Spherical micron-sized polymer particles coated with magnetite have been prepared by a new method consisting of a two step process, in which iron compounds are coated on a micron-sized polymer core by controlled hydrolysis of FeCl 3 in the presence of urea, poly(vinylpyrrolidone) and hydrochloric acid. The thickness of the coating layer could be altered by adjusting the amount of FeCl 3 added to the system. Magnetic particles were obtained by treating the coated powders with hydrogen at moderate temperatures. The chemical composition and morphology of the coating layer, and the resulting magnetic properties varied with the calcination conditions.
Ti15V7Al alloy, which has a composition exhibiting peculiar phenomena upon tempering, was prepared and its martensitic transformation behavior during cooling and heating was investigated. The structure of the quenched specimen mostly consisted of the ¢-phase and a small quantity of ¡AA martensite near the grain boundaries. When heating an elastically bent strip of the specimen, spontaneous bending occurred, as reported for some other alloys. Subzero treatment using LN 2 newly induced some martensites around the prior martensites formed by quenching, but no martensites were formed in the single ¢ region. The formation of the martensites by the subzero treatment exhibited time dependence. Even tempering at 550°C for 3 s, induced the formation of coarse martensites throughout the specimen. All martensites formed by the quenching, the subzero treatment or the tempering disappeared completely upon heat treatment at 200°C for 300 s, resulting in a single ¢phase. However, the coarse martensites were regenerated from the single ¢-phase by tempering at 550°C for a short time, which means that the martensite behavior in the range of 200550°C is reversible. Continuous isothermal aging at 550°C led to marked hardening through the process ¢ ¼ coarse ¡AA ¼ fine ¡AA ¼ ¢ + fine ¡. Both an M S curve and a free-energy model, which can explain the martensite formation at low and high temperatures and the annihilation at 200°C, are proposed.
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