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Microstructure evolution and hardening by spinodal decomposition in Fe-Ni-Mn-Al Alloys
R. K. Zheng a, David Saxey a, Satoko Kuwano a, James A. Hanna a, Markus K. Wittmann b, John Loudis b, Ian Baker b, Zongwen Liu a, Ross Marceau a, Simon P. Ringer a
a Australian Key Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australia b Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755 Abstract
Fe-Ni-Mn-Al alloys was cast and then aged at 550 °C. Transmission electron
microscopy and atom probe studies clearly show the occurrence of spinodal
decomposition. The microstructure and mechanical properties evolution with ageing was
carefully investigated and compared with theoretical models. The relationships between
microstructure and mechanical properties therefore concluded.
Keywords:
Spinodal Decomposition; Hardness; microstructure; Atom probe; Transmission electron
microscopy (TEM)
1
1 Introduction
The decomposition of unstable solutions has two mechanisms: (i) instability with
respect to local perturbations that are large in degree but small in extent (nucleation and
growth), and (ii) instability with respect to a nonlocal perturbation that is small in degree
but large in extent (spinodal decomposition) [1]. There has been much work on the effect
of former mechanism on mechanical properties [2, 3]. Whereas, it deserves much effort
how the mechanical properties are affected by the long-range coherent composition
fluctuations resulting from spinodal decomposition.
The quaternary metallic Fe-Ni-Mn-Al system is of interest for its rich assortment
of possible phase transformations and, therefore, wide array of potential microstructures
and properties [4]. The strength of the alloys is comparable to the strongest maraged
aircraft steels and hardest bearing steels available on the market, but with a better
strength-to-weight ratio. The high aluminum content not only contributes to oxidation
resistance, but makes Fe-Ni-Mn-Al alloys lighter than any steels [4, 5]. The
Fe35Ni15Mn25Al25 alloys display interesting hardening behaviors with annealing. At
the early stage of annealing, the hardness increases significantly. After that, the hardness
decreases. And finally the hardness increases again with further annealing. The
interesting hardness behavior should be pertinent to the microstructure evolution with
annealing. In order to reveal the microstructure-properties relationship in the spinodal
decomposition, Atom probe and transmission electron microscopy (TEM) including
energy dispersive x-ray spectrometry (EDS) was used to characterize the resulting
microstructure in the as cast, and annealed condition, and room temperature hardness
measurements were performed to survey the mechanical properties.
2
2 Experiments
The Fe35Ni15Mn25Al25 alloys were arc melted in a water-cooled copper mold
under argon from constituent elements that were of 99.9% purity or better. Ingots were
flipped and melted four times to ensure mixing. The alloys were cooled down from the
melt in 1-2 minutes to avoid the cracking caused by rapid quench. Subsequent
measurements on a Cameca SX50 electron probe microanalyzer showed that final
compositions were all within 0.4 at.% of the nominal value. Some variation in the scale
of as-cast microstructures was present between castings, although properties and behavior
were similar. Subsequently annealing was performed at 550 °C in air for up to 72 hours.
Hardness measurements were performed in air at room temperature using a Leitz
MINIload tester with a Vickers-type indenter using a 200g load and a 12 s drop time.
Reported values are the average of at least 5 measurements. Transmission electron
microscopy (TEM) was used to characterize the microstructure. Small pieces were first
cut with water cooled diamond saw. The small pieces (0.4×1×3 mm) were mechanical
grinded using tripods on diamond papers. Finally the grinded wedges supported by
copper grids were thinned by a Gatan precision ion polish system. The specimens were
studied with Philips CM12 TEM and JEOL 3000F TEM equipped with X-ray energy
dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Imago
local electrode atom probe (LEAP) was also employed to investigate the spinodal
decomposition in the alloys. Blanks of 0.4×0.4×10 mm were cut by diamond saw. Then
the blanks were electropolished in 20% and 2% perchloric acid in butoxyethanol for the
rough and fine stages respectively.
3
3 Results and discussion
3.1 Transmission electron microscopy
Transmission electron microscopy study revealed that the Fe30Ni20Mn25Al25
alloys have a periodic coherent microstructure, which suggests the occurrence of spinodal
decomposition [6, 7]. The interconnected spinodal decomposition of elemental
segregation is visible by EELS elemental mapping. For example, Fe, Ni, Mn, Al maps of
one specimen were shown in Fig. 1. It is obvious that Ni co-segregates with Al and Fe
co-segregates with Mn. The Ni/Al and Fe/Mn rich regions form spinodal wave top and
bottom, respectively. Phases and possible ordering transformations in the systems with
the presence of more than two elements have been discussed [8, 9]. Spinodal
decomposition has also been ob served in the ternary Fe-Ni-Al [10] and Fe-Ni-Mn [11]
systems. In the latter, and possibly the former, the relatively rapid decomposition leads to
a metastable two-phase structure [4, 5]. The same situation also occurs in the
Fe35Ni15Mn25Al25 alloys. In this system, it is worthy of noting that the immiscibility of
Fe and Ni at low temperatures, the strong ordering tendencies of Ni and Al, and the high
solubility of Mn in both Fe and Ni [4]. The last of three factors can provide enough
flexibility in elemental segregation to allow for close lattice matching between the two
phases. Such segregation of spinodal decomposition involves a very low coherency strain
barrier and, hence, can proceed much more rapidly than phase separation by nucleation
and growth [12].
The wavelength (λ) and amplitude (A) are the two characterization factors of
spinodal decomposition. From the TEM images shown in Figs. 2(a-d), the wavelength
can be easily determined directly. To obtain the average wavelengths of spinodal
4
decomposition, we did fast Fourier Transformation (FFT) of images and determined the
wavelength by measuring the FFT spots. Results are summarized in Table 1. The
evolution of wavelength with ageing may also be derived. It is clear that the wavelength
increases with ageing time monotonically. This indicates that spinodal decomposition
significantly develops as aging proceeds. EDS has the capability to measure the
composition fluctuation and amplitude can be, therefore, obtained. We would not like to
adopt EDS to determine the amplitude from the EDS analysis, because both the elemental
composition and spatial resolution of EDS are not as high as atom probe. Higher spatial
resolution requires smaller electron beam, as a consequence, the elemental composition
resolution gets lower.
The structures of the Fe30Ni20Mn25Al25 alloys can be inferred from the electron
diffraction patterns of [001] [Figs. 1(e-h)] and [101] [Figs. 2(i-l)] orientations. The parent
phase has been believed to be is B2-structured [4]. The 2 h annealed specimen displays
B2 (NiAl) and bcc (Fe, Mn) structures clearly according to Figs. 2(f, j). However, another
phase besides B2 appears in the as-cast and 22 h annealed specimens, as shown in Figs.
2(e, g, i, k), which indicate the occurrence of L21 structure [13]. In the 72 h annealed
specimen, third-phase precipitates were observed and determined to be β-Mn structured,
as shown in Fig. 2(h). Precipitates are not included in the diffraction pattern of Fig. 2(l).
The initially insuppressible concentration waves, which define the beginnings of a
regular, periodic, and often interconnected array of two coherent phases, is characteristic
of the spinodal decomposition of a solid solution [6, 7]. The orientation of concentration
waves, i.e., the spinodal phases, is determined by the elastic anisotropy of the lattice, so It
favorers the weak cube directions of low elastic anisotropy [6]. To find out the spinodal
5
decomposition directions of the Fe30Ni20Mn25Al25 alloys, we investigate the alloys by
high resolution TEM (HRTEM). From the HRTEM images shown in Figs. 1(j-m), we can
see the spinodal decompositions waves and crystallographic lattices. The insets are the
corresponding FFT patterns of lattices. It is clear that the spinodal decomposition of the
Fe30Ni20Mn25Al25 alloys propagates along <100> directions, as previous reported [1].
3.2 Atom probe
TEM is one of the best-adapted investigation techniques to study the
microstructural evolution takes place at nanometer scale. Unfortunately, the combination
of the similarity of atomic scattering factors of these elements and the high coherency of
the forming phases limits the capability of TEM to characterize the fine microstructural
evolution in spinodal decomposition systems. Conversely, 3-dimensional atom probe
(3DAP) is very suitable to study the fine scale phase separation because it provides both
microstructural and microchemical analysis at near atomic scale. Therefore, it is possible
to characterize the spinodal decomposition of Fe-Ni-Mn-Al alloys quantitatively with
3DAP.
In contrast EDS in TEM, mass spectrometer is used to distinguish different
elements in atom probe. The identification of Fe, Ni, Mn and Al is not a problem for the
atom probe analysis. The only minor problem is the overlap of 58Fe and 58Ni. But this
overlap can usually be accounted for, since we know the natural abundance of the
elements isotopes. In fact, the natural abundance 58Fe is so low that we can ignore it. A
typical mass spectrum was indexed and shown in Fig. 3. Another aspect is the different
evaporation field at the specimen surface. As a consequence, elements with low
evaporation field may evaporate between the evaporation pulses, and thus not be detected,
6
resulting in underestimated measured concentration. A pulse fraction (pulse voltage over
standing voltage) of 20%, and lower specimen temperature are commonly used to
minimize the effect.
The 3D reconstructions of the spinodal alloys obtained with the topographic atom
probe are shown Fig. 4 (a-d). Each dot is a single atom, and different elements are labeled
with different colors. The spinodal decomposition is evidenced with the exclusive Ni/Al-
rich and Fe/Mn-rich regions. The 3D reconstruction provides good representation of the
morphology of the microstructure.
The 1-dimensional concentration profiles along the spinodal decomposition
direction give a possible way to determine the wavelength and amplitude of spinodal
decomposition. The first important information provided by the concentration profiles is
the characteristic length of the decomposition, i.e., the wavelength. But it is difficult to
align the 1D profile along the spinodal propagation. The spatial resolution of atom probe
is not better than TEM. So we adopted the wavelength values given by TEM.
The solute distribution in the alloys can be determined with a frequency
distribution. A frequency distribution is a plot of frequency of occurrence of data blocks
with a given number of solute atoms against that number. Alternatively, the frequency
distribution may also be plotted in terms of concentration [14]. The frequency
distribution of Fe of the alloys was shown in Figs. 4(e-h). The presence of more than one
peak indicates the presence of phase separation, here spinodal decomposition, in the
material. A variety of models had been derived to determine the compositions of the
phases. The solute concentrations in a pair of coexisting phases may be estimated with
the Pa or sinusoidal method. This method is based on Cahn's sinusoidal or linear theory
7
of spinodal decomposition [15]. Another method to determine the solute concentrations
of the coexisting phases has been developed based on the Langer, Bar-on, Miller (LBM)
treatment of non-linear spinodal decomposition [16]. In this model, Langer et al.
considered a probability distribution consisting of a pair of Gaussian distributions. The
results of LBM model were plotted in Figs. 4(i-l) for comparison with the experimental
frequency distributions. The Fe compositions at spinodal wave top and bottom are
indicated by the two peaks in frequency distribution. The amplitudes of spinodal
decomposition were derived and summarized in Table 1.
3.3 Mechanical properties
The hardness dependence on annealing time was shown in Fig. 5. This alloy
displays the early hardening and then softening clearly. Hardness increases from 540
Kg/mm2 without annealing to 578 Kg/mm2 with 2 hour anneal. But hardness decreases to
541 Kg/mm2 with 22 hour anneal. Further anneal increase the hardness again. Hardness
reaches 666 Kg/mm2 after 72 hour anneal. Annealing in 10 min – 3 hours range and
headiness measurements were repeated several times, and we found them to be repeatable.
Significant rapid strengthening has been observed in spinodal materials by heat
treatment has been attributed to the amplitude increase of the composition waves and the
associated strain field [17-19]. With amplitude and wavelength information, the
relationships between the fine-scale microstructure and the mechanical properties can be
derived from mechanical data. The first model was proposed by Cahn [20]. He studied
how the mechanical properties of a cubic crystal should be affected by the long-range
coherent composition fluctuations resulting from spinodal decompositio. He then
assumed a sinusoidal shape of concentration waves and determined the yield stress of the
8
material. He found that the yield stress should increase in proportion to A2λ. The
magnitudes of yield stress of Cahn model usually are lower than experimental values,
because wave squaring was not taken into account [19]. In fact, Cahn assumed that
temperature of aging primarily affects the wavelength, while time of ageing primarily
affects amplitude, which is not consistent with our experiments. Cahn also assumed that
the spinodal decomposition propagated along <100> directions, which is not consistent
with our experiments either. However, in spite of many consistencies, Chan’s model
interprets our results very well. As shown in Fig. 6, the hardness of the alloys is
proportional to A2λ.
A different coherency stress model was proposed by Dahlgren [21], who assumed
a lamellae square-wave structure in contrast to Cahn’s. Unlike Cahn, he did not put
constraints on the dislocation shape, nor did he include line tension forces. He found that
yield stress is proportional to A and independent of λ. While Cahn’s result predicted a
yield stress smaller than the experimentally observed, generally the Dahlgren equation
predicts a too large yield stress. Although the spinodal decomposition of Fe-Ni-Mn-Al
alloys has nearly square-wave shape, the spinodal decomposition of Fe35Ni15Mn25Al25
alloys does not follow this model.
Kato et al [22, 23] studied the hardening mechanism by spinodally modulated
structure in body centered cubic (bcc) systems. They found that both the misfit effect due
to the coherent internal stress and the modulus effect due to the special variation of
elastic modulus contributed significantly to the increment in yield stress. Yield strength is
determined to be proportional to A and inversely proportional to λ for misfit effect and
9
modulus effect respectively. Although the Fe35Ni15Mn25Al25 alloys are bcc-strucutred,
the experimental results are not consistent with this model.
The strength and hardness of as-cast and annealed Fe35Ni15Mn25Al25 alloys
likely arise from other factors besides spinodal decomposition too. The B2/bcc modulated
structure seems to lead to high hardness, since 2 h and 72 h annealed specimens with this
structure display higher hardness than the other two. The occurrence of high-ordered L21
phase is related to the low hardness of as-cast and 22 h annealed specimens. The
precipitate of disordered β-Mn phase in 72 h annealed specimens may also be responsible
for the hardening in the further annealed specimens.
4 Conclusion
Spinodal decomposition of Fe35Ni15Mn25Al25 alloys was clearly observed with
TEM and atom probe. Chemical analysis revealed that the elements tended to decompose
into a Fe/Mn rich bcc phase and Ni/Al rich B2 phase. Cahn’s model of spinodal
decomposition can successfully account for the hardness behavior. Ordered L21 phase
appears in some specimen and leads to low hardness. The β-Mn precipitate observed in
72 hour annealed alloy may contribute to the high hardness.
Acknowledgement This work was supported by….
10
References [1] Ditchek B, Schwartz LH. Applications of Spinodal Alloys. Annu Rev Mater Sci
1979;9:219.
[2] NABARRO RN. Adv. Phys. 1952;1:269.
[3] Ringer SP, Hono K, Polmear IJ, Sakurai T. Nucleation of precipitates in aged Al-
Cu-Mg-(Ag) alloys with high Cu:Mg ratios. Acta Mater 1996;44:1883.
[4] Hanna JA, Baker I, Wittmann MW, Munroe PR. A new high-strength spinodal
alloy. Journal of Materials Research 2005;20:791.
[5] Baker I, Wittmann MW, Hanna J, Munroe PR. Microstructure and mechanical
behavior of Fe-20Ni-25Mn-25Al. JOM 2004;56:150.
[6] Cahn JW. Spinodal Decomposition in Cubic Crystals. Acta Metallurgica
1962;10:179.
[7] Cahn JW. Phase Separation by Spinodal Decomposition in Isotropic Systems.
Journal of Chemical Physics 1965;42:93.
[8] Soffa WA, Laughlin DE. Decomposition and ordering processes involving
thermodynamically first-order order -> disorder transformations. Acta Metallurgica
1989;37:3019.
[9] Soffa WA, Laughlin DE. High-strength age hardening copper-titanium alloys:
redivivus. Progress in Materials Science 2004;49:347.
[10] Misra A, Gibala R, Noebe RD. Optimization of toughness and strength in
multiphase intermetallics. Intermetallics 2001;9:971.
[11] Singh J, Wayman CM. Age-Hardening Characteristics of a Martensitic Fe-Ni-Mn
Alloy. Materials Science and Engineering 1987;94:233.
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[12] Cahn JW. On Spinodal Decomposition. Acta Metallurgica 1961;9:795.
[13] Wittmann M, Baker I, Munroe PR. The structure and mechanical properties of
Fe2AlMn single crystals. Philos Mag 2004;84:3169.
[14] Miller MK. Atom probe tomography : analysis at the atomic level. New York:
Kluwer Academic / Plenum Publishers, 2000.
[15] Cahn JW. 1967 Institute of Metals Lecture Spinodal Decomposition. T Metall Soc
Aime 1968;242:166.
[16] Langer JS, Baron M, Miller HD. New Computational Method in Theory of
Spinodal Decomposition. Phys Rev A 1975;11:1417.
[17] Kato M, Mori T, Schwartz LH. Hardening by Spinodal Modulated Structure. Acta
Metallurgica 1980;28:285.
[18] Schwartz LH, Mahajan S, Plewes JT. Spinodal Decomposition in a Cu-9 Wt
Percent Ni-6 Wt Percent Sn Alloy. Acta Metallurgica 1974;22:601.
[19] Schwartz LH, Plewes JT. Spinodal Decomposition in Cu-9Wt Percent Ni-6Wt
Percent Sn .2. Critical Examination of Mechanical Strength of Spinodal Alloys. Acta
Metallurgica 1974;22:911.
[20] Cahn JW. Hardening by Spinodal Decomposition. Acta Metallurgica
1963;11:1275.
[21] Dahlgren SD. Correlation of Yield Strength with Internal Coherency Strains for
Age-Hardened Cu-Ni-Fe Alloys. Metallurgical Transactions a-Physical Metallurgy and
Materials Science 1977;8:347.
[22] Kato M. Hardening by Spinodally Modulated Structure in Bcc Alloys. Acta
Metallurgica 1981;29:79.
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[23] Park KH, Lasalle JC, Schwartz LH, Kato M. Mechanical-Properties of Spinodally
Decomposed Fe-30 Wt-Percent Cr Alloys - Yield Strength and Aging Embrittlement.
Acta Metallurgica 1986;34:1853.
13
Table captions Table 1 The phases, amplitude, wavelength and hardness of as-cast and annealed alloys.
alloy phases Amplitude (%) wavelength (nm) hardness As-cast B2, L21 38.3 17 540 2 hours B2, bcc 45.5 27 573 20 hours B2, L21 27 39 541 72 hours B2, bcc, β-Mn 50 ~50 666
14
Figure Captions Fig. 1 EELS maps of Fe, Ni, Mn, Al of 2 h annealed specimens. Fig. 2 (a-d) TEM images reveal spinodal decomposition; (e-h) [001] direction diffraction patterns; (i-l) [101] direction diffraction patterns Fig. 3 Typical mass spectrum of atom probe. Fig. 4 (a-d) Atom probe 3-dimensional reconstructions; (e-h) frequency distributions of as-cast and annealed specimens. Fig. 5 The hardness dependence on annealing time at 550 °C. Fig. 6 The relationship between hardness and amplitude and wavelength predicted by Cahn’s model
15
16
(b) (d)(a) (c)
(k) (m)(l)(j)
17
0 20 40 60 80 100
0
300
600
900
1200
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1800
2100 Observed Binomial LBM
Freq
uenc
yComposition (%)
0 20 40 60 80 100
0
400
800
1200
1600
2000
2400
2800 Observed Binomial LBM
Freq
uenc
y
Composition (%)
0 20 40 60 80 100
0
300
600
900
1200
1500
1800
2100
2400
Freq
uenc
y
Observed Binomial LBM model
Composition (%)
0 20 40 60 80 100
0
100
200
300
400
500
600 Observed Binomial LBM
Freq
uenc
y
Composition (%)
(e)
(f)
(g)
(h)
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0.1 1 10520
540
560
580
600
620
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680H
ardn
ess
(Kg/
mm
2 )
Annealing Time (hours)
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540 560 580 600 620 640 660 68023456789
10111213
A2 λ
Hardness (Kg/mm2)
20