The transition mode of dilute binary alloys during directional solidification near to the absolute stability limit

The morphological evolution near the absolute stability limit during directional solidification has been studied systematically on dilute Al–Mn alloys. It is found that the interfacial morphology of Al–0.52wt%Mn and Al–1.2wt%Mn alloys changes from coarse cellular structure to fine cells, and then again to be coarsened with the increase of velocity to near the absolute stability limit. This indicates that there exists a minimum cell spacing corresponding to the maximum effective constitutional supercooling. As the growth rate approximates to or exceeds the critical velocity of absolute stability by calculation according to M–S theory, the interfacial morphology of Al–0.52wt%Mn alloy may still retain a cellular structure. For Al–1.2wt%Mn alloy, when the growth velocity is near the absolute stability limit, the fine cells may change to a band or grain-like structure which in some cases takes an oscillating manner, which possibly implies the existence of a non-linear effect during high growth rate.     

The influence of the T-stress on the directional stability of cracks

The sign of the T-stress has been widely used for deciding whether directional stability prevails for straight cracks subjected to mode I loading under small scale yielding. However, there is little evidence for the reliability of such a criterion. On the contrary, it is shown that a local criterion is not applicable and that directional stability generally depends on body and load geometry as well as on material parameters, whereas the sign of the T-stress is totally irrelevant in most cases.     

Evolution of the microstructure and solute distribution of Sn-10wt% Bi alloys during electromagnetic field-assisted directional solidification

The effects of forced flows at different velocities on microstructure and solute distribution during the directional solidification of Sn-10 wt% Bi alloys under a simultaneous imposition of a transverse static magnetic field(TSMF) and an external direct current(DC) have been investigated experimentally and numerically. The experimental results show that the solid-liquid interface will gradually become sloping with the increase of the forced flow velocity when the thermoelectric magnetic convection(TEMC)dominates the forced flow at solidification front. However, the interface will gradually become planar as the flow velocity further increases when the electromagnetic convection(EMC) dominates the forced flow. Moreover, when the flow velocity gradually increases, the primary dendrite spacing decreases from384 to 105 μm accordingly. The simulation results show that the solute distribution at the two sides of the sample can be significantly changed by the forced flow at solidification front. The rejected solute will be unidirectionally transported to one side of the sample along the TEMC(a low-velocity forced flow),thereby causing the formation of a sloping interface. However, the rejected solute will be returned back along the EMC(a higher-velocity force flow), which results in a planar interface. Furthermore, the solute content at the two sides of the sample under the forced flows at different velocities was measured. The results are in good agreement with the simulation results, which shows that the solute content difference between the two sides of the sample reaches the maximum when a 0.5 T TSMF is applied, while the solute content difference decreases to zero with a simultaneous application of a 0.5 T TSMF and a 1.6 × 10~5 A/m~2 external DC.     

用AEAM模型研究Ti与过渡族bcc金属二元合金的结构

运用AEAM模型理论,计算了Ti金属的总能量,并和Rose方程的结果作了比较;同时系统地研究了Ti与bcc金属组成的二元合金系统形成焓,并对计算结果进行了分析,给出了Ti与bcc金属组成的各合金结构稳定性与成分关系,为AEAM模型理论研究二元合金结构及稳定性提供了思路.     

The effect of the second-phase volume fraction on the grain size stability and flow stress during superplastic flow of binary alloys

This paper considers to what extent the second-phase volume fraction in superplastic binary alloys affect the matrix grain size stability during deformation and, through it, the flow stress at constant temperature and strain rate. It is shown for five different superplastic binary alloy systems, that at constant temperature and strain rate the flow stress will increase with the deviation of the second-phase volume fraction in the alloys from that required for maximum matrix grain size stability. A new parameter (Z) which quantifies these deviations has been introduced in this paper. The possible errors in determining the pertinent parameters in the rate equation for superplastic flow by testing alloys withZ is discussed.     

羧甲基纤维素化学结构对乳蛋白稳定性影响

采用异丙醇/乙醇高浴比混合介质体系和单一乙醇介质低浴比体系,分别制备了两种乳蛋白稳定剂羧甲基纤维素CMCm和CMCe。通过乳液体系耐酸、耐盐、稳定性等测试分析,系统研究了两种稳定剂对乳液蛋白的稳定性影响。结果表明,异丙醇/乙醇高浴比混合介质体系得到的CMCm的耐酸、耐盐性好,沉淀率低,乳液稳定期长。通过SEM对乳液形态观察分析表明,添加CMCm的乳液蛋白间形成较均匀连续网络形貌,而添加CMCe的乳液体系则有明显的网路缺陷,蛋白有团聚趋势。为了深入了解两种稳定剂结构对乳液性能的影响,采用核磁共振测试对比研究了CMCm和CMCe大分子链上—CH2COO-基团的分布。研究发现,异丙醇/乙醇高浴比混合体系制备的CMCm,其葡萄糖残基上C-6位羟基的羧甲基化程度较高,而单一乙醇介质低浴比体系得到的CMCe则是C-2位上羟基的羧甲基化程度高。     

The contribution of the nucleation process to grain formation in calculating solidification microstructure by CA-DFD

To predict solidification macrostructures, the direct finite difference (DFD) method for heat transfer calculation was coupled with cellular automaton (CA). In the CA-DFD, the nucleation process as the initial condition of calculations was determined according to Gaussian distribution or being instantaneous. In the case of Gaussian distribution, there are six nucleation-related parameters. In this work, the inspection of relationships of preset nuclei to grain, preset nuclei to born nuclei, and born nuclei to grain was carried out by comparison of each ratio. However, these relationships are only valid under the same solidification environments. In contrast to the preset method, the successive nuclei setting method (SNS) was carried out. This method manages the nucleus settings by every time step. The microstructures by SNS were compared with those by preset.     

球磨Mg0.97La0.03Ni合金的热稳定性及电性能研究

采用XRD、DTA、SEM及电池性能测试仪等对球磨Mg0.97La0.03Ni合金的结构、形貌、活化性能、热稳定性、电化学稳定性及容量衰减机理等进行了详细的研究。结果表明：样品的热稳定性及循环稳定性随着球磨时间的延长而增加。经400r／min球磨50h的样品在第二次活化时即达到最大值450mAh／g．经25次循环充放电后．该样品的容量与其最大值相比下降了53％．容量衰减的主要原因有：在循环充放电过程中．非晶体逐渐分解生成Mg2NiH4和Ni等晶体相，同时在颗粒表面形成腐蚀产物Mg(OH)2等。     

钴铝\镁铝水滑石的热稳定性能及其氧化物的结构研究

采用溶液共沉淀法合成了CoAl-HT和MgAl-HT水滑石,在800℃下煅烧4h得到了钴铝和镁铝尖晶石氧化物粉体。利用XRD、FT-IR和TG对水滑石及其氧化物粉体的结构和热稳定性能进行了表征分析。从XRD和FT-IR分析结果可知,所得产品为层板水滑石物质,且结晶性良好,从CoAl-HT和MgAl-HT结果对比可知,镁铝组合更容易形成水滑石结构;由氧化物粉体的XRD分析结果可知,在800℃下两者都形成了尖晶石结构,且钴铝氧化粉体尖晶石无杂相,结晶性更好;TG曲线呈现了两个样品的水滑石热学性能,通过对比可知,MgAl-HT在216和372℃下分解而CoAl-HT在197和243℃下分解说明MgAl-HT比CoAl-HT具有更好的热稳定性能。     

The dimensionless analysis of the stability of composite superconductors with respect to thermal disturbances

The stability of superconductors with respect to thermal disturbances is determined by many factors: external conditions, properties of the composite, and nature of the heat exchange with the environment. To simulate the thermal processes mathematically, it was necessary to consider non-linear heat conduction equations which take into account the temperature dependences of the specified thermophysical parameters. The results obtained in this way depend on many parameters, which hinder establishing any general regularities. The solution of the corresponding equations in a dimensionless form reduces the number of variables and hence a generalized analysis of the phenomena can be performed     

Riccati传递矩阵法在潜艇结构强度和稳定性分析中的应用

运用Riccati传递矩阵方法，建立了旋转壳单元的场传递矩阵，推导了肋骨和母线倾角不连续位置的点传递矩阵，在推导中考虑了肋骨各方向可能的变形，编制了用于分析组合加肋旋转壳应力和稳定性的计算机程序（应力程序SAPRi，稳定性程序BAPRi）。利用SAPRi程序和BAPRi程序对潜艇耐压结构的典型结构算例进行了应力和稳定性分析，并将计算结果与理论结果或商用软件（MSC／NASTRAN）的计算结果进行比较，表明该程序结果正确可信，计算速度快，适合于工程应用。     

Use of fluorescence to probe the surface dynamics during disorder-to-order transition and cluster formation in dihalonaphthalene-water thin films on Al2O3(0001)

Amorphous dihalonaphthalenes that are prepared by vacuum deposition onto a cold Al₂O₃ surface form electronically excited dimers when optically pumped, and their emission is characteristically red-shifted, broad and featureless compared to the monomeric fluorescence. If the surface is heated, the adlayer undergoes a disorder-to-order transition at a temperature characteristic of the molecule. Since pure crystalline dihalonaphthalenes typically fluoresce and do not exhibit excimeric features, the transition was studied by taking advantage of the changes in the spectral characteristics of the adlayer. These included transmittance, and emission from fluorescence and excimer. The combination of these methods allowed a close look at the surface dynamics of molecules on the surface of Al₂O₃ as the adlayer was heated from the deposition temperature to desorption.If a bilayer is formed by depositing water onto the surface with the organic adlayer on top, water, with its lower desorption energy, can be made to percolate into the organic layer. The optical probes indicate that the water clearly associates with the organic molecules while the excess water desorbs. By varying the coverage of either the water or the dihalonaphthalene, the stoichiometric composition of the cluster can be determined and are reported here.