Nanocrystalline structures obtained by the crystallization of an amorphous Fe40Ni38B18Mo4 soft magnetic alloy

The kinetics of crystallization of a soft magnetic amorphous Fe40Ni38B18Mo4 alloy was studied by TEM, EDX and resistivity methods. The kinetic parameters were measured from TEM studies and resistivity measurements. Nanocrystals of the fcc FeNi phase were found to crystallize by a primary crystallization mechanism followed by slow growth kinetics. The volume fraction measured by TEM matches well with that calculated from resistivity results and a TTT diagram was constructed. Quantitative measurements of the nucleation and growth rates as a function of temperature and time were performed. The nucleation rate was found to decrease with an increase in heat treatment time due to an increase in boron content in the amorphous matrix as the crystallization took place. The crystal growth was found to slow down considerably due to the presence of Mo. The crystal size was calculated according to the Michels model and compared to our experimental results. Molybdenum was found to dramatically alter the energetics of crystallization, the morphology of the crystals and particularly the kinetics of crystallization. These results offer a method of creating new families of nanostructured magnetic materials by suitable molybdenum addition.