Creep induced anisotropy in Fe- base nanocrystalline alloys.
Dr. Giselher Herzer
Date & Time: 10:30 - 11:30, July 13th (Wed), 2016.
Place: 7F Small Seminar Room, Sengen
Abstract:
The talk reviews the basic features of creep induced anisotropy in nanocrystalline Fe-Cu-Nb-Si-B alloys. The material highlighted will be flash annealed Fe73.5Cu1Nb3Si15.5B7. This composition is of particular interest for application because of its virtually vanishing magnetostriction constant. Crystallization of Fe73.5Cu1Nb3Si15.5B7 under tensile stress yields an almost perfectly linear hysteresis loop and a domain structure characteristic for a magnetic easy plane perpendicular to the stress axis. This creep induced anisotropy originates from the local magneto-elastic anisotropy of the FeSi crystallites associated with their elongation induced by stress annealing. The magnetic anisotropy constant Ku is proportional to the stress applied during annealing and can amount to be several thousands of J/m³ which is two orders of magnitude higher than the typical magnitude of field induced anisotropies. Such strong and well-controlled anisotropies are of great interest e.g. for magnetic energy storage cores which require a low permeability of typically a few hundreds being constant over a wide magnetic field range. The attainable maximum anisotropy is limited by the yield strength at the annealing temperature. It is about Ku ≈ 12 kJ/m³ for Fe73.5Cu1Nb3Si15.5B7 and corresponds to a permeability of μ ≈ 50 being constant up to a magnetic field of 20 kA/m. Such a high induced anisotropy even exceeds the local magneto-crystalline anisotropy constant K1 of the constituent crystalline FeSi phase. Nonetheless, the coercivity still stays as small as Hc ≈ 8 A/m. The coercivity mechanism can be well understood within the framework of the random anisotropy model extended to the case of mixed random and uniform anisotropy.