Atomistic Modeling of Multicomponent Systems

The need to accelerate materials design programs based on economical and efficient modeling techniques provides the framework for the introduction of approximations in otherwise rigorous theoretical schemes. Several quantum approximate methods have been introduced through the years, bringing new opportunities for the efficient understanding of complex multicomponent alloys at the atomic level. As a promising example of the role that these methods might have in the development of complex systems, in this work we discuss the Bozzolo-Ferrante-Smith (BFS) method for alloys and its application to a variety of multicomponent systems for a detailed analysis of their defect and phase structure and their properties. Examples include the study of the phase structure of new Ru-rich Ni-base superalloys, the role of multiple alloying additions in high temperature intermetallic alloys, and interfacial phenomena in nuclear materials, highlighting the benefits that can be obtained from introducing simple modeling techniques to the investigation of complex systems. This article was presented at the Multi-Component Alloy Thermodynamics Symposium sponsored by the Alloy Phase Committee of the joint EMPMD/SMD of The Minerals, Metals, and Materials Society (TMS), held in San Antonio, Texas, March 12-16, 2006, to honor the 2006 William Hume-Rothery Award recipient, Professor W. Alan Oates of the University of Salford, UK. The symposium was organized by Y. Austin Chang of the University of Wisconsin, Madison, WI, Patrice Turchi of the Lawrence Livermore National Laboratory, Livermore, CA, and Rainer Schmid-Fetzer of the Technische Universitat Clausthal, Clauthal-Zellerfeld, Germany.     

Solidification Simulation Using Scheil Model in Multicomponent Systems

Solidification paths simulated by Scheil model have been analyzed. Since the Scheil model assumes that the already solidified solid phases are “frozen,” the simulated solidification paths must follow the eutectic type paths and cross over the peritectic type boundaries to satisfy the mass balance requirement. Examples are given for comparison between the Scheil simulation results and the experimental observations for alloys in Mo-Ti-Si system.     

Calculation of Two-Dimensional Sections of Liquidus Projections in Multicomponent Systems

A two-dimensional (2D) section of liquidus projection consists of univariant lines of three-phase equilibria between liquid and other phases. It is useful not only for people’s easy visualization but also for retaining important information about the multicomponent liquidus surface, which is critical for understanding and controlling the solidification microstructure. An algorithm is presented for calculating two-dimensional sections of liquidus projections for multicomponent systems. It can project a liquidus surface onto any given two-dimensional section in the compositional space. A hypothetical quaternary system is designed to illustrate the features of the 2D sectional liquidus projections. Then the Nb-Ti-Si-Cr system is used as an example to demonstrate how to apply a 2D sectional liquidus projection to determine the amount of Cr addition into Nb-Ti-Si alloy to affect the liquidus surface of the quaternary system.     

钛硅共晶系合金结晶过程与微观形貌

观察和分析了钛硅共晶系合金的共晶结晶过程以及微观组织中Ti5Si3相的典型形貌.结果表明,亚共晶与共晶钛硅合金中Ti5Si3相在共晶析出时均生长成棒状甚至颗粒状.更细致的观察发现,Ti5Si3相的截面形貌为六角形,具有光滑的120°夹角界面,而且中心部位嵌有非共格的球形α-Ti颗粒.同时,Miedema理论计算结果表明,Ti5Si3相比β-Ti相具有更负的生成热,是钛硅共晶系合金中两相共晶时的领先相.     

Preparation and Performance of Ceria Doped Two-Component deNOx Monolithic Catalysts at Low Temperature

Abstract: The (V-W-Ti-O)x (Cu-Al-O)1-x (Ce-O)0.03, (x=0.9, 0.8) deNOx monolithic catalysts were prepared by low temperature co-sintering (LTC) within the sintering temperature range of 650-850 °C. The effects of sintering temperature on the mechanical strength, porosity and water absorption of the monolithic catalysts and the effect of reaction temperature on catalytic activity were investigated. The microscopic structure of the catalysts was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and fourier infrared spectroscopic analysis (FT-IR). Results show that catalysts are porous in micro-structure and grains are distributed uniformly. The crystallographic forms are mainly TiO2 (rutile), TiVO4, WV2O6, CeVO4, Cu2V2O7, AlV2O4, CuAlMnO4 and Cu1.9V12O29. For the reforming monolithic catalysts sintered at 700 °C for 2 h, the two-component catalysts exhibit high catalytic activities in the low temperature range of 100-200 °C and their bending strength reaches to 45.5 MPa. The NO conversion of (V-W-Ti-O)0.8 (Cu-Al-O)0.2 (Ce-O)0.03 monolithic catalyst corresponds to 75.9% at reactor temperature of 150 °C.     

模具温度对Al-Si合金中共晶Si形貌的影响

研究了模具温度对Al-Si合金中共晶Si形貌的影响。结果表明：模具温度改变了合金铸态组织中Si相形貌,影响了其在随后固溶处理中的变化进程。对于经过适当变质处理的Al-Si共晶合金,在模具温度较低时,铸态共晶Si相为细小的纤维状,经短时固溶处理就能得到良好的共晶Si相形貌;而在模具温度较高时,铸态Si相表现为较粗的纤维或蠕虫状,固溶处理后,Si相的尺寸较大,形态较差。     

Au-Ag-Si新型中温共晶钎料的研究

根据合金相图的基本原理,分析了Au-Ag-Si系相图,研制出熔化温度在400℃～500℃的共晶钎料合金.通过钎料合金与Ni板的润湿性测试和润湿后界面的微观组织分析.结果表明:Au-Ag-Si系钎料对于纯Ni具有良好的漫流性和润湿性,在450℃～500℃范围内,可以对Ni板进行钎焊.     

On Negative Diagonal Elements in the Diffusion Coefficient Matrix of Multicomponent Systems

In multicomponent systems the diffusion coefficient turns into a matrix. The diagonal elements represent diffusion of a species caused by its own concentration gradient. In a thermodynamically stable binary alloy it is easy to see that this diagonal element must be positive but in a multicomponent system it is less obvious. The sign of the diagonal elements in the general case is discussed in this report. It is shown that the sign of an individual diagonal element has no physical meaning but can be changed by changing the dependent concentration variable. Only the sum of all the diagonal elements need to be positive in a stable system.     

低温气体多元共渗与复合表面处理技术的对比研究

低温气体多元共渗技术在提高材料表面硬度、耐磨性和耐蚀性等性能的应用方面取得一定的进展,但单一的表面技术往往具有一定的局限性,因此采用低温气体多元共渗与高频感应淬火技术相结合的复合表面处理处理技术对20钢进行表面处理,并与低温气体多元共渗技术进行比较,分析复合表面处理技术对20钢显微硬度、耐磨性和耐蚀性的影响。结果表明,经过复合表面处理后,20钢表面硬度达到830HV,耐磨性有显著提高,但是耐蚀性有所下降。     

NiAl-28Cr-6Mo共晶合金的高温氧化行为

本文采用恒温氧化实验方法,在900～1150℃下测试了NiAl-28Cr-6Mo共晶合金的氧化性能,分析了合金的氧化动力学,SEM观测了合金表面以及横截面的形貌.研究表明,NiAl-28Cr-6Mo共晶合金在900～1100℃下合金表面生成了连续的Al2O3氧化膜,具有较好的抗氧化性能;900～1000℃氧化膜主要由θ...     

Al-Si合金中Si相的团粒化研究

利用金相和电子探针等手段研究了AC8C、ZL104和ZL109牌号的亚共晶及共晶Al-Si合金中共晶和初晶Si相的团粒化。结果表明，用Sr变质处理附以热处理手段相结合的方法可促使共晶Si相呈团粒状。并均匀分布干铝基体上，加入TiB2粒子可进一步促进共晶Si趋于圆滑，甚至使之团球化。使用杆状Al-P中间合金后，可使共晶Al-Si合金析出大量初晶Si，获得过共晶组织，并在一定条件下可促使初晶Si晶粒团粒化，提高活塞的金相等级。另外，在Si、Cu和Mg成分一定的条件下可形成一个球状复合相，为获得真正的球状Si相开辟了新思路。     

NiAl-31Cr-3Mo共晶合金的高温氧化行为

本文采用恒温氧化实验方法,在900 ～ 1150℃下测试了NiAl-31Cr-3Mo共晶合金的氧化性能,分析了合金的氧化动力学;SEM观测了合金表面以及横截面的形貌.研究表明,共晶合金表面在900～ 1100℃下形成了连续的Al2O3氧化膜,表现出一定的抗氧化性能;共晶合金表面在1150℃下未形成完整的Al2O3氧化膜...     

电迁移引发Cu/SnBi/Cu焊点组织形貌的演变

主要研究电流密度为5×103 A/cm2,室温和高温(100 ℃)条件下共晶SnBi焊点的电迁移特性.结果表明:室温条件通电465 h后,阳极界面处出现Bi的挤出,阴极界面处出现裂纹;而在高温条件下通电115 h后,组织形貌发生了很大的变化.高温加速了阴极钎料的损耗,导致电流密度在局部区域高度集中,从而产生更多的焦耳热,最终引发焊点的熔化.熔融状态下Sn原子与Cu反应,在基体形成大量块状的Cu6Sn5金属间化合物,严重降低焊点的可靠性.     

单向凝固中温度梯度和凝固速度对铝硅合金共晶生长形态的影响

对Al-13Si-0.2Sr-0.35Mg合金进行了一系列单向凝固实验,研究了界面前沿温度梯度GL和生长速度R对铝硅合金共晶生长形态的影响。结果表明,GL和R对Sr变质的非小面-小面Al-Si体系共晶生长的影响符合界面稳定性理论,随着R的增大和GL的减小,界面前沿成分过冷变大,共晶体生长形貌经历了从平界面失稳到胞状晶、胞状树枝晶、柱状树枝晶和等轴晶转变。     

合金成分及熔炼工艺对多元铌基超高温合金组织的影响

采用真空自耗电弧熔炼和真空非自耗电弧熔炼2种工艺熔炼了4种不同成分的多元铌基超高温合金,研究了合金成分及熔炼工艺对组织的影响规律.结果表明:4种成分的合金均由初生Nb基固溶体枝晶或初生(Nb,x)5Si3(x代表Ti, Cr和Hf元素)块和板条以及Nbss/(Nb,x)5Si3共晶团组成.随着Si含量的升高,组织中Nbss/(Nb,x)5Si3共晶团的含量增多且初生相也从Nbss枝晶转变为(Nb,x)5Si3块或板条.铌硅化物相为γ-(Nb,x)5Si3及β-(Nb,x)5Si3.大块或板条状硅化物内有微裂纹出现.     

回火温度对铁基多元合金堆焊层组织及耐磨性的影响

研究了回火温度对铁基多元合金堆焊层组织及耐磨性的影响。结果表明,该堆焊层回火后具有二次硬化现象;当回火温度低于400℃时,其硬度变化不明显,超过600℃后,硬度明显下降;在560℃回火2h,具有优异的耐磨性,且组织与耐磨性有良好的对应。     

回火温度对铁基多元合金堆焊层组织及耐磨性的影响

研究了回火温度对铁基多元合金堆焊层组织及耐磨性的影响。结果表明，该堆焊层回火后具有二次硬化现象；当回火温度低于400℃，其硬度变化不明显，超过600℃，硬度明显下降；在560℃回火2h．具有优异的耐磨性，且组织与耐磨性有良好的对应。     

共晶和过共晶铝硅合金活塞尺寸稳定性的研究

研究了两种共晶和两种过共晶铝硅合金活塞在２００℃的长期时效过程中的尺寸稳定性。研究发现，随着时效时间的延长，所有活塞顶部的直径都呈现不可逆长大；过共晶铝合金活塞的尺寸稳定性高于共晶铝合金活塞；共晶铝硅合金活塞顶部直径有两次突然长大，而过共晶铝合金活塞仅有一次。作者初步解释了上述现象     

自蔓延高温渗硼共晶化机理及渗层组织研究

分析了45钢自蔓延高温渗硼共晶化渗层的组织形态及相结构,探讨了渗层形成机理。结果表明,自蔓延高温渗硼共晶化可在工件表面形成共晶组织渗层,渗层与基体呈冶金结合;渗层与基体结合处,硼含量陡降,不存在明显的过渡区。     

加热温度对低合金耐磨铸铁中碳化物形态和冲击疲劳性能的影响

采用金相显微镜、扫描电镜及冲击疲劳试验机研究了加热温度对低合金耐磨铸铁中共晶碳化物形态和冲击疲劳性能的影响.结果表明:随加热温度的升高,耐磨铸铁中的网状共晶碳化物陆续溶断,块状碳化物逐渐增多,当铸铁在960℃保温3 h空冷后,可获得较高的冲击疲劳性能.