Nanocomposites and nanomaterials

Mechanisms and kinetics of the formation of a metastable bcc-phase upon heating of rapidly quenched La73.5Ag26.5 amorphous alloy 
A.B. Lysenko1, T.V. Kalinina1, O.A. Zadorozhnia1, I.V. Zagorulko2
1Dniprovsky State Technical University, Dniprobudivska, str., 2, Kamyanske, 51918, e-mail: zadorozhnialena@gmail.com
2 G.V. Kurdymov Institute for Metal Physics of NAS of Ukraine, 03680, Kyiv-142

The structure of the amorphizing alloy La73.5Ag26.5 was investigated. Amorphous alloy was obtained by quenching from a liquid state at cooling rates υ–~ (5·104–107) K/s and then it was subjected to heat treatment by continuous heating from room temperature to 673 K, as well as isothermal holdings in the temperature range of crystallization of amorphous samples. It is shown that at υ– ~105 K/s, an amorphous structure typical for metallic glasses is fixed in the alloy under study. Three diffuse intensity maxima are revealed in the corresponding X-ray diffraction patterns. The effective sizes of the coherent scattering regions, estimated from the width of the first halo, are ~ 1.6 nm.
Upon subsequent heating at a rate of υ+ ~ 0,08 K/s, the structure of rapidly quenched foils undergoes two transformations stages. At the first stage, a metastable phase with a bcc-lattice and crystallite sizes of ~ (8 – 9) nm is polymorphically crystallized. The single-phase nanocrystalline state is retained in a fairly wide (453 – 433) K temperature range. At higher temperatures, the metastable bcc-phase decomposes into a mixture of equilibrium eutectic phases:   β-La and LaAg compound with an ordered CsCl-type lattice.
In a special set of experiments, we studied the dependences of the electrical resistivity (ρ) of rapidly quenched amorphous foils on the time t of isothermal treatment at temperatures close to the onset of crystallization of the bcc-phase, which were converted to the traditional kinetic form х(t), where х is the fraction of the crystallized volume. It is shown that the kinetic equation x(t) has an exponential character with the Avrami exponent n = 1. This value of n excludes the possibility of three-dimensional crystal growth processes and leads to the conclusion about the implementation of some other mechanism of crystallization of the bcc-phase. Based on the results of previous investigations, it can be assumed that this process occurs through cooperative displacements of atomic ensembles at distances shorter than the shortest diffusion paths in amorphous materials. As a result of such displacements, crystals with bcc-coordination are almost instantly formed. In this case, layers with a low atomic density are formed along the boundaries of the crystallization centers, which leads to the termination of the action of the shear mechanism of the crystals formation, the final size of which in the alloy under study does not exceed 10 nm.