Influence of thermal treatment on structure and microhardness of Fe 75 Ni 2 Si 8 B 13 C 2 amorphous alloy Vladimir A. Blagojevi  c a , Du  san M. Mini  c b , Tomá  s  Zák c , Dragica M. Mini  c a, * a Faculty of Physical Chemistry, University of Belgrade, Serbia b Military Technical Institut in Belgrade, Serbia c Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic article info Article history: Received 4 May 2011 Received in revised form 27 June 2011 Accepted 27 July 2011 Available online 28 August 2011 Keywords: A. magnetic intermetallics B. mechanical properties at ambient temperature D. microstructure F. diffraction abstract Correlation between hardness of amorphous Fe 75 Ni 2 Si 8 B 13 C 2 alloy and thermally induced structural transformations has been investigated by measuring microhardness in a series of samples heated at different temperatures from 25 to 1000  C. The alloy has a relatively high hardness in the amorphous state, due to its chemical composition involving silicon, boron and carbon. As the alloy begins to crys- tallize, microhardness increased and reached a plateau in 500e650  C temperature region, due to formation composite structure involving the small nanocrystals of a-Fe(Si) and Fe 2 B phases dispersed in the amorphous matrix. After treatment at higher temperatures, the nanocomposite structure is replaced by a more granulated structure, leading to decline in microhardness. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Iron-based amorphous alloys have been a focus of considerable scientific interest in recent times. Their main features are homoge- nous and isotropic structure and isotropic properties. Their good soft magnetic properties are mainly determined by magneto-elastic and annealing-induced anisotropies [1], and they are also characterized by high corrosion resistance and good mechanical properties [2], making them suitable for use in a variety of applications, such as power devices [3,4], information handling technology, magnetic sensors [5] and anti-theft security systems [6]. Addition of metalloid amorphizers like B, Si, P or C and the substitution of Fe by Co or Ni (or a mixture of both) enhance their glass forming ability [7,8], while elevated temperature or prolonged performance could induce a transformation into a crystalline state, which could lead to a loss of their advantageous physical properties [9], limiting them to single- use applications. On the other hand, the magnetic properties of amorphous Fe-based alloys can improve significantly after crystal- lization, if nanocrystalline phases are formed [10,11], producing functional materials with targeted properties. Commercial soft magnetic nanocrystalline materials have recently been successfully obtained by crystallization of amorphous precursors [12]. These materials are characterized by a microstructure of nanocrystals embedded into an amorphous matrix, exhibiting superior soft magnetic and mechanical properties to both amorphous and crys- talline magnetic alloys. There has been a lot of interest lately in the mechanical prop- erties of iron-based amorphous alloys [13e15] and their hardness, in particular. A study of iron-based alloy powders [16] revealed that hardness of the alloy is at maximum when the sample is composed of a mixture of crystalline nanoparticles and amorphous phase. The authors attribute this, in part, to the fact that amorphous/crystal interface has lower interfacial energy than crystal/crystal interface [17] and this structure suppresses propagation of shear bands [18] and cracks along these interfaces. Additionally, the dispersion of the nanoparticles probably suppresses the deformation of the amorphous phase through shear sliding. Recent theoretical studies of iron-based binary systems predict existence of short-range ordering in iron-based amorphous alloys [19]. A theoretical inves- tigation of nanoscale phase separation in amorphous FeeB alloys indicates that, in amorphous Fe-based alloys (Fe 80 B 20 and Fe 83 B 17 ), Fe-pure regions are formed in parallel with Fe-rich regions (which contain around 9% B) and B-rich regions [20]. Recently conducted ab initio molecular dynamics simulations [21] of liquid and amor- phous Fe 78 Si 9 B 13 alloys showed that FeeSi bonding should be stronger than FeeB bonding and there should be no bonding states * Corresponding author. Tel.: þ381 11 3336 689. E-mail address: dminic@ffh.bg.ac.rs (D.M. Mini c). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.07.027 Intermetallics 19 (2011) 1780e1785