Thursday, 11. January 2018, 13:00

Structural evolution of Fe-based glassy alloys: modern methods of analysis

Dr. Mihai Stoica

Fe-based BMGs are well known for their attractive combination of structural and magnetic properties. Moreover, these alloys are widely studied and considered for potential applications because of low price and good glass forming ability (GFA). Fe-Co-B-Si-Nb system was first introduced by Inoue et al. in 2004; among this class of BMGs the [(Fe0.5Co0.5)0.75Si0.05B0.20]96Nb4 shows the highest GFA, high mechanical strength and excellent soft magnetic properties. A drawback, which may limit the possible industrial applications, is the brittleness of the monolithic BMGs, as well as their relatively low saturation magnetization. It was demonstrated that in the case of Fe-based glassy ribbons with small contents of Nb, minor addition of Cu might be a good way to trigger nanocrystallization of Fe(Si), averaging out the magnetic anisotropy and therefore increasing the soft magnetic properties, as in case of FINEMET alloys.

Very recently [1] we have shown that fully amorphous rods with diameters of up to 2 mm can be obtained upon 0.5 at.% Cu addition to the base [(Fe0.5Co0.5)0.75Si0.05B0.20]96Nb4 alloy. The new Cu-added glass shows very good thermal stability and a drastically changed crystallization behavior compared to the Cu-free base alloy. On heating, the (Fe36Co36B19.2Si4.8Nb4)99.5Cu0.5 BMG samples show two glass transition-like events separated by an interval of more than 100 K, with a bcc-(Fe,Co) solid solution forming in-between. Further [2], we have shown that the primary precipitation of bcc-(Fe,Co) is promoted by the formation of medium-range order clusters. The addition of Cu also improves the plastic deformability of the BMG samples, and the combination of a high fracture strength of 3913 MPa and 2.45% fracture strain make these glasses attractive for applications.

The Cu-added glassy samples remain magnetically very soft, with 1.1 T saturation, less than 2 A/m coercivity, and 712 K Curie temperature. By controlling the annealing temperatures one can also tailor the magnetic properties, and the precipitation of bcc-(Fe,Co) grains changes continuously the composition of the remaining amorphous matrix. The variation of the saturation magnetization and the coercivity with temperature is in very good agreement with the crystallization behavior as observed by calorimetric studies. Time-resolved synchrotron X-ray diffraction in transmission mode also allowed us to fully investigate the mechanism of phase formation upon heating. The variation of the lattice parameter revealed that the precipitated phase contains mostly Fe, while the matrix is continuously enriched in Co upon heating. The soft-magnetic properties are also changed by the precipitation of the cubic ferromagnetic crystals. The present talk brings into discussion the crystallization behavior and kinetics in detail, in context with the variations of the soft-magnetic properties.

1        M. Stoica, P. Ramasamy, I. Kaban, S. Scudino, M. Nicoara, G. B. M. Vaughan, J. Wright, R. Kumar, and J. Eckert, Acta Mater. 95, 335 (2015).
2        P. Ramasamy, M. Stoica, A. H. Taghvaei, K.G. Prashanth, Ravi Kumar, J. Eckert, J. Appl. Phys. 119, 073908 (2016).