Microstructure and magnetic properties of amorphous/nanocrystalline Co40Fe22Ta8B30 alloy produced by mechanical alloying

Multicomponent Co40Fe22Ta8B30 powders were synthesized by mechanical alloying (MA) of the elemental powder mixture. The microstructure of the powders at the initial stage of milling mainly consists of nanocrystalline bcc-(Fe,Co) phase and unreacted tantalum along with a minor amount of amorphous phase. The amorphous phase becomes the dominant phase with the progression of the MA process, reaching a weight fraction of ∼96% after milling for 200 h. Thermal and structural analysis reveals that the amorphous phase crystallizes through a single crystallization event to form the metastable complex fcc (Fe,Co,Ta)23B6 and the (Co,Fe)3B2 phases. The magnetic measurements show that the saturation magnetization of the powders decreases continuously with increasing the milling time. On the other hand, the coercivity and the squareness ratio of the hysteresis loop first increase and after milling for 22 h they decrease continuously. The evolution of the coercivity with milling time is discussed with respect to different phenomena, such as residual stress, grain refinement, amorphous phase formation, surface anisotropy and particles surface irregularities.