Tsuyoshi Masumoto1 and Hiroshi Kimura2
1The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai
The crystallization process of an iron-base amorphous alloy (Fe-P-C) was studied by means of electrical resistivity and differential specific heat measurements, transmission electron microscopy and X-ray diffraction, and the transformation sequence from the amorphous state to the crystalline state was clarified together with the precipitation morphology of the metastable and stable phases.
The main results obtained are as follows:
(1) The crystallization process of the amorphous Fe-P-C alloy is divided into four stages: (a) the stage of incipient crystallization where the short range ordering of atoms is considered to occur in the amorphous phase, (b) the formation of a metastable bcc phase (MS-I) with lattice constant a0=2.87 Å, (c) the formation of second metastable phase (MS-II) with complex structure, and (d) the formation of stable phase consisting of a mixture of α -Fe, Fe3P and Fe3C.
(2) A number of microcrystallites of the MS-I phase disperse homogeneously in the amorphous matrix. They have a cubic shape with the size 100 to 500 Å at temperatures above about 350°C. The precipitation is induced by the homogeneous nucleation and the crystal growth is extremely slow. After prolonged aging the size and number of the crystallites remain constant. The crystals of MS-II phase appear in the amorphous matrix with an oval shape at about 430°C and has a lath martensite-like internal structure. They grow rapidly from a few nuclei and completely spread over the amorphous matrix containing the MS-I phase. Above about 450°C, the MS-II phase transforms to the stable phase.
(3) The prolonged aging at a temperature lower than about 350°C or a relatively lower cooling rate from the liquid state changes the amorphous structure to a bcc structure.
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