CRYSTAL-STRUCTURE AND MAGNETIC-BEHAVIOR 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 Fe77Nb4Cu1Si12 B6 (B6), prepared with the B-11 isotope, have been analysed using data obtained by means of bl situ neutron diffraction. This technique allo ws 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.% S i in the phase when it is annealed at 500 degrees C for one hour. An i ncrease to 800 degrees C favours the growth of Fe(Si) grains and the c rystallization of other phases, mostly Fe borides. A Rietveld analysis of these phases results in a good reproduction of the nominal composi tion of the alloy. It also elucidates the crystallographic structure o f the Fe(Si) phase. This is similar to the Fe3Si <Fm(3)over bar m> str ucture, but with some of the Fe atoms occupying some (45%) of the Si 4 a sites. The compositions and amounts of the phases derived are in agr eement with Mossbauer spectroscopy results for the same sample. Knowle dge of the Fe(Si) composition enables one to compare the different mag netic behaviours observed for bulk and nanocrystalline alloys. By cont rast, B6 alloy does not show the presence of a Fe(Si) DO3 structure, p resumably due to the lower amount of Si in the Fe(Si) phase. The therm al expansion of the phases that appear is fairly linear and the corres ponding thermal expansion coefficients for the different phases have b een extracted. The magnetic structure of the Fe(Si) phase is ferromagn etic collinear, without any trace of antiferromagnetic ordering. The t hermal variation of the (1, 1, 1) magnetic peak intensity of the Fe(Si ) phase matches well with reported DC magnetization results.