Thermal stability and crystallization of two rapidly quenched Mg-based amorphous alloys (Mg83Ni17 and Mg83Ni9.5Y7.5) were studied. The influence of yttrium on the crystallization as well as on the microstructure after crystallization was analysed. The binary alloy crystallizes at about 180 °C to a metastable Mg6Ni phase with a grain size of about 30 nm. It is shown that the transformation proceeds by nucleation and three dimensional linear growth until the end of the process. At higher temperatures, above 300 °C, the metastable phase transforms to the equilibrium αMg and Mg2Ni phases with a very small thermal effect and slow kinetics, being hard to detect by DSC. The ternary alloy (Ni in Mg83Ni17 is partially replaced by Y) reveals rather different crystallization behavior. Nanocrystallization takes place in the range 175-225 °C as a two-step process. The resulting microstructure is extremely fine with an average nanocrystalline size of about 5-6 nm. Kinetic analysis of the first nanocrystallization reaction shows that most probably nucleation with a very high rate leads to high initial density of nuclei, which grow with a diffusion-controlled rate. Impingement of the diffusion fields of the growing nanocrystals occurs at later stages of the transformation and additionally decreases the kinetics. The nanostructure is stable and transforms to the equilibrium Mg24Y5, Mg2Ni and Ni2Y3 phases with coarser microstructure at higher temperatures (250-290 °C). The formation of Mg-Y and Ni-Y intermetallics during crystallization is expected to impede the grain growth of the Mg2Ni phase and results in finer microstructure of the fully transformed ternary alloy compared to the binary Mg-Ni alloy. © 2002 Elsevier Science B.V. All rights reserved.
|Journal||Journal of Alloys and Compounds|
|Publication status||Published - 28 Oct 2002|
- Amorphous materials
- Rapid solidification
- Rare earth alloys
- X-ray diffraction