Complex Microstructural Changes Research Articles

Nano-Grain Formation in a Fully Ferritic SUS 316L Austenitic Stainless Steel Produced by HS-PM Process

In this paper, the high strain powder metallurgy (HS-PM) process was applied to an SUS 316L stainless steel powder, and the microstructural changes during the process are discussed. The HS-PM process is applied to an SUS 316L austenitic stainless steel. In the case of the SUS 316L stainless steel, room temperature recrystallization and recovery of an austenite phase take place because of the increased high angle grain boundary area and the existence of excess vacancies, which are stored during the milling process. Very fine ferrite grains are formed in the early stage of the milling. Although the amount of the α' martensite is not so large even after 98% cold rolling of the parent material, the extremely high density of defects introduced by the milling process increased free energy of the austenite phase, and thereby stabilized the α phase. In the late stage of milling, or in the case of higher energy milling, fully ferritic nano grain structure with an average grain size of approximately 20nm is formed. During heating, the fully ferritic powder showed complex microstructural changes. The powder compact sintered at 1173K for 3.6ks showed (α'+ M 2 C) microduplex structure with an average grain size of 250nm.

Modeling the microstructural changes during hot tandem rolling of AA5XXX aluminum alloys: Part II. Textural evolution

A comprehensive mathematical model of the hot tandem rolling process for aluminum alloys has been developed. Reflecting the complex thermomechanical and microstructural changes effected in the alloys during rolling, the model incorporated heat flow, plastic deformation, kinetics of static recrystallization, final recrystallized grain size, and texture evolution. The results of this microstructural engineering study, combining computer modeling, laboratory tests, and industrial measurements, are presented in three parts. In this Part I, laboratory measurements of static recrystallization kinetics and final recrystallized grain size are described for AA5182 and AA5052 aluminum alloys and expressed quantitatively by semiempirical equations. In Part II, laboratory measurements of the texture evolution during static recrystallization are described for each of the alloys and expressed mathematically using a modified form of the Avrami equation. Finally, Part III of this article describes the development of an overall mathematical model for an industrial aluminum hot tandem rolling process which incorporates the microstructure and texture equations developed and the model validation using industrial data. The laboratory measurements for the microstructural evolution were carried out using industrially rolled material and a state-of-the-art plane strain compression tester at Alcan International. Each sample was given a single deformation and heat treated in a salt bath at 400 °C for various lengths of time to effect different levels of recrystallization in the samples. The range of hot-working conditions used for the laboratory study was chosen to represent conditions typically seen in industrial aluminum hot tandem rolling processes, i.e., deformation temperatures of 350 °C to 500 °C, strain rates of 0.5 to 100 seconds and total strains of 0.5 to 2.0. The semiempirical equations developed indicated that both the recrystallization kinetics and the final recrystallized grain size were dependent on the deformation history of the material i.e., total strain and Zener-Hollomon parameter (Z), where \(Z = \dot \varepsilon exp \left( {\frac{{Q_{def} }}{{RT_{def} }}} \right)\) and time at the recrystallization temperature.

Evaluation of the nucleation and coarsening kinetic behavior of the secondary hardening carbide of Fe-HCo-10Ni-1Mo-0.16C steel at two chromium levels, using an analytical and modeling approach: Part II

The effect of 1 % Cr addition on the resistivity during preaging and peak aging of an AF1410 based steel was examined by analytical and modeling approaches. The increased kinetics of aging resulting from a chromium addition, due to a variety of complex microstructural changes,was analyzed by Avrami-Mehl and Wert-Zener formulations, which were modified so that the exponent was a function of time. Using a eneralized Avrami equation, it was shown that the nucleation rate, N(t), at short aging times was large but became a rapidly declining function as N(t) approaches zero in a supersaturated system. A mathematical solution, using AF1410 steel carbide growth data at 510 °C, confirmed reported experimental evidence of second-stage carbide nucleation at 1 to 2 h.

Dynamic imaging of structural changes in silver catalysts by environmental scanning electron microscopy.

Polycrystalline silver catalysts are used extensively for the partial oxidation of methanol to formaldehyde, which is then primarily incorporated in the synthesis process for adhesives and resins. In order to maximize formaldehyde production it is essential to gain a comprehensive understanding of the complex microstructural changes which occur in the catalyst during reaction conditions. However, conventional electron microscopic techniques are incapable of imaging catalysts at high temperatures and in the presence of a gaseous atmosphere. Therefore, an environmental scanning electron microscope (ESEM) has been used to image polycrystalline silver catalysts during simulated industrial conditions. The most dramatic effect of heating various catalysts to 700 degrees C in the ESEM chamber was the formation of "pinholes" in the silver surface. These "pinholes" occur at specific temperatures and are inherently associated with the catalytic process, resulting from near-surface explosions caused by subsurface hydroxyl recombination. Of particular interest was the nature and location of the holes, which preferentially occur in the vicinity of surface defects such as platelets and edge structures. To the best of our knowledge, this study represents the first time that the progress of a catalytic reaction has been observed under in situ conditions by scanning electron microscopy.

Measurement of Lateral Dopant Diffusion in Rapid Thermal Annealed W-Polycide Gate Structures

AbstractLateral dopant diffusion is a well known problem in dual-gate W-polycide CMOS devices. We have recently demonstrated that RTA processing helps to alleviate this problem and at the same time ensures sufficient dopant activation. However, due to the complex micro-structural changes in both poly-Si and WSix (x˜2.5) layers during the RTA process, the time dependence of the diffusion processes and dopant distribution are difficult to predict. Consequently, the process optimization and device simulations are rather unreliable. We describe a new experimental technique to measure lateral dopant diffusion and 2-dimensional dopant distribution in RTA processed W-polycide structures using conventional SIMS analysis of lithographically defined test structures. Our experiments show that the technique is capable of measuring lateral dopant diffusion over distances between one and tens of microns without losing the vertical resolution of conventional SIMS profiling. The technique can be used to study diffusion processes in a variety of materials and multi-layer structures.

Sintering and Heat Treatment of Steels Made from A Partially Prealloyed Iron Powder

Abstract Compacts of partially prealloyed steels, containing 1·75%Ni, 1·5%Cu, 0·5%Mo, and 0, 0·3, or 0·5%C, with initial porosities of about 6 or 15%, were sintered at 1150°C for times ranging from 0·25 to 128 h and cooled through the critical range at about 30 K min −1. The chemical heterogeneity of the compacts, which was very marked after short sintering times, gradually decreased with increasing sintering time, but complete homogeneity was not achieved in 128 h. The changes in microchemistry with elapse of sintering time brought about complex microstructural changes in the relatively slowly cooled compacts. These caused the mechanical properties of the compacts to vary in a complex manner with sintering time. Optimum properties of sintered and relatively slowly cooled compacts were obtained after sintering times of ∼ 1 h, when the compacts were still very heterogeneous. Other compacts that had been sintered for 1 h were subjected to conventional austenitizing, quenching, and tempering treatments. The ...

Thermal Properties of Frozen Peas, Clams and Ice Cream

Thermal properties of ice cream and bulk-stored frozen peas were determined experimentally. Thermal conductivity was determined for each product as a function of temperature and bulk density. Enthalpy-temperature data were collected for each product using calorimetric procedures. The freezing energy requirement for stuffed clams under commercial operating conditions was also determined. Thermal conductivity of ice cream was found to increase with increasing temperature and bulk density. Conductivity results for frozen peas showed no significant trend with either variable. Frozen foods of porous or granular bulk-stored nature, appear to undergo complex microstructure changes with temperature, and further research on these effects is appropriate. Enthalpy-temperature data showed characteristic trends. Energy requirement for freezing stuffed clams was low due to a low overall moisture content.

Phenothiazine : Food Industries, Oct. 1935, P. 492

The complex microstructural changes following superplastic deformation in statically and continually recrystallizing Al-alloys are examined. Al alloy, AA 2004, exhibits continual recrystallization during deformation, whereas in two other alloys, AA 5456 and AA 7020, static recrystallization leads to the formation of equiaxed grains prior to the onset of superplastic deformation. Other features such as grain growth, grain rotation, dislocation accumulation and their influence on texture evolution during superplastic deformation are investigated. The observations clearly suggest that grain boundary sliding is the rate controlling process during steady state superplastic deformation.