The structure, physical and mechanical properties and peculiarities of nanocrystallization of a number of ribbon and bulk amorphous metallic alloys (AMAs) on the basis of the Fe42–71(Ni,Со,Cr,Мо,W,Nb,V,Mn,Al)10–32Y1,8–2 (Si,C,B,Р)17–24 system were studied by X-ray diffraction, electron microscopy, differential scanning calorimetry, resistometry, durometry, corrosion resistance test and three-point bending strength measurements.
The technique of preparation was developed and ribbon and bulk samples of amorphous, amorphous-nanocrystalline and 100% nanocrystalline alloys close to eutectic compositions with different degree of doping at ultrahigh (up to 106 K/s) and moderate (from 90 to 1000 K/s) cooling rates of the melt, respectively, were obtained. The sizes of crystals in the initial amorphous crystalline or completely nanocrystalline, predominantly multiphase samples, ranged from 10 to 32 nm.
It has been experimentally established that alloys most doped with weakly soluble in iron Cr, W, Mo (about 30 at.% in total), carbon (9,8-15 ат.%), with a low content of Y (1.8...2 at.%) and Al (0,1 ... 1 at.%) are characterized by the highest tendency to amorphization and thermal stability. The structure of these alloys after crystallization consists of a mixture of α-Fe solid solution nanocrystals metastable borides Me3(B), Cr23C6 and Fe7С3 carbon compounds.
The analysis of the process of crystallization of the AMA
Fe42–71(Ni,Со,Cr,Мо,W,Nb,V,Mn,Al)10–32Y1,8–2(Si,C,B,Р)17–24 showed that the presence of copper and niobium in its composition contributes to increase of nucleation rate and inhibit growth of nanocrystals based on α-Fe solid solution. These factors are as a precondition for the formation of nanoscale α-Fe crystals not only during heating the amorphous phase, but also during cooling the melt. In turn, the dispersion nature of Me2B boride crystals in high speed cooled castings is due to their low growth rate, which is connected to the low (~18%) concentration of boron (and carbon) atoms in relation to the stoichiometry of this phase occurs with a significant redistribution of elements in the supercooled melt.
Experimentally established regularities of nanocrystallization during thermal treatment of bulk amorphous alloys of different chemical composition, the similarities and differences from the process of nanocrystallization of amorphous ribbons obtained from the same alloys (with the same chemical composition) were determined. In particular, it has been shown that the activation energy of each stage of the nanocrystallization process for bulk samples is 18...50% lower than the corresponding values for amorphous ribbons. The increase in the difference in activation energies on the late stages of nanocrystallization shows a significantly higher content of cluster (frozen nuclei) of borocarbide compounds in bulk amorphous samples, which is a consequence of smaller by 4 orders of their cooling rate compared to amorphous ribbons.
It has been shown for the first time that partial replacement of Fe in a known Fe80B14Si6 alloy by a number of transition metals, among which the chromium and molybdenum that are weakly soluble in iron (on the example of Fe45Ni19.4Co8.5Cr5.7Mo1,9B14Si5.5 alloy), leads to the formation of a nanocrystalline structure at the first stage of the two-stage process crystallization of AMA with a decrease of more than twice the size of α-Fe nanocrystals and an increase in their volume fraction by almost an order of magnitude. The enhanced nucleation in a multicomponent amorphous alloy is due to a lower value of activation energy for the formation of critical nuclei size. A possible reason for the increased rate of nucleation in the amorphous Fe45Ni19.4Co8.5Cr5.7Мо1.9B14Si5.5 alloy is less liquid-solid phase interface energy. This, in turn, may be due to an increase in the volume fraction of clusters based on α-Fe in multicomponent alloys.
It was first established that formed in the initial bulk samples, as well as during nanocrystallization process (after heating to 1120 K and, consequently, after finishing of all stages of crystallization) of amorphous ribbons of new Fe42-71 (TM, Al)10-32Y1,8- 2(Si, C, B, P)17-24 alloys, multiphase structure is characterized by record values of microhardness of 15-20 GPa.
For the practical application of Fe45Ni19.4Co8.5Cr5.7Мо1.9B14Si5.5 amorphous ribbons is important an obtained, due to doping, increase of microhardness, corrosion resistance, specific electrical resistance, and a significant reduction in the thermal coefficient of resistance (TCR) from 1,6×10-4 to 4,4×10-5 K-1 in the initial amorphous state. In particular. the high efficiency of using this alloy as a high-strength corrosion-resistant low temperature tape heater in household and industrial equipment has been proved. This material may be an alternative of non-chrome, kanthal and carbon fiber materials imported into Ukraine.
