Nanocrystallization of Al‐based glasses via nucleation and growth under “soft impingement” conditions

Abstract

The nanocrystallization behaviour of Al86Ni6Co2Gd3Y2Tb1 amorphous alloy has been studied by X‐ray diffraction methods involving small angle scattering, resistance measurements and differential scanning calorimetry. It has been established that the transformation in amorphous alloy investigated occurs mainly by the growth process at initial state of transformation while at final stage nucleation contributes into formation of nanocomposite structure. In order to describe the nanocrystallization kinetics of the Al‐based glasses the analytical kinetic equations in the frames of Kolmogorov‐Johnson‐Mehl‐Avrami model have been proposed and the effective volume diffusion coefficient values which govern the diffusion‐limited growth nanocrystals have been determined. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Estimation of diffusivity governing primary nanocrystallisation and its relation to thermal stability of amorphous phases

“…3 consist of (111) and (200) diffraction peaks corresponding to Al and overlapping with a broad halo from the residual amorphous phase. Analysis of the deconvoluted XRD patterns shows that nanophase composites formed at the first crystallisation stage have the crystallised volume fractions (X), the average sizes of Al nanocrystals (L) and the volume densities (N) in the ranges of (0.21-0.44), (13)(14)(15)(16)(17)(18)(19)(20)(21)…”

“…The volume fraction of Al nanocrystals was calculated using the relative areas of the convoluted amorphous halo, A am , and crystalline, A cr , peaks (111) and (200) as X=A cr /(A cr +A am ) [23]. As it has been shown earlier [18], an underestimation of X calculated by this technique [24] has no influence on the effective diffusion coefficient values. The average grain size, L, in the partially crystallised samples was estimated from the half-width of the XRD peaks (neglecting possible strain contributions) using Scherrer's equation.…”

“…The model was tested by analysing the primary crystallisation of the Fe 85 B 15 glass, which occurred by formation of relatively large α-Fe(B) nanocrystals (46-75 nm) with somewhat lower volume density (about 10 21 m −3 ) compared with the parameters of nanocomposite structures formed in the Finemet-type and Al-based amorphous alloys. Later [18], it was demonstrated that these equations can be used to describe the nanocrystallisation process in the Al 86 Ni 6 Co 2 Gd 3 Y 2 Tb 1 amorphous alloy.…”

“…However, the existing theoretical models of the atomic transport across the liquid (glassy)-crystal interfaces do not allow a quantitative estimation of the mobility even for pure metals [19]. Therefore, the so far reported values of the diffusion coefficients that control the primary nanocrystallisation process in metallic glasses were obtained by indirect methods as fitting parameters of the appropriate models describing the crystal growth [18,[20][21][22] or the transformation kinetics [12,17]. However, no systematic research on this topic has been reported in the literature to date.…”

“…[17,18] has allowed us, in particular, to determine the values of the effective diffusivity which controls the growth of Fe-and Al nanocrystals, respectively. It is evident that diffusion plays a key role in crystallisation because it determines the rates of crystal nucleation and growth.…”