Microstructure Formation Process Research Articles

Fractal-type relations and extensions suitable for systems of evolving polycrystalline microstructures

We report on two possible physically justified situations concerning the evolution of polycrystalline microstructures. The first is an observation that a polycrystalline microstructure formation process, considered as a random walk in the space of the crystallites’ sizes, can be equivalent to the anomalous kinetic problem in a continuum percolation space. The second, in turn, regards some extension of the random walk process while occurring on a fractal substrate and/or when a single grain boundary can presumably be treated as a random walk trajectory, i.e. a situation quite acceptable when dealing with, e.g. quasicrystals. One may find possible applications of the modeling offered in the areas of biophysics and physical metallurgy.

Development of the microstructure of the silicon nitride based ceramics

Basic regularities of silicon nitride based materials microstructure formation and development in interrelation with processing conditions, type of sintering additives, and starting powders properties are discussed. Models of abnormal or exaggerated grain growth are critically reassessed. Results of several model experiments conducted in order to determine the most important factors directing the microstructure formation processes in RE-fluxed Si3N4 ceramics are reviewed. Existing data on the mechanisms governing the microstructure development of Si3N4-based ceramics are analyzed and several principles of microstructure tailoring are formulated.

Extension of the primary solidification region of Nd2Fe14B by levitation of undercooled melts

Undercooled Nd–Fe–B melts with compositions near the Nd2Fe14B phase (Φ phase) were studied by the electromagnetic levitation technique. In situ measurements of the solidification kinetics were accomplished and melt drops with controlled undercooling were quenched onto chill substrates in order to reveal the solidification sequence and microstructure formation processes. Crystal growth velocities of the Φ phase of the order of 1 mm s−1 were estimated from in situ measurements. Consistent with the nucleation theory, which predicts primary γ-Fe phase crystallization up to 650 K undercooling for the stoichiometric alloy, γ-Fe nucleation could not be avoided during undercooling but the quenching of the melt drops led to dissolution of the γ-Fe nuclei during Φ-phase formation. The γ-Fe phase growth was suppressed in a seam adjacent to the chill surface, whereas Fe dendrites occurred in the remaining parts of the quenched samples. Reduced solute rejection on growth is believed to be responsible for the preferred metastable Φ-phase formation from the undercooled melt on chill substrates. The investigation gives some clues to understand different microstructures and magnetic properties of powder precursors for permanent magnets obtained from different rapid solidification routes.

Manganese-Zinc-Ferrites with Improved Magnetic and Mechanical Properties

The 3-component system of manganese-zinc-ferrites is being thoroughly investigated as regards its intrinsic properties. The macroscopic properties of these spinel are variable within broad limits over the process of microstructure formation. Oxides of the 3d and 4d elements are frequently used as additives to determine the temperature dependence of the initial permeability and the electrical conductivity of manganese-zinc-ferrites. Additives for acceleration of sintering based on oxides of the elements of the third, fourth and fifth main group of the periodic system require an improved knowledge about the process of microstructure formation of 8 - 10 components systems especially as regards grain growth, microchemistry and the structure of the grain boundary. Due to the increased application of silicon power semiconductors (IGBT, power MOSFET) high demands are made as well on the mechanical properties of ferrite components, especially on tensile strengths, crack toughness and thermal shock resistivity. Proceedings from the basics of linearly elastic fracture mechanics experimental results are presented as regards increase of tensile strength, reduction of the mechanical stress dependence of the initial permeability and improvement of the thermal shock resistivity of manganese-zinc-ferrites for commercial applications. An interpretation of the microstructure-properties ratio is carried out by means of microstructural and microchemical investigations using the high-resolving transmission electron microscopy.

へプタン添加Ｔｉ‐Ａｌメカニカルアロイング材の組織形成過程と結晶粒微細化

へプタン添加Ｔｉ‐Ａｌメカニカルアロイング材の組織形成過程と結晶粒微細化

Microstructure formation during crystallization of immiscible phases

For composite materials such as metal-oxide, metal-ceramics, etc., the formation process of grain microstructure has been theoretically considered from the macrokinetic point of view. The interaction (mainly a thermal one) between two immiscible phases during crystallization was studied. Various types of grain-size distribution were found and distinguished. A comparison with an experimental metallographic specimen of a metal-oxide system was made. Self-propagating high-temperature synthesis was used to produce the material ingots. Limits to the use of the model proposed are revealed.

Microstructure formation in polyblends containing liquid crystalline polymers

Based on rheological data, two polyblends of poly(ether sulfone) (PES) and polycarbonate (PC) with a liquid crystalline polymer (LCP) as the dispersed phase were prepared by extrusion. The process of LCP microstructure formation was investigated by studying the morphology of the blend obtained from different sections of the extruder. The deformation of LCP domains is controlled by the viscosity ratio of the dispersed phase to the continuous phase (ηdηm). LCP domains in LCP/PES blend have been deformed into oriented fibrils, due to a viscosity ratio of 0.01. A quantitative comparison of LCP droplet sizes before and after the extruder die shows that the morphology of extrudates has been determined before the melt enters the die. After the die, the diameter and length to diameter ratio of LCP fibrils were increased by coalescence and further deformation by the action of extensional flow. The morphology of LCP/PES extrudates after remaining at quiescence within the die for different time intervals indicates that even after a 60 s residence, LCP domains retained their fibrillar structure in the blend and no significant recoiling and breakup of LCP fibril occurred.

Structure formation mechanisms of titanium inflammation in a nitrogen atmosphere

Experimental results on the structure- and phase-formation dynamics in rapid pulse heating of titanium powder in a nitrogen atmosphere are presented. A relation between the main macrokinetic ignition parameters and the microstructure formation process in the system studied is found.

Microstructure changes in high-speed hot rolling of steel rods

Transmission electron microscopy (TEM), quantitative texture analysis by means of neutron diffraction (QTA) and X-ray diffraction profile analysis (XDPA) were used to study the microstructure changes occurring in high-speed hot rolling (\(\dot \varphi \)≦1500 s−1;T=1073 K) of high-alloy stainless steel X8CrTi17 (ferrite) and X8CrNiTi18.10 (austenite). The investigations indicate that at higher deformation rates recrystallization due to adiabatic overheating becomes an important process of microstructure formation.