Study of nanocrystallization in FINEMET alloy by active screen plasma nitriding

Abstract The nanocrystallization process of amorphous Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 was investigated by active screen plasma nitriding (ASPN) treatment at temperatures ranging from 410 °C to 560 °C for 3 h in two gas mixtures of 75% N 2 –25% H 2 and 25% N 2 –75% H 2 at 5 mbar atmosphere. The amorphous ribbons were then annealed under vacuum at the same time and temperatures mentioned above. The structure of the samples was analyzed using various techniques such as X-ray diffraction (XRD), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Microhardness measurements, electrical resistivity and Vibrating Sample Magnetometer (VSM) were used to study mechanical, electrical and magnetic properties of the samples, respectively. It was observed that the ASPN treatment leads to finer grain size and higher crystalline volume fraction and modifies the structural features of Fe(Si) phase. The Fe(Si) lattice parameter for the nitrided samples decreased up to 520 °C, indicating that Si atoms form a solid solution in α-Fe and it was raised at 560 °C, due to Si ex-solution in the Fe(Si) phase and the formation of more nitrided phases. In ASPN treatment, the lowest magnitude of coercivity and the maximum saturated magnetization were obtained at 440 °C in 75% N 2 –25% H 2 and at 560 °C in 25% N 2 –75% H 2 gas mixtures, respectively. Our proposed method (ASPN treatment) increased the microhardness and electrical resistivity of the samples.