KENJI MATSUDA, SUSUMU IKENO, HIROAKI MATSUI, TATSUO SATO, KIYOSHI TERAYAMA, and YASUHIRO UETANI Differential scanning calorimetry (DSC) curves were obtained from Al-1.0 mass pct Mg 2 Si (balanced) and Al-1.0 mass pct Mg 2 Si Ϫ0.4 mass pct Si (excess Si) alloys, and precipitates corresponding to each peak at the DSC curve were interpreted by means of high-resolution transmission electron microscopy (HRTEM) observation in order to understand the precipitation sequence of metastable phases. Five peaks were obtained on the DSC curves, from which four were exothermic (A, C, D, and E) and one endothermic (B). Upon HRTEM observation, the peaks for the excess Si alloy were explained as follows: peak A-B: Guinier-Preston (GP) zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the ЈЈ phase; peak C: ЈЈ phase and precipitation of type B; peak D: dissolution of the ЈЈ phase; precipitation of type A and Ј phase; and peak E: dissolution of the type B, type A, and Ј precipitation of the ( ϩ Si) phase. This result is quite different from that in the balanced alloy as follows: peak A-B: GP zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the parallelogram-type precipitate; peak C: parallelogram-type precipitate and precipitation of Ј phase; peak D: Ј phase, dissolution of parallelogram-type precipitate; and peak E: the -(Mg 2 Si).
The present study is concerned with the kinetics of carbon reduction of MnO and the identification of Mn carbide formed in this reduction process by the effluent gas analysis method. Mn carbide (Mn7C3) was formed at an earlier stage of reduction, and the activation energy of 217 kJ/mol nearly equal to that for the Boudouard's reaction was obtained. When carbon was consumed entirely, the reaction between MnO and Mn7C3 occurred to yield metallic manganese, and the activation energy of 259 kJ/mol corresponding to the reaction of CO2 with Mn7C3 was obtained. Mn carbide formed in the reduction process of MnO was identified to be Mn7C3 phase, by the effluent gas analysis method using the He-O2 mixture. In addition, EPMA and X-ray diffraction examinations showed the presence of Mn7C3 phase in this reduction product.
The internal structure of different types of graphite commonly found in the ductile cast iron were investigated by TEM technique for TEM samples prepared by FIB method to understand their nucleation and growth. Spheroidal and vermicular graphite were observed in the ductile cast iron with spheroidizing treatment. Flake graphite was observed in the same cast iron without the spheroidizing treatment. The spheroidal graphite had a three-fold internal structure, with an amorphous-like central region, annual rings of a layered intermediate region, and an outer region made up of large polygonal crystalline platelets in a mosaic-like structure. The vermicular and flake graphite had a similar to that of the outer region of the spheroidal graphite, in that it consisted of similar crystalline platelets.
Precipitation sequence in an Al-1.6 massMg 2 Si alloy was investigated by DSC measurement and HRTEM observation. Four exothermic peaks (A, C, D and E) and one endothermic peak (B) were obtained when samples were heated at 10 K/min. Precipitates were confirmed when samples aged to the temperature of each peak were observed using HRTEM, and were classified into several types. Products in the region between peaks A and B were classified as GP zones. Precipitates between peaks B and C were classified as the random-type precipitates. Precipitates at peak C were classified as the parallelogram-type precipitates. Precipitates at peak D were classified as metastable phase b′ . Precipitates at peak E were classified as equilibrium phase b. Although there have been many reports that the b″ phase is formed at peak C, this was not observed in the present study, rather random-type precipitates were observed. The hardness of samples heated to various temperatures was measured. The temperature that resulted in maximum hardness corresponded to the temperature that yielded the maximum amount of random-type precipitates.
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