High performance soft magnetic materials play an important role in improving the
energy efficiency, performance and miniaturization of electro-technical devices such as
smartphones, hard disk, electric motors, alternators, transformer actuators, relays etc. By
virtue of large consumption of energy due to the modern lifestyle, even a small improvement
in material performance can have a huge impact on the savings and the overall economy.
Currently, non-oriented Si steel, known as “electrical steels” with magnetic induction in the
range of 1.8 – 2.0 T is widely used on account of their acceptable electrical and magnetic
properties. Similar to Si-steels, Fe-P based alloys also have promising characteristics; the
addition of small percentages of P to Fe enhances the electrical resistivity without affecting
much the saturation magnetization.
Manufacturing of Fe-P soft magnetic components through powder metallurgy is well
known but with the compromise in magnetic properties. In recent times, the wrought
metallurgical processing of these Fe-P alloys is gaining importance due to the possibility of
metalworking and achieving a good combination of soft magnetic properties as well as
realizing an alternative and cost-effective alloy to conventional electrical steels. The reason
for this achievement is the optimization of the microstructure through carefully designed heat
treatment. However, the quantum of R&D work on wrought metallurgically processed bulk
Fe-P alloys in terms of composition-process-structure-properties correlations is not much,
which prompted the author to take up this thesis work. The multi-pronged studies 10 kg level
also have additional aims to compare the properties of Fe-P based alloys with commercial
soft magnetic materials in order to explore this alloy as an alternate to the conventional nonorientated Si-steel for soft magnetic applications. The alloys were exhaustively characterized
using different techniques like XRD, SEM, EDS, EBSD, TEM, SAXS, VSM, PPMS, B-H
Loop tracer, coercimeter, Vickers microhardness and four-probe potentiometric technique to
comprehensively understand the underlying scientific correlations among the various alloy
process parameters.
Initial studies were carried out on a series of Fe-xP (x = 0.4, 1.0, 1.5, 2.0, 3.0, 4.0
wt.%) binary alloys of each 200 g charge prepared using arc melting technique. The samples
were given a two-step heat treatment at 1000 C for 3 hours followed by 500 C for 30
minutes in a vacuum atmosphere and characterized. The TEM investigations reveal the
presence of Fe3P precipitates in the α-Fe(P) matrix for compositions below 3%, whereas 4
wt.% P alloy shows the dendritic structure. The Fe-4P alloy exhibited high electrical
resistivity, ρ value of 740 nΩ m and high saturation magnetization, µ0Ms value of 1.94 T
which is comparable to 6.5 % Si steel (µ0Ms = 1.8 T, ρ =820 nΩ m). It may be possible to
fine-tune the heat treatment to realize the required values of coercivity and hardness, which is
beyond the purview of this work. Based on the magnetic properties, Fe-0.4P alloy exhibiting
lowest coercivity (Hc ~ 38 A/m) and highest magnetization (µ0Ms ~ 2.11 T) was chosen for
scaled up process and detailed analysis. The conventional wrought alloy processing
(induction melting, forging, rolling) was adopted for the 10 kg level ingots.
Fe-0.4 wt.% P alloys were vacuum induction melted and poured into cast-iron moulds
and the ingots were forged and rolled into sheets of 0.5 mm thickness. The samples prepared
out of the sheets were heat treated at various temperatures (500 – 1000 °C) to study the
recovery and recrystallization behaviour and were correlated with the microstructural
parameters and mechanical/magnetic properties. Heat treatment at 1000 °C/1h resulted in the
formation of a fully recrystallized coarse grain structure with µ0Ms value of ~ 2.13 T, Hc ~ 80
A/m and core loss of 387 W/kg (1 kHz, 1 T). Upon further aging at 300 - 500 °C/0.5 h, the Hc
value reduced to 43 A/m with a core loss of 347 W/kg. Transmission electron microscopy
studies revealed the formation of Fe3P nanoprecipitates (2-4 nm) leading to a reduction in
lattice strain as confirmed from X-Ray diffraction data analysis. The soft magnetic properties
obtained is attributed to the combination of coarse micron sized strain free grains and fine
nano scale precipitate favoring low coercivity and high resistivity respectively The results
were also compared with commercial non-oriented Si-steels (M700-50A: ~0.87 wt% Si and
M400-50A: ~2.2 wt% Si) and found to exhibit comparable/better properties at low
frequencies (DC to  50 Hz). The inferior AC magnetic properties of Fe-0.4P binary alloy to
those of Si-steel are attributed to the low resistivity of ~205 nΩ m. The addition of Si is
known to increase resistivity. Hence, three ternary alloys {(Fe-0.15P-0.25 Si); (Fe-0.15P085Si) and (Fe-0.4P-0.85 Si)} of Fe-P-Si were studied using the wrought alloy route to
understand the effect of Si and P independently. The elaborate studies resulted in the
identification of Fe-0.4P-0.85Si as a promising soft magnetic material with high saturation
magnetization (~2.11 T), low coercivity (~ 47 A/m), high resistivity (410 nΩ m) and low core
loss (~184 W/kg at 1 T/1kHz). The addition of Si to the binary Fe-P was found to increases
the resistivity and concomitantly decreases the core loss. This is attributed to the presence of
Si both in the matrix as well as in some of the Fe3P/(Fe3P,Si) precipitates. To rationalize the
influence of various microstructural parameters on the magnetic and electrical properties of
the alloys, we have used the existing mathematical equations based on theoretical models.
The theoretical calculations suggest that the coercivity value predominantly depends on grain
size than any other parameter. In addition, it is understood that the presence of
nanoprecipitates did not impede the domain wall motion due to their size effects. The
stability of the magnetic properties was confirmed by long time aging studies. It is also found
that the properties of Fe-0.4P-0.8Si ternary alloy are at par with those of high Si-containing
commercial alloys (M530-50A, M400-50A), which signifies the scientific and technological
importance of this work. Based on this work, the extended scope for further R&D work has
also been chalked out, which may be taken up at a later stage.