Успехи физики металлов (Mar 2020)
Conditions for the Fabrication of Metallic Glasses and Truly Amorphous Materials
Abstract
A review of the literature data on the problem of the disordered structural-states’ formation during the rapid cooling of metallic melts is performed. The author’s approach novelty consists in the fact that, in addition to the generally accepted model of metallic glasses (MG), which are structurally amorphous–nanocrystalline composites, the conditions for fixing single-phase amorphous structures lacking inclusions of the crystalline component are analysed. The criteria for the tendency of melts to noncrystalline solidification are considered: ‘deep eutectic’, thermodynamic, structural (topological), physicochemical, and kinetic ones. Particular attention is paid to the works, in which the critical values of the thickness and cooling rate of melts providing the MG formation are defined through a consistent solution of thermal and kinetic problems formulated regarding the quenching from the liquid state (QLS) process. As shown, using the model of effective rates of nucleation and crystals’ growth in studies of mass-crystallization kinetics allows analysing the complex transformations, which occur under conditions of competition between several crystalline phases and/or transformation mechanisms, and simplifies the calculations of the microstructure parameters of the QLS products. The authors propose a detailed description of the thermokinetic analysis of amorphization conditions for metal melts, which is also contained in the present work. As proved, the probability of metal glasses structure formation is primarily determined by the crystal-growth rate, the value of which is controlled by the ratio of the difference between the free energies of the liquid and crystalline phases, ΔGV, to the viscosity of the melt, η. The intervals of the ΔGV/η criterion values are determined for materials of four groups with significantly different vitrification ability. The final section contains the results of a computational analysis of the conditions for suppressing the nucleation processes and fixation of truly amorphous structural states. As follows from the relevant literature data, only alloys, which solidify as MG when casting into a metal mould, demonstrate the real prospect for obtaining the fully amorphous structures. As concluded, the main factors increasing the ability to absolute amorphization are relatively low (up to 10^20 1/(m3·s) values of the stationary frequency of crystallization-centres’ formation and a pronounced tendency to retardation of the nucleation processes with increasing cooling rate of the melts.
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