Ti-45at.% Al合金定向凝固过程中显微组织演化的计算机模拟

采用基于溶质扩散控制模型CA方法对Ti-45at.% Al合金定向凝固过程中显微组织演化过程进行了数值模拟.模拟结果表明,在温度梯度较高时,初生晶核间距无论大小,均可通过分枝或竞争生长而使一次胞晶/枝晶间距得到调整.在抽拉速度一定情况下,随着温度梯度增加,胞晶/枝晶间距减小,在温度梯度一定情况下,随着抽拉速度增大,观察到胞晶/枝晶混合结构,混合结构区随温度梯增大而增大,模拟结果与实验观察相符合. 关键词： Ti-45at.% Al合金 定向凝固 组织演化 数值模拟     

二元合金高速定向凝固过程的相场法数值模拟

利用相场法对二元合金高速定向凝固过程进行了模拟，为简化计算，采用了温度冻结近似.模拟了定向凝固时平界面失稳、界面形态演化、溶质浓度分布以及高速绝对稳定性条件下胞晶向平面晶的转变等，得到了不同抽拉速度下胞晶间距、胞晶尖端温度以及有效溶质分配系数等参数，显示了高速生长条件下的溶质截留效应. 关键词： 定向凝固 相场法 高速绝对稳定性 溶质截留     

铸锭凝固组织对相应非晶合金晶化过程中二十面体准晶相形成动力学的影响

通过在真空电弧熔炼炉内对合金铸锭进行反复熔炼处理，获得到了凝固组织不同的Zr₆₅Al_7.5Cu_12.5Ni₁₀Ag₅合金铸锭.在相同的制备条件下，由凝固组织不同的合金铸锭通过吸铸法制备得到了薄片非晶合金.利用差示扫描量热法(DSC)对非晶合金的晶化动力学进行了分析.x射线衍射谱表明，在Zr₆₅Al_7.5Cu_12.5Ni₁₀Ag₅非晶合金晶化过程中，二十面体准晶相(I相)作为初生相析出.Kissinger分析结果表明，合金铸锭的凝固组织细化，相对应的非晶合金发生晶化时，I相形成与分解的有效激活能都增大，说明非晶合金及析出的I相的热稳定性都提高.从结构的遗传性角度就合金铸锭凝固组织对相应非晶合金晶化过程中二十面体准晶相的形成动力学的影响进行了讨论. 关键词： 二十面体准晶相 晶化动力学 凝固组织     

单相合金凝固过程时间相关的界面稳定性(Ⅱ)实验对比

采用典型的透明模型合金，即丁二腈-1．5at％水杨酸苯酯(SCN-1．5at％Salol)合金，对界面前沿溶质扩散边界层和界面稳定性进行了实时观测，并结合丁二腈-1．1wt％乙醇(SCN-1.1wt％Eth)、丁二腈0.43wt％氧杂萘邻酮152(SCN-0.43wt％C152)的实验结果，对纯扩散和存在对流情况下的定向凝固界面形态稳定性进行了系统的分析.发现以往所进行的界面稳定性分析能够准确描述实验中所观测到的界面失稳的时间相关特性，并发现界面失稳演化存在两个时间结点，即界面失稳孕育时间和平胞转变孕育时间，所获得的时间相关解能够准确预测实验中观测到的平胞转变孕育时间，而对流效应则明显缩短了界面失稳和平胞转变的孕育时间.实验中获得的单位扰动振幅发展速率比稳态理论预言值小了将近一个数量级，而与时间相关解较为接近. 关键词： 界面稳定性 凝固 扩散 对流     

Diffusion and precipitation phenomena across layer interfaces in a roll bonding composite of Al(Si) and Al(Cu)

Layered composite structures can be generated in metallic sheets by roll bonding of dissimilar metals/alloys. In this investigation, heat treatable (Al(Cu)) and non-heat treatable (Al(Si)) aluminium alloys are roll bonded in sheet form. Large hardness differences between layers poses significant bonding challenges in the form of multiple necking within the hard Al(Cu) layers. For successful processing, it is vital to choose the candidate materials in a state of marginal hardness differences during rolling, but being capable of altering properties through subsequent heat treatments. Atomic diffusion takes place during heat treatment of the composite sheet and results in gradual hardness variation across sheet thickness. The Al(Cu) layers contribute to strength, whereas the Al(Si) layers provide protection from corrosion/wear-related degradation in the newly developed hybrid sheet. The overall mechanical properties of the heat-treated composite fall between the base alloys. The bonding interfaces are noted as the potential spots for initiating failure.     

Microstructure and properties of Ni–Ni3Si composites by directional solidification

Ni–Ni₃Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni₃Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni–Ni₃Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni–Ni₃Si eutectic composites display regular lamellar eutectic structure at the solidification rate R=6.0–40.0 μm/s and the lamellar spacing is decreased with the increase of the solidification rate. Moreover, the Ni–Ni₃Si hypoeutectic composites present lower micro-hardness than pure Ni₃Si, which indicate Ni–Ni₃Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni–Ni₃Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni₃₁Si₁₂ phase in the Ni–Ni₃Si eutectic composites.     

Investigation on the microstructure of a τ-Mg32(Al,Zn)49 strengthened Mg–Zn–Al alloy with relatively low Zn content

In the present work different orthogonal tight-binding molecular dynamics methods have been employed for describing small silicon clusters. The cohesive energies calculated using these methods have been compared with those found from the first principles Car-Parrinello method. The comparison shows that the orthogonal tight binding matrix elements and repulsive potentials need to include the radial cutoff up to fourth neighbor distance in diamond structure in order to reproduce ab initio results. The environmental correction is not needed in this orthogonal tight-binding method.     

Hard magnetic properties of nanosized Sr(Fe,Al)12O19 hexaferrites obtained by Pechini method

In this work, we report the magnetic properties of isotropic M-type SrFe_12−xAl_xO₁₉ (x=0.0,1.5) hexaferrites synthesized by means of Pechini method. A polycrystalline distribution of fine grains was verified by Transmission Electron Microscopy for both compositions, with average sizes below 60 nm. Remarkable coercivity values within the range 500–850 kA/m were attained as a consequence of a combined effect of grain size refinement together with an enhancement of the anisotropy field afforded by the incorporation of the Al³⁺ cations into the hexagonal crystal structure.     

Improved LIBS limit of detection of Be, Mg, Si, Mn, Fe and Cu in aluminum alloy samples using a portable Echelle spectrometer with ICCD camera

Laser-induced breakdown spectroscopy (LIBS) is a laser-based technique that can provide non-intrusive, qualitative and quantitative measurement of metals in various environments. LIBS uses the plasma generated by a high-energy laser beam to prepare and excite the sample in one step. In the present work, LIBS has been applied to perform elemental analysis of six trace elements simultaneously in aluminum alloy targets. The plasma is generated by focusing a pulsed Nd:YAG laser on the target in air at atmospheric pressure. LIBS limit of detection (LOD) is affected by many experimental parameters such as interferences, self-absorption, spectral overlap and matrix effect. We aimed to improve the LIBS LOD by optimizing these experimental parameters as possible. In doing so, a portable Echelle spectrometer with intensified CCD camera was used to detect the LIBS plasma emission. This advanced Echelle spectrometer provides a constant spectral resolution (CSR) of 7500 corresponding to 4 pixels FWHM over a wavelength range 200–1000 nm displayable in a single spectrum. Then, the calibration curves for iron, beryllium, magnesium, silicon, manganese and copper as minor elements were achieved with linear regression coefficients between 98–99% on average in aluminum standard sample alloys. New LOD values were achieved in the ppm range with high precision (RSD 3–8%). From the application view point, improving LIBS LOD is very important in the on-line industrial process control to follow-up multi-elements for the correct alloying in metals.     

Preparation of F-doped titania nanoparticles with a highly thermally stable anatase phase by alcoholysis of TiCl4

Pure anatase is a metastable phase and inclined to (transform) be transformed into rutile structure under heating over than 500 °C, which limits its suitability for high-temperature applications. Hitherto much research efforts have been made to increase the stability temperature of anatase structure. However, metallic doping usually introduced metallic oxides into titania at high temperature, and many nonmetallic doping are not competent for increasing the stability temperature of anatase structure up to 900 °C. In this study, F-doped anatase TiO₂ nanoparticles were conveniently prepared via the alcoholysis of TiCl₄ and the as-prepared product shows very high stability temperature up to 1000 °C before being transformed into rutile structure phase. On the basis of XPS results of F-doped titania annealed at different temperature, it is learned that the F atoms were anchored on the crystal planes of anatase in favor of decreasing the energy faces of anatase and stabilizing the anatase structure till annealed at 1300 °C all the anatase were transformed into rutile phase.