Materials Science and Engineering A 375–377 (2004) 785–788 Crystallization behavior of (Fe 100-x Co x ) 62 Nb 8 B 30 bulk amorphous alloy M. Shapaan a , J. Gubicza a , J. Lendvai a , L. K. Varga b,∗ a Department of General Physics, Eötvös University, P.O. Box 32, H-1518 Budapest, Hungary b KFKI Research Institute for Solid state Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary Abstract The effect of Co addition on the glass-forming ability (GFA) has been investigated in the (Fe 100-x Co x ) 62 Nb 8 B 30 (x = 0, 33, and 50) bulk amorphous alloy system by differential scanning calorimeter (DSC). The thermal stability measured by the apparent activation energies decreased from about 5.5 to 4.5 eV upon Co addition. The glass-forming ability measured by the temperature interval T x - T g was decreased substantially compared to Fe 62 Nb 8 B 30 alloy, but it is still large enough for practical applications. The DSC and DTA parameters, T g , T x , T m and T l were combined in different parameters, T g /T m , T x /T l and T x /(T g + T l ) and T x /(T l - T g ) to express the glass-forming ability. The cystallization products were studied by XRD. The metastable Fe 23 B 6 crystallization product was found in all the alloys. © 2003 Elsevier B.V. All rights reserved. Keywords: Bulk glass-forming ability; Differential thermal analysis; Crystallization 1. Introduction Experimental works carried out by Itoi and Inoue [1] and Inoue [2] revealed that the increase of boron content from about 20% to 30 at % for Fe(Co,Ni)–Nb–B amorphous al- loys caused an extension of the supercooled liquid region up to 80 K. The high boron content confers also high electrical resisitivity above 200  cm, which is very advantageous for high frequency applications of these soft magnetic alloys. While ribbon samples can be easily prepared by melt spinning with a fully amorphous structure, the critical thickness of bulk amorphous alloys depends strongly on the bulk glass-forming ability (BGFA). This property can be studied in terms of a number of parameters like reduced glass-transition temperature, T g /T m , reduced crystallization temperature, T x /T l or T x /(T g + T l ) [3,4] and a new param- eter G = T x /(T l - T g ), where T l is the offset temperature of melting and T l - T g is the cooling increment [5]. The intention of this paper to present a comparative study of the BFGA for the (Fe 100-x Co x ) 62 Nb 8 B 30 alloys and to study the crystallization products after different crystallization steps. ∗ Corresponding author. Fax: +36-1-3922220. E-mail address: varga@szfki.hu (L.K. Varga). 2. Experimental procedure Multi-component amorphous (Fe 100-x Co x ) 62 Nb 8 B 30 (x = 33 and 50) ribbons were prepared by melt spinning technique using Cu disk with tangential velocity of 25 m/s. The ribbon sample was about 35 m thick and 4 mm wide. The amorphicity of the ribbons was examined by X-ray diffractometry (XRD). A Perkin-Elmer DSC-2 differential scanning calorimeter (DSC) was employed to determine the glass-transition temperature, crystallization tempera- ture and crystallization enthalpy H x . All the calorimetric measurements were carried out in an argon flux in order to protect the sample against oxidation. The thermal transfor- mation data above 1000K were determined by DTA using SETARAM equipment. The structure of the as cast and heat treated ribbons were examined by X-ray diffractometry us- ing a Bruker D8 advanced diffractometer with Cu radiation. 3. Results and discussion Fig. 1a shows the X-ray diffraction pattern of the as cast (Fe 50 Co 50 ) 62 Nb 8 B 30 ribbon, indicating the formation of a mostly single amorphous phase. The wave vector (K p = 4π sin θ/λ) at the maximum position of the main broad peak is measured as 30.85 nm -1 . Fig. 2 shows the DSC curves of 0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.181