Influence of microstructure on the magnetic and mechanical behaviour of amorphous and nanocrystalline FeNbB alloy

The formation of a nanocrystalline structure and its influence on the magnetic and mechanical properties in a ternary Fe80.5Nb7B12.5 alloy has been investigated using a variety of complementary methods. The crystallization studies performed by DSC calorimetry, magnetization and electrical resistivity measurements have confirmed a two-stage nature of the primary crystallization process. The microstructure in the series of heat-treated amorphous and nanocrystalline specimens with different volume fractions of crystalline phase was examined by transmission electron microscopy, x-ray diffraction and 57 Fe Mossbauer spectrometry. The results obtained by a combination of static magnetic measurements and Mossbauer spectrometry have indicated a higher degree of structural and magnetic inhomogeneity of the residual amorphous phase after nanocrystallization. Striking differences in the magnetic hardening regime at elevated temperatures have been observed for the samples with different volume fractions of nanocrystalline particles. The strongest magnetic hardening effects are visible for the samples exhibiting a medium degree of crystallinity, while the best soft-magnetic properties are obtained for the samples where the primary crystallization process is nearly finished. The ductility tests have revealed that the transition from ductile to brittle behaviour develops predominantly in an amorphous phase just before crystallization and the subsequent crystallization causes only slight changes in the embrittlement level. On the other hand, the hardness is rather insensitive to the structural relaxation processes before crystallization and its value increases proportionally to the volume fraction of precipitated nanocrystalline grains.