking faults in the carbides.     

Revealing and characterising solidification structure of ductile cast iron

Abstract

This study deals with the solidification and microsegregation processes of near eutectic ductile cast iron. A detailed analysis of old and new solidification models was made. Metallographic techniques, developed elsewhere by the authors, were used to reveal the solidification microstructure and macrostructure. The results allow a new solidification model to be proposed, where each solidification unit is a grain of eutectic austenite that has a dendritic substructure and contains a very large number of graphite nodules. A pattern of microsegregation exists inside each solidification grain, while no intergranular microsegregation has been detected. A procedure to characterise the solidification refinement was developed considering the location of the last to freeze areas.     

Transmission electron microscopy of a transformation toughened white cast iron

Transmission electron microscopy has been used to study the microstructure of an experimental white cast iron, in which a combination of modified alloy composition and unconventional heat treatment has resulted in a fracture toughness of 40 MPa m-1/2. Microstructural features of the alloy that contribute to the toughness improvement and hence distinguish it from conventional white irons have been investigated. In the as-cast condition the dendrites are fully austenitic and the eutectic consists of M7C3 carbides and martensite. During heat treatment at 1130 °C the austenite is partially destabilized by precipitation of chromium-rich M7C3 carbides. This results in a dendritic microconstituent consisting of bulk retained austenite and secondary carbides which are sheathed with martensite. The martensite sheaths, which contain interlath films of retained austenite, are irregular in shape with some laths extending into the bulk retained austenite. Emphasis has been placed on the morphology, distribution, and stability of the retained austenite and its transformation products in the dendrites. The implications of these findings on the transformation toughening mechanism in this alloy are discussed.