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УкраинаМетодами рентгенографического анализа, дифференциальной сканирующей калориметрии и измерения микротвёрдости исследованы склонность к аморфизации (СА) и механические свойства ряда новых многокомпонентных сплавов на основе железа.Сплавы Fe 69 Mn 1 Mo 4 Cr 2 C 7 P 10 B 5 Si 2 и Fe 55 Ni 8 Co 6 Mo 4 Cr 2 V 1 Al 2 P 9 C 6 B 5 Si 2 были получены в аморфном состоянии литьём в медный кокиль в форме пластинок толщиной 0,5 и 2 мм соответственно, а кристаллизация сплава Fe 50,0 Ni 19,0 Cr 6,5 Мо 1,5 V 1 B 14,1 С 2,5 Р 4,4 Si 1 подавляется только в тонких лентах, полученных методом спиннингования расплава.Анализ СА был проведён в рамках известных термодинамических критериев, основанных на температурах перехода стеклования (T g ), начала кристаллизации (T ons ) и ликвидуса (T l ).Установлено, что критические скорости охлаждения, необходимые для объёмной аморфизации первых двух сплавов, предсказываемые критерием  m  (2T ons  T g )/T l (1010 и 262 К/с соответственно), хорошо согласуются со значениями

Structural changes of rapidly cooled ribbons of the amorphous alloys Al 88–86 (Ni,Co,Fe) 6–8 (Y,Gd,Nd,La) 5–6 , which occur during heating at a rate of 10 K/min and lead to a loss of ductility, have been investigated experimentally. It has been shown that samples of the studied alloys are divided into two groups, in the first of which the loss of ductility is due to the formation of a nanocomposite structure, whereas the embrittlement of samples in the second group is caused by processes of structural relaxation in the amorphous phase (decrease in the concentration of a free volume). It has been established for the first time that there is an empirical correlation between the dynamic temperature, after heating to which the alloys lose their ductility at room temperature, and the ratio of the shear modulus to the elastic modulus of the alloys, which is calculated from the nominal chemical composition.

Both structure and mechanical characteristics of melt-spun glassy ribbons with a thickness of 20 mu m and bulk plates of 2 and 3 mm thick with nanocrystalline structure fabricated by the ejection copper mould casting method of Fe61.7Cr13.4Mo1.9Nb1.9Mn0.5Cu0.5Al1Si1C1.9B16.2 alloy as well as the transition of the amorphous phase into crystalline state are examined by x-ray diffraction analysis, differential scanning calorimetry and microhardness measurements. Crystallization of the glassy phase occurred via two stage by sequential formation of alpha-Fe solid solution and Me2B nanocrystals, respectively. The analysis of glass forming ability and crystallization of amorphous phase show that formation of two-phase (alpha-Fe + Me2B) nanocrystalline structure with grain sizes in the range of 16-43 nm in bulk samples is a result of both the enhanced nucleation rate of alpha-Fe solid-solution crystals caused by the Cu clustering as well as the retardation of the growth processes due to formation of Nb-rich shells around the alpha-Fe nanocrystals and the necessity of essential atomic rearrangements for growth of the boride crystals. The strength characteristics of both the as-cast amorphous ribbons and the nanocrystalline plates estimated from their hardness (10.3 and 10.9-11.2 GPa, respectively) correspond to the highest ultimate strength reported for the crystalline Fe-based alloys (3.3 GPa) while the maximum value of hardness for the alloy investigated (17.7 GPa) is reached in the fully-crystallized ribbon with a nanocrystalline structure.

The changes of the Al-nanocrystals' sizes, L(t), and kinetics of nanocomposite-structure formation, X(t), at the first crystallization stage of amorphous Al87Ni8Y5 alloy during annealing at 460 K, 501 K and 536 K are investigated by x-ray diffraction and electrical-resistivity measurements' techniques. The experimentally measured L(t) curves are described by the analytical model accounting the diffusion-fields' impingement, and values of the effective diffusion coefficient governing the growth are determined. Using the L(t) and dX/dt changes, the nucleation rates for Al nanocrystals are estimated, and it is established that they are essentially decreasing from 3.36.10(20), 1.15.10(21) and 1.67.10(21)m(-3).s(-1) to 1.0.10(18), 1.2.10(18) and 1.4.10(18) m(-3).s(-1), respectively, during nanocrystallization at the selected annealing temperatures. As shown, the observed decrease of the nucleation rates can be quantitatively interpreted by substitution of the thermodynamic driving force, calculated within the regular-solutions' model and dependent on the concentration of alloying elements within the amorphous matrix, into classical equation of homogeneous-nucleation rate.

Thermal stability and changes of both the average size of Al nanocrystals and their crystallised volume fraction formed in a series of the amorphous Al–(Ni,Co,Fe)–(Gd,Y,Tb) alloys as well as of α-Fe(Si) in the Fe73.5Si13.5B9Cu1Nb3 alloy under isothermal conditions have been experimentally studied by a combination of X-ray diffraction (XRD) analysis, differential scanning calorimetry (DSC) and electrical resistance measurements. The experimental data have been fitted with the analytical models describing the diffusion-limited growth of nanocrystals and nanocrystallisation kinetics accounting for impingement of diffusion fields and the values of the effective diffusivity governing this process have been estimated. The obtained values of the diffusivity in Al-based amorphous alloys follow the Arrhenius-type dependencies with correlation between the pre-exponential factors and the activation energies somewhat different from that found for impurity diffusion coefficients in amorphous alloys. It has been established that at the onset crystallisation temperatures varying from 453 to 778K, the values of the effective diffusivity in the investigated amorphous alloys are in the narrow range of 1.7–4.7×10−20m2s−1, which indicates a crucial role of the effective diffusivity for the thermal stability of nanocrystals forming amorphous alloys and the possible reason for this is discussed.

The changes of structure and microhardness of amorphous Al87Ni8Y5 Al87Ni8Gd5 Al86Ni6Co2Gd6 and Al86Ni2Co6Gd6 alloys during constant-rate heating are studied by differential scanning calorimetry, x-ray analysis, and by measurements of electrical resistance and microhardness. As shown, the full replacement of Gd with Y results in the increase of the onset of the first crystallization stage by 25 K, while change of the Ni/Co ratio in Al86Ni8-xCoxGd6 alloys from 3/1 to inverse one leads both to enhancement of thermal stability of amorphous phase by 44 K and to decrease of the number of crystallization stages from three to two. As revealed, the microhardness of the alloys at issue essentially increases (from 3000-3500 MPa to 4500-5500 MPa) due to formation of nanoscale Al crystallites (of 16-19 nm) and intermetallic compounds and decreases to congruent to 3000-4000 MPa when the structure coarsens at the final crystallization stages. As found, the average grain size of the crystal phases is the main factor, which governs the changes in microhardness of partially and fully crystallized alloys.

Kinetics of the first crystallization stage of Al86Ni2Co5.8Gd5.7Si0.5 amorphous alloy and structure of the partially crystallized specimens were investigated by measurements of electrical resistance, X-ray diffraction and transmission electron microscopy. The presence of Al nanocrystals and eutectic colonies consisted of mutually oriented crystals of Al and metastable phase was found. The transient nucleation and slowing-down diffusion-limited growth and the interface-controlled growth of the quenched-in nuclei were identified as mechanisms of formation of the Al nanocrystals and eutectic colonies, respectively.

Effect of the quenching wheel velocity in the range 20.7-26.5 m/s on the cooling rate as well as on the structure and microtopology of the contact surfaces of the glass-forming FeNiPB melt-spun ribbons has been experimentally studied. Both the values of the cooling rate and heat transfer coefficient at the wheel-ribbon interface estimated from the temperature vs. time curves recorded during melt spinning runs are in the ranges (1.6-5.2)×106 K/s and (2.8-5.2)×105 Wm-2K-1, respectively, for ribbon thicknesses of 31.4-22.0 μm. It was found that the density of the air pockets at the underside surface of ribbons decreases while its average depth remains essentially unchanged with the wheel velocity. Using the surface quality parameters the values of the heat transfer coefficient in the areas of direct ribbon-wheel contact were evaluated to be ranging from 5.75 to 6.65×105 Wm-2K-1.

The two-stage crystallization behavior has been experimentally identified in Fe85B15 metallic glass at isothermal and non-isothermal conditions involved appearance of primary α-Fe-based nanoscale particles in the first stage and polymorphic formation of Fe3B crystals in the second step. An analytical model for description of two simultaneous processes in the Kolmogorov-Johnson-Mehl-Avrami formalism frameworks has been developed both for isothermal and constant rate heating conditions. The model is based on additivity of the extended volumes and on the assumption that growth of α-Fe nanoscale crystals is retarded due to diffusion field impingement. The calculated two-stage crystallization kinetic curves agree with those experimentally measured and the deduced values of free parameters are in reasonable accordance with the available ones.