Experimental evaluation of three single-particle dissolution models

The dissolution of the 60-85-mesh fraction os recording, flow-through dissolution apparatus equipped with a dissolution cell; it was particularly suitable for kinetic analysis of multiparticulate systems. By using a time-scaling approach, experimental data are compared with theoretical calculations to evaluate, quantitatively, which of three single-particle dissolution models best describes the data and how well the multiparticulate kinetics can be explained mathematically. The nonspherical tolbutamide particles are replaced in the calculations by a hypothetical system of spherical particles that appears to be log-normally distributed. This procedure permits the calculation of the intrinsic dissolution profile, considering both size distribution and particle shape effects.     

Dissolution of particles in binary alloys: part II. experimental investigation on an Al-Si alloy

A detailed experimental study of the dissolution kinetics of Si particles in an Al-Si alloy has been carried out in order to test the validity of the two models presented in the accompanying article.[1] In these models, the dissolution kinetics are dependent on the particle size distribution of the alloy. An alloy with composition C_o = 0.77 at. pct Si was heat-treated in order to obtain rather coarse spherical particles (1 to 10 μn). The size distribution of the particles was found to be close to the log-normal distribution. At high temperatures, when the solvus concentration was well above C_o, the experimental values were very close to the values of the model which predicted the highest dissolution rates. At lower temperatures, when the solvus concentration was closer toC _o, the experimental values lay in between the values predicted by the two models. The results clearly demonstrate that a size distribution of particles must be included in the model if an accurate prediction of the dissolution kinetics is to be achieved.     

Particle growth by coalescence during liquid phase sintering of Fe-Cu

Particle growth during liquid phase sintering of Fe-30 wt% Cu and Fe-60 wt% Cu was measured and compared with a recently developed coalescence theory. Measured normalized particle size distribution, time-dependence of average particle size and dependence of growth rate on solid phase volume fraction agree well with calculations based on the assumption of a diffusion-controlled coalescence process where all component particles are assumed to have equal coalescence frequency. Slight discrepancies can be explained as the results of nonspherical particle shape and of favored coalescence opportunity for larger particles.     

Analysis of softening alloys of the Al-Ni system containing particles of variable dispersity

Regularities of the deformation strengthening and softening of aluminum alloys containing second-phase Al₃Ni particles 0.3 to 2.2 μm in size with a volume fraction from 0.03 to 0.1 are investigated during cold deformation and subsequent annealing at 0.6t _m. It is shown that the largest hardness increment is observed for alloys with a maximal fraction of fine particles (d = 0.3 μm) after rolling deformation larger than 0.4. Fine particles prevent the development of crystallization upon true deformation up to 2.3, thereby effectively inhibiting softening. An increase in the particle size to 1.2–2.2 μm stimulates nucleation during recrystallization, substantially accelerating this process. For example, in order to ensure recrystallization uniformly over the entire sheet volume at d = 2.2 μm, cold deformation with ? = 0.4 is sufficient.     

A Note on the Misuse of Area Images to Obtain Particle Size Information in Solid-Solid Systems

Abstract

Multiphase solids (slag + matte, metals + inclusions) are frequently sectioned and the areas of the phases visible at the resulting surfaces are obtained using one of a number of microscopic techniques. Number concentrations of particles can be obtained unambiguously from these images. However, misinterpretation of these area distributions as particle size distributions for the dispersed phases arises frequently and in diverse contexts. When particles are binned coarsely and the particles are nearly spherical, the impact of such misinterpretations on mean particle size and interfacial area per unit volume and even relative dispersed phase volume fraction is shown to be small. In other cases the errors can be significant. In all cases, the number fraction of small particles is overstated. Thus, interfacial area per unit volume of dispersed phase tends to be overstated, the mean particle size and relative dispersed phase volume fraction underestimated. In this note, an analytical method for the deconvolution of area distributions for dispersed spherical particles is rehearsed and numerical results for randomly oriented axially-symmetric particles and analogues for disc-like and needle-like particles are presented which illustrate the significance of the impact of such misinterpretations. Other limitations on the interpretation of area distributions are also discussed.     

α相含量对AA3104铝合金热轧组织及退火行为的影响

通过对AA3104铝合金均匀化时间的控制,获得α相含量分别为18%和68%的初始材料,并对其进行轧制温度约为480℃~420℃、形变量为95%的热轧形变。利用扫描电镜(SEM)及能谱仪(EDS)对第二相含量、平均尺寸以及尺寸分布进行定量表征。利用显微硬度测定了不同α相含量热轧板的再结晶温度,利用X射线衍射技术(XRD)对再结晶前后织构进行研究,并对比分析织构差异。结果表明:热轧后,α相含量为18%的第二相粒子中尺寸接近5μm的粒子数量较少,其完全再结晶温度约为380℃,且立方织构的含量低;α相含量为68%的试样热轧后含有更多尺寸接近5μm的第二相粒子,其完全再结晶温度约为340℃,且立方织构含量较前者高,结果表明含有第二相粒子的AA3104铝合金热轧组织及退火行为与α相含量有关。     

Coarsening and morphology of β′ particles in Fe-Ni-Al-Mo ferritic alloys

Studies have been carried out of the effect of aging at 973 K on the ordered NiAl β′ precipitate particles in a series of Fe-Ni-Al alloys with and without the addition of Mo. The lattice parameter disregistry between matrix and precipitates is affected by the Mo content. The rate of precipitate coarsening during isothermal aging is slowest when the misfit is minimized. The dependence of coarsening upon misfit is more pronounced in alloys with low precipitate volume fractions. Raising the precipitate volume fraction increases β′ coarsening rates more than predicted by theory. The lattice parameters of both the β′ particles and the matrix are functions of aging time. Compositional changes, segregation of Mo to the particle-matrix interface and, at long aging times, loss of coherency are proposed as causes of the observed lattice parameter changes. The addition of Mo to the Fe-Ni-Al alloys causes the β′ particles to remain spherical even after extended periods of aging whereas the particles become somewhat cubic or rectangular parallelepiped in form in the Mo-free alloys. The size distributions of the β′ particles agree in a general way with the predictions of the Brailsford and Wynblatt and Voorhees and Glicksman models, but are somewhat less skewed and do not show the predicted sharp cutoff in particle size on the high radius side.     

Studies on addition of inclusions to molten aluminum using a novel technique

Considerable initiatives have often been taken to introduce specific solid particles directly into molten metals in a desirable quantity, but little success has been achieved, particularly in the case of reactive particles. Therefore, alternative routes such as production of solid particles through chemical reactions within the melt are often used. While these methods are capable of producing solid particles within the melt, the chemistry of the melt changes and control of particle size and chemistry is difficult. In the present study, a direct addition technique has been used to introduce many types of inclusions into liquid aluminum and Al-Si alloys, irrespective of their wettability and chemical reactivity, while preserving the surface characteristics and melt chemistry. A uniform particle distribution can be obtained even at low volume fraction of addition and with particle sizes of the order of 2 to 5 μm. This technique allows valuable information regarding the behavior of many inclusions, such as TiB₂, TiC, SrO, and Sr(OH)₂, in liquid aluminum to be studied. Several such examples are presented.     

Direct observation of coarsening in AlCu alloys

Observations have been made on coarsening of θ-CuAl₂ precipitates at the surface of AlCu alloys. One specific area was examined over extended periods so that dissolution and growth of individual precipitates could be followed. It was found that nearly all precipitates shrank on ageing, even those larger than the average particle size. Only the largest 8% increased in dimension. A simple theory was used which gave good agreement with the observed dissolution kinetics of all precipitates except the very largest. This theory assumes a solute flux from the precipitate to a larger precipitate located a significant distance away. The theory gave a mean value for interfacial energy of 0.81 J/m². Particles were observed to spheroidise during ageing, and by the time their dimension had decreased 50%, they were completely spherical. Particle coalescence took place in a few cases but was generally of minor importance in the overall coarsening process.     

微生物分解难处理金矿的动力学研究

在均匀混合的间歇式反应器中用氧化亚铁硫杆菌分解难处理金矿,研究了不同粒度矿粒分解的动力学。结果表明,细菌在矿粒表面的吸附符合兰格穆尔等温吸附式,等温吸附常数KA不受粒度大小的影响,而单位质量矿粒的最大吸收量XAm随着粒度的增大而减小。建立了用于描述细菌生长和矿石分解的动力学方程式,其与试验数据能很好的吻合,矿石的初始粒度对吸附在矿粒表面的细菌的生长以及矿石的分解都有很大的影响。     

The recrystallisation of aluminium-silicon alloys containing a bimodal particle distribution

The recrystallisation behaviour of Al-Si alloys containing bimodal dispersions of silicon particles was studied using optical and transmission electron microscopy. This behaviour was compared with that of several unimodal alloys with different particle dispersions. For the bimodal alloys, the large particles were observed to act as nucleation sites. The overall kinetics of recrystallisation however, were retarded due to the influence of the fine dispersion of small particles, and were similar to those of an alloy containing small particles only. It was concluded that the fine dispersion primarily affected the early stages of nucleus growth. The recrystallised grain size and shape were found to be dependent on the annealing temperature, and the fine dispersion level.     

Influence of particle size and volume percent of flaky mo particles on the mechanical properties of AI2O3/Mo composites

The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling process and three-point flexural strength and fracture toughness of A1₂O₃ composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with A1₂O₃ powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these Mo particles in the A1₂O₃ matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small spherical shape.     

The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—II. Tensile properties

The effect of chromium and iron additions and of annealing and working on the microstructure and tensile properties of vapour quenched AlCr and AlCrFe alloys has been determined. Tensile strengths of the worked AlCrFe alloys were in the range 568–831 MPa. Chromium in solid solution or iron present as iron-rich precipitates increased the yield stress by 44.7 MPa/at.%Cr and 333 MPa/at.%Fe respectively. The contributions to the yield strength of AlCr alloys were solid solution 40% and dislocation density/cell size 60% and to the yield strength of AlCrFe alloys were solid solution 25%, iron-rich precipitates 42% and dislocation density/cell size 33%. Vapour quenching may allow the more efficient use of alloying elements in the strengthening of Al-alloys and greater flexibility in obtaining the desired combination of solute concentration, particle volume fraction and particle size.     

Microstructural evolution and thermal stability

High-pressure inert gas atomization (HPGA) has been used to produce rapidly solidified Cu-21Nb-2Mo (weight percent) powders with a range of particle sizes and microstructures. The associated microstructures depend on particle size. Specifically, fine-scale particles (\s15 μm@#@) are characterized by a predominance of multiphase spheroids and a small population of Nb-based dendrites in an almost pure matrix of Cu. In contrast, large particles (45 to 75 μm@#@) contain only Nb-based dendrites in a Cu matrix. The volume fraction of the dispersed constituent is much higher in the former instance than in the latter. The change in microstructure with particle size is analyzed in terms of both the amount of undercooling and cooling rate of the liquid droplets prior to and during solidification. In particular, the large undercoolings in the fine particles are believed to induce a nonequilibrium liquid phase separation which results in a high volume fraction of spheroidal, multiphase Nb-Cu particles within a Cu-rich matrix containing Nb-rich dendrites. High-temperature (900 °C) isothermal annealing treatments have also been performed on consolidated material to determine the inherent thermal stability of these microstructures.     

Behavior of ceramic particles at the solid- liquid metal interface in metal matrix composites

Directional solidification experiments have been conducted to document SiC particle behavior at the solid-liquid interface in Al-2 pct Mg (cellular interface) and Al-6.1 pct Ni (eutectic interface) alloys. Particle size ranged from 20 to 150 μm diameter. Although predictions based on the thermodynamic approach suggest that no engulfment is possible, it was demonstrated that particles can be entrapped in the solid if adequate solidification rates and temperature gradients are used. The main factors responsible for this behavior are considered to be the difference between the thermal conductivities of particles and metal, the buildup of volume fraction of particles at the interface, and the morphological instability of the interface induced by the particles. A model including the contribution of drag and thermal conductivity has been proposed. Calculation with this model produced numbers for the critical velocity slightly higher than those evaluated experimentally. Various factors which can account for this discrepancy are discussed.     

取向电工钢加工过程中第二相粒子的析出行为

利用场发射扫描电镜观察了以MnS为主要抑制剂的普通取向电工钢加工过程中第二相粒子的分布状态,统计了粒子面密度、平均尺寸以及相应的尺寸分布.结果显示,热轧加工造成了大量第二相粒子弥散、细小地析出,同时基体仍保持过饱和状态.冷轧变形会造成第二相粒子的回溶行为,而基体的过饱和状态会减弱回溶现象.中间退火与脱碳退火过程中会同时存在新粒子的形核及已析出粒子的粗化两个过程,而在最终二次再结晶升温阶段则以第二相粒子明显粗化为主.     

The kinetics of composite particle formation during mechanical alloying

The kinetics of composite particle formation during attritor milling of insoluble binary elemental powders have been examined. The effects of processing conditions(i.e., mill power, temperature, and charge ratio) on these kinetics were studied. Particle size distributions and fractions of elemental and composite particles were determined as functions of milling time and processing conditions. This allowed the deduction of phenomenological rate constants describing the propensity for fracture and welding during processing. For the mill-operating conditions investigated, the number of particles in the mill generally decreased with milling time, indicating a greater tendency for particle welding than fracture. Moreover, a bimodal size distribution is often obtained as a result of preferential welding. Copper and chromium “alloy” primarily by encapsulation of Cr particles within Cu. This form of alloying also occurs in Cu-Nb alloys processed at low mill power and/or for short milling times. For other conditions, however, Cu-Nb alloys develop a lamellar morphology characteristic of mechanically alloyed two-phase ductile metals. Increasing mill power or charge (ball-to-powder weight) ratio (CR) increases the rate of composite particle formation. B.J.M. AIKIN, formerly Graduate Student, Materials Science and Engineering Department, University of Virginia, Charlottesville, VA. T.H. COURTNEY, formerly Professor, Materials Science and Engineering Department, University of Virginia.     

The effect of ternary additions on the decompositon of metastable beta-phase titanium alloys

The effect of ternary additions of Al, O, Sn, and Zr on the decomposition of metastable Ti-Mo and Ti-V Β-phase alloys has been studied. It is shown that all these additions reduce the volume fraction, upper temperature limit of formation, and time of stability of the Ω phase. These results have been contrasted to earlier work in binary alloys in which misfit was shown to be a primary factor in determining the morphology and range of stability of the Ω phase. In the ternary alloys the particle morphology can still be explained in terms of misfit, but other considerations appear to be predominant in determining the range of stability. Results are also presented on a phase separation reaction which occurs at higher alloy contents and which has an important influence on the morphology of α-phase precipitation. Formerly with North American Rockwell Science Center.