High-temperature Magnetic Properties Of Ni-contained FeCo-based Nanocrystalline Alloys

The structure and magnetic properties of as-quenched and nanocrystalline Nix(Fe0.5Co0.5)73.5-xSi13.5B9Nb3Cu(x=5,10,15,20,25,30) alloys were investigated. The influence of Ni content on crystallization process, Curie temperature and high-temperature magnetic properties was mainly studied.DSC curves of the as-quenched alloys were measured to determine the two-stage crystallization process. It indicated that the Ni addition has remarkable effect on the crystallization temperature. With Ni content increasing, the primary crystallization temperature enhanced and the secondary crystallization temperature decreased. Consequently, the interval temperature between the two crystallization temperatures exhibited a gradual reduced tendency with the Ni content increasing.The structural analysis of as-quenched and annealed samples at different temperatures has been performed by the evolution of x-ray diffraction patterns(XRD). It was found that all the510℃-annealed samples gave rise to an ultrafine structure composed of the residual amorphous matrix,(FeCoNi)3Si and α-FeCo crystals(tens of nanometers in size). The annealing temperature has a significant influence on the microstructure of the samples, including the lattice parameter, thickness of the intergranular amorphous layer and crystalline volume fraction.Magnetic properties at the elevated temperatures were mainly studied by the evolution of initial permeability μi with the temperatures T (μi-T curves). It was shown that the room-temperature initial permeability was enhanced significantly by the partial substitution of Ni for Fe and Co in (Fe0.5Co0.5)73.5Si13.5B9Nb3Cu alloy. However, the Curie temperature of amorphous phase(Tcam) decreased with Ni content increasing.For the high Ni content alloys(x≥20), μi achieved a larger value at the room temperature but decreased rapidly when the temperature approached to Tcam. However, the low Ni content alloys(x≤15) exhibited the optimal thermal stabitity. Especially for the alloy with x=5, when annealing at600℃under vacuum atmosphere, μi remained a considerable value at a wide temperature range above Tcam. The evolution of μi as a function of annealing temperature and Ni content was systematically analysed in terms of the effective anisotropy model as well as exchange-coupling interaction between the adjacent nanograins in nanocrystalline alloys. The as-quenched Ni2o(Feo.5Coo.5)53.5Si13.5B9Nb3Cu alloy was also submitted to isothermal treatment for successive heating-cooling cycles. It was observed that the room-temperature μi value was much larger than that of samples annealed under vacuum atmosphere, even at the same annealing temperature. Moreover, the sample annealed under heating-cooling cycles achieved much more stable magnetic properties. It was suggested that the alloy with x=20was sensitive to the annealing conditions, and maybe the successive annealing but a short time prefered for the better magnetic properties in our present alloy.