6.3 Electrodeposited Nanocrystalline Alloys
205
way as for Ni (sulphur segregation at the grain boundaries, possibility of anomalous
grain growth).
The systematic treatment of the nc-Fe–Co–Ni system [140, 157–161] is rather
difficult due to the interplay of the variation of the composition and the grain size,
and the separation of the two variables is often controversial. From structural point
of view, various phase transitions take place in this ternary system (fcc, bcc and hcp
for high Ni, Fe and Co mole fractions, respectively). However, the compositions
associated with the phase transitions cannot be determined unambiguously, and the
practical phase transition ranges strongly depend on the deposition conditions [162].
The importance of the Fe–Co–Ni system stems from the optimization opportunity
of the magnetic properties. While the saturation magnetization is unambiguously
determined by the deposit composition and hence, is insensitive to the bath type used,
the coercivity strongly varies with the deposition conditions [140]. The minimum
coercivity of electrodeposited nc-Fe–Co–Ni alloys is well below 10 Oe [140, 157,
158]. It was commonly found that the minimum of the coercivity occurs for deposits
that are composed of a phase mixture in the fcc–bcc transition zone, which is the
indirect consequence of the grain size minimum. It is common that the grain size
minimum occurs in the phase mixture composition range because the neighbouring
crystals with incompatible structure mutually impede the growth of each other.
Although zinc does not belong to the iron group, it is reasonable to discuss the
deposition of nc-Ni–Zn alloys in this chapter since the anomalous nature of the
codeposition of Ni and Zn. Although Zn is the metal with high deposition preference,
the bath composition for the Ni–Zn deposition differs from the rest of the system
mentioned above in the sense that the Zn
2+ concentration is usually larger than that
of Ni
2+ . Due to the phase formation properties of the Ni–Zn system, alloys with
Ni content less than 30 at.% can be deposited with three different crystal structures
belonging to this composition range. Higher Ni content can be achieved with a
pulse-reverse deposition mode, but the danger of the formation of a porous deposit is
high [163]. Various baths have been tested for electrodeposition of nc-Ni–Zn alloys
[163–166], and the trends found for the deposit properties varied much with the bath
type.
6.3.3 Alloys of Iron Group Metals with Molybdenum
or Tungsten
The induced codeposition mechanism is a common feature of the deposition of iron
group metals with oxoanion-forming metals of the fifth and sixth line of the periodic
table (Mo and W, respectively).This means that the iron group metals can be deposited
alone, but pure Mo and W (and also Re) cannot be obtained by electrodeposition.
However, when two metals from the two groups are deposited together, an alloy can
be formed with continuously varying composition from the pure iron group metal
to a certain composition which depends on the system studied. From chemical point
205
way as for Ni (sulphur segregation at the grain boundaries, possibility of anomalous
grain growth).
The systematic treatment of the nc-Fe–Co–Ni system [140, 157–161] is rather
difficult due to the interplay of the variation of the composition and the grain size,
and the separation of the two variables is often controversial. From structural point
of view, various phase transitions take place in this ternary system (fcc, bcc and hcp
for high Ni, Fe and Co mole fractions, respectively). However, the compositions
associated with the phase transitions cannot be determined unambiguously, and the
practical phase transition ranges strongly depend on the deposition conditions [162].
The importance of the Fe–Co–Ni system stems from the optimization opportunity
of the magnetic properties. While the saturation magnetization is unambiguously
determined by the deposit composition and hence, is insensitive to the bath type used,
the coercivity strongly varies with the deposition conditions [140]. The minimum
coercivity of electrodeposited nc-Fe–Co–Ni alloys is well below 10 Oe [140, 157,
158]. It was commonly found that the minimum of the coercivity occurs for deposits
that are composed of a phase mixture in the fcc–bcc transition zone, which is the
indirect consequence of the grain size minimum. It is common that the grain size
minimum occurs in the phase mixture composition range because the neighbouring
crystals with incompatible structure mutually impede the growth of each other.
Although zinc does not belong to the iron group, it is reasonable to discuss the
deposition of nc-Ni–Zn alloys in this chapter since the anomalous nature of the
codeposition of Ni and Zn. Although Zn is the metal with high deposition preference,
the bath composition for the Ni–Zn deposition differs from the rest of the system
mentioned above in the sense that the Zn
2+ concentration is usually larger than that
of Ni
2+ . Due to the phase formation properties of the Ni–Zn system, alloys with
Ni content less than 30 at.% can be deposited with three different crystal structures
belonging to this composition range. Higher Ni content can be achieved with a
pulse-reverse deposition mode, but the danger of the formation of a porous deposit is
high [163]. Various baths have been tested for electrodeposition of nc-Ni–Zn alloys
[163–166], and the trends found for the deposit properties varied much with the bath
type.
6.3.3 Alloys of Iron Group Metals with Molybdenum
or Tungsten
The induced codeposition mechanism is a common feature of the deposition of iron
group metals with oxoanion-forming metals of the fifth and sixth line of the periodic
table (Mo and W, respectively).This means that the iron group metals can be deposited
alone, but pure Mo and W (and also Re) cannot be obtained by electrodeposition.
However, when two metals from the two groups are deposited together, an alloy can
be formed with continuously varying composition from the pure iron group metal
to a certain composition which depends on the system studied. From chemical point