Spinodal decomposition in the binary Fe-Cr system Saeed Baghsheikhi Master’s degree project School of Industrial Engineering and Management Department of Materials Science and Engineering Royal Institute of Technology SE-100 44 Stockholm Sweden August 2009
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Spinodal decomposition in the binary
Fe-Cr system
Saeed Baghsheikhi
Master’s degree project
School of Industrial Engineering and Management
Department of Materials Science and Engineering
Royal Institute of Technology
SE-100 44 Stockholm
Sweden
August 2009
ii
iii
Abstract Spinodal decomposition is a phase separation mechanism within the miscibility
gap. Its importance in case of Fe-Cr system, the basis of the whole stainless steel
family, stems from a phenomenon known as the “475oC embrittlement” which
results in a ruin of mechanical properties of ferritic, martensitic and duplex stainless
steels. This work is aimed at a better understanding of the phase separation process
in the Fe-Cr system.
Alloys of 10 to 55 wt.% Cr , each five percent, were homogenized to achieve
fully ferritic microstructure and then isothermally aged at 400, 500 and 600oC for
different periods of time ranging from 30min to 1500 hours. Hardness of both
homogenized and aged samples were measured by the Vickers micro-hardness
method and then selected samples were studied by means of Transmission
Electron Microscopy (TEM).
It was observed that hardness of homogenized samples increased monotonically
with increasing Cr content up to 55 wt.% which can be attributed to solution
hardening as well as higher hardness of pure chromium compared to pure iron. At
400oC no significant change in hardness was detected for aging up to 1500h,
therefore we believe that phase separation effects at 400oC are very small up to this
time. Sluggish kinetics is imputed to lower diffusion rate at lower temperatures. At
500oC even after 10h a noticeable change in hardness, for alloys containing 25 wt.%
Cr and higher, was observed which indicates occurrence of phase separation. The
alloy with 10 wt.% Cr did not show change in hardness up to 200h which suggests
that this composition falls outside the miscibility gap at 500oC. For compositions of
15 and 20 wt.% Cr only a small increase in hardness was detected even after 200h
of aging at 500oC, which could be due to the small amounts of α´ formed.
However, it means that alloys of 15 wt.% Cr and higher are suffering phase
separation. For compositions inside the miscibility gap, hardening effect is a result
of phase separation either by nucleation and growth or spinodal decomposition. To
distinguish between these two mechanisms, TEM studies were performed and we
found evidence that at 500oC the Fe-25 wt.% Cr sample decomposes by nucleation
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and growth while that of 35 wt.% Cr shows characteristics of the spinodal
mechanism. For compositions inside the miscibility gap, with increasing Cr content
up to 40% the change in hardness generally increased and for 45% and higher it
always decreased. This suggests that the composition range corresponding to the
spinodal region at 500oC is biased towards the Fe-rich side of the phase diagram. At
600oC only samples of 25, 30 and 35 wt.% Cr were studied because according to
the previous studies, the spinodal boundary is most probably located in this
composition range. However, no change in hardness was observed even up to 24h.
We believe that this means the miscibility line lies below 600oC for alloys
containing 35 wt.% Cr and lower. Further investigations are needed to confirm and
Fig. 4.7: Hardness vs. aging time at 500oC for different compositions
4.3.2 TEM micrographs Results from electron microscopy are presented in figures 4.8 and 4.9. Figures
4.8a to 4.8c correspond to the Fe-35wt.%Cr sample aged at 500oC for 10,100 and
200h respectively.
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Fig. 4.8: TEM micrographs of Fe-35wt.% Cr alloy aged for (a) 10h and (b) 100h and (c) 200h. Scale bar is 10nm in all cases. Zone axis is close to [100]. No diffraction pattern corresponding to a second phase was observed.
Fe-35 wt.% Cr alloy aged for 10h at 500oC does not show a modulated contrast
(fig. 4.8a) while such an effect is observable after 100h of aging at this temperature
(fig. 4.8b). These modulations are amplified with increase in aging time while the
observed domain size, which was about 5-10nm, is more or less unchanged.
Modulation in contrast which is a characteristic of spinodal mode of
decomposition was not detected in Fe-25 wt.% Cr alloy aged for 200h, instead fine
particles in the order of 10nm in size were detected. See figure 4.9 and compare
with figures 4.8b and 4.8c.
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Fig. 4.9: TEM micrograph of Fe-25 wt.% Cr sample aged for 200h at
500oC, scale bar is 200 nm. Zone axis is close to [111] and no superimposed
diffraction pattern corresponding to a second phase was detected.
4.4 Aging at 600oC
Hardness measurements did not show any notable change for aging times up to
24 hours (see figure 4.10). Fluctuations could be attributed to the experimental
scatter as the error bars of each composition overlap.
aging at 600C
100
125
150
175
200
225
250
0 5 10 15 20 25 30
time (h)
hard
ness
(HV0
.1)
25% Cr30% Cr35% Cr
Fig. 4.10: Hardness versus aging time at 600oC
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4.5 Comparison between experiments and literature phase diagrams
In figures 4.11 and 4.12 results of this survey are summarized and compared with
other studies. Figure 4.11 also contains miscibility gap and spinodal lines calculated
by Xiong [23] as well as that of Andersson and Sundman. These calculations were
performed using the Thermo-calc software and are based on optimization of the
experimental data available by their time. At 500oC results of this work are in
complete agreement with Williams, Chandra and Xiong but regarding the position
of spinodal line it disagrees with Andersson and Sundman. At 600oC our results are
consistent with Williams and Chandra, see figure 4.12, but oppose with that due to
Xiong.
Figure 4.11: Miscibility gap and spinodal lines calculated by
Thermocalc and compared with experimental studies. [23]
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Figure 4.12: Miscibility gap due to Williams and Paxton and spinodal due to Chandra and Schwartz
Compared with the results of this study.
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Chapter 5
DISCUSSION The increase in hardness with increasing Cr content up to 55 wt.% for
homogenized alloys can be attributed to solution hardening as well as higher
hardness of pure chromium compared to pure iron (see table 2.1), the latter being a
reason for increase in hardness while composition exceeds 50 wt.% Cr.
At 400oC lack of a noticeable increase in hardness up to 1500h suggests that
reaction development is small up to this time. Although a slight increase is
detectable, especially for compositions between 30 to 50 wt.% Cr, since error bars
are always overlapping it cannot prove any phase separation effect. Sluggish
progression of phase separation at 400oC is due to slower diffusion rate compared
to the higher temperatures. Therefore to detect the decomposition, longer aging
times are necessary. Result of this study for 400oC is in agreement with phase field
calculations by Grönhagen and Ågren [24] which states that “no obvious phase
separation is seen during the first 2000h”.
At 500oC lack of hardness change for Fe-10 wt.% Cr sample up to 200h suggests
that this composition/temperature lies outside the miscibility gap. A small, but
statistically acceptable, increase in hardness for 15 and 20 wt.% Cr compositions is
observed which implies that these alloys have suffered phase separation and
therefore they are inside the miscibility gap. Since these two compositions are close
to the miscibility boundary, amount of α´ that can form is small and accordingly it
cannot result in a remarkable hardness increase. For alloys of 25 wt.% Cr and
higher phase separation effect is quite significant. In case of Fe-25 wt.% Cr alloy
TEM analysis suggests that phase separation mechanism is nucleation because
although hardness shows that this sample has started to decompose, there is still
no modulation in contrast and instead fine particles are present. Lack of any
superimposed pattern in all diffraction images can be attributed to similar lattice
parameter for α and α´ phases as well as the small amount of α´. In case of Fe-35
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wt.% Cr alloy, TEM images of 100h and 200h aged samples are showing modulated
microstructure and therefore spinodal decomposition is the dominating
mechanism. Based on this result we conclude that spinodal boundary at this
temperature lies between 25 and 35 wt.% Cr. TEM micrographs also suggest that a
domain size of about 5-10nm is formed in case of Fe-35 wt.% Cr which has the
same order of magnitude with results of Brenner and Miller[17]. This domain size or
wavelength of modulations was more or less unchanged up to 200h which is not
far from expectation because of low interfacial energy between α and α´ phases.
However, amplitude of modulations i.e. contrast between domains was intensified
as aging time was extended. This is in accordance with the theory discussed in
section 2.5. Since inside the spinodal )βR( is positive and, according to eq. 12,
amplitude grows exponentially with time, discretion between domains in figure 4.8c
is easier than figure 4.8b.
At 500oC decomposition was noticeable already for 10 hours of aging which is
due to faster diffusion compared to 400oC. Another interesting observation at
500oC was that phase separation, which is directly related to hardness change (∆H),
generally increased up to 40 wt.% Cr and always decreased after 45. This suggests
that the spinodal center, a composition for which reaction development is highest
at a certain temperature and time, lies between 40 and 45 wt.% Cr, i.e. it is biased
towards Fe-rich side of the phase diagram. There are both thermodynamic and
kinetic descriptions to explain why reaction development is highest at an
intermediate composition inside the spinodal. At a constant temperature as
composition moves from spinodal boundary, 0G/dXd 22 = , towards the apex of
Gibbs energy-composition curve, 0dG/dX = , driving force for precipitation
increases and decomposition is promoted, see figure 2.1b. Also value of 22G/dXd
becomes more negative, thus critical wavelength decreases according to eq. 3 and
probability of thermal fluctuations to create a concentration variation of a
wavelength larger than cλ increases which translates to faster kinetics.
Regarding hardness-aging time curves at 500oC increasing trend of our curves are
in agreement with previous studies [3,5,6,17], but their curvature is different. Such
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discrepancy will diminish to a great extent if we draw time axis in logarithmic scale
and allow a smooth line to pass through data points, instead of connecting
successive points by straight lines, just as other researchers did. However, since we
have smaller number of data points and rather shorter investigated times it is still
preferred to demonstrate the data with a linear time scale.
At 600oC no change in hardness was observed which indicates that no phase
separation takes place up to 24h. Since at 500oC a noticeable hardness increase after
10h was detected, it was expected that any nucleation or spinodal reaction at 600oC
would initiate earlier because the temperature is higher. Therefore absence of any
decomposition effect up to 24h indicates that this temperature is above the
miscibility boundary for alloys of 35 wt.% Cr and lower.
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Chapter 6
CONCLUSIONS
1- In a binary solid solution of Fe and Cr, the hardness of the alloy increases
with increasing Cr content up to 55 wt.%.
2- At 400oC the hardness does not increase markedly up to 1500h, which means
that no notable phase separation using the hardness method can be detected.
3- At 500oC alloy of 10 wt.% Cr does not show hardening effect up to the
investigated time of 500h. However, those of 15 wt.% Cr and higher are
demonstrating phase separation.
4- Alloys of 15 to 25 wt.% Cr decompose by nucleation and growth at 500oC
while those of 35 wt.% Cr and higher are showing characteristics of spinodal
mechanism.
5- Within the miscibility gap at 500oC increasing Cr content up to 40 wt.% is
generally accompanied by increase in reaction development, but from 45%
onwards it shows a decreasing trend. This suggests that the centre of spinodal at
500oC lies between 40 and 45% Cr contents, in other words it is biased towards
Fe-rich side of the phase diagram.
6- At 500oC for compositions inside the spinodal of Fe-Cr system hardness
increased monotonically up to 200h.
7- At 600oC no phase separation effect was observed by hardness measurements
up to 24h. It indicates that this temperature is beyond the miscibility gap for
alloys of 35 wt.% Cr and lower.
8- Hardness method is able to show the phase separation effects for
compositions well inside the miscibility gap. For those close to the immiscibility
boundary more sensitive methods are suggested.
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Future work
There is still much research necessary to achieve a comprehensive understanding
of the Fe-Cr system, the following topics seemed interesting to the author’s
opinion as a means to approach this goal:
1- Aging at intermediate temperatures for longer times, especially alloys of 10-35
wt.% Cr.
2- Following the microstructural changes by means of APFIM, STEM or EDS unit
loaded in TEM.
3- Effect of inclusions, applied stress or magnetic field on spinodal decomposition
in Fe-Cr binary system.
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