Effects of annealing temperature and heating rate on microstructure, magnetic, and mechanical properties of high-Bs Fe81.7−xSi4B13NbxCu1.3 nanocrystalline alloys

The effects of annealing temperature and heating rate on the microstructure, magnetic, and mechanical properties of melt-spun Fe81.7−xSi4B13Cu1.3Nbx (x = 0–4) alloy ribbons have been investigated. With increasing the annealing temperature, a ductile–brittle transition occurs during amorphous structure relaxation, the brittleness becomes severe with more α-Fe precipitation, and the hardness rises continuously. After annealing at respective optimum temperatures under the heating rate of 20 K/min, as the Nb content increases from 0 to 4 at.%, average grain size (Dα-Fe) and volume fraction (Vα-Fe) of the α-Fe in the nanocrystalline alloys decrease gradually from 53.3 nm and 52% to 8.7 nm and 42%, respectively; the strain at fracture (ef) representing ductile level increases from 1.33 to 1.72%; and the coercivity (Hc), saturation magnetic flux densities (Bs), and Vickers hardness (Hv) all decrease gradually. As the heating rate rises from 10 to 400 K/min, the Dα-Fe of the Fe81.7Si4B13Cu1.3 alloy decreases from 45.7 to 28.4 nm without considerable variation of the Vα-Fe; the Hc lowers from 235 to 25 A/m, the ef increases from 1.10 to 1.66%, and the Bs and Hv change slightly. Enriching of Nb weakens the dependence of nanostructure, magnetic softness, and annealing embrittlement on the heating rate. A correlation of ef ∝ Dα-Fen is found for the nanocrystalline alloys, where the n rises from − 1 to − 1/2 with enriching of Nb from 0 to 4 at.%. The mechanisms by which nanostructure affects magnetic and mechanical properties have been discussed.