Effect of Gd on the devitrification and magnetic properties of [(Fe0.6Co0.4)0.75Si0.05B0.20]96-xNb4Gdx metallic glasses

• Detrimental effect on the glass forming ability of microalloying with Gd in FeCoBSiNb alloys. • Classical GFA criteria cannot describe the glass formation in Gd- alloyed FeCoBSiNb alloys. • Increasing Gd amount produce a change in the primary crystallization phases. • Combined XRD and Mössbauer Spectroscopy is a powerful method to characterize the crystallization route in Fe-based metallic glasses. The influence of microalloying with Gd on the thermal stability, crystallization and magnetic properties of [Fe 0.6 Co 0.4 ) 0.75 Si 0.05 B 0.20 ] 96-x Nb 4 Gd x (x=0,1,2,5) metallic glasses was examined. Various glass-forming ability criteria were calculated based on thermal characteristics obtained by differential scanning calorimetry for melt-spun ribbons and compared to the maximum size achieved of the alloys via direct rod casting. Although thermodynamic-based criteria predict larger glass-forming ability for the alloy with higher Gd content, x=5 alloy showed the lowest ability to form full glassy rods. The structural evolution after first crystallization event was analyzed by x-ray diffraction and Transmission Mössbauer spectroscopy. The observed changes in the precipitated phases in x=5 alloy compared to x=0,1,2 alloys are associated to the decrease in the glass-forming ability. The formation of phases other than (FeCo) 23 B 6 phase in x=5 alloy is responsible for not achieving fully glassy rods. Additionally, the magnetic properties of glassy ribbons are significantly affected by the addition of Gd. Saturation magnetization and Curie temperature decrease as Gd content increases.