Application of the Mechanical Alloying Technique to the Construction of Low-temperature Phase Diagrams

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A technique, so called mechanical alloying, has attracted much attention as a new method to produce metastable materials through not only the solid-solid but also solid-gas reactions. Powders subjected to ball milling are heavily cold worked and energized to form metastable phases. The energy accumulated during ball milling is believed to be stored in the grain boundaries and lattice defects. These lattice imperfections would play a role in accelerating atomic diffusion upon annealing of ball milled powders, even at low temperatures, where atomic diffusion no longer effectively proceeds in ordinary bulk samples. The aim of present research is to investigate the applicability of the mechanical alloying technique (hereafter abbreviated as MA) to the sample preparation for the determination of the phase diagrams in that temperature range. The major sesults obtained are summarized below :1) The phase transformations occurring in the MA sample were found to proceed at a several order higher … More rate as compared to that in bulk samples. This offers us an opportunity to shed some light on the determination of equilibrium phase diagrams at relatively low temperature regions.2) This rapid reaction is found to be attributed to the small grain size and the accumulation of the lattice strain.3) The MA provides an opportunity to produce a large amount of the sample in single phase much easier than the DC sputtering method.4) Precise control of the milling condition results in reducing the impurities coming from a ballmill vial, balls and the ambient gas.5) The sigma* (Cr) + (alpha-Fe) eutectoid temperature in the Fe-Cr system is lower than 480ﾟC.The two-phase separation occurring this system proceeds much faster than the formation of the sigma phase. In other words, the sigma-phase is formed after the completion of the two-phase separation.6) The alpha-phase is obtained by MA in the Fe-V and Fe-Cr systems starting from both a mixture of the respective elemental metal powders and the sigma-phase powders.7) We have determined, for the first time, the Curie temperature in a composition range 34<X<53 in the Fe_<100-X>V_X system.8) A guide line was established to trace the MA process for a combination fo ferromagnetic and nonferromagnetic elements by means of magnetic measurements. Less