In situ structural investigation of amorphous and nanocrystalline Fe40Co38Mo4B18 microwires

Thermal evolution of the structure of glass-coated nanocrystalline FeCoMoB microwire during its devitrification has been studied. It is shown that annealing at the temperature above 411 °C leads to the formation of crystalline α-FeCo grains with diameter ∼12 nm. Annealing at higher temperature increases the crystalline weight fraction up to 40% at 565 °C. However, crystalline grains size increases very weakly to ∼13 nm. The thermal expansion coefficient of nanocrystalline microwire decreases by one half comparing to that of the amorphous precursor.