ON THE INFLUENCE OF Zr AND Hf ADDITION ON THERMAL AND MAGNETIC PROPERTIES OF NANOCRYSTALLINE Fe-Co-Hf-Zr-Cu-B ALLOYS

M. Kowalczyk – J. Ferenc – T. Kulik

   One of the most rapidly developing groups of magnetically soft materials are the nanocrystalline, iron-based alloys. Special interest concerns the materials obtained by partial crystallization of metallic glasses, with better magnetic properties than those found for the amorphous counterparts. Nanocrystalline magnetically soft materials are divided into three main groups: Finemets, Nanoperms and Hitperms. The first two groups of materials exhibit very soft magnetic behavior, however, their application is limited to relatively low temperatures, due to low situated Curie point of their amorphous matrix. The useful method of increasing the Curie temperature of both phases of the alloys is the addition of cobalt. A partial replacement of Fe by Co results in an extension of application temperature to 500-600°C. The modification with Co is the most effective for NANOPERM, and hence these Co-modified alloys bear the name of HITPERM. The disadvantage of this modification is that the excellent magnetically soft properties of Nanoperm's alloys are slightly deteriorated. Magnetically soft nanomaterials for high-temperature applications have to fulfill two basic requirements: (i) very soft magnetic behavior at elevated temperatures, and (ii) stable performance at elevated temperatures for the time of application. This paper describes the results of the study of (Fe_0,5Co_0,5)_93-x(Hf_1-vZr_v)_xCu₁B₆ alloys, where x = 5, 6, 7 and 9 (at %) and v = 0, 0.5 and 1. The influence of Hf and Zr on the crystallization process and magnetic properties was investigated. Differential thermal analysis (DTA), X-ray diffraction (XRD) and quasistatic hysteresis loop tracing were used. Isothermal annealing at various temperatures for 1 h was carried out and magnetic properties of the alloys after such heat treatment were studied. Partial replacement of Hf by Zr reduces the temperature of the 2-nd crystallization stage, reducing the thermal stability of nanocrystalline alloys. All the alloys, if annealed between 450 and 600°C for 1 h exhibit the desired two-phase structure, and if the annealing temperature exceeds 600°C, other phases appear and magnetically soft properties are lost. The lowest coercive field, Hc, of 23 A/m, was obtained for the nanocrystalline alloy containing 7% of Hf. An increase of the Hf or Zr total content increases the coercive field, Hc, of the nanocrystalline alloys. Zr used instead of Hf also increases Hc of the studied materials. However, the content of Zr or Hf does not have a significant influence on the annealing temperature that enables obtaining the lowest coercive field. Such common optimum annealing temperature is 550°C.

Keywords: hitperm, nanocrystalline alloys, soft magnetic alloys, coercive field, thermal stability

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