Crystallization kinetics of the amorphous magnetic material 2605 CO (Fe 67Co 18B 14Si 1)

The reaction kinetics of Fe 67Co 18B 14Si 1 have been studied with continuous heating differential scanning calorimetry (both peaks) and isothermal resistivity measurements (first peak only). The first and second peaks can be characterised by constant values of empirical activation energies (2.5 and 4.02 eV, respectively) and pre-exponential factors (In K 0 = 40.65 and 57.35 min -1, respectively). The first transformation involving the formation of α-iron dendrites does not appear to conform to a simple Johnson-Mehl-Avrami relation insofar as it shows a non-linear behaviour in the ln ln(1- x 2) -1 against ln t plot. Two quite distinct slopes, hence two values of Avrami exponent are observed (average values of n 1 = 1 n 2 = 5). The behaviour is confirmed by both measurement techniques. Various possibilities, e.g. surface crystallization or incubation time are not considered responsible for the observed behaviour. Explanations in terms of the morphology of the growing dendrites are briefly discussed.