Formation of bcc nanocrystals in Co-based amorphous alloys

•doping with components with a bcc lattice leads to formation of bcc crystals.•nanocrystal size decreases when bcc element concentration increases.•heterogeneous nanocrystal formation with nucleation on ordered regions. The crystallization of amorphous alloys of Co-Fe-B-(Ti, Nb) system was studied by differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. It was determined that the alloying of amorphous alloys of Co-Fe-B system with elements having a bcc lattice promotes the formation of a metastable phase with this lattice during crystallization. Nanocrystal size and the fraction of the phase with a bcc lattice depend on the concentration of alloying components. When the concentration of bcc components increases, nanocrystal size decreases. When Nb (an element with a bcc lattice) is replaced by Ti (an element with an hcp lattice), the fraction of the bcc phase decreases, and multiphase crystallization (simultaneous formation of several crystalline phases) occurs. In Co56Fe16B20Ti8 alloy, the first crystallization stage consists of two steps. During the seemingly simultaneous formation of several crystalline phases, first a small quantity of the bcc phase and then Co23B6 and fcc-Cо crystals are formed sequentially. All the results obtained indicate heterogeneous nanocrystal formation with the nucleation on structurally related ordered regions (on ordered clusters which consist of alloy components with the structure related to the crystallizing phase).