Single- and Multistage Crystallization of Amorphous Alloys
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CTURE, PHASE TRANSFORMATIONS, AND DIFFUSION
Single- and Multistage Crystallization of Amorphous Alloys S. V. Terekhov* Donetsk Institute for Physics and Engineering Named after O.O. Galkin, National Academy of Sciences of Ukraine, Donetsk, 83114 Ukraine *e-mail: [email protected] Received August 21, 2019; revised February 12, 2020; accepted March 10, 2020
Abstract—A thermodynamic approach to the description of single- and multistage crystallization of iron- and aluminum-based amorphous alloys is suggested in this work. The adequacy of theoretical calculation of the volume fraction of the crystalline phase to experimental data is demonstrated for the case of precipitation of one or several phases in disordered systems. The parameters of the model depend on the rate of heating of amorphous alloy. In the case of multistage crystallization, the theoretical calculations indicate the occurrence of thermal processes in the system, which affect the crystallite growth. These processes are the decelerated growth of crystallites of the same phase; enrichment of the amorphous matrix in alloy components, which do not crystallize, as a result of thermal diffusion; overlapping thermal effects of different processes; etc. Keywords: phase transition, crystallization, multistaging, amorphous alloy DOI: 10.1134/S0031918X20070108
INTRODUCTION Unique physico-mechanical properties of amorphous alloys [1–6] are controlled by the absence of short-range order in these alloys, their ability to resist changes in the temperature and other factors of environment, and the mechanism and degree of crystallization. Theoretical knowledge on the kinetics of the crystallization process was developed in the 20th century [7–9] and was named the Kolmogorov–Johnson– Melh–Avrami (KJMA) model. The KJMA model is used to calculate the volume fraction of the crystalline phase in iron- and aluminum-based amorphous alloys [10–16]. The application of the KJMA model in practice showed that upon multistage crystallization (only crystals of the same type grow) and sufficiently high heating rates of amorphous alloys, a deviation of theoretical curve from experimental data is observed [16]. Moreover, it is difficult to use the model in the case of multistage crystallization when in the amorphous matrix there grow matrix phase regions differing in the composition. Phase transitions, which are due to peculiarities of the behavior of thermodynamic characteristic function at the singular (critical) point, can be called point (abrupt) phase transitions [17]. Upon first-order phase transition, the heat release (absorption) and jumps of the specific entropy and volume are observed [18, p. 106]. Upon second-order phase transition, the latent heat of transition is equal to zero, the specific entropy and volume vary continuously, and the specific heat exhibits a (singular or finite) jump [18, pp. 117–123].
However, many real phase transformations (for example, in disordered systems) occur within certain temperature, time, and other ranges [19–21]. Because of this, they were c
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