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Characterization of the microstructure of high-chromium cast irons using Mössbauer spectroscopy
(2001)
The finish grinding process for cement materials uses ball mills with differently sized grinding balls. Because the grinding takes place through the impact and friction of the balls, balls with a high wear resistance are desirable and the materials of choice are the high-chromium white cast irons. This article examines the behavior of these cast irons when subjected to various heat treatments. The emphasis is on the dependence of the microstructure properties on the chemical composition of the alloys and the heat treatment parameters. The content of retained austenite and that of carbon in the martensite phase were determined by the use of Mössbauer spectroscopy. The results verify that a higher proportion of retained austenite with low carbon content and a martensite with higher hardness produce a material with good fracture toughness and resistance to abrasive wear.
Stable isotope ratios and trace element concentrations of fossil bones and teeth are important geochemical proxies for the reconstruction of diet and past environment in archaeology and palaeontology. However, since diagenesis can significantly alter in vivo incorporated isotope signatures and elemental compositions, it is important to understand alteration processes. Here, we present the results of in vitro alteration experiments of dental tissues from a modern African elephant molar in aqueous solutions at 30 °C and 90 °C for 4 to 63 days each. Dental cubes with ≈3 mm edge length, comprising both enamel and dentin, were placed into 2 ml of an acidic (HNO3) aqueous solution (pH ≈1) enriched with different isotopes (25Mg, 44Ca, 67Zn, 86Sr). Element and isotope distribution profiles across the different dental cubes were measured with LA-(MC-)ICP-MS and EMPA, while potential changes of the bioapatite crystal structure were characterised by Raman spectroscopy. Isotope ratios measured by LA-(MC-)ICP-MS revealed an alteration of the outer ≈200-300 μm of the enamel in all experiments. Dentin was fully altered after one week (at 90 °C) and the tracer solution started to penetrate through the dentin even into the innermost enamel. However, the central part of the enamel remained unaltered. The Raman spectra suggest a strong recrystallization in the dentin and in the outer ≈40 μm of the enamel and a partial demineralisation of the outer rim of the cubes. Our results indicate that independent of time, temperature or low initial pH, enamel apatite shows a high resistance against the experimental alteration in structure and isotopic composition, in contrast to dentin apatite.
Stable isotope ratios and trace element concentrations of fossil bones and teeth are important geochemical proxies for the reconstruction of diet and past environment in archaeology and palaeontology. However, since diagenesis can significantly alter primary diet-related isotope signatures and elemental compositions, it is important to understand and quantify alteration processes. Here, we present the results of in-vitro Alteration experiments of dental tissues from a modern African elephant molar reacted in aqueous solutions at 30 °C and 90 °C for 4 to 63 days. Dental cubes with ≈ 3 mm edge length, comprising both enamel and dentin, were placed into 2 mL of acidic aqueous solution enriched in different isotopes (25Mg, 44Ca, 67Zn, 86Sr, initial pH 1). Element and isotope distribution profiles across the reacted cubes were measured with LA-(MC-)ICP-MS and EMPA, while potential effects on the bioapatite crystal structure were characterised by Raman spectroscopy. In all experiments isotope ratios measured by LA-(MC-)ICP-MS revealed an alteration of the enamel in the outer ≈ 200–300 μm. In contrast, dentin was fully altered (≈ 1.4 mm) after one week at 90 °C while the alteration did not exceed a depth of 150–200 μm during the 30 °C experiments. Then, the tracer solution started also to penetrate through the enamel-dentin junction into the innermost enamel, however, leaving the central part of the enamel unaltered, even after three months. The Raman spectra suggest an initial demineralisation in the acidic environment while organic matter (i.e. collagen) is still preserved. In the 90 °C experiment, Raman spectra of the v1 PO4) band of the dentin shift over time towards synthetic hydroxylapatite patterns and the Ca (and Sr) concentrations in the respective solutions decrease. This indicates precipitation of newly formed apatite. Isotope and element concentration profiles across the dental tissues reveal different exchange mechanisms for different isotope systems. Magnesium is leached from enamel and dentin, while Zn is incorporated into the apatite crystal structure. However, the distribution of both elements is not affected in the innermost enamel where their concentrations do not change over the whole duration of the experiments. We found no correlation of reaction depth in the cubes and experimental duration, which might be caused by natural variability of the dental material already at the beginning of the experiment. Our alteration experiments in a closed system at high temperatures ≤90 °C and low initial pH demonstrate that at least the central part of mm-thick mammalian enamel apatite seems to be resistant against alteration preserving its pristine bioapatite mineral structure as well as its in-vivo elemental and isotopic composition. The experiments assess diagenetic alteration in a novel multi-proxy approach using in-situ analyses in high spatial resolution. It is demonstrated that the isotopes of Ca, Sr, Zn and
Mg in the dentin are prone for diagenetic alteration, while enamel is more resistant against alteration and could be used for dietary and physiological reconstructions in fossil teeth.