The characterization of structure, thermal stability and magnetic properties of Fe–Co–B–Si–Nb bulk amorphous and nanocrystalline alloys

Recently bulk amorphous alloys have attracted great attention due to their excellent magnetic properties. The glass-forming ability of bulk amorphous alloys depends on the temperature difference (ΔTx) between glass transition temperature (Tg) and crystallization temperature (Tx). The increase of ΔTx causes a decrease of the critical cooling rate (Vc) and growth of the maximum casting thickness of bulk amorphous alloys. The aim of the present paper is to characterize the structure, the thermal stability and magnetic properties of Fe36Co36B19Si5Nb4 bulk amorphous alloys using XRD, Mössbauer spectroscopy, DSC and VSM methods. Additionally the magnetic permeability μi (at force H ≈ 0.5 A/m and frequency f ≈ 1 kHz) and the intensity of disaccommodation of magnetic permeability Δμ/μ(t1) (Δμ = μ(t1 = 30 s) − μ(t2 = 1800 s)), have been measured, where μ is the initial magnetic permeability measured at time t after demagnetisation, the Curie temperature TC and coercive force Hc of rods are also determined with the use of a magnetic balance and coercivemeter, respectively.Fe–Co–B–Si–Nb bulk amorphous alloys were produced by pressure die casting with the maximum diameters of 1 mm, 2 mm and 3 mm.The glass transition temperature (Tg) of studied amorphous alloys increases from 807 K for a rod with a diameter of 1 mm to 811 K concerning a sample with a diameter of 3 mm. The crystallization temperature (Tx) has the value of 838 K and 839 K for rods with the diameters of 1 mm and 3 mm, respectively. The supercooled liquid region (ΔTx = Tx − Tg) has the value of about 30 K. These values are presumed to be the origin for the achievement of a good glass-forming ability of the Fe–Co–B–Si–Nb bulk amorphous alloy. The investigated amorphous alloys in the form of rods have good soft magnetic properties (e.g. Ms = 1.18–1.24 T). The changes of crystallization temperatures and magnetic properties as a function of the diameter of the rods (time of solidification) have been stated.

Research highlights▶ The X-ray diffraction proves the amorphous structure of the as-quenched alloy. ▶ The Mössbauer studies haven’t confirmed amorphous structure in rods with higher diameter. ▶ The structure of the sample with 2mm diameter consist of two amorphous matrix with various Fe concentration and small amount of frozen nuclei. ▶ In the sample with highest diameter a fine α-FeCo crystalline phase was detected. ▶ The alloy display relatively good functional properties.