Abstract
An electron microscope study is made on isothermal decomposition in Alnico 8 alloy as annealed around 800°C for 10–90 min in a magnetic field. The results obtained are found to agree well in many essential aspects with Cahn’s theoretical predictions on spinodal decomposition in a ferromagnetic cubic alloy. In the case of 10-min annealing, the predicted linear relationship can be confirmed between a square of the wavenumber of the modulated structure and the decomposition temperature, and an extrapolation of the straight line yields a reasonable spinodal temperature Ts of 860°C. Furthermore, some interesting facts are found on the later stage decomposition: Within a period of 10 to 90 min, the wavelength, i.e. the particle size grows linearly with annealing time. And, when annealed at a higher temperature near the Ts, it decomposes into a three-phase structure, among which the least ferromagnetic phase only possesses a peculiar substructure. A tentative explanation is given for these facts.
