Papers by Keyword: Coarsening

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Authors: Felipe Hernandez-Santiago, Victor M. Lopez-Hirata, Maribel L. Saucedo-Muñoz
Abstract: A study of the coarsening process of the decomposed phases was carried out in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100> Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the morphology of the Ni-rich phase changed to ellipsoidal. The variation of mean radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the modified LSW theory for ternary alloys.
Authors: Yancy W. Riddle, Thomas H. Jr. Sanders
Authors: Merton C. Flemings
Abstract: A summary is given of the history of our understanding of dendrite coarsening, including particularly fragmentation. Much is now understood about this process as it takes place in directional solidification of a quiescent melt. Much less is understood about it in the rapidly cooled, turbulent environment of semi-solid casting. The importance of dendrite fragmentation in semi-solid processing is that it is key to obtaining fine final grain size, grain spheroidicity and rapid production rate. I have chosen in this keynote paper to talk about the fundamentals of an important part of the semisolid casting process ... that of “dendrite fragmentation.” The paper is written with an eye to its possible practical usefulness to researchers in process innovation. If we understood the dendrite fragmentation mechanism better, could we achieve finer, more numerous, grains than we do now? Could fully non dendritic structures be obtained industrially in short processing times?
Authors: Andrew Godfrey, O.V. Mishin, T.B. Yu
Abstract: Some methods for quantitative characterization of the microstructures deformed to large plastic strains both before and after annealing are discussed and illustrated using examples of samples after equal channel angular extrusion and cold-rolling. It is emphasized that the microstructures in such deformed samples exhibit a heterogeneity in the microstructural refinement by high angle boundaries. Based on this, a new parameter describing the fraction of regions containing predominantly low angle boundaries is introduced. This parameter has some advantages over the simpler high angle boundary fraction parameter, in particular with regard to data collected from electron-backscatter diffraction investigations, where boundaries with very low misorientation angles cannot be reliably detected. It is shown how this parameter can be related to the recrystallization behavior. Another parameter, based on mode of the distribution of dislocation cell sizes is outlined, and it is demonstrated how this parameter can be used to investigate the uniformity, or otherwise, of the restoration processes occurring during annealing of metals deformed to large plastic strains.
Authors: Francisco Carlos Robles Hernandez, Leonardo González-Reyes, I. Hernández-Pérez
Abstract: In the present paper TiO2 (anatase) nanoparticles were synthesized by ultrasonic radiation proving the potential of this method. The synthesized anatase is heat treated at a temperature of 500°C in open air to coarse it using times from 1 h to 72 h. The heat treatment conditions were selected to prevent phase transformation and to solely sponsor coarsening of anatase from 6.2 nm to 28.3 nm. The synthesized and heat treated anatase were characterized using Electron Microscopy (Transmission and Scanning), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method, UV-vis, Raman and Infrared spectroscopy. In the preset work is presented an analysis of anatase that is complemented by the agreement in the different characterization methods. This helps in the understanding of anatase coarsening as a function of heat treatment time and grain size, being the late a more suitable approach. This work opens up new perspectives to produce synthetic nanoparticulate anatase with potential for various applications.
Authors: Suk Won Kang, Dock Young Lee, Ki Bae Kim
Abstract: In order to produce a high quality and cost effective part in the rheocasting process for an automobile industry it has been important to develop a slurry-on-demand process, which can manufacture the semi-solid slurry having a fine and globular structure of primary solid phase. The morphology of primary solid phase of semi-solid slurry is coarsened and globularized during an isothermal holding process in a solid-liquid region. Accordingly, in this paper, the microstructural evolution of semi-solid slurry during the isothermal holding at a various temperature in solid-liquid region was investigated to examine a coarsening and globularization behavior of primary solid phase. The semi-solid slurry of Al alloy was produced in a slurry maker using a multiplex-type slurry cup that we developed recently. A size of primary solid phase was the finer at the higher holding temperature, but under a constant holding time a roundness degree of primary solid phase was the lower at the lower holding temperature. Also, a coarsening rate of primary solid phase was not considered to be affected significantly by a holding temperature even though a little lower coarsening rate was obtained at the higher holding temperature.
Authors: S. Ashouri, Mahmoud Nili-Ahmadabadi
Abstract: Semi-solid forming (SSF) involves alloys with non-dendrite microstructure that contain spherical solid particles in the liquid matrix. This process is generally divided into three main steps: feedstock manufacturing, reheating and forming. Feedstock has the main effect on the cost and quality of product. Many researches have been carried out to reduce the cost of feedstock manufacturing. Slope plate is a simple semi-solid process that can reduce the cost of feedstock. In this study a Cu plate with water and without water circulation was used to investigate the effect of pouring temperature, cooling rate during casting and solidification in the mold on the microstructure. Cast ingots with optimized microstructure were reheated in various time and temperatures in semi-solid region to obtain kinetics of globularization and solid grain growth.
Authors: S. Hossein Nedjad, Mahmoud Nili-Ahmadabadi, Tadashi Furuhara, Tadashi Maki
Abstract: Fe-Ni-Mn maraging alloys show discontinuous coarsening of fct θ-NiMn precipitates along prior austenite grain boundaries (PAGBs) during isothermal aging. Heavy cold rolling of a solution annealed Fe-10Ni-7Mn (wt. %) maraging alloy and subsequent aging treatment at 773 K led to the formation of a nanostructured material. Hardness measurement and transmission electron microscopy were used to study the aging behaviour and microstructural evolution of the nanostructured alloy. At the early stage of aging, an ultrafine grained structure was observed with fine matrix precipitates. However, at later stages of aging, severe coarsening of precipitates was found. At that stage, a lamellar microstructure composed of finely precipitated ferrite laths and ultrafine ferrite grains were identified. Coarse fct θ-NiMn precipitates were identified at the ultrafine ferritic grain boundaries. Development of the coarsening reaction was found to increase the ultrafine grained ferritic regions with prolonged aging.
Authors: Siri Harboe, Vijenthan Sothyratnam, Michael Modigell, Annalisa Pola
Abstract: In this work, the effect of stirring conditions on agglomeration and coalescence-coarsening in isothermal, globulitic, semisolid AlCu10%wt was investigated. It is shown that for the investigated system, a shear rate regime exists which promotes crystallographic alignment of the agglomerated grains, leading to a rapid coalescence process, thereby the formation of rosettes. It is also experimentally validated that for lower shear rates, the size and porosity of aggregates are increased compared to that at higher level of shear.
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