Electron microscopy characterization of an as-fabricated research reactor fuel plate comprised of U–7Mo particles dispersed in an Al–2Si alloy matrix

To understand the microstructural development of nuclear fuel plates during irradiation, it is imperative to know the microstructure of a fuel plate after all the fabrication steps have been completed and before it is inserted into the reactor. To this end, a U–7 wt.% Mo alloy research reactor dispersion fuel plate with Al–2 wt.% Si matrix was destructively examined using scanning and transmission electron microscopy to characterize the developed microstructure after fabrication. Of particular interest for this study was how the Si that was added to the fuel matrix partitioned between the various fuel plate phases during fabrication. Si was added to the matrix so that the microstructure that developed during fuel fabrication would exhibit good irradiation behavior. SEM analysis was used to identify the representative microstructure, the compositions of the various phases, and the partitioning behavior of the fuel and matrix constituents. TEM analysis was employed to definitively identify the phases in the U–7Mo alloy and the phases that formed due to diffusional interactions between the fuel particles and matrix during fuel plate fabrication. The TEM results are the first reported for an as-fabricated U–7 wt.% Mo dispersion fuel plate with an Al alloy matrix. SEM results showed that a significant portion of the original γ-(U–Mo) fuel particles had transformed to a lamellar microstructure, comprised of α-U and either γ or γ' phases, and the fuel/matrix interaction layers were enriched in Si. TEM analysis identified an ordered fcc (U–Mo)(Al–Si)₃ type of phase, which formed at the decomposed U–7Mo/matrix interface and extended into the lamellar microstructure. Some regions of the U–7Mo particles retained the single-phase γ-(U–Mo). Small precipitate phases were observed in the fuel meat matrix that contained Fe, Al, and Si. The Si that is added to the matrix of a U–Mo dispersion fuel plate to improve irradiation performance appears to result in the creation of a Si-rich (U–Mo)(Al–Si)₃ type of fuel/matrix interaction layer during fabrication that appears to exhibit favorable behavior during irradiation compared to the behavior of the layers that form in U–Mo dispersion fuel plates without Si in the matrix.     

Flux-assisted infiltration of liquid Al-6063 into TiC beds in air

This study details trials to produce aluminium metal matrix composites reinforced with TiC particles by means of a flux-assisted infiltration technique. Whilst no infiltration of TiC beds occurred, by using a K-Al-F flux infiltration was successful at temperatures as low as 680°C. Some reaction of TiC with the Al matrix, forming TiAl_2.3Si_0.1 and Al₄C₃, was observed in the microstructure along with flux trapped within the Al-6063 matrix. DSC showed exothermic oxidation of TiC to occur, until the flux melts at 545°C arresting and preventing further oxidation by spreading over, coating and cleaning the particle surfaces. As soon as the flux melts, it also starts dissolving the oxide layer on the Al alloy and prevents any re-oxidation by isolating the surface from the surrounding atmosphere. Sessile drop experiments suggest that when the alloy melts and the oxide layer has been dissolved by the flux, intimate contact occurs between the liquid and the particles. The low tensions for the solid/flux and liquid metal/flux interfaces facilitates spreading and wetting of liquid Al on the TiC particles, followed by infiltration of the bed and the displacement of the flux to the outer surfaces of the sample.     

Corrosion behavior of Al–60 vol.% SiCp composites in NaCl solution

In this study, corrosion behavior of pure Al and Al–4 wt.% Mg alloy matrix composites, comprising 60 vol.% SiC particles, has been investigated. Composites were produced by pressure infiltration technique at 750 °C. The corrosion tests were carried out in 3.5 wt.% NaCl environment up to 28 days. The weight loss of the composites increased with increasing duration time up to 3–5 days then remained constant. Scanning electron microscopy (SEM) analysis showed that Al–4 wt.% Mg alloyed matrix composite exhibited higher corrosion resistance than pure Al matrix composite although potentiodynamic polarisation measurements showed higher i_corr values of Al–4 wt.% Mg alloyed matrix composites than pure Al matrix composites. Experimental results revealed that precipitation of Mg₂Si as a result of reaction between Al–Mg alloy and SiC particle has a beneficial effect on corrosion resistance of Al–4Mg alloy matrix composites due to interruption of the continuity of the matrix channels within the pressure infiltrated composites.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

Products of interaction in the Al-Si alloy-carbon fibre system

Samples of a composite material were obtained by pressure infiltration with AL2 melt of a carbon tape. Products of interaction forming on the fibre-matrix interfacial boundary were investigated. It has been found that Al₄C₃ and SiC phases and silicon crystals are formed. An increase of the contact time between a fibre and the melt, and melt temperature and the infiltration pressure leads an increase in the Al₄C₃ carbide phase quantity and to the bond strength on the fibre-matrix interfacial boundary. Here with tensile strength tests, the character of fractures changes from tensile (with noticeable fibre pull-out from the matrix) — to ductile; shear strength is increased.     

In situ production of Al–TiB<Subscript>2</Subscript> nanocomposite by double-step mechanical alloying

An in situ Al–TiB₂ nanocomposite was synthesized by mechanical alloying (MA) of pure Ti, B and Al powder mixture in a planetary ball mill. A double-step process was used to prevent the formation of undesirable phases like Al₃Ti intermetallic compound. In the first step, a powder mixture was tailored to obtain nominal Al–90 wt% TiB₂ composition and the second step involved the addition of Al to the mixture in order to achieve Al–20 wt% TiB₂. The structural and thermal characteristics of powder particles were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), and transmission electron microscopy (TEM). The results showed that the MA process leads to the in situ formation of nanosized TiB₂ particles in an Al matrix with a uniform distribution. It was also found that the double stage addition of aluminum can prevent the formation of undesirable compounds even after annealing at high temperatures_.     

Impurity effects on the nucleation and growth of primary Al3(Sc,Zr) phase in Al alloys

Impurity effects on the nucleation and growth of primary Al₃(Sc,Zr) phase have been investigated in high purity Al alloys and commercial purity Al alloys, respectively. In the case of high purity Al alloys, primary Al₃(Sc,Zr) phases were found to be pushed to grain boundaries ahead of the solidification front. Such type of primary Al₃(Sc,Zr) phase did not contribute to the heterogeneous nucleation, and thereby the grain refinement of Al alloys. In the case of commercial purity Al alloys, the presence of Fe, Si, Cu, Mg, Ti, and other impurities significantly enhanced the heterogeneous nucleation of primary Al₃(Sc,Zr) phase. Most primary Al₃(Sc,Zr) phases were found to be located within the α-Al matrix, and kept an identical orientation relationship with the α-Al matrix. Furthermore, the presence of the impurities also changed the growth mode on the primary Al₃(Sc,Zr) phase. In the case of commercial purity Al alloys, a peritectic to eutectic reaction was induced due to the presence of the impurities. A layered growth was observed leading to a narrow particle size distribution. In contrast, in the case of high purity Al alloys, a featureless structure was observed. This investigation demonstrates that impurities and their concentrations are important factors affecting the nucleation and growth of primary Al₃(Sc,Zr) phases, and thereby for the successful grain refinement in Al-based alloys.     

Solid-state reactions underlying mechanochemical synthesis in the Fe-Al system

The sequence of solid-state transformations in the mechanochemical synthesis of Fe-Al intermetallics according to the reactions Fe + Al = FeAl, FeAl + 2Fe = Fe₃Al, and 2FeAl + 3Al = Fe₂Al₅ in powder mixtures has been studied by X-ray diffraction, differential scanning calorimetry, and Mössbauer spectroscopy. The results indicate that the process involves the formation of atomic configurations that may become nuclei of stable or metastable intermediate phases. Prolonged milling leads to homogenization of the synthesis product and the formation of a solid solution or an intermetallic phase with a low degree of long-range order. Complete ordering of the intermetallic phase can as a rule be achieved by heating.     

New Al–AlN composites fabricated by squeeze casting: interfacial phenomena

Three Al–AlN composites (2024, 6060, 5754 with ∼45 vol% AlN) fabricated by squeeze casting were studied by TEM. Chemical reactions occurring at the matrix–AlN interfaces have been investigated. MgAl₂O₄ spinel crystals were found in 6060 and 5754 composites. The magnesium element of the matrix reacts with a very thin alumina layer which is deposited on the AlN surfaces during the liquid infiltration step. The 5754 composites exhibited a stronger reaction leading to the formation of MgO phases with the spinel. Degradation of the mechanical properties was clearly shown in that case. A faceting of the AlN surfaces was observed in all composites.     

SiCp颗粒增强铝基复合材料工艺与组织研究

对无压渗透制备碳化硅颗粒增强铝基复合材料工艺进行了探索，并利用光学显微镜、扫描电镜对其组织进行了观察，用ｘ射线衍射仪对复合材料组成相进行了分析。结果表明：采用无压渗透技术，可以制备碳化硅颗粒增强铝基复合材料；熔融的基体合金对碳化硅颗粒预制体渗透完全；其过程存在Ａｌ与ＳｉＣ的化学反应，产物为Ｓｉ和碳化铝（Ａ１４Ｃ３），其中Ｓｉ进入基体中，Ａｌ４Ｃ３能与大气中水汽发生化学反应，结果使碳化硅铝基复合材料存放一定时间后发生龟裂和粉化。为限制Ａｌ与Ｓｉ反应，可向基体中加入适量的Ｓｉ元素，可使电裂与粉化问题得到解决。     

Effects of Sip size and volume fraction on properties of Al/Sip composites

Al–12 wt.% Si alloy matrix composites reinforced with high volume fraction of Si_p were fabricated by squeeze infiltration. The effects of the compacting pressure on the volume fraction of Si_p in preforms, and the influences of Si_p size and volume fraction on the properties of Al/Si_p composites were examined through this study. Si particles were compacted at different pressure of 40–130 MPa followed by sintered at 1000 °C for 7 h to obtain preforms containing 60–70 volume fraction (vol.%) of Si_p. The sintered preforms were then infiltrated with Al–12 wt.% Si alloy at 750 °C under a 75 MPa squeeze infiltration pressure. It was found that lower coefficient of thermal expansion (CTE) and smaller density may be obtained with higher Si_p volume fraction, yet increasing Si_p volume fraction leads to higher amount of porosities in the composites and thus lowers the thermal conductivity (TC) and flexural strength. Besides, with the same Si_p volume fraction, coarse Si particles result in higher CTE and TC, while finer Si particles may lower CTE and enhance the flexural strength of the composites effectively. From the results obtained in this study, it is expected that the high volume fraction Si_p reinforced Al/Si_p composites posses good potential in electronic packaging applications.     

高体积分数SiC_p/Al复合材料中界面现象研究

利用 XRD及 TEM方法测定了真空气压渗流法制备的四种基体复合材料及其界面区域的相组成。结果表明 ,复合材料中界面处出现了新相 ,认为新相的出现是界面反应的结果。应用热力学及动力学方法分析讨论了界面反应及其产物的形成过程。指出 ,通过控制基体中的添加元素含量便可控制复合材料中界面反应及出现的相 ,界面反应对材料的制备过程及其性能有重要的影响。     

In‐situ composite coating on powder metallurgy master alloy fabricated by vacuum hot‐pressing sintering technology

A material consisting of an in‐situ titanium carbide reinforced nickel‐aluminide (Ni₃Al) coating and a powder metallurgy master alloy was fabricated by vacuum hot‐pressing sintering technology. A metallurgical bonded, pores‐free interface between composite coating and powder metallurgy master alloy was formed at the sintering temperature of 1050 °C, pressure of 10^‐4 Pa and pressing pressure of 40 MPa. The phase, microstructure and wear behavior of composite coating were investigated. The results showed that polygonal titanium carbide particulates with various sizes were homogeneously distributed in nickel‐aluminide matrix. The sintering temperature, pressing pressure and heat from as‐reactions‐formed coating green compact facilitated the pore infiltration with transiently generated liquid phases and ensured the high‐intensity metallurgical bonding between composite coating and powder metallurgy master alloy. Due to the abnormal elevated‐temperature properties of nickel‐aluminide matrix, titanium carbide particulates reinforcement and the mechanically mixed layer protection, TiC/Ni₃Al‐coated parts demonstrated superior wear resistance and lower friction coefficient while compared with Ni₃Al‐coated parts and H13 steel.     

Activated pressureless infiltration of metal-matrix composites with graded activator content

Metal-matrix composites (MMCs) were produced by activated pressureless infiltration of porous Al₂O₃ compacts with the presence of elemental titanium as an activator and steel (X38CrMoV5-1) as the metal matrix. The quality of infiltration was subsequently investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results show that poor quality of infiltration is associated with blocking of infiltration channels due to the formation of Ti-rich phases which are accumulated over infiltration depth. To prevent the pores blocking, a layer-graded activator green body in which the activator quantity is decreased with infiltration depth was used. Furthermore, it could be shown that the mechanism based on evaporation/condensation of metal onto activator particles is in agreement with the results of this study.     

Fracture chemistry of δ-Al2O3 (Saffil) fibres in an MgLi matrix environment

Fracture surfaces of δ-Al₂O₃ (Saffil) fibres embedded in an Mg-8 wt% Li matrix by the pressure infiltration process were investigated by in situ Auger electron spectroscopy to study the chemistry of embrittlement of the former, resulting from a cross-section attack by the molten Mg-8 wt% Li matrix. The unaffected fibres failed transgranularly without any indications of silica being the crack-controlling flaws. Displacement reductive reactions between fibre constituents (δ-Al₂O₃, silica) and penetrating lithium and magnesium species produce the phases that disturb the structural coherency of fibres and provide the paths for the crack propagation. In the early fibre/matrix reaction stage (slightly affected fibres) there are elemental silicon and aluminium and, most likely, also Li₂O that cause the intergranular fracture of fibres, whereas in the advanced reaction stage (strongly affected fibres), MgO is predominantly formed and the fracture propagates throughout the fibres via the MgO-enriched regions. This revised version was published online in November 2006 with corrections to the Cover Date.     

无压浸渗制备的SiC/Al复合材料的微观组织研究

利用SRD,OM,SEM,TEM等微观结构分析手段,对无压浸渗制备的SiCp/Al复合材料的微观结构进行了研究。结果表明,SiCp/Al复合材料中存在SiC,Al,MgAl2O4,Si和Mgi2Si诸相。在组织中没有粗大的铝硅共晶体针条,铝基体被众多SiC颗粒分割,成为细小的连续的空间网络。在铝基体中分布着Si相及Mg2Si相。透射电子显微镜高分辨像表明,在SiC与铝合金的界面上存在镁铝尖晶石（MgAl2O4）相,没有出现Al4C3相。     

Phase transformations in Al–Mg–Zn alloys during high pressure torsion and subsequent heating

The structure, phase composition, and their thermal evolution were studied in case of ternary Al–Zn–Mg alloys before and after high-pressure torsion (HPT) in Bridgman anvils. The as-cast non-deformed alloys contained the fine particles of Mg₃₂(Al,Zn)₄₉ (τ phase), MgZn₂ (η phase), AlMg₄Zn₁₁ (η′ phase), and Mg₇Zn₃ phases embedded in the matrix of Al-based solid solution. During heating in differential scanning calorimeter (DSC), all these phases dissolved around 148 °C. The τ nanoparticles coherent with (Al) matrix-formed instead around 222 °C. HPT of the as-cast alloys strongly refined the grains of (Al) solid solution from 500 μm to 120–150 nm. The particles of τ, η, η′, and Mg₇Zn₃ phases fully dissolved in the (Al) matrix. During the following DSC-heating, particles of η phase appeared and grew. Their amount became maximal around 166 °C. The growth of η phase in the fine-grained HPT-treated alloys instead of τ phase in the coarse-grained ones is explained by the shift of the (Al) + η/(Al) + η + τ/(Al) + τ lines in the Al–Zn–Mg ternary phase diagram due to the grain boundary (GB) adsorption. At 166 °C the η phase formed the continuous flat layers in numerous (Al)/(Al) GBs. This corresponds to the complete GB wetting by the η phase. Other (Al)/(Al) GBs contain separated lenticular η particles (incomplete GB wetting). Increasing the temperature from 166 to 320 °C led to the disappearance of the completely wetted (Al)/(Al) GBs. In other words, the transition from complete to the incomplete wetting of (Al)/(Al) GBs by the η phase proceeds between 166 °C and 320 °C.     

Quasicrystal-reinforced Mg alloys

The formation of the icosahedral phase (I-phase) as a secondary solidification phase in Mg–Zn–Y and Mg–Zn–Al base systems provides useful advantages in designing high performance wrought magnesium alloys. The strengthening in two-phase composites (I-phase + α-Mg) can be explained by dispersion hardening due to the presence of I-phase particles and by the strong bonding property at the I-phase/matrix interface. The presence of an additional secondary solidification phase can further enhance formability and mechanical properties. In Mg–Zn–Y alloys, the co-presence of I and Ca₂Mg₆Zn₃ phases by addition of Ca can significantly enhance formability, while in Mg–Zn–Al alloys, the co-presence of the I-phase and Mg₂Sn phase leads to the enhancement of mechanical properties. Dynamic and static recrystallization are significantly accelerated by addition of Ca in Mg–Zn–Y alloy, resulting in much smaller grain size and more random texture. The high strength of Mg–Zn–Al–Sn alloys is attributed to the presence of finely distributed Mg₂Sn and I-phase particles embedded in the α-Mg matrix.     

Mechanical behavior of pure Al and Al–Mg syntactic foam composites containing glass cenospheres

Glass cenospheres were used as space holders for making aluminum matrix syntactic foams by pressure infiltration technique. The mechanical properties and failure behavior of cenospheres/Al syntactic foams with pure Al and Al–Mg alloys were investigated in the present work. The failure behavior of cenospheres in two syntactic foams was similar. However, the mechanical behavior of these two syntactic foams was different. Under compression process, the cenospheres/pure Al showed discontinuous shear band and drum shape, while cenospheres/Al–Mg exhibited continuous shear band and was divided by main shear zone. At the tensile state, the cenospheres in pure Al matrix syntactic foam debonded from the matrix, while the cenospheres in Al–Mg matrix syntactic foam was well-bonded and appeared to lamellar tearing. It is suggested that the difference of mechanical deformation behavior could be attributed to the matrix ductility and the forming of interfacial reaction product MgAl₂O₄ coatings.     

Effect of Nd content on microstructure and mechanical properties of Grf/Al composite

M40 graphite fibre reinforced Al-17Mg matrix composites with different neodymium (Nd) content (Al-17Mg, Al-17Mg-0.2Nd, Al-17Mg-0.5Nd and Al-17Mg-2Nd) were fabricated by pressure infiltration method. Microstructure of Gr_f/Al composites was investigated by XRD, SEM, TEM and HRTEM. Effect of Nd on microstructure and mechanical properties of Gr_f/Al composites were deeply discussed. Al₃Mg₂ and Al₁₁Nd₃ phases followed by segregation of Nd and Mg at carbon-aluminum interface were detected in composites containing Nd. The size and amount of Al₄C₃ phase were increased with Nd content. Bending strength of Gr_f/Al composites were decreased sharply from 1463 MPa (Gr_f/Al-17Mg) to 791 MPa (Gr_f/Al-17Mg-2Nd) after the addition of Nd. The increased Nd content decreased the pull-out of single fibre and bundles, which was due to the high interfacial bonding strength with formation of Al₄C₃, Al₃Mg₂, Al₁₁Nd₃ phases and the transition layer.