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….

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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

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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

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