Evolution of fcc Cu clusters and their structure changes in the soft magnetic Fe_85.2Si₁B₉P₄Cu_0.8 (NANOMET) and FINEMET alloys observed by X-ray absorption fine structure

It is known that Cu plays an essential role in reducing the grain size of precipitated bcc Fe(Si) nanocrystallites in a nanocrystalline soft-magnetic Fe_85.2Si₁B₉P₄Cu_0.8 (NANOMET^®) alloys like as an Fe_73.5Si_13.5B₉Nb₃Cu₁ (FINEMET^®). However, significant differences are there between two alloys; NANOMET has much higher iron content (∼85%) than FINEMET (73.5%) and the former contains P instead of Nb for the latter. In the present work, the local structure around Cu in FINEMET was measured by X-ray absorption fine structure (XAFS) at 20 K and compared with those of NANOMET during nanocrystallization. Definite differences between NANOMET and FINEMET are found in the way of the evolution of Cu clusters during nanocrystallization. In FINEMET, an fcc structure of Cu is recognized in an as-quenched ribbon indicating existence of a small number of Cu clusters or a very small size of Cu clusters which is stable up to 450 °C, while the fcc Cu clusters are developed rapidly above 450 °C. An fcc structure of the Cu clusters in FINEMET is retained all the way to the end of the nanocrystallization. On the contrary, for NANOMET the local structure around Cu changes in a sequence as “amorphous → fcc → bcc → fcc” by annealing. The reasons of such different behaviors of the local structure around Cu during nanocrystallization are discussed in terms of different contributions of Cu clusters in bcc Fe precipitation between FINEMET and NANOMET. A significantly fast crystallization process with an extraordinary large heat release can be another reason for the transition of the local structure around Cu from fcc to bcc for NANOMET.