Il Sohn

Prediction models for the yield strength of particle-reinforced unimodal pure magnesium (Mg) metal matrix nanocomposites (MMNCs)

Particle-reinforced metal matrix nanocomposites (MMNCs) have been lauded for their potentially superior mechanical properties such as modulus, yield strength, and ultimate tensile strength. Though these materials have been synthesized using several modern solid- or liquid-phase processes, the relationships between material types, contents, processing conditions, and the resultant mechanical properties are not well understood. In this paper, we examine the yield strength of particle-reinforced MMNCs by considering individual strengthening mechanism candidates and yield strength prediction models. We first introduce several strengthening mechanisms that can account for increase in the yield strength in MMNC materials, and address the features of currently available yield strength superposition methods. We then apply these prediction models to the existing dataset of magnesium MMNCs. Through a series of quantitative analyses, it is demonstrated that grain refinement plays a significant role in determining the overall yield strength of most of the MMNCs developed to date. Also, it is found that the incorporation of the coefficient of thermal expansion mismatch and modulus mismatch strengthening mechanisms will considerably overestimate the experimental yield strength. Finally, it is shown that work-hardening during post-processing of MMNCs employed by many researchers is in part responsible for improvement to the yield strength of these materials.

Crystallization control for remediation of an FetO-rich CaO-SiO2-Al2O3-MgO EAF waste slag.

In this work, the crystallization behavior of synthesized FetO-rich electric arc furnace (EAF) waste slags with a basicity range of 0.7 to 1.08 was investigated. Crystal growth in the melts was observed in situ using a confocal laser scanning microscope, and a delayed crystallization for higher-basicity samples was observed in the continuous cooling transformation and time temperature transformation diagrams. This result is likely due to the polymerization of the melt structure as a result of the increased number of network-forming FeO4 and AlO4 units, as suggested by Raman analysis. The complex incorporation of Al and Fe ions in the form of AlO4 and FeO4 tetrahedral units dominant in the melt structure at a higher basicity constrained the precipitation of a magnetic, nonstoichiometric, and Fe-rich MgAlFeO4 primary phase. The growth of this spinel phase caused a clear compositional separation from amorphous phase during isothermal cooling at 1473 K leading to a clear separation between the primary and amorphous phases, allowing an efficient magnetic separation of Fe compounds from the slag for effective remediation and recycling of synthesized EAF waste slags for use in higher value-added ordinary Portland cement.

Simulation of low carbon steel solidification and mold flux crystallization in continuous casting using a multi-mold simulator

An inverted water-cooled multi-mold continuous casting simulator was used to investigate initial solidification of low-carbon steels and crystallization of mold flux. Embedded mold thermocouples showed characteristic temperature profiles dependent on parameters including casting speed, oscillation frequency, and stroke. Higher maximum temperatures for thermocouples at higher casting speeds, higher frequencies, and lower stroke lengths were observed. The surface of the as-cast steel strips showed oscillation marks similar to those of industrially cast slabs and higher casting speeds resulted in shallower oscillation marks. The measured pitch agreed well with the theoretical pitch suggesting the multi-mold simulator to be a cost-effective alternative to pursue fundamental studies on initial solidification in the mold. Analysis of the mold flux taken between the copper mold and solidified steel shell showed highly dendritic uni-directional crystallization occurring within the flux film suggesting that the heat transfer direction is dominantly horizontal towards the water-cooled copper mold. In addition, the solidified flux located at the upper to lower part of the mold suggested morphological differences in the size and shape of the crystalline phases indicating that crystallization ratio can increase depending upon the retention in the mold and subsequently decrease radiative heat transfer as the flux traverses down the mold.

Valorization of electric arc furnace primary steelmaking slags for cement applications.

To produce supplementary cementitious materials from electric arc furnace (EAF) slags, FeO was reduced using a two-stage reduction process that included an Al-dross reduction reaction followed by direct carbon reduction. A decrease in FeO was observed on tapping after the first-stage reduction, and further reduction with a stirred carbon rod in the second-stage reduction resulted in final FeO content below 5wt%, which is compatible with cement clinker applications. The reduced electric arc furnace slags (REAFS) mixed with cement at a unit ratio exhibited physical properties comparable to those of commercialized ground granulated blast furnace slags (GGBFS). Confocal laser scanning microscopy (CLSM) was used to obtain fundamental information on the cooling characteristics and conditions required to obtain amorphous REAFS. REAFS can be applied in cement mixtures to achieve the hydraulic properties needed for commercial use.

Heat-Transfer Phenomena Across Mold Flux by Using the Inferred Emitter Technique

An investigation was carried out to study the heat-transfer phenomena across mold flux film by using infrared emitter technique (IET). With IET, it is possible to develop the mold fluxes with a liquid layer at the top and a solid layer in contact with copper mold with the degree of varying crystallization. The dynamic crystallization and melting process of the mold fluxes as well as their effects on the overall heat-transfer rate in the mold were successfully conducted. The single hot thermocouple technique (SHTT) was also employed in this investigation to study the melting and crystallization behaviors of mold fluxes for the interpretation of IET results. The results suggested that the interfacial thermal resistance between the solidified mold flux and copper mold would significantly influence the heat-transfer rate in continuous casting and the melting of the mold flux tends to enhance the overall heat-transfer rate. The technique established in this article by utilizing the IET can be well applied to the investigation of mold flux thermal properties, which in turn gives guidelines for the design of new mold flux for continuous casting.

Characteristics of medium carbon steel solidification and mold flux crystallization using the multi-mold simulator

An oscillating multi-mold simulator with embedded thermocouples was used to study the initial solidification of medium carbon steels and crystallization characteristics of the mold flux. Casting speed variations in the simulator from 0.7 m/min to 1.4 m/min at fixed oscillation frequency and stroke resulted in higher copper mold temperatures. Frequency modifications from 2.5 Hz to 5.0 Hz and stroke changes from 8.1 mm to 5.4 mm at fixed casting speeds also resulted in higher copper mold temperatures. Surface profile analysis of as-cast steel strips showed characteristic oscillation marks comparable to the narrow faces of the industrial cast slabs. The apparent effect of casting variables on the temperature and surface profiles during the solidification of the medium carbon steels could be correlated to the variations in the negative strip time and subsequent changes in the extent of mold flux infiltration. Back scattered scanning electron microscope analysis of the full length of the retrieved flux film after casting showed cuspidine crystallization ratio that increased from the upper to lower portion of the flux film. This dynamic crystallization and growth of the cuspidine phase increases as the flux is sustained at high temperatures for longer periods. Additional experiments with industrial fluxes designed for soft cooling of medium carbon steel grades showed comparable infiltration thickness of the flux, but the crystallization characteristics were significantly different, which could have a significant impact on the heat transfer rate and mechanism through the flux film.

In-Situ Observation of Crystallization and Growth in High-Temperature Melts Using the Confocal Laser Microscope

This review discusses the innovative efforts initiated by Emi and co-workers for in-situ observation of phase transformations at high temperatures for materials. By using the high-temperature confocal laser-scanning microscope (CLSM), a robust database of the phase transformation behavior during heating and cooling of slags, fluxes, and steel can be developed. The rate of solidification and the progression of solid-state phase transformations can be readily investigated under a variety of atmospheric conditions and be correlated with theoretical predictions. The various research efforts following the work of Emi and co-workers have allowed a deeper fundamental understanding of the elusive solidification and phase transformation mechanisms in materials beyond the ambit of steels. This technique continues to evolve in terms of its methodology, application to other materials, and its contribution to technology.

Understanding the Magnesiothermic Reduction Mechanism of TiO2 to Produce Ti

Titanium dioxide (TiO2) powders in the mineral form of rutile were reduced to metallic and an intermediate phase via a magnesiothermic reaction in molten Mg at temperatures between 973 K and 1173 K (700 °C and 900 °C) under high-purity Ar atmosphere. The reaction behavior and pathway indicated intermediate phase formation during the magnesiothermic reduction of TiO2 using XRD (X-ray diffraction), SEM (scanning electron microscope), and TEM (transmission electron microscope). Mg/TiO2 = 2 resulted in various intermediate phases of oxygen containing titanium, including Ti6O, Ti3O, and Ti2O, with metallic Ti present. MgTi2O4 ternary intermediate phases could also be observed, but they were dependent on the excess Mg present in the sample. Nevertheless, even with excessive amounts of Mg at Mg/TiO2 = 10, complete reduction to metallic Ti could not be obtained and some Ti6O intermediate phases were present. Although thermodynamics do not predict the formation of the MgTi2O4 spinel phase, detailed phase identification through XRD, SEM, and TEM showed significant amounts of this intermediate ternary phase even at excess Mg additions. Considering the stepwise reduction of TiO2 by Mg and the pronounced amounts of MgTi2O4 phase observed, the rate-limiting reaction is likely the reduction of MgTi2O4 to the TitO phase. Thus, an additional reduction step beyond thermodynamic predictions was developed.

Microstructural evolution in semisolid forging of A356 alloy

A semisolid forging study using A356 alloy was carried out to investigate the evolution of microstructures affected by the solid fraction, the forging pressure and the addition of Sr. The semisolid slurry for forging was made by an electromagnetic stirring method. Microstructures were evaluated at two typical positions of the semisolid forged specimens: one is the region, where direct forging pressure is applied, and the other is the region, where the slurry was squeezed and extruded indirectly. Microstructural characteristics, such as the morphology of the primary Al particles and the eutectic Si, were changed with the positions observed. As the solid fraction increased, the morphology of the primary Al particles was transformed from rosette-like to globular. As the forging pressure increased, the size of eutectic Si became fine. An increase of the forging pressure is speculated to improve the interfacial heat transfer coefficient between the semisolid slurry and the mould wall, leading to an increase of the cooling rate on the growth of eutectic Si. The addition of Sr is effective in modifying the morphology for eutectic Si, especially when the forging pressure was low.

Potential utilization of hazardous mining wastes in the production of lightweight aggregates

ABSTRACT Lightweight aggregates from mixtures of gold tailing, red mud and waste limestone were produced using a high-temperature sintering process. Raw materials were quantitatively analyzed, and hazardous materials were identified by chemical analysis. An oxide system corresponding to the major components of the mining waste was carefully designed based on the CaO–FeOx–Al2O3–SiO2 quaternary system similar to lightweight aggregates. Sintering, softening and melting behaviour of the sample was investigated to optimize the sintering conditions. After heating, specimens were analyzed to clarify its sintering mechanism. Phase change during sintering was observed by X-ray diffraction, and the effect of temperature on the structural unit changes of the specimen was confirmed by Fourier transform-infrared spectroscopy. Microscopic analysis by scanning electron microscope-energy dispersive X-ray analysis was carried out to identify the morphological changes of the sample. Physical properties of the specimen were measured, and the compressive strength was improved with less water absorption.