Effects of grain growth blocking in annealed metalloid-poor Fe–M–Cu–B–Si ribbons (M = Nb, Mo, V)

•Metalloid-poor Fe–M–Cu–B–Si (M = Nb, Mo, V) annealed ribbons were investigated.•Only Nb blocks the bcc-Fe grain growth to preserve magnetic softness at annealing.•Prolonged higher temperature annealing is unusable without grain-growth blocking.•None of M hinders effectively the preferred surface bcc-Fe crystallization.

Omitting of the Fe–Nb–Cu–B–Si Finemet standard grain-growth blocker (Nb, Mo) in modern high-induction very Fe-rich nanocrystalline ribbons limits the necessary thermal treatment to a narrow range. To see why is this necessary, the grain-growth blocking has been investigated in materials between modern high-induction and Finemet – in metalloid-poor Fe–M–Cu–B–Si (M = Nb, Mo, V) alloys. Only Nb has been found to allow wide range annealing and preserve fine grain, Mo does not and V also enables precipitation of Fe boride. Mo and V containing alloys thus lose their magnetic softness at annealing at and above 480 °C and 1 h mostly due to grain growth. Neither of M effectively hinders preferred surface bcc-Fe crystallization leading to macroscopic internal stress and slant loops when annealing in Ar at T ≥ 480 °C.

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