Structural changes during heating or aging in a wide temperature range were examined with three amorphous alloys of Fe₇₈Si₁₀B₁₂, Ni₇₅Si₈B₁₇ and Co₇₅Si₁₅B₁₀ by measurements of electrical resistance, differential scanning calorie and Vickers hardness and also by transmission electron microscopy and X-ray diffraction.
The results obtained are summarized as follows:
(1) The crystallization process of three amorphous alloys is divided into four stages: (a) the incipient stage of crystallization where a certain short range ordering of atoms occurs, (b) the formation of primary metastable phases (MS-I), (c) the formation of secondary metastable phase (MS-II) with complex single structures, and (d) the formation of a stable phase consisting of a mixture of each equilibrium phase. The MS-I phase appears in the amorphous matrix in a manner of homogeneous nucleation and gradual growth, while the MS-II phase grows rapidly from a few nuclei and completely spreads over the amorphous matrix containing the MS-I phases. At higher temperatures, these MS-II phases transform finally to stable phases.
(2) The temperature-time-transformation diagrams of three alloys were constructed. In these diagrams, distinct differences in the transformation sequence and the mode are observable in the upper and lower ranges of the critical temperature (Tx′). Above this boundary, crystallization by way of nucleation and growth proceeds through two metastable phases and finally to a stable phase. Below that temperature, on the other hand, progressive aging gradually changes the amorphous structure to the assembly of fine grains (about 100∼200Å) with a simple structure such as bcc, fcc, or hcp.