The role of surface texture on friction and transfer layer formation during repeated sliding of Al-4Mg against steel

In the present investigation, various kinds of textures, namely, unidirectional, 8-ground, and random were attained on the die surfaces. Roughness of the textures was varied using different grits of emery papers or polishing powders. Then pins made of Al-4Mg alloys were slid against steel plates at various numbers of cycles, namely, 1, 3, 5, 10 and 20 using pin-on-plate reciprocating sliding tester. Tests were conducted at a sliding velocity of 2 minis in ambient conditions under both dry and lubricated conditions. A constant normal load of 35 N was applied in the tests. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. Surface roughness parameters of the plates were measured using an optical profilometer. In the experiments, it was observed that the coefficient of friction and formation of the transfer layer depend on the die surface textures under both dry and lubricated conditions. More specifically, the coefficient of friction decreases for unidirectional and 8-ground surfaces while for random surfaces it increases with number of cycles. However, the coefficient of friction is highest for the sliding perpendicular to the unidirectional textures and least for the random textures under both dry and lubricated conditions. The difference in friction values between these two surfaces decreases with increasing number of cycles. The variation in the coefficient of friction under both dry and lubrication conditions is attributed to the change in texture of the surfaces during sliding. (C) 2011 Elsevier B.V. All rights reserved.

Thermodynamics of Aluminium -Strontium Alloys

The activity of strontium in liquid Al-Sr alloys (X(Sr) less-than-or-equal-to 0.17) at 1323 K has been determined using the Knudsen effusion-mass loss technique. At higher concentrations (X(Sr) greater-than-or-equal-to 0.28), the activity of strontium has been determined by the pseudoisopiestic technique. Activity of aluminium has been derived by Gibbs-Duhem integration. The concentration - concentration structure factor of Bhatia and Thornton at zero wave vector has been computed from the thermodynamic data. The behaviour of the mean square thermal fluctuation in composition and the thermodynamic mixing functions suggest association tendencies in the liquid state. The associated solution model with Al2Sr as the predominant complex can account for the properties of the liquid alloy. Thermodynamic data for the intermetallic compunds in the Al-Sr system have been derived using the phase diagram and the Gibbs' energy and enthalpy of mixing of liquid alloys. The data indicate the need for redetermination of the phase diagram near the strontium-rich corner.

Pattern-Formation During Self-organization in Three-Phase Eutectic Solidification

Eutectic growth is an interesting example for exploring the topic of pattern-formation in multi-phase systems, where the growth of the phases is coupled with the diffusive transport of one or more components in the melt. While in the case of binary alloys, the number of possibilities are limited (lamellae, rods, labyrinth etc.), their number rapidly increases with the number of components and phases. In this paper, we will investigate pattern formation during three-phase eutectic solidification using a state-of-the art phase-field method based on the grand-canonical density formulation. The major aim of the study is to highlight the role of two properties, which are the volume fraction of the solid phases and the solid-liquid interfacial energies, in the self-organization of the solid phases during directional growth. Thereafter, we will show representative phase-field simulations of a micro-structure in a real alloy (Ag-Al-Cu) using an asymmetric phase diagram as well as interfacial properties.

原位生成TiC粒子在快凝Al-Fe合金中变化行为的研究

通过OM，SEM和TEM等手段研究了在常规铸态以及快速凝固Al-8wt％Fe合金中原位生成TiC粒子时棚组成及显微组织的变化。同时结合利用外加法制备的常规铸态和快速凝固Al-8wt％Fe/TiC合金的娃微组织变化，得出了原位反应TiC粒子的生成及其在合金熔体中的变化行为：在常舰铸态下生成的尺寸较大的八岍体TiC颗粒在快速凝固过程中将发生全部或部分溶解，在随后的怏冷过程中将重新析出并形成细小的TiC粒子，原位生成的TiC粒子在快速凝固过程中同样可以对α-Al起到促发形核的作用。

The mechanism of PEO process on Al–Si alloys with the bulk primary silicon

This study focuses on mechanism of ceramic coating on Al-Si alloys with bulk primary Si using plasma electrolytic oxidation (PEO) technology. Al-Si alloys with 27-32% Si in weight were used as substrates. The morphologies, composition and microstructure of PEO coatings were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray system (EDX). Results showed that the PEO process had four different stages. The effect of bulk Si is greatly on the morphology and composition of coatings at first three stages. Anodic oxide films formed on Al and Si phases, respectively. When the voltage exceeded 40 V, glow appeared and concentrated on the localized zone of interface of Al and Si phase. Al-Si-O compounds formed and covered on the dendrite Si phase surface, and the coating on bulk Si, which was silicon oxide, was rougher than that on other phase. If the treatment time was long enough, the coatings with uniform surface morphologies and elements distribution will be obtained but the microstructure of inner layer is looser due to the bulk Si.

Materiais tipo hidrotalcita contendo Cu como precursores de catalisadores para a redução de NO pelo CO

Dentre os óxidos de nitrogênio, o N2O é um gás do efeito estufa altamente nocivo. Devido ao potencial contaminante que este possui, torna-se importante a implementação de processos capazes de reduzir a sua emissão, bem como a dos NOx. Tradicionalmente, têm-se empregado catalisadores baseados em metais nobres, porém estes apresentam como principal desvantagem o elevado custo. Desse modo, sempre houve o interesse pelo uso de outros tipos de catalisadores e metais neste sistema de reação. Nesse contexto, na presente dissertação procurou-se sintetizar precursores de catalisadores tipo hidrotalcita Cu-AlCO3 e avaliar o seu desempenho na reação de redução do NO pelo CO, visando melhorar a atividade e a seletividade a N2. Foram estudados diversos parâmetros de síntese e diferentes composições. Os parâmetros mais influentes na síntese foram a relação molar H2O/(Al+Cu) e a temperatura de secagem do sólido, cujos melhores valores foram 434 e 25C, respectivamente. Testaram-se dois sólidos, o primeiro composto pela fase hidrotalcita quase pura e o segundo com uma clara mistura entre fases hidrotalcita e malaquita. As análises térmica e química revelaram presença da fase malaquita em ambos os materiais com porcentagens de 14 e 40%, respectivamente. Os resultados de difração de raios X indicaram a presença da fase CuO para os catalisadores provenientes da calcinação dos materiais tipo hidrotalcita, porém a espectroscopia Raman evidenciou a presença de Cu2O no catalisador proveniente do material com maior mistura de fases. Os ciclos redox mostraram uma melhora na redutibilidade dos catalisadores após um ciclo de oxidação-redução. Além disso, foi estudado o impacto do envelhecimento térmico a 900C por 12 h no desempenho dos catalisadores. Pelos resultados de teste catalítico os melhores desempenhos foram alcançados pelos catalisadores envelhecidos, contudo o catalisador proveniente do precursor mais puro apresentou-se melhor tanto novo como envelhecido em termos de menor rendimento de N2O. Uma comparação com catalisadores à base de metal nobre mostrou um bom desempenho dos catalisadores à base de cobre, com a vantagem destes apresentarem menor emissão de N2O em temperaturas menores

熔盐电解法制备Al-Li合金的应用基础研究及其工艺初探

Al-Li合金是近年发展起来的新型航空材料，具有低密度、高强度的特点。目前世界上正就如何进一步提高其断裂韧性，使Al-Li合金尽早走出实验室，获得实际用进行广泛，深入的研究。熔盐电解法就是在这种情况下发展起来的。虽然此法现在尚处于研究的初级阶段，但已以其能够在较简单的设备上制备低Na高纯Al-Li合金的特点受到广泛的重视，是一种很有前途的发展方向。针对我国的技术、设备现状，在参考国外研究结果的基础上，采用熔盐电解法制备Al-Li母合金 → 应用合金的制备方法是可以尽快赶上世界发步伐的有效途径。因此，本文作为整个熔盐电解制备Al-Li合金系统研究的一部分，针对目前在此领域中很多应用基础问题，诸如：作为新的电解体系正在探索中的LiCl-KCl-LiF三元相图，Li在液体Al阴极中的扩散系数，利用熔盐电解法制备低Na高纯Al-Li合金的热力学基础及如何克服LiCl强烈的吸水性给电解工艺带来的种种不便等均未得到系统研究的现状，设计完成了一系列有关熔盐电化学和热力学实验，填补了本领域的一些研究空白，并为进一步系统研究工艺条件提供了重要的参考数据。1．用NH_4Cl氯化Li_2CO_3的研究 根据热力学从理论上论证了在200 ℃左右下述氯化反应：Li_2CO_3 + 2NN_4Cl = 2LiCl + 2NH_3~↑ + H_2O~↑ + CO_2~↑可以进行完全。利用DSC方法测定反应产物LiCl的纯度，并用离子色谱法分析反应产物中CO_3~=的含量，结果均证明：在Li_2CO_3:NH_4Cl = 1:4 (mol)时，Li_2CO_3可以定量转化为LiCl，剩余的NH_4Cl完全分解。根据热重分析结果推测NH_4Cl氯化Li_2CO_3的反应历程为：Li_2CO_3 + 2NH_4Cl = 2LiCl + 2NH_3~↑ + H_2O~↑ + CO_2~↑ NH_4Cl = HCl~↑ + NH_3~↑ 而并非是想像中的：NH_4Cl = HCl~↑ + NH_3~↑ 2HCl + Li_2CO_3 = 2LiCl + H2O~↑ + CO_2~↑利用X－射线衍射方法分析产物，结果亦说明氯化可以成功。将1:4（mol）= Li_2CO_3:NH_4Cl混合样品在差热分析反应炉中直接加热测定反应产物LiCl的溶点，并与纯LiCl样品熔点的测定结果相比较，二者完全一致。以上结果说明：不仅可用此氯化反应产物代替LiCl应用于熔盐电解制备Al-Li合金中，而且还可将其应用于LiCl体系的相图测定中。因Li_2CO_3，NH_4Cl均不吸水，极易处理，因此以上研究结果无论对于Al-Li合金的工艺研究还是其他有关LiCl体系的基础研究都是很有价值的。2．直接氯化法制备LiCl-KCl-LiF三元相图的研究。LiCl-KCl-LiF三元相图是研究此体系电解机制的重要基础。为将以上直接氯化法应用于差热分析中制作此三元体系相图，首先用直接氯化法测定了三个已知二元LiCl体系相图：LiCl－KCl；LiCl-LiF；LiCl-NaCl。与文献结果吻合很好。说明将此法应用于差热分析中制作LiCl体系相图结果是可靠的。在LiCl-KCl-LiF三元相图的测定中共做出七个垂直截面，在各截面上读出等温条件下的相界点投影到浓度三角形中，得到等温投影图。结果说明LiCl-KCl-LiF是固态完全不互溶的三元共晶体系，共有三个液－固两相区；三个液－固－固三相区；一个液－固－固－固四相区，（三元共晶平面）和一个固－固－固三相区。四相点温度为348 ℃，其组成在三相平衡线的交点处，在实验上测出近似等于：41.4KCl + 57.3LiCl + 1.3LiF (mol)。3．氯化物体系中Li~+, Na~+析出电位的比较及其去极化作用的研究。在正常电化序中，Li~+应先于Na~+析出。但在以Al作阴极电解LiCl体系时，则由于Li~+在Al上有较强的去极化作用而提前析出。这是熔盐电解法可以制备低Na，高纯Al-Li合金的基础。本文在理论上对此问题进行了较深入的研究。具体内容包括（1）。测定Li~+, Na~+在二元氯化物体系中的析出电位。通过在Al阴极，Mo阴极上二离子析出电位的比较，确认了Li~+在Al阴极上产生很强的去极化作用是能利用电解法制备低Na高纯Al-Li合金的根本原因。并为在工艺研究中选择合适的电流密度提供了参考依据。（2）．根据热力学理论推导出合金化反应产生的自由焓变化与去极化作用的关系：ΔG_x + ΔG_m = -nFΔE。揭示了产生去极化作用的原因。并根据ΔG_x（偏摩尔过剩自由焓）与合金结构的关系提出可以利用二元合金相图推测极化类型及极化大小。并根据动力学原理对温度对极化的影响提出了自己的看法。（3）．求出合金化反应的热效应，认为在一定条件下亦可利用此值作为判断去极化作用大小的标准。（4）．测定Li~+, Na~+在Al-Cu, -Al-RE合金上的析出电位。结果表明Cu，RE的存在均可加强Li~+在阴极上的去极化作用，进一步加大了Li~+, Na~+析出电位之间的差别，有利于制备更纯的Al-Li 使金。为直接生产Al-Cu-Li, Al-RE-Li三元母合金奠定了基础。（5）测定Li~+在不同组成配比的LiCl-KCl熔体中，在Al阴极上的析出电位，并求出LiCl的离子平均活度系数γ＝0.71 (T = 740 ℃), 熔体对理想状态产生负偏离。4．利用阳极计时电位法测定T＝720 ℃时Li在液体Al中的扩散系数D_(Li/Al) = 4.94 * 10~(-5)cm~2·s~(-1)，与利用Stocks-Einstan公式计算出的理论值D_(Li/Al) = 4.85 * 10~(-5)cm~2·s~(-1)吻合较好。5．在上述理论研究的基础之上进行了工艺初探，所得初步结论有：（1）．加入LiF可以提高电效。（2）．采用电流密度为1 A/cm~2时，不加搅拌亦可制备出成份均匀，含Li量为10％（w.f）的Al-Li合金。（3）．根据实验结果提出 Li在熔体中的熔解可能是影响电流效果的主要原因。

Crystallographic and electrochemical characteristics of Ti45Zr35Ni13Pd7 melt-spun alloys

Ti45Zr35Ni13Pd7 alloys are prepared by melt spinning at different cooling rates (v). The phase structure and electrochemical hydrogen storage performance are investigated. When U is 10 m/s, the alloy consists of icosahedral quasicrystalline phase (I-phase), C14 Laves phase and a little amorphous phase. When v increases to 20 or 30 m/s, a mixed structure of I-phase and amorphous phase is formed. Maximum discharge capacity of alloy electrode decreases from 156 mAh/g (v = 10 m/s) to 139 mAh/g (v = 30 m/s) with increasing v. High-rate discharge ability at the discharge current density of 240 mA/g decreases monotonically from 61.2% (v = 10 m/s) to 56.8% (v = 30 m/s).

Microstructures, mechanical properties and corrosion behavior of high-pressure die-cast Mg-4Al-0.4Mn-xPr (x=1, 2, 4, 6) alloys

Mg-4Al-0.4Mn-xPr (x = 1, 2, 4 and 6 wt.%) magnesium alloys were prepared successfully by the high-pressure die-casting technique. The microstructures, mechanical properties, corrosion behavior as well as strengthening mechanism were investigated. The die-cast alloys were mainly composed of small equiaxed dendrites and the matrix. The fine rigid skin region was related to the high cooling rate and the aggregation of alloying elements, such as Pr. With the Pr content increasing, the alpha-Mg grain sizes were reduced gradually and the amounts of the Al2Pr phase and All, Pr-3 phase which mainly concentrated along the grain boundaries were increased and the relative volume ratio of above two phases was changed. Considering the performance-price ratio, the Pr content added around 4 wt.% was suitable to obtain the optimal mechanical properties which can keep well until 200 degrees C as well as good corrosion resistance. The outstanding mechanical properties were mainly attributed to the rigid casting surface layer, grain refinement, grain boundary strengthening obtained by an amount of precipitates as well as solid solution strengthening.

Catalytic reduction of NO, by CO on hydrotalcites-derived mixed oxides CoAlM and MgAlM (M = Cr, Mn, Fe, Co, Ni, Cu)

Two series of mixed oxides, CoAlM and MgAlM (M = Cr, Mn, Fe, Co, Ni, Cu), were prepared by calcining their corresponding hydrotalcite-like compounds (HTLc). The ratio of Mg: Al: M (or Co: Al: hi) was 3:1:1. The catalytic activity of all samples for the reaction of NO + CO was investigated. The results showed that the activity of CoAlM was much higher than that of MgAlM. The structure and the property of redox were characterized by XRD and H-2-TPR. The results indicated that only MgO phase was observed after calcining MgAlM hydrotalcites, and the transition metals became more stable. The spinel-like phase appeared in all of CoAlM samples after the calcination, and the transition metals were changed to be more active, and easily reduced. The activities of three series of mixed oxides CoAlCu obtained from different preparation methods, different ratio of Co:Al: Cu and at different calcination temperatures, were studied in detail for proposing the mechanism of reaction. The ability of adsorption of NO and CO were investigated respectively for supporting the mechanism.

The effect of chromate on the pitting susceptibility of AA7075-T651 studied using potentiostatic transients

The effect of chromate on metastable pitting of AA7075-T651 as determined via potentiostatic polarisation is reported. A systematic study of metastable pitting and its correlation with stable pits was conducted in various concentrations of sodium chromate (Na2CrO4), revealing the metastable pitting rate was able to provide a quantitative metric for pitting corrosion. The size and number of metastable pits decreased significantly in the presence of chromate. The present study is intended as a general baseline for the assessment of future chromate replacement technologies, as elaborated herein. © 2014 Elsevier Ltd.

A metallurgical study of aluminium alloys used as aircraft components

The effects of heat treatment on morphologies and microstructures of Al 2024 and Al 7050 alloys, used as aircraft components, were studied by metallographic techniques. Light microscopy (LM) and quantitative image analysis were used to characterize the precipitate dispersion and morphology for these alloys. The application of the scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) combined techniques for studying these multiphase systems makes it possible to distinguish and quantify the different phases in the surface structure. Xray diffraction also permitted a qualitative comparison of the structures before and after heat treatments.

Order-disorder transformation in the Ni-4.49 at.% Al alloy

The order-disorder transformation in the Ni-4.49 at.% Al alloy was studied using electrical resistivity measurements, microhardness measurements, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results confirmed the ordering behavior expected for Ni-Al dilute alloys and the suggested relation between resistivity changes and microhardness changes with antiferromagnetic spin ordering. The higher value obtained for the activation energy of vacancy migration was associated with a decrease in the Al concentration gradient near solute-depleted regions. (c) 2005 Elsevier B.V. All rights reserved.

Ligas Ti-Al-Nb como biomaterial: elaboração, caracterização, modificação eletroquímica da superfície e estudo da biocompatibilidade utilizando culturas primárias de células da medula óssea humana

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Determinação do calor específico de ligas AlCu produzidas em um dispositivo de solidificação unidirecional vertical ascendente

O conhecimento das propriedades termofísicas é de fundamental importância para o estudo de ligas metálicas obtidas por solidificação,uma vez que esta se relaciona de forma direta com o coeficiente de transferência de calor na interface metal/molde. Assim, o Grupo de Pesquisa em Metalurgia e de Meio Ambiente – GAPEMM da Universidade Federal do Pará desenvolve uma linha de pesquisa que propõe um conjunto de técnicas e procedimentos que visa determiná-las. Por outro lado, sabe-se que existe uma correlação significativa entre processo, estrutura e propriedades de um material obtido por solidificação, visto que a distribuição de soluto em uma liga metálica ocorre de maneira não uniforme. A maneira como ocorre solidificação e a quantificação das variáveis envolvidas no processo têm influência fundamental nas propriedades do material. O presente trabalho utilizou ligas Al-Cu (Al-2%Cu, Al-5%Cu e Al-8%Cu) obtidas por solidificação unidirecional vertical ascendente, realizado através de um dispositivo projetado, construído e aferido pelo GAPEMM. Através destas, pretende-se fazer um estudo do calor específico à medida que a frente de solidificação se afasta da chapa molde bem como com o aumento do teor de soluto. Para isso, foi utilizada uma técnica conhecida na literatura como Lei de Resfriamento de Newton, a qual possibilita através das curvas de temperatura x tempo determinar as temperaturas necessárias para o cálculo do calor específico.