Nanocomposites and nanomaterials

Modeling of Fe80B20 alloy crystallization process during quenching from the melt and heating the amorphous ribbons
Kozoriz V.S., Kosynska O.L. 
Dniprovsk State Technical University
Dniprostroevska str., 2a, Kamianske, 52818, Ukraine
E-mail: krao@ua.fm

The algorithm joint solutions thermal and kinetic problems, allowing to investigate crystallization processes in metal alloys during cooling of the melt and heating of the amorphous ribbon, has been developed. A computational program developed on the basis of this algorithm, makes it possible to calculate a temperature field of the foils, crystallization parameters and a crystallized volume fraction formed at cooling x, a contribution to a crystal phase at the expense of growth „frozen-in” crystallizations centers at heat of foils x+ and the contribution of crystals formed from an amorphous matrix die in the heat x+. According to the results, in terms of the melt quenching, depending on the thickness of the foils l1, can be fixed different structural state of the Fe80B20: amorphous, amorphous-crystalline and fully crystalline structures with average crystal size  from 46 to 316 nm. Such structures differ in the volume fraction of the crystalline phase, in the density of the crystallization centers and in their average dimensions, and also make it possible to analyze the effect of the structural state on the crystallization kinetics upon heating of quenched samples. It is established that when the foils are heated at a rate of 0.67 K/s, depending on l1, the nucleation and growth processes of the crystals can make a different contribution to the total transformed volume fraction x. At crystallization of thin foils, l1=12 μm, the crystallization is carried out mainly by the mechanism of the new crystals formation (x+=1.510-1, x+=8.410-1). And with the growth of l1, the ratio of the quantities x+ and x+ changes and, starting from a certain critical thickness , the crystallization is carried out due to the enlargement of the crystals that appeared at the melt cooling stage. Foils microstructure analysis showed that in the entire thickness range under study, the density of the crystallization centers formed in the amorphous matrix upon heating is higher than the number of crystals formed upon cooling of the melt. In this case, the crystals formed in the heating stage grow at low rates and get much smaller dimensions during the whole transformation time (3.7-1.8)10-7m, in comparison with the "frozen-in" transformed crystals (18.8-6.8)10-7m. Thus, by selecting the cooling and heating modes for amorphous foils, it is possible to control the parameters of the microstructure, achieving a given set of material properties.