Interactions between iron, manganese, and the Al-Si eutectic in hypoeutectic Al-Si alloys

Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pet Si-0.5 pet Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pet Fe, but reduced porosity in alloys with 0.6 and I pet Fe. Thermal analysis and quenching were undertaken to determine the effect of iron and manganese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large beta-Al5FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation of alpha-Al15Mn3Si2 upon addition of manganese reversed these effects. It is proposed that this interaction is due to beta-Al5FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms.

Phase equilibria in high MgO ferro- and silico-manganese smelting slags

Phase equilibria have been determined experimentally for pseudo-ternary sections of the form “MnO”- (CaO+MgO)-(SiO2+Al2O3) with a fixed Al2O3/SiO2 weight ratio of 0.17 and MgO/CaO weight ratios of 0.25 and 0.17 respectively for temperatures in the range 1473-1673 K. The primary phase fields present for the MgO/CaO weight ratio of 0.17 include manganosite (Mn,Mg,Ca)O; dicalcium silicate α-2(Ca,Mg,Mn)O·SiO2; merwinite 3CaO⋅(Mg,Mn)O⋅2SiO2; wollastonite [(Ca,Mg,Mn)O·SiO2]; diopside [(CaO,MgO,MnO,Al2O3)·SiO2]; tridymite (SiO2); tephroite [2(Mn,Mg)O·SiO2]; rhodonite [(Mn,Mg)O·SiO2] and melilite [2CaO·(MgO,MnO,Al2O3)·2(SiO2,Al2O3)]. For the section with MgO/CaO weight ratio of 0.25 the anorthite phase (CaO⋅Al2O3⋅2SiO2) is also present. The liquidus temperatures of ferro- and silico-manganese smelting slags have been determined. The liquidus temperatures at low MnO concentrations are shown to be principally dependent on the modified basicity weight ratio (CaO+MgO)/(SiO2+Al2O3).

Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia

40Ar/39Ar laser incremental heating analyses of individual grains of supergene jarosite, alunite, and cryptomelane from weathering profiles in the Dugald River area, Queensland, Australia, show a strong positive correlation between a sample’s age and its elevation. We analyzed 125 grains extracted from 35 hand specimens collected from weathering profiles at 11 sites located at 3 distinct elevations. The highest elevation profile hosts the oldest supergene minerals, whereas progressively younger samples occur at lower positions in the landscape. The highest elevation sampling sites (three sites), located on top of an elongated mesa (255 to 275 m elevation), yield ages in the 16 to 12 Ma range. Samples from an intermediate elevation site (225 to 230 m elevation) yield ages in the 6 to 4 Ma range. Samples collected at the lowest elevation sites (200 to 220 m elevation) yield ages in the 2.2 to 0.8 Ma interval. Grains of supergene alunite, jarosite, and cryptomelane analyzed from individual single hand specimens yield reproducible results, confirming the suitability of these minerals to 40Ar/39Ar geochronology. Multiple samples collected from the same site also yield reproducible results, indicating that the ages measured are true precipitation ages for the samples analyzed. Different sites, up to 3 km apart, sampled from weathering profiles at the same elevation again yield reproducible results. The consistency of results confirms that 40Ar/39Ar geochronology of supergene jarosite, alunite, and cryptomelane yields ages of formation of weathering profiles, providing a reliable numerical basis for differentiating and correlating these profiles. The age versus elevation relationship obtained suggest that the stepped landscapes in the Dugald River area record a progressive downward migration of a relatively flat weathering front. The steps in the landscape result from differential erosion of previously weathered bedrock displaying different susceptibility to weathering and contrasting resistance to erosion. Combined, the age versus elevation relationships measured yield a weathering rate of 3.8 m. Myr−1 (for the past 15 Ma) if a descending subhorizontal weathering front is assumed. The results also permit the calculation of the erosion rate of the more easily weathered and eroded lithologies, assuming an initially flat landscape as proposed in models of episodic landscape development. The average erosion rate for the past 15 Ma is 3.3 m. Myr−1, consistent with erosion rates obtained by cosmogenic isotope studies in the region.

Effect of Mn deficiency and legume inoculation on rhizosphere pH in highly alkaline soils

Although plant growth is often limited at high pH, little is known about root-induced changes in the rhizospheres of plants growing in alkaline soils. The effect of Mn deficiency in Rhodes grass (Chloris gayana cv. Pioneer) and of legume inoculation in lucerne (Medicago sativa L. cv. Hunter River), on the rhizosphere pH of plants grown in highly alkaline bauxite residue was investigated. Rhizosphere pH was measured quantitatively, with a micro pH electrode, and qualitatively, with an agar/pH indicator solution. Manganese deficiency in Rhodes grass increased root-induced acidification of the rhizosphere in a soil profile in which N was supplied entirely as NO3-. Rhizosphere pH in the Mn deficient plants was up to 1.22 pH units lower than that of the bulk soil, while only 0.90 to 0.62 pH units lower in plants supplied with adequate Mn. When soil N was supplied entirely as NO3-, rhizosphere acidification was more efficient in inoculated lucerne (1.75 pH unit decrease) than in non-inoculated lucerne (1.16 pH unit decrease). This difference in capacity to lower rhizosphere pH is attributable to the ability of the inoculated lucerne to fix atmospheric N2 rather than relying on the soil N (NO3 ) reserves as the non-inoculated plants. Rhizosphere acidification in both Rhodes grass and lucerne was greatest in the meristematic root zone and least in the maturation root zone.

Preparation, characterization, and catalytic properties of clay-based nickel catalysts for methane reforming

Naturally occurring clays and pillared clays are used as supports of nickel catalysts for the methane reforming reaction with carbon dioxide to synthesis gas. The structural and textural characteristics of the supports and catalysts are systematically examined by N-2 adsorption/desorption and X-ray diffraction (XRD) techniques. It is found that the pore structures and surface properties of supports greatly affect the catalytic activities of the catalysts prepared. The catalysts supported on the mesoporous clays or pillared clays are obviously superior to those on microporous supports because the mesoporous supports are highly thermal stable compared to the microporous ones. It is found that introducing lanthanum to the supports can improve the catalyst basicity and thus enhance the catalytic activities of these catalysts. Deactivation of catalysts prepared and factors influencing their stability are also discussed. (C) 1998 Academic Press.

Measurement of grain boundary composition for X52 pipeline steel

Analytical electron microscopy was used to measure the composition of grain boundaries (GBs) and interconstituent boundaries (IBs) of X52 pipeline steel using specimens about 40-60 nm in thickness. All elements of interest were examined with the exception of carbon. With this caveat; there was no segregation at proeutectoid ferrite GBs. This indicated that the commonly expected species S and P are not responsible for preferential corrosion of GBs during intergranular stress corrosion cracking of pipeline steels. Manganese was the only species measured to segregate at the IBs. Manganese segregated to the IBs between proeutectoid ferrite and pearlitic cementite, and desegregated from IBs between proeutectoid ferrite and pearlitic ferrite. The pearlitic cementite was Mn rich. There was no Mn segregation at the IBs between pearlitic ferrite and pearlitic cementite. The pattern of Mn segregation could be explained in terms of diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.