Characterization of a {(Fe60Co40)75B20Si5}96Nb4 impulse atomized glassy powder by Neutron Diffraction and Differential Scanning Calorimetry

This paper investigates FeCo-based alloy glass formability by high undercooling and high cooling rate solidification technique and the hardness evaluation of the generated samples. For this investigation, Impulse Atomization was used to generate {(Fe60Co40)75B20Si5}96Nb4 powders of different sizes that correspond to different cooling rates and undercoolings. The amorphous fraction and kinetic crystallization properties of the investigated powders were determined by means of Differential Scanning Calorimetry and Neutron Diffraction. The enthalpy of crystallization of a close to amorphous powder produced by gas atomization was used as a reference for the calculation of amorphous fraction by calorimetry in atomized powders. Thus, a quantitative estimation of cooling rate corresponding to each powder size and the variation of amorphous fraction with cooling rate are presented. Higher cooling rate promoted by smaller powder size and helium atomization atmosphere is found to yield higher amorphous fraction. At relatively high cooling rates Neutron Diffraction technique yields higher amorphous fractions than the Differential Scanning Calorimetry technique. The critical cooling rate for amorphous phase formation under unconstrained solidification conditions using Impulse Atomization is found to be ~15,000Ks-1 which corresponds to a powder size of about 100 μm atomized in nitrogen or about 200 μm atomized in helium.