Comments on `Character of transformations in Fe-Co system'
Department of Materials Science and Metallurgy, University of Cambridge
Pembroke Street, Cambridge CB2 3QZ, U.K.
Scripta Mater., 52:2005, p1347-1351
downloaded from www.thomas-sourmail.net
The existence of an ordering reaction in FeCo alloys has recently been
challenged. The compatibility of new and published results with the
alternative explanation is examined. It is shown that the existence of an
ordering reaction remains the best explanation for most observations to date.
Near equiatomic FeCo based alloys are face-centred cubic (fcc) ()
above 983 C, and body-centred cubic (bcc) () below this
temperature. It is generally accepted that orders to a B2 structure
() below 730 C (figure 1).
The existence of an ordering reaction in Fe1-xCox alloys (with x in
the range 29-70 wt% ) has been widely accepted for more than 50
Early evidence for ordering were presented by Kussmann in 1932 ,
Rodgers and Maddocks in 1939  and Shull and Siegel in 1949 .
The ordering reaction, and particularly its kinetics, subsequently attracted
because of the presumed relationship between order and the severe intergranular
brittleness of these alloys. Such studies have used a variety of methods to
follow the evolution of the long range order parameter S as a function of
time and/or temperature. These methods have been summarised in a recent review,
co-written by the author .
Recently however, Ustinovshikov and Tresheva  have suggested that the ordering reaction does
not occur in the bulk, but rather at the surface of samples, provided that these
are heat-treated in air.
They further suggested that the heat-capacity peak observed in all
studies at 730 C relates to a change in the sign of the
interaction between Fe and Co rather than to the ordering
reaction. The elusive 550 C secondary peak
, sometimes referred to as `550 C anomaly', would
on the contrary correspond to an as yet undocumented phase-transition.
In the present work, we examine the evidence put forward by Ustinovshikov and Tresheva in the light
of a number of studies overlooked by these authors, and new results obtained so
as to test directly the validity of their alternative explanation.
As mentioned in the introduction, Ustinovshikov and Tresheva proposed that the ordered
phase only forms in a thin surface layer, provided that
heat-treatments are carried out in air. In an attempt to make this
claim compatible with the vast number of studies reporting the formation of
, the authors emphasised that most of these studies involved
heat-treatments in air (for example, Ref. ), and used X-ray diffraction
(for example, Ref. ) as a method to estimate the long-range order
parameter. Because this method only provides a relative measure of the order
parameter (that is, a sample of known degree of order is required), it can only
estimate the relative extent of the transformation (that is, the extent of the
transformation compared to that in the sample assumed to be fully ordered).
While these arguments are certainly valid, they only concern a few of
the majority of results reporting the existence of .
For example, Smith and Rawlings  carried out heat-treatments in a
salt bath and used neutron diffraction, which allows an estimation of
the absolute value of the long-range order parameter. Additional evidence of
ordering using neutron diffraction is provided by Yu et al. [21,24,25] or
Zhu et al. , for example. Neutron diffraction does not suffer from the
problems associated with X-rays: low penetration, and very weak superlattice
peaks (as discussed later). For example, while for X-rays, the ratio of
is 1/1390, it is
1/6 in neutron diffraction .
Clegg and Buckley [6,17] also reported ordering with samples
heat-treated in salt bath. Other methods have involved heat-treatments
in Pyrex capsules filled with argon , and most studies have
used at least two different experimental methods to establish the
degree of order.
It seems clear, therefore, that Ustinovshikov and Tresheva's hypothesis faces considerable
difficulties to fit past studies. Nevertheless, there remains the need to explain
their own experimental results.
In their study, Ustinovshikov and Tresheva used rectangular plates of 3 mm thickness
which they aged at 700 C in air and in vacuum.
X-ray diffraction showed that a surface layer produced peaks `consistent' with the
ordered phase , only for the samples heat-treated in air.
Once this layer was removed, the peaks disappeared. The authors reported a
logarithmic time dependency for the penetration into the bulk, and an increased
amount of in Fe-49Co-2V when compared to Fe-50Co.
The above authors also noted that this surface B2 structure has a
lattice parameter which is hardly dependent on the composition of the
alloy, with 0.2902 nm for Fe-35Co, 0.2905 nm for Fe-50Co and 0.2909 for Fe-65Co.
In addition, its hardness was found to be different (lower) than that of the bulk.
It is proposed that the observed layer was an oxide, previously reported by
Rogers et al. . Rogers underlined that the peaks
corresponding to this oxide are close enough to the peaks of the
phase so as to make its identification impossible if the
order is low.
The rate of penetration and the absence of the layer from samples heat-treated
in vacuum also strongly support this hypothesis. In addition, the
growth of a stoichiometric oxide could result in a phase of lattice
parameter relatively independent of the bulk composition, as
observed by Ustinovshikov and Tresheva.
The lattice parameter of the ordered phase, on the contrary,
is expected to depend on composition as follows :
The binary phase diagram for FeCo, after .
After heat-treatments at 700 C, the authors indicate that diffraction
peaks for the B2 structure never appear in the vacuum heat-treated samples, and
only in a surface layer in those heat-treated in air. The choice of 700 C
to perform the comparison is rather surprising as the degree of order is known to
be low at this temperature (figure 2).
It is therefore not impossible that the order parameter was too low in both
cases to give clear superlattice diffraction peaks.
The similarity between the X-ray atomic factors of Fe and Co means
that the superlattice peaks are very weak, even when anomalous
scattering effects are used to enhance their intensity (by using Fe or
Co radiations) : a value of
has been reported for Fe-50Co at its maximum degree of order
, using cobalt radiations as in the study of
Ustinovshikov and Tresheva. Other authors have underlined the impossibility of distinguishing
any superlattice line except for the
In contrast, in the results reported by Ustinovshikov and Tresheva , the peaks interpreted
as superlattice peaks (for example,
) have intensities
similar to those of the structure.
Therefore, neither the position nor the intensity of the peaks reported by Ustinovshikov and Tresheva
are in agreement with what they identify as . It is surprising that
the comparison between air/vacuum heat-treatment was not repeated at
temperatures lower than 700 C where the degree of order is expected to be
Finally, increased vacancy supersaturation have been observed in vanadium
containing alloys when compared to binary alloys . This could explain
the faster oxidation of the former as the vacancy supersaturation would enhance
Having postulated that there is no bulk ordering in FeCo alloys, Ustinovshikov and Tresheva attempted
to explain the transition observed at 730 C. For this purpose, they
proposed that the interactions between Fe and Co become repulsive
above 730 C. That is to say, the interaction parameter changes sign.
This, in turn, is argued to lead to separation in pure elements.
In samples held 4 h at 900 C or 1000 C and water quenched, Ustinovshikov and Tresheva
reported disappearance of the original solid solution peaks and apparition of two
sets of peaks consistent with an fcc phase of lattice parameter 0.3564 nm and
a bcc phase of lattice parameter 0.2848 nm. These are argued to correspond to
`pure' cobalt and `pure' iron, respectively.
Evidence based on X-ray measurements in Ustinovshikov and Tresheva's work are discussed
with an accuracy of about 0.0001 nm. However, as for the
identification of , the parameters reported differ
significantly from previously reported values (0.287 nm for
bcc iron and 0.3544 nm for fcc cobalt).
The lattice parameters of the and high-temperature Fe-Co solid
solutions have been reported as :
The long range order parameter as a function of
temperature in FeCo, after .
While these appear to provide a slightly better match for the results of Ustinovshikov and Tresheva,
The existence of an fcc phase at high temperature is well
documented (for example, [1,2]), but it is equally well established
that this phase cannot be retained at room temperature, and undergoes a
martensitic transformation to the bcc structure (for example, see Ref.
If a sample is quenched from the high temperature two-phase domain, it is
possible to distinguish between the martensite (
), and the high temperature because of their different
This has been used to estimate the width of the field
at high temperature (for example, ), by measurement of the
and compositions. Some of these works used identical
procedures as those described by Ustinovshikov and Tresheva. For example, Bennett and Pinnel
[29,30] or Mahajan et al.  aged samples of FeCo-2V for 6 h at 900,
925 and 950 C, in flowing Ar-10%H2 then quenched the samples in
ice-brine before measuring the composition.
None of these results report separation in two components at these temperatures.
Therefore, Ustinovshikov and Tresheva's observations cannot be explained by supposing that the
high-temperature heat-treatment fell in the two-phase region, not only because
of the narrowness of this domain, but also because the austenite would in any
case not have been retained at room temperature where the measurements were
performed. On the other hand, previous measurements of the composition at high
temperature do not support the hypothesis of a separation in pure components.
According to Ustinovshikov and Tresheva, the ordering reaction occurs only on the surface
of samples that have been heat-treated in air, and therefore cannot
be of any consequence for the magnetic or mechanical properties.
On the contrary, the authors suggest that the magnetic
properties are `grossly changed' by the formation of the hypothetical
-structure at 550 C. This, however, is only supported
by measurement of the saturation at 700 and 500 C (which are
reported as points in figure 3).
Interestingly, past experiments have never claimed that the ordering
reaction had a strong impact on the saturation magnetisation, and the
exact influence of the ordering reaction upon the mechanical
properties is still discussed .
Measurements of the saturation magnetisation show an increase of about 4%
upon ordering [32,6,33,34], as illustrated in figure
3. It is clear that, while one of the measurements by
Ustinovshikov and Tresheva is in good agreement with typical values, the saturation
at 700 C is widely different from any previously reported
Rather than examining whether details of past experiments could explain the
discrepancies, it was decided to perform a direct test of Ustinovshikov and Tresheva's hypothesis.
For this purpose, a sample of Rotelloy 8 was used. This is an equiatomic FeCo
alloy with 1V, 0.2 Ta wt%, made by Carpenter Ltd, and was provided as a
sheet of 150 m thickness. The sample had been cold-rolled and was therefore
given a recrystallisation heat-treatment of 2 h at 800 C. The length of
the heat-treatment ensures that there are no residues of texture
[35,36,37]. The heat-treatment was performed in a silica capsule
filled with a partial pressure of argon.
A sample 2 x 2 mm was then obtained by electro-erosion, to avoid any
deformation during the cutting process, and the magnesium oxide surface layer
(used as an insulator in laminate products) was removed by careful polishing.
A vibrating sample magnetometer was used to measure the saturation moment
as a function of temperature. The sample temperature is controlled by a flow of
helium, therefore protecting the sample from oxidation.
The variation of the saturation magnetisation
for FeCo-2V as a function of temperature, after . The two points
obtained by Ustinovshikov and Tresheva  are superimposed. Note that the measurement at 700
C is of 0.5 T.
In a first experiment, the saturation moment was measured after maintaining
the sample at 750 C for a few minutes, then decreasing the
temperature at a rate of 8 C/s.
In a second experiment, the sample was held a few minutes at 750 C, then
quenched to 560 C where the saturation moment was measured and
found to be similar to that obtained in the previous experiment.
The sample was then quenched to 540 C to follow the evolution of
the saturation moment as a function of time for 10 minutes.
This is because, according to Ustinovshikov and Tresheva, the transformation to the
-structure only happens after 5 minutes. Because no significant
variation occurred, the measurement was repeated at 500 C. The results are
shown in figure 4.
Clearly, neither of the measurements undertaken in the present work
support the existence of a phase-transition at 550 C.
The hypotheses put forward by Ustinovshikov and Tresheva are difficult to reconcile with a number of
studies overlooked by these authors. In addition, part of their results can be
understood without rejecting ordering, while another part could not be
reproduced. For example, Ustinovshikov and Tresheva report a saturation magnetisation of
0.5 T at 700 C, but 2.20 T at 500 C. All of the published
literature and our present results fail to confirm such a large change
between 500 and 700 C.
Furthermore, the alternative explanation put forward for the 730
C transition is dubious: if this peak was the result of
separation into pure elements, one would expect a significant cooling
rate dependency, and relatively slow overall kinetics, as the process
would involve long-range diffusion. However, regardless of the nature
of the transformation occurring at 730 C, its kinetics
 are clearly not compatible with a reaction involving
The author is grateful to N. Mathur for help with the magnetisation
measurements, to Rolls-Royce, in particular S. W. Hill, for provision of the
samples and to Pr. H. K. D. H. Bhadeshia for support and discussions.
(A) The saturation moment as a function of
temperature for a sample cooled at 8C/s. The first derivative
is also shown as points, without scale. (B) The saturation moment at 540 and
500 C as a function of time, showing no significant evolution.
Note that the peak in (A) is slightly below the temperature
generally agreed for the onset of ordering, which is to be expected
given the relatively high cooling rate used.
I. Ohnuma, H. Enoki, O. Ikeda, R. Kainuma, H. Ohtani, B. Sundman, K. Ishida,
Acta Mater. 50 (2002) 379-393.
A. S. Normanton, P. E. Bloomfield, F. R. Sale, B. B. Argent, Metal Sci. 9
L. Kussmann, Zeit. Techn. Physik 13 (1932) 449-460.
J. W. Rodgers, W. R. Maddocks, Tech. rep., Second alloy steels report-Section
C. G. Shull, S. Siegel, Phys. Rev. 75 (1949) 1008-1010.
D. W. Clegg, R. A. Buckley, Metal Sci. J. 7 (1973) 48-54.
N. S. Stoloff, R. G. Davies, Acta Metall. 12 (1964) 473-485.
P. Grosbras, J. P. Eymery, P. Moine, Acta Metall. 24 (1976) 189-196.
M. Rajkovic, R. A. Buckley, Metal Sci. (1981) 21-29.
P. Grosbras, J. P. Eymery, P. Moine, Scripta Metall. 7.
A. W. Smith, R. D. Rawlings, Phys. Stat. Sol. A 34 (1976) 117-123.
R. A. Buckley, Metal Sci 19 (1975) 243-247.
J. P. Eymery, P. Grosbras, P. Moine, Phys. Stat. Sol. A 21 (1974) 517-528.
A. T. English, Trans. Met. Soc. AIME 236 (1966) 14-18.
J. A. Rogers, H. M. Flower, R. D. Rawlings 9 (1975) 32-35.
J. F. Dinhut, H. Garem, J. P. Eymery, P. Moine, Scripta Metall. 8 (1974)
R. A. Buckley, Metal Sci. 13 (1979) 67-72.
R. V. Major, C. M. Orrock, IEEE Trans. Magn. 24 (1988) 1856-1858.
A. I. C. Persiano, R. D. Rawlings, J. Mat. Sci. 26 (1991) 4026-4032.
A. I. C. Persiano, R. D. Rawlings, J. Mater. Eng. 12 (1990) 21-27.
R. H. Yu, S. Basu, Y. Zhang, J. Q. Xiao, J. Appl. Phys. 85 (8) (1999)
T. Sourmail, H. K. D. H. Bhadeshia, Microstructure, magnetic and mechanical
properties of FeCo based alloys, Tech. rep., Advanced Aerospace Materials
DARP, Rolls-Royce (2004).
Y. Ustinovshikov, S. Tresheva, Mat. Sci. Eng. A A248 (1998) 238-244.
R. H. Yu, S. Basu, Y. Zhang, A. Parvizi-Majidi, J. Q. Xiao, J. Appl. Phys.
85 (9) (1999) 6655-6659.
R. H. Yu, S. Basu, R. Y. Zhang, A. Parvizi-Marjidi, K. M. Unruh, J. Q. Xiao,
IEEE Trans. Magn. 36 (5) (2000) 3388-3393.
Q. Zhu, L. Li, M. S. Masteller, G. J. D. Corso, Appl. Phys. Lett. 69 (25)
J. A. Ashby, H. M. Flower, R. D. Rawlings, Metal Sci 11 (1977) 91-96.
K. Kawahara, J. Mat. Sci. 18 (1983) 3427-3436.
J. E. Bennett, M. R. Pinnel, J. Mater. Sci 9 (1974) 1083-1090.
M. R. Pinnel, J. E. Bennett, Metall. Trans. 5 (1974) 1273-1283.
S. Mahajan, M. R. Pinnel, J. E. Bennett, Metall. Trans. 5 (1974) 1263-1272.
C. W. Chen, J. Appl. Phys. 32 (3) (1961) 348S-355S.
L. Li, J. Appl. Phys. 79 (8) (1996) 4578-4580.
J. E. Goldman, R. Smoluchowski, Phys. Rev. 75 (1949) 310-311.
M. M. Borodkina, E. I. Detalf, Y. P. Selisskiy, Fiz. Metal. Metalloved. 7
A. A. Goldenberg, Y. P. Selliskiy, Fiz. Metal. Metalloved. 15 (5) (1963)
Y. P. Selliskiy, M. N. Tolochko, Fiz. Metal. Metalloved. 13 (4) (1962)
This document was generated using the
LaTeX2HTML translator Version 2002 (1.62)
Copyright © 1993, 1994, 1995, 1996,
Computer Based Learning Unit, University of Leeds.
Copyright © 1997, 1998, 1999,
Mathematics Department, Macquarie University, Sydney.
The command line arguments were:
latex2html -split 1 -title 'Comments on `Character of transformations in Fe-Co systems`.' -white -noparbox_images -math_parsing -notop_navigation -nonavigation -noreuse -dir ./ index.tex
The translation was initiated by on 2005-02-07