Crystal structure and magnetic behaviour of nanocrystalline Fe-Nb-Cu-Si-B alloys studied by means of in situ neutron diffraction

Two Fe-Nb-Cu-Si-B alloys, Fe73.5Nb3Cu1Si13.5B9 (B9) and Fe77Nb4Cu1Si12B6 (B6), prepared with the B-11 isotope, have been analysed using data obtained by means of bl situ neutron diffraction. This technique allows one to scrutinize crystallographic phases during thermal treatments, avoiding problems due to sample handling. The B9 sample develops Fe(Si) <Fm(3)over bar m> (DO3) nanometric crystals (10 nm) with 19 at.% Si in the phase when it is annealed at 500 degrees C for one hour. An increase to 800 degrees C favours the growth of Fe(Si) grains and the crystallization of other phases, mostly Fe borides. A Rietveld analysis of these phases results in a good reproduction of the nominal composition of the alloy. It also elucidates the crystallographic structure of the Fe(Si) phase. This is similar to the Fe3Si <Fm(3)over bar m> structure, but with some of the Fe atoms occupying some (45%) of the Si 4a sites. The compositions and amounts of the phases derived are in agreement with Mossbauer spectroscopy results for the same sample. Knowledge of the Fe(Si) composition enables one to compare the different magnetic behaviours observed for bulk and nanocrystalline alloys. By contrast, B6 alloy does not show the presence of a Fe(Si) DO3 structure, presumably due to the lower amount of Si in the Fe(Si) phase. The thermal expansion of the phases that appear is fairly linear and the corresponding thermal expansion coefficients for the different phases have been extracted. The magnetic structure of the Fe(Si) phase is ferromagnetic collinear, without any trace of antiferromagnetic ordering. The thermal variation of the (1, 1, 1) magnetic peak intensity of the Fe(Si) phase matches well with reported DC magnetization results.