title: Structural and magnetic characterization of Fe-Si-B-P-Cu alloys
Creators: name: Kuhnt, Markus
Publisher: name: UNSPECIFIED
Year: 2020

description: Nanocrystalline Fe-Si-B-P-Cu alloys have attracted a lot of attention due to a high saturation polarization of about 1.8T combined with low coercivity. Low glass-forming ability however hindered large-scale-application. Recently, it has been reported that Fe81.2Co4Si0.5B9.5P4Cu0.8 and (Fe85.2Si0.5B9.5P4Cu0.8)99 C1 combines good magnetic properties with sufﬁcient glass-forming ability. However, reports on Fe-Si-B-P-Cu alloys could not be reproduced by any other group. In this thesis, we have studied the material’s properties of the base alloy Fe85.2Si0.5B9.5P4Cu0.8 based from that investigated the inﬂuence of carbonand cobalt addition. Magnetic and mechanical properties as well as electrical resistivity of Fe85.2Si0.5B9.5P4Cu0.8 have been measured in dependence on annealing temperature Ta by using ﬂash-annealing. In agreement with literature we found in the nanocrystalline state, which is formed at about 400 ◦C, a high saturation polarization of about Js = 1.82 T and low coercivity Hc = 4−5 Acm−1. Saturation magnetostriction is as high as λs = 14 ppm, which can be well understood from the relatively low crystalline fraction νcr = 50 %. Mechanical tests reveal a typical behavior for nanocrystalline Fe-based soft magnetic materials. Due to stress relaxation and the reduction of the free volume, the alloy embrittles well before the formation of the nanocrystalline state. Upon nanocrystallization both hardness and Young’s modulus increase, which can be correlated with an increasing volume fraction of the crystalline α-Fe phase. The electrical resistivity was found dropping from about 124 Ωcm in the as-cast state to about 61 Ωcm in the nanocrystalline state, which is much lower than for Fe-Si-B-Nb-Cu alloys. The low electrical resistivity compared to Fe-Si-B-Nb-Cu alloys causes higher eddy current losses and thus rather limits the use of Fe-Si-B-P-Cu alloys to low frequency applications. For studying the inﬂuence of cobalt addition, magnetic and electric properties of nanocrystalline Fe85.2CoxSi0.5B9.5P4Cu0.8 (x = 0, 4, 10, 15, 20, 25, 35, 40, 50, 57 at%) have been investigated and were correlated with their structural properties. Co addition doesn’t effect the crystallization temperatures much and a nanocrystalline phase, comprising a bcc phase and a residual amorphous matrix, is formed for all compositions at alloys at about 400 ◦C. In the nanocrystalline state, both coercivity and magnetostriction increase with Co content, from Hc = 4 A m−1 to Hc = 29 A m−1 and from λs = 14 ppm to λs = 55 ppm, respectively. The saturation polarization reaches a maximum of about 1.88 T at 25 at% Co. Electrical resistivity show a distinct maximum of ρel = 68 Ωcm at 10 at% Co. Both, the dependence of magnetic and electric properties, can be understood from the changing composition of the constituent phases. The inﬂuence of carbon addition on magnetic and electrical properties was investigated for Fe85.2−xCxSi0.5B9.5P4Cu0.8 for carbon contents up to x = 3 at%. In the as-cast state, the inter-atomic distance of the iron atoms increases, which results in an increased Curie-temperature TC and eventually in an enhanced saturation polarization Js, but also an enhanced saturation magnetostriction λs. In the nanocrystallie state, carbon addition reduces the grain size of the bcc grains and thus coercivity can be lowered from Hc = 4 A m−1 down to Hc = 2 Am−1. On the other hand, a reduced crystalline volume fraction results in slightly lower saturation polarization and increased saturation magnetostriction. However, the lower volume fraction also enhances electrical resistivity up to about 75 Ωcm for x = 3 at%, which might prove useful for high frequency applications.