The complex primary crystallization kinetics of the amorphous Finemet soft magnetic alloys has been analyzed by non-isothermal DSC measurements. The local activation energies E c ( α ) were determined by an isoconversional method without assuming the kinetic model function and its average value was about 383 kJ/mol. The nucleation activation energy E n and growth activation energy E g were 425 and 333 kJ/mol, respectively. And the apparent local activation energies E c can be expressed by E n and E g as follows: E c  =  aE n  +  bE g . The local Avrami exponents lies between 1 and 2 in a wide range of 0.2  α E c ( α ) and local Avrami exponents n ( α ) are applicable and correct in describing the primary crystallization process of the amorphous Finemet alloy according to the theoretical DSC curve simulation.

Complex primary crystallization kinetics of amorphous Finemet alloy

A striking feature of nanocrystalline (NC) materials prepared by different techniques is the variation of lattice strain present in the crystallites. A simple model is proposed to account for this phenomenon, which allows the establishment of a quantitative relationship describing the dependence of the crystal lattice strain on the grain size. Good agreement between theoretical and experimental data indicates that the stress field, induced by the excess volume in grain boundaries, is the dominant source of the lattice strain.

Origin of lattice strain in nanocrystalline materials

A lattice distortion, represented by microstrain and lattice parameter change, is frequently observed in nanocrystalline (NC) materials. In this letter, we develop a quantitative model to account for this phenomenon. The internal stress induced by the excess volume at grain boundaries is suggested to be the main reason for the lattice distortion. NC materials, prepared by the milling and crystallization method, were used for analysis and comparison. Theoretical calculation shows a reasonable agreement with experimental observations in the grain-size range from 5 to 100 nm. Our results indicate that there is an inherent relationship between microstrain and lattice parameter change in NC materials.

Relationship between microstrain and lattice parameter change in nanocrystalline materials

Crystallization behavior of ZrAlNiCu bulk metallic glass with wide supercooled liquid region

The Kolmogorov–Johnson–Mehl–Avrami model for isothermal transformation kinetics is universal under specific assumptions. However, the experimental Avrami exponent deviates from the universal value. In this context, we study the effect of transient heterogeneous nucleation on the Avrami exponent for bulk materials and also for transformations leading to nanostructured materials. All transformations are assumed to be polymorphic. A discrete version of the KJMA model is modified for this purpose. Scaling relations for transformations under different conditions are reported.

K. Zhang

Papers

Nanocrystallization of amorphous Fe33Zr67 alloy