MoSi2和WSi2的价电子结构及性能分析

根据固体与分子经验电子理论,对MoSi2和wSi2的价电子结构进行了定量的分析,通过键距差方法计算了MoSi2和WSi2晶体中各键上的共价电子数．结果表明：在MoSi2和WSi2晶体中,沿（331）位向分布的Mo-Si和W-Si原子键最强,这些键上的共价电子数和键能分别影响化合物的硬度和熔点．晶体中晶格电子数影响其导电性和塑性,MoSi2晶体中含有较高密度的晶格电子,因此MoSi2的导电性和塑性比WSi2好．并从键络分布的不均匀性解释了MoSi2和WSi2脆性产生的原因．     

Ni在熔融(Li,K)_2CO_3中的腐蚀行为

采用电化学阻抗谱技术研究了Ｎｉ在６５０℃熔融（Ｌｉ,Ｋ）２ＣＯ３中的腐蚀行为．在自腐蚀电位下,Ｎｉ的腐蚀电化学阻抗谱呈典型的扩散控制特征,表面形成疏松的氧化膜,腐蚀速度较快．提出了相应的等效电路,求得相关电化学参数     

TiC-TiB2增强MoSi2复合材料的力学性能及抗氧化行为

以MoSi2、Ti和B4C粉为原料,采用高温热压技术合成不同体积分数TiC-TiB2增强MoSi2复合材料,研究TiC-TiB2颗粒对MoSi2基体材料显微组织、力学性能和高温氧化性能的影响.结果表明:30%TiC-TiB2/MoSi2(体积分数)复合材料的抗弯强度和维氏硬度分别达到468.3 MPa和17.07 GPa,比纯MoSi2的分别增加了63.2%和83.5%.随着TiC-TiB2体积分数的增加,复合材料的断裂方式由以沿晶断裂为主向以穿晶断裂为主转变,强化机制是细晶强化和弥散强化.在800～1 200 ℃氧化192 h时,30%TiC-TiB2复合材料的增质是10%TiC-TiB2复合材料的2.38～3.23倍.氧化层中没有发现低熔点的B2O3,而TiO2和SiO2的存在使材料具有较好的抗氧化性.     

On the crystallographic characteristics of ion beam synthesised β–FeSi2

Nanometre-scale β–FeSi₂ precipitates were introduced in a single crystal silicon substrate by low-dose ion-beam synthesis (IBS). The crystallographic relationship between these nanometre β precipitates and the silicon substrate has been studied by high resolution electron microscopy (HREM). The results show that the orientation relationship (OR) between the nanometre β precipitates and the silicon substrate is [100]_β//[110]_Si and (001)_β//(1 1)_Si, with abnormally large strain between the precipitates and the substrate. This OR is important for the formation of 90°-OD boundaries within β–FeSi₂ grains. Also, the relationship between various reported low-index ORs has been analysed and a new low-index OR is predicted.     

TiAl金属间化合物的熔盐热腐蚀行为

研究了ＴｉＡｌ金属间化合物在熔融盐中的热腐蚀行为。ＴｉＡｌ在９００℃（Ｎａ，Ｋ）２ＳＯ４中呈现良好的抗热腐蚀行为，形成的腐蚀产物具有分层结构，外层为ＴｉＯ２与Ａｌ２Ｏ３的混合氧化物，和二层由致密的Ａｌ２Ｏ３组成，内层是Ｔｉ或Ａｌ的硫化物。在Ｎａ２ＳＯ４＋ＮａＣｌ的混合盐中，ＴｉＡｌ遭受严重的热腐蚀，不能形成保护性的Ａｌ２Ｏ３层，形成的Ａｌ２Ｏ３和ＴｉＯ２混合膜的粘附性差，较易剥落。     

Role of chlorides in hot corrosion of a cast Fe–Cr–Ni alloy. Part II: thermochemical model studies

In many high temperature applications severe degradation of alloys is caused by thin deposits of molten salts, especially alkali metal sulfates, alkali metal chlorides and mixtures of these salts. Calculations of multi-component thermochemical equilibria in systems involving (initially) metal/salt/gas as a function of local oxygen activity can help identify the important hot corrosion reactions. Such calculations for four pure salts (KCl, NaCl, Na₂SO₄, and K₂SO₄) and a mixture of these salts in contact with an Fe–20%Cr alloy at 800 °C are presented. The results predict that the compositions of gas, oxide, (sulfide, when sulfate was input) and salt phases depend strongly upon the salt chosen and upon the local oxygen activity. In some cases the equilibrium salt composition is significantly changed by reactions with metal or oxide phases. The calculated results for the salt mixture were compared with experimental data from part I of this paper. The model calculations have led to the identification of two new factors that support faster hot corrosion rates in an alkali chloride + sulfate salt compared to that in alkali sulfate alone. First, alkali chlorides, unlike sulfates, support a continuous salt pathway from ambient to the metal/scale interface, allowing oxidant to be efficiently transported to oxidize metal. Secondly, under oxidizing conditions alkali chlorides have a higher solubility of dissolved Fe- and Cr-containing species than that in alkali sulfates. Both of these factors support higher transport rates, which according to the fluxing theory of hot corrosion will lead to faster corrosion.     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.     

碳纳米管强韧化二硅化钼复合材料

利用机械合金化（MA）方法合成MoSi2纳米先驱粉体，并对碳纳米管（CNT）进行超声分散，将MoSi2和CNT湿法球磨混合后，采用热压烧结方法制备了CNT／MoSi2复合材料。结果表明，Mo-Si粉末按原子比1：2混合，以转速510r／min球磨24h得到杂质含量较低的MoSi2纳米粉体。烧结后材料的相组成分析结果显示，不含CNT的MoSi2材料主要为MoSi2相，同时含有少量Mo5Si3；添加CNT后，复合材料中新增了少量的SiC，Mo5Si3的含量也比非增强MoSi2中高。CNT／MoSi2复合材料强度和韧性较纯MoSi2材料均有提高，含2．5％（质量分数，下同）CNT复合材料的抗弯强度提高了72％，添加I％CNT复合材料的断裂韧性提高T43％。对CNT／MoSi2复合材料显微结构分析发现，CNT细化材料晶粒，CNT的拔出，CNT使裂纹偏转、分支和桥联等机制综合作用提高了复合材料的韧性。细晶强化和弥散强化作用提高了材料强度。     

Hot corrosion behaviour of Fe-Al based intermetallic in molten NaVO3 salt

The hot corrosion of sprayed Fe-40 (at.%)Al intermetallic alloy with additions of boron and alumina whiskers in molten NaVO₃ at 700 °C has been evaluated by potentiodynamic polarization curves and electrochemical impedance spectroscopy, EIS, techniques. For short exposure times, the corrosion mechanism under these conditions was observed to be controlled by an activation process, whereas for longer exposure times, the corrosion process was under diffusion control due to the growing of an Al₂O₃ oxide scale, which made the diffusion of both reducing and oxidizing species through the scale to the alloy or scale surfaces more difficult. Equivalent electric model used to simulate the EIS data showed that a finite length Warburg diffusion could properly characterize the diffusion process, which confirmed the formation of a compact corrosion product scales containing rich aluminium oxide. Thus, the corrosion process was under diffusion control of aggressive ions through the formed scale. The electrochemical study was complemented by scanning electronic microscopy characterization and micro chemical analysis.     

La2O3-Mo5Si3/MoSi2复合材料的力学性能和高温氧化行为

通过自蔓延高温合成了稀土协同Mo5Si3复合强韧化MoSi2的复合粉末,研究了La2O3-Mo5Si3/MoSi2复合材料的室温力学性能和高温氧化特性。结果表明:与纯MoSi2相比,稀土和Mo5Si3细化了材料的晶粒,提高材料的室温弯曲强度和断裂韧性,其强化机制为细晶强化,韧化机制为细晶韧化、裂纹偏转、裂纹分支和微桥接;当Mo5Si3含量不超过30%(摩尔分数)时,随着Mo5Si3含量的增加,材料的抗氧化性能降低,而RE-40%Mo5Si3/MoSi2(摩尔分数)复合材料出现粉化现象;RE-Mo5Si3/MoSi2复合材料抗氧化性的降低,主要是由于Mo5Si3较差的抗氧化性、材料致密度的降低以及晶粒细化的结果;0.8%稀土(质量分数)协同5%Mo5Si3(摩尔分数)的RE-Mo5Si3/MoSi2复合材料具有较好的综合力学性能和高温抗氧化特性。     

添加15％Mo对反应熔渗Al制备MoSi2复合材料组织和力学性能的影响

采用在常规模压生坯中浸渗Al的方法,研究了MoSi2+Mo反应熔渗Al后的显微组织和力学性能的变化情况.研究表明MoSi2坯体在1350℃反应浸渗Al可使抗弯强度高达737 MPa,其断裂韧性也可达到4.3 MPa·m1/2,远远高于热压单相材料;然而其相成分中存在残余的Si相和Al相将会阻碍其高温应用;在MoSi2坯体中加入Mo粉可以消除残余硅相;经过计算,当Mo添加量为15%(质量分数,下同)时,可以完全消除Si相,并使Al相达到最低,而其强度并不降低.同时SEM观察表明,添加15%Mo后其断口显示晶粒间絮状夹层变成颗粒状黑色相面积较未加入Mo粉时减小.从而进一步证实添加15%Mo产生的组织变化将有利于该材料的高温力学性能.     

二硅化钼材料在CO和N2混合气氛条件下使用时的破坏机理

考察了二硅化钼电热材料在CO和N2混合气氛条件下使用破坏机理。结果表明，在这种非氧化气份条件下，二硅化钼材料表面很难生成氧化物保护膜；二硅化钼材料表面发生脱硅反应，生成低硅化物，甚至MoC，在表面形成变质层。在频繁的升降温过程中，由于表面变质层与基体的热膨胀系数不同，发生剥落。从而导致二硅化钼材料在这种特出气氛条件下使用寿命的降低。     

Effect of surface oxidation on corrosion mechanism of Co–20·8Ni–6Al–10Cu–11Fe cobalt alloy in molten cryolite

In this paper, the effect of surface oxidation on corrosion behaviour of Co–20·8Ni–6Al–10Cu–11Fe alloy in molten cryolite is investigated. The samples were produced by casting and then were oxidised at 1000°C for 10, 30 and 70 h respectively. The oxide layers were studied by scanning electron microscope (SEM) and X-ray diffraction (XRD). To determine the corrosion behaviour of the oxidised samples, they were exposed to molten cryolite at 930°C for 20 h. After corrosion, the samples were studied by SEM. The results showed that all the samples corroded, but the sample oxidised for 70 h, was more stable than the other and the other one that oxidised for 10 h, corroded severely.     

Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C. Part I: Literature review

In the first part of this paper the results of a literature review are presented. An overview of the oxidation behaviour in air and in combustion environments of both pure MoSi₂ and MoSi₂ composites in the temperature range from 400 to 1650°C is given. The second part of this paper reports about our results from oxidation tests with selected MoSi₂ composites (containing 15 vol.‐% Al₂O₃, Y₂O₃, ZrO₂, HfO₂, SiC, TiB₂, ZrB₂, or HfB₂, respectively) from different development stages at temperatures in the pest region as well as up to 1700°C. The third part describes the oxidation behaviour of the optimised MoSi₂ composites developed on the basis of the results from part II.     

“A study on the effect of Co,Cr and Ti on the corrosion of FE40AL intermetallic in molten NaCl–KCl mixture”

The effect of the addition of 3 at% Cr, Co or Ti on the corrosion behavior of Fe40Al intermetallic alloy in 1 M NaCl:1 M KCl mixture at 750 °C has been carried out by using electrochemical techniques. Techniques included potentiodynamic polarization curves, linear polarization resistance and electrochemical impedance spectroscopy measurements. It has been found that the addition of either Cr, Co or Ti decreased the corrosion rate of Fe40Al intermetallic by almost five times due to the formation of a passive layer, absent in the base alloy. This was because the diffusion of Al towards the top of the alloy for the establishment of a protective Al₂O₃ layer was enhanced by the presence of these alloying elements. The corrosion rate for the base alloy was under diffusion control of the reactants, whereas that for alloyed intermetallic was under charge transfer control.     

Isothermal section at 1673 K of the Mo–Ru–Si diagram and crystallographic structures of ternary phases

Efforts to improve the high temperature behavior of MoSi₂ in oxidizing environments led to the investigation of the Mo–Ru–Si phase diagram. The isothermal section at 1673 K was determined by X-ray diffraction, optical and scanning electron microscopies and EPMA. Five new silicides were identified and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diffraction. Mo₁₅Ru₃₅Si₅₀, denoted α-phase, is of FeSi-type structure, space group P2₁3, a=4.7535 (5) Å, D_x=7.90 g. cm⁻³, Bragg R=7.13. Mo₆₀Ru₃₀Si₁₀ is the ordered extension of the Mo₇₀Ru₃₀ σ-phase with space group P4₂/mnm, a=9.45940(8) Å, c=4.94273(5) Å, D_x=6.14 g. cm⁻³, Bragg R=5.75.     

High-temperature wear behaviors of MoSi2 under different loads

MoSi2 was prepared by SHS, pressed at room-temperature and then vacuum sintered at 1 500 ℃ for 1 h. The tribological properties of MoSi2 against Al2O3 were investigated by using an XP-5 type High Temperature Friction and Wear Tester. Micrographs and phases of the worn surface of MoSi2 were observed by SEM with EDS and X-ray diffraction. The results show that the wearing process of MoSi2 at high temperature exists three stages： running-in, interim and steady periods. MoSi2 exhibits preferable wear resistance when the load is lower than 50 N. Adhesion and oxidation wear exists widely at elevated temperature; however besides these, with increasing the load, the main wear mechanisms of MoSi2 could be changed from adhesion, plastic forming to fatigue fracture in turn.     

MoSi_2加热元件的高温碳化

采用XRD、SEM和EPMA技术研究MoSi2棒材的高温碳化行为。结果显示:试样心部仍为MoSi2相,碳化产物为Mo5Si3C和Mo2C;碳化层组织疏松,有大量圆形孔洞存在,厚度约为500～800μm,心部组织表现为MoSi2沿晶和穿晶脆性断裂特征;碳化产物位于三元Mo-Si-C平衡相图中Mo5Si3、Mo5Si3C和Mo2C三相区内,试样由内到外,Mo2C含量逐渐升高;碳化产物主要由Mo5Si3碳化形成,而并非MoSi2的直接碳化产物。     

MoSi2高温氧化层的微观结构

采用SEM,TEM和XRD方法研究了MoSi2在1200－1600℃的氧化层微观结构。在1240℃以下,氧化层由SiO2和其它氧化物混合而成,致密度较差。1240－1520℃区间氧化层表面存在针状、扇状或羽状的低温石英,氧化层较薄。在1520℃以上,氧化层中含有块状、粒状或蜂巢状的方石英,氧化层致密而均匀,增强了材料的抗氧化性能。