Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-'%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 mum s(-1,) in a temperature gradient between 10 and 20 K mm(-1). After directional solidification, the composition of each specimen varied dramatically, from 32'%Al in the region that had remained solid to 18%Al (32 mum s(-1) specimen) and 13%Al (580 mum s(-1) specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. (C) 2003 Maney Publishing.

Volcanic resurfacing and the early terrestrial crust: Zircon U-Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland

The oldest known bona fide succession of elastic metasediments Occurs in the Isua Greenstone Belt. SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for 30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon, Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3,71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age. The mica schist hosted zircons have U/Pb ages. Th/U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed, Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking oft Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt Outpouring,;, This process Causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope corn posit ion,, potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments. (C) 2005 Elsevier B.V.All rights reserved.

An evaluation of thermal plasmas in pyrometallurgy

The development of reliable, high powered plasma generators has resulted in many plasma processes being proposed as alternatives to existing pyrometallurgical technologies. This work evaluates the advantages and disadvantages of plasma systems by reviewing plasma generators, their integration with reactors and the process economics. Many plasma systems were shown to be technically and economically superior to existing technologies, but some of the plasma system advantages quoted in the literature were found to be impractical because of other system constraints. Process applications were limited by the power inputs available from plasma generators compared to AC electric furnaces. A series of trials were conducted where chromite and steelplant baghouse dusts were smelted in the Tetronics' 2.0 MW transferred arc/open bath reactor to confirm the operating characteristics of the plasma system and its economics. Chromite smelting was technical superior to submerged arc furnace technology, but the economics were unfavourable because of the limited power available from the water-cooled plasma torch and the high electrical energy consumption. A DC graphite electrode plasma furnace using preheated and prereduced chromite concentrates will compete economically with the submerged arc furnace. Ni, Cr and Mo were economically recovered from high alloy content steelplant dusts for recycling. Five Electric Arc Furnace dusts were smelted to produce a non-toxic residue and recover the contained zinc to an enriched zinc oxide product for recycling. It should be possible to condense the zinc vapour directly in a zinc splash condenser to increase the value of the product. Because of the limited power available from plasma generators, plasma processes will be most suitable for treating high and medium value materials such as Au, Pt, Mo, Ni, Ti, V, Cr etc at small production rates, heating metals in tundishes and ladles and remelting superalloy scrap. The treatment of environmentally hazardous waste materials is a particularly interesting application because of the additional financial incentives. Non-transferred arc plasma generators will be used for air and gas preheating in blast furnaces to reduce metallurgical coke consumptions.

Influência do número de refundições da liga Ni-Cr-Mo-Ti sobre as propriedades de próteses metalocerâmicas

A frequently encountered difficulty in oral prosthetics is associated with the loss of metallic alloys during the melting stage of the production of metal-ceramic replacement systems. Remelting such materials could impar their use in oral rehabilitation due to loss in esthetics, as well as in the chemical, physical, electrochemical and mechanical properties. Nowadays, the Ni-Cr-Mo-Ti alloy is widely used in metal-ceramic systems. Manufacturers state that this material can be remelted without significant alterations in its behavior, however little has been established as to the changes in the performance of this alloy after successive remelting, which is common practice in oral prosthetics. Therefore, the objective of this study was to evaluate possible changes in the esthetics and associated properties of metalceramic samples consisting of Ni-Cr-Mo-Ti and dental porcelain. Three to five remelting steps were carried out. The results revealed that Ni-Cr-Mo-Ti can be safely used even after three remelting steps. Further remelting significantly affect the characteristics of the alloys and should not be recommended for the manufacture of metal-ceramic systems

Efeito das variáveis laboratoriais protéticas na adesão da porcelana com ligas de níquel-cromo

The metalceramic crowns are usually used in dentistry because they provide a resistant structure due to its metallic base and its aesthetics from the porcelain that recovers this structure. To manufacture these crowns, a series of stages should be accomplished in the prosthetic laboratories, and many variables can influence its success. Changes in these variables cause alterations in the metallic alloy and in the porcelain, so, as consequence, in the adhesion between them. The composition of the metal alloy can be modified by recasting alloys, a common practice in some prosthetic laboratories. The aim of this paper is to make a systematic study investigating metalceramic crowns as well as analyzing the effect of recasting Ni-Cr alloys. Another variable which can influence the mechanism of metalceramic union is the temperature used in firing porcelain procedure. Each porcelain has to be fired in a fixed temperature which is determined by the manufacturer and its change can cause serious damages. This research simulate situations that may occur on laboratory procedures and observe their consequences in the quality of the metalceramic union. A scanning eletron microscopy and an optic microscopy were accomplish to analyse the metal-ceramic interface. No differences have been found when remelting alloys were used. The microhardness were similar in Ni-Cr alloys casted once, twice and three times. A wettability test was accomplished using a software developed at the Laboratório de Processamento de Materiais por Plasma, on the Universidade Federal do Rio Grande do Norte. No differences were found in the contact angle between the solid surface (metallic substratum) and the tangencial plane to the liquid surface (opaque). To analyse if the temperature of porcelain firing procedure could influence the contact area between metal and porcelain, a variation in its final temperature was achieve from 980° to 955°C. Once more, no differences have been found