Nanocomposites and nanomaterials

Crystallization kinetics of Mg65Cu25Y10 alloy from liquid and amorphous phases
Lysenko А.B.1, Zagorulko I.V.2, Kalinina T.V.1
1 Dniprovsk State Technical University, Dniprobudivska str., 2,  Kamyanske – 51918, Ukraine. 
2 G. V. Kurdyumov Institute for Metal Physics of the N.A.S. of Ukraine, Academician Vernadsky Boulevard, 36, Kyiv-03142, Ukraine.
E-mail: zagorylko@ukr.net

The calculated analysis of the crystallization kinetics of the bulk-amorphizing Mg65Cu25Y10 alloy under conditions of quenching from the liquid state (QLS) and the subsequent heating of rapidly quenched samples,  X-ray diffraction patterns of which do not contain signs of crystallinity, is performed. It was shown that depending on the thickness of the melt layers l, the structures of different types are formed during solidification: from truly amorphous, without of the nanocrystalline component inclusions (so-called “quenching nuclei”), to polycrystalline. Intermediate segments of this structural spectrum are metallic glasses, in which the fraction of the crystallized volume, xе, does not exceed the sensitivity of the X-ray phase analysis (~10-2), and amorphous-crystalline structures (10-2<xе<100). The critical (maximum) thicknesses of the QLS products, which ensure the production of metallic glasses (lMG = 7,4·103 μm) and truly amorphous states (lА = 10 μm), are determined.
In the second block of calculations for rapidly quenched samples with a thickness of l ≤ lА and lА < l ≤ lMG, the temperature of the beginning of crystallization, Tbc, which was identified with the temperature to achieve the volume fraction of the crystalline phase of ~10-2, was determined at different heating rates υ+.  It was assumed that in the first case, the transformation is carried out by the formation of new crystals in an amorphous matrix mechanism, and in the second, both through this mechanism and through the growth of quenching nuclei. It is shown that under conditions of relatively slow (υ+ = 1 K/s) heating, higher values of Tbc are achieved if the process is carried out according to the mechanism of quenching nuclei growth. When using high-speed (υ+ > 100 K/s) heating modes, a greater thermal stability is demonstrated by truly amorphous structures, which crystallize by nucleation and growth of new crystals.