Optical dilatometry for the control of microstructure development during sintering

An optical method for the in situ measurement of sintering shrinkage has been developed. It relies on a simple and robust CCD camera technique and is applicable up to high temperatures (2000 °C) and heating rates (10 K/min) in industrial furnace atmospheres. A resolution of less than 1 μm for a sample diameter of 10 mm has been achieved using a special geometrical arrangement and a smart image analysis algorithm. Systematic errors are small (<0.1%) and can be corrected by an additional measurement. A high reproducibility of shrinkage measurement has been observed in various sintering experiments. The optical dilatometer is very flexible: it has been used also for an accurate temperature calibration and for wetting experiments and it can be adjusted to different furnace types. Received: 17 May 2000 / Reviewed and accepted: 26 June 2000     

On the mathematical theory of the microstructure evolution during the final stage of liquid phase sintering

A new mathematical theory of the microstructure evolution during the final isothermal stage of liquid phase sintering has been developed. The basis of this theory has been formed by materials science and continuum mechanics. The microstructure evolution has been described by the system of non-linear differential equations, which contain more than ten parameters. Finite difference analysis has been performed for a heavy alloy cylindrical specimen. The results of the calculations have been compared with experimental data.     

铝粉固相烧结过程中的微结构演化

在铝粉末样品的固相烧结过程中使用同步辐射CT(SR-CT)技术对其进行实时投影成像,用滤波反投影算法重建得到了铝粉末在烧结过程中不同时刻的内部微结构的二维断层重建图像,并用数字图像处理技术得到了三维重建图像;通过重建图像清晰地观测到样品在3个烧结阶段中颗粒的接触、烧结颈的形成、晶粒和气孔的长大以及气孔球化并收缩等现象,从而实现了对铝粉末烧结过程的实时和无损观测.统计了样品在不同烧结时刻的孔隙率、孔隙率随着烧结时间和烧结时间对数的变化曲线,并根据曲线分析了样品在不同烧结时刻致密化速率的变化,得到了烧结中期孔隙率与时间对数的线性关系,结果与现有烧结理论吻合.     

Development of the microstructure during gradient sintering of a cemented carbide

A gradient sintered WC–Ti(C,N)–Co-based cemented carbide has been studied. The material was sintered in a nitrogen-free atmosphere, resulting in an outward diffusion of nitrogen. Due to the strong thermodynamic coupling between nitrogen and titanium, an inward diffusion of titanium is created. As a result of the diffusion processes the material develops a cubic carbide depleted and binder phase enriched surface zone, and inside this zone a region enriched in cubic carbide. The structure of the gradient surface zone has been studied, where variations in volume fractions of the different phases present have been investigated by scanning electron microscopy (SEM) and image analysis, and elemental variations have been studied with electron probe microanalysis (EPMA). Computer simulations of the gradient formation, based on diffusion and thermodynamic properties, show good agreement with the experimental results, and the gradient formation can be reasonably well predicted. Also studies of grain composition have been performed. In the rim of the cubic carbide grains, variations in composition are found which can be related to the sintering process. The composition of cubic carbide grains has been studied with SEM and analytical electron microscopy.     

Simulating microstructure evolution during passive mixing

The prediction of microstructure evolution during passive mixing is of major interest in order to qualify and quantify mixing devices as well as to predict the final morphology of the resulting blend. Direct numerical simulation fails because of the different characteristic lengths of the microstructure and the process itself. Micro-macro approaches could be a valuable alternative but the computational cost remains tremendous. For this reason many authors proposed the introduction of some microstructural variables able to qualify and quantify the mixing process at a mesoscale level. Some proposals considered only the effects induced by the flow kinematics, other introduced also the effects of shape relaxation due to the surface tension and coalescence. The most advanced integrate also the break-up process. However, the derivation of the evolution equations governing the evolution of such microstructural variables needs the introduction of some closure relations whose impact on the computed solution should be evaluated before applying it for simulating complex mixing flows. In this work we consider the Lee and Park??s model that considers the flow kinematics, the surface tension, the coalescence and the break-up mechanisms in the evolution of the area tensor. The accuracy of both a quadratic closure and an orthotropic relations will be analyzed in the first part of this work, and then the resulting closed model by using a quadratic closure will be used for simulating complex mixing flows.     

Effects of milling and particle size distribution on the sintering behavior and the evolution of the microstructure in sintering powder compacts

In this investigation, it was observed that the contamination arising from the milling medium has a great influence on the sintering behavior. The densification rate of the powder milled with ZrO₂ balls is largely inhibited, and the activation energy of ZrO₂-doped alumina is significantly enhanced. The possible reasons for the enhancement of the activation energy were discussed. The effects of particle size distribution of powder compacts on the microstructural evolution of alumina during sintering were proposed and discussed.     

Sintering behavior and microstructure evolution in cp-titanium processed by spark plasma sintering

This paper investigates the effect of various sintering temperatures from 800 to 1500?°C on the microstructure evolution of cp-titanium processed by Spark Plasma Sintering (SPS). The material processing conditions under consideration may change the obtained atomic order and microstructure. The different relation of the mechanical and corrosion resistance properties observed in the analyzed results could at the same time be shaped in the expected range. The SPS procedure allows the obtainment of nearly theoretical densities of the compacts and excellent mechanical properties (UTS?=?892?MPa, CS?=?1442?MPa). The results confirm that the contact angle measurement could support the process control, particularly if a microstructure feature is considered.     

Pore evolution model of ceramic membrane during constrained sintering

Pore size has been found to strongly depend on the sintering program in the preparation of porous ceramic membranes. In this paper, a model was developed to predict the variation in pore size and porosity of membranes during the sintering process. A comparison of shrinkage characteristics was made between the sintering processes of supported membranes and unsupported membranes. For supported membranes, the effect of restriction coming from a rigid substrate on the sintering behavior has been taken into account in the calculation. It is predicted that the pore size increases in supported membranes and decreases in unsupported membranes as the sintering temperature is increased. Calculations also showed that the loss of porosity in the supported membranes was less than that in the unsupported membranes. In order to verify reliability of this model, unsupported and supported membranes were prepared with α-Al₂O₃ powders at the sintering temperatures ranging from 1125 °C to 1325 °C. The pore size and porosity were measured by gas permeation technique and Archimedes’s method. The experimental results for the unsupported and supported α-Al₂O₃ membranes showed a good agreement with those calculated from the model. Therefore, this model provides an effective tool in predicting the porosity and the pore size of ceramic membranes at the different sintering temperatures.     

Microstructural evolution of Y123/Ag composites during sintering

The grain-growth kinetics of YBa2Cu3O7–xX (Y123) and the coarsening kinetics of silver inclusions in Y123/Ag composites during sintering were investigated. The sintering was carried out in the temperature range 900–950°C. The addition of silver lowered the formation of a liquid phase and the grain growth of Y123 in Y123/Ag composites was thus enhanced at the beginning of sintering. However, as the effective silver content was increased, more silver inclusions became interconnected, and the grain growth kinetics and coarsening kinetics slowed down significantly. This may be due to the mass transportation paths progressively changing from the grain boundaries to interfacial boundaries as the amount of interconnected silver networks is increased. The grain growth kinetic constant was calculated and compared with other published data. Because the inclusion was ripened with the growth of matrix grains, the coarsening of silver inclusions was deduced to be a coalescence process.     

Temperature and stress fields evolution during spark plasma sintering processes

Numerical modelling of Spark Plasma Sintering (SPS) processes is essential to evaluate temperature and stress distributions that can result in sample inhomogeneities. Most of the available literature, however, produced analysis in static conditions. In this work, we focused our attention on the time evolution of current density, temperature and stress distribution during a SPS process using a new approach that includes a PID control in the algorithm, allowing a realistic simulation of experiments performed using a temperature controller. Controlled temperature experiments have been simulated and discussed, with special interest focused on the time evolution of the process. The results showed that stress gradients inside the samples (~40%) are much greater than the temperature gradients (~2%), suggesting that heterogeneities in the microstructure can also be caused by the stress gradient. During the evolution of the process, a peak in stresses is experienced by the alumina sample at the beginning of the cooling stage, caused by differences in contraction between the sample and the die. It has been proved that, using a controlled cooling stage, these peaks in the stresses can be easily eliminated.     

Microstructural evolution during the supersolidus liquid phase sintering of nickel-based prealloyed powder mixtures

A novel concept for full-density sintering is described. Two prealloyed powders with slight compositional differences are tailored to separate the solidus temperatures into high-melt and low-melt compositions. A mixture of these two powder compositions allows full-density sintering at a temperature between the two solidus temperatures. For these experiments, the two powders were nickel-based alloys, where the low-melt powder contained boron. The mixed powders were sintered at temperatures above the solidus of the low-melt powder to form a transient liquid that promoted rapid densification of the mixture. Microstructure evolution during sintering was assisted using quenching experiments. Variables in this study included the heating rate, peak temperature, hold time, and powder ratio. Interdiffusion between the two powders controls microstructure evolution, with a dominant role associated with boron diffusion and reaction. The transient liquid phase responsible for densification is linked to boron diffusion and subsequent compound precipitation.     

Effect of metal oxide precursor on sintering shrinkage, microstructure evolution and electrical properties of silver-based pastes

Since the shrinkage behavior of silver-based films has been correlated with the characteristics of oxide additives used, the relative role of metal oxides and metal-organic precursors in sintering shrinkage and microstructure evolution of silver films was investigated and compared in this work. Films with an oxide powder additive exhibit two-stage shrinkage behavior in contrast to one-stage continuous shrinkage, which occurs in silver films with metal-organic precursors, added. Furthermore, metal-organic precursors are less effective than metal oxide powders in reducing shrinkage of silver-based films. That can be reasonably explained that metal-organic precursors can be effectively decorated around silver powder to inhibit the sintering densification. The Zr-based organic precursor among the metal-organic precursors exhibits optimal retardation in sintering densification of silver film, which is probably interrelated to refractory characteristics of ZrO₂. The unique conductivity and grain growth of silver film with 1.0 wt% tungsten-organic precursor added was possibly due to the partial dissolution of W into Ag.     

Oxygen evolution during the formation and sintering of nickel-manganese oxide spinels for thermistor applications

Nickel-manganese spinel, prepared from 20 wt % NiO and 80 wt % Mn₂O₃, forms at 950°C by the intermediate formation of Mn₃O₄ with evolution of oxygen, determined by mass-spectrometric evolved gas analysis. On heating to higher temperatures, further oxygen is evolved resulting in pore formation and bloating of pressed sintered samples and anomalies in their densification and electrical properties. Thermodynamic considerations and X-ray diffraction intensity measurements suggest that all the Mn²⁺ is located in the tetrahedral spinel sites, this cation configuration remaining unchanged by higher-temperature treatments such as sintering.     

Phase transformation and microstructure evolution of pearlite heat-resistant steel during heating

ABSTRACT

The phase transformation and microstructure evolution of pearlite heat-resistant steel during heating were observed with an ultra-high temperature confocal scanning laser microscope. The α-ferrite completely disappeared earlier than Fe₃C during the formation of γ-austenite, which is inconsistent with the fact that the Fe₃C should disappear completely earlier under equilibrium conditions. After the Fe₃C?+?α→γ transformation, static recrystallisation of γ-austenite occurred, accompanied by the dissolution of cementite. During the γ→δ transformation, the δ-cell first precipitated at the triple point of the γ-austenite grain boundaries, and then the δ-cell platelet with one tip appeared in the γ-austenite grain. The law of δ/γ inter-phase boundaries was analysed based on inter-phase boundary types and element diffusion.     

Structural evolution in ZrO2-Al2O3 gels during sintering

The crystallization sequence and phase transformation of inorganic ZrO₂-Al₂O₃ gels during heating has been investigated. The effects of incorporating CaO, MgO and Al₂O₃ into gels are reported. The microstructural evolution and crystallization of the gels during firing were observed by TEM, and the gradual disappearance of homogeneity in the gel was found to be related to thermal processing conditions.     

Ni-Al喷涂层反应烧结的组织演变与结合机制

为了改进热喷涂层组织结构和结合性能，在不引起反应的条件下，将Ni-Al混合粉（93%Ni 7?）喷涂于A3钢基材表面，通过中温反应烧结处理，制备了具有良好结合性能的涂层试样，利用SEM和XRD研究了涂层烧结前后显微组织和相结构的变化。结果表明：反应烧结前后涂层显微组织发生了明显变化，烧结后涂层所固有的层状结构消失；烧结时涂层中Al首先与Ni反应生成Ni2Al3，随处理温度增高而转化为NiAl3，基材界面出现冶金结合，并在界面愈合区间发现了Ni-Fe固溶体。     

Simulation of microstructure evolution during pulsed-laser annealing and ablation of solid mnterinlc

Summary Microstructure evolution in an amorphous layer of Si (a-Si) irradiated by pulsed lasers has been investigated using a two-dimensional heat-conduction, phase-change, and microstructure simulation model. The approach has several unique features for describing nonequilibrium phase transformations under the extreme conditions of pulsed-laser annealing and ablation, e.g., the concepts of the state diagram and the state array. The approach has been used to study several problems related to the annealing and ablation of solids by pulsed lasers, e.g., (i) dynamics of explosively propagating buried liquid layers during pulsed-laser annealing of an a-Si layer on a c-Si substrate; (ii) dynamics of Si flake nucleation and growth and the resulting final microstructures for various laser energy densities;, (iii) ablation due to absorption by localized but extended defects such as occurs in the case of MgO; (iv) particulate ejection during laser ablation and matrix-assisted laser desorption ionization (MALDI). The results from the modeling and simulation are compared to experimental measurements and observations. Quantitative and qualitative agreement between experiment and theory demonstrate that the approach can be applied to a variety of problems and for many different materials.     

Molecular dynamics modeling of microstructure evolution during growth of amorphous carbon films

Summary Classical molecular dynamics simulations, using Brenner's bond-order interatomic potential model, is used to study the bonding microstructure formation during quench from liquid and during growth on a diamond surface. For a 64-atom quench simulation we find 56 sp³- and 8 sp²-bonded carbon atoms, in qualitative agreement with tight-binding simulations. The growth of amorphous carbon films was simulated by depositing carbon and hydrogen atoms onto a diamond surface at energies up to 100 eV The simulated films are amorphous with a maximal density near the deposition energies (20–40 eV) used to grow films on magnetic disks. Lower deposition energies yield open graphitic structures, while much higher deposition energies cause the surface to ablate, leading to a poorly defined interface. The hardness calculated from the densest simulated films is about twice that found experimentally.