Metastable Phase Diagram and Precipitation Kinetics of Magnetic
Nanocrystals in FINEMET Alloys
Article
Jha, Rajesh, Diercks, David R, Stebner, Aaron P et al. (2017). Metastable Phase Diagram and Precipitation Kinetics of Magnetic
Nanocrystals in FINEMET Alloys
.
Jha, Rajesh, Diercks, David R, Stebner, Aaron P et al. (2017). Metastable Phase Diagram and Precipitation Kinetics of Magnetic
Nanocrystals in FINEMET Alloys
.
Research over the years has shown that the formation of the Fe$_3$Si phase in
FINEMET (Fe-Si-Nb-B-Cu) alloys leads to superior soft magnetic properties. In
this work, we use a CALPHAD approach to derive Fe-Si phase diagrams to identify
the composition-temperature domain where the Fe$_3$Si phase can be stabilized.
Thereafter, we have developed a precipitation model capable of simulating the
nucleation and growth of Fe$_3$Si nanocrystals via Langer-Schwartz theory. For
optimum magnetic properties, prior work suggests that it is desirable to
precipitate Fe$_3$Si nanocrystals with 10-15 nm diameter and with the
crystalline volume fraction of about 70 \%. Based on our parameterized model,
we simulated the nucleation and growth of Fe$_3$Si nanocrystals by isothermal
annealing of Fe$_{72.89}$Si$_{16.21}$B$_{6.90}$Nb$_{3}$Cu$_{1}$ (composition in
atomic \%). In numerical experiments, the alloys were annealed at a series of
temperatures from 490 to 550 \degree C for two hours to study the effect of
holding time on mean radius, volume fraction, size distribution, nucleation
rate, number density, and driving force for the growth of Fe$_3$Si
nanocrystals. With increasing annealing temperature, the mean radius of
Fe$_3$Si nanocrystals increases, while the volume fraction decreases. We have
also studied the effect of composition variations on the nucleation and growth
of Fe$_3$Si nanocrystals. As Fe content decreases, it is possible to achieve
the desired mean radius and volume fraction within one hour holding time. The
CALPHAD approach presented here can provide efficient exploration of the
nanocrystalline morphology for most FINEMET systems, for cases in which the
optimization of one or more material properties or process variables are
desired.
