Role of Si on structure and soft magnetic properties of Fe87−xSixB9Nb3Cu1 ribbons

•Complete amorphous phase is obtained in as spun ribbons upto 25at.% Si alloy.•18% Si alloy exhibits highest glass forming ability as it is at eutectic point.•Annealing of all ribbons yields the precipitation of Nanocrystalline Fe3Si phase.•Si in Fe3Si phase increases from 21 to 25at.% for 11 to 25at.% Si alloys.•Coercivity decreases with Si content upto 18% Si and then increases further. Role of Si content on the structural, thermal and soft magnetic properties of melt spun and annealed Fe87−xSixB9Nb3Cu1 (x=11,13,18,20,25 and 30at.%) alloys has been investigated. Processing of ribbons at a higher wheel speed (47m/s) during melt spinning results in the formation of amorphous phase in all the alloys expect for 30at.% Si whereas at lower wheel speed (34m/s), amorphous phase has been observed only in 18at.% Si alloy ribbon. Detailed thermal analyses revealed that higher glass forming ability of 18at.% Si alloy is due to the near eutectic point in these alloy series and higher ratio between the crystallization and liquidus temperature (Tx/TL). On annealing, nanocrystalline Fe3Si phase was found to be precipitated out in amorphous matrix in alloys up to 20at.% Si ribbons whereas paramagnetic Fe4.9Si2B phase along with Fe3Si phase was observed in 25% Si ribbon. Detailed Mössbauer spectroscopy of the annealed ribbons revealed that the composition of the nanocrystalline Fe3Si phase increases from Fe-21at.% Si towards the stoichiometric composition (Fe-25at.% Si) with Si content. The results are also corroborated with precision lattice parameter measurements. The coercivity of the annealed ribbons initially decreases, attains a minimum at 18at.% Si and then increases abruptly with Si content.