In situ XRD studies of nanocrystallization of Fe-based metallic glass: a comparative study by reciprocal and direct space methods

Structural changes occurring in an Fe72.5Cu1Nb2Mo2Si15.5B7 alloy during a combination of constant rate heating (20 K min(-1)) and isothermal holding at 500 and 520 degrees C were investigated using in situ high-energy X-ray diffraction. We found that the ferromagnetic-to-paramagnetic transition of the amorphous phase is revealed as a change in the slope of the thermal expansion curve when heating a sample at a constant rate up to 520 degrees C. Real space analysis by means of the atomic pair distribution function (PDF) demonstrated that the rate and extent of the thermal expansion strongly depend on the interatomic separation. The PDF proved to be a reliable method for the description of crystallization kinetics. Further it allows determination of sizes of ultrafine nanocrystals with grain sizes well below 8 nm and thus makes observation of early stages of nanocrystallization possible. Following grain growth kinetics during isothermal annealing at 500 and 520 degrees C we found that the activation energy of the process is 357 +/- 12 kJ mol(-1).