Effect of Annealing on Magnetic and Mechanical Behavior of NANOPERM-type Alloys

Summary form only given. NANOPERM-type metallic glasses [1, 2] are still attractive candidates for variety of practical applications, because of their unique magnetic [3, 4] as well as mechanical properties [5]. Their annealing above crystallization temperature induces partial crystallization that is characterized by formation of α-Fe grains with typical sizes of up to tens of nanometers. Soft magnetic properties and mechanical properties of such nanocrystalline materials can be tailored not only with the aid of their chemical composition, but also by controlling the size the nanocrystalline grains created by heat treatment of amorphous precursor, their morphology, and structural composition. In order to understand magnetic and mechanical properties of NANOPERM-type alloys it is necessary to understand their microstructure and physical consequences of transformation from amorphous to nanocrystaline state. It is well known that structural changes demonstrate themselves also via macroscopic magnetic and mechanical parameters such as temperature dependence of magnetization, Curie temperature, AC and DC hysteresis loops as well as microhardness. We have studied a NANOPERM-type Fe76Mo8Cu1B15 alloy in form of ribbon 0.025 mm thick and 10 mm wide, produced by a rapid quenching method, with emphasis upon its magnetic and mechanical properties. All measurements were performed for the sample in the as-quenched state and after annealing at 837 K for 30 min. (above the primary crystalization temperature i.e. 723 K). Microstructure of the samples was studied by X-ray diffraction and AFM/LFM investigations. Temperature evolution of DC magnetization measured under zero-field cooling conditions using amorphous and nanocrystalline samples with different amounts of nanocrystalline grains was performed in temperature range from 50 K up to 400 K. In addition, DC hysteresis loops were also taken with the help of a VersaLab system (Quantum Design). Room tempertaure AC soft magnetic properties versus frequency and maximum induction were studied for toroidal samples by a hysteresisgraph (AMH-50K-S, Laboratorio ElletroFisico Engineering) at frequency up to 20 kHz. The Steinmetz coefficients for the amorphous and nanocrystaline alloys were calculated in the frequency range 50 Hz - 20 kHz. Mechanical characterization of investigated materials was performed by nanoindentation tests with respect to Oliver-Pharr method for maximum load of 250 mN. Sample of the Fe76Mo8Cu1B15 alloy in the as-quenched state was fully amorphous, which was confirmed by a broad halo in X-ray diffraction pattern and absence of visible precipitates on AFM/LFM images recorded in contact mode. The peaks present in X-ray diffraction pattern confirm two-phase behaviour of annealed material. It was also seen in measurements of microhardnesas for different regions in samples. The average values of hardness obtained for the amorphous matrix and nanocrystalline bcc grains are equal to 1221 HV and 1600 HV, respectively. The magnetic behavior described by DC hysteresis loops for the Fe76Mo8Cu1B15 alloy in the as-quenched state and after annealing at 837 K for 30 min. measured in the temperature range 200-400 K presented in Fig. 1 show a completely different behavior. The Curie temperature of the amorphous alloy equals 313 K, whereas for the annealed sample the higher value of the Curie point due to the nanocrystalline α-Fe phase was observed. For designing process and simulation of inductive elements (e.g. transformers, chokes) the AC magnetic characteristics are very important. Fig. 2 shows core losses as a function of maximum induction for the Fe76Mo8Cu1B15 alloy. It is seen that nanocrystalline material with low core losses in wide range of frequency and Curie point above 400 K (Fig. 1) is excellent candidate for high temperature and frequency soft magnetic material. Moreover, the Steinmetz coefficients K and α [6] calculated for the nanocrystalline Fe76Mo8Cu1B15 alloy increase from 0.0381 to 0.5840 and from 1.71 to 2.42, respectively for with frequency range 500 Hz - 20 kHz.