Microstructure evolution and hardening by spinodal decomposition in Fe-Ni-Mn-Al Alloys

Transcription

1 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 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

2 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

3 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 ( 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 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

4 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

5 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 L2 1 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

6 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 58 Fe and 58 Ni. But this overlap can usually be accounted for, since we know the natural abundance of the elements isotopes. In fact, the natural abundance 58 Fe 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

7 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/Alrich 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

8 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 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/mm 2 without annealing to 578 Kg/mm 2 with 2 hour anneal. But hardness decreases to 541 Kg/mm 2 with 22 hour anneal. Further anneal increase the hardness again. Hardness reaches 666 Kg/mm 2 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

9 material. He found that the yield stress should increase in proportion to A 2 λ. 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 A 2 λ. 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

10 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 L2 1 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 L2 1 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

11 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 -&gt 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:

12 [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, [15] Cahn JW 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. 12

13 [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:

14 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, L hours B2, bcc hours B2, L hours B2, bcc, β-mn 50 ~

15 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 16

17 (a) (b) (c) (d) (j) (k) (l) (m) 17

18 Frequency (e) Observed Binomial LBM Composition (%) (f) Observed Binomial LBM Frequency Composition (%) (g) Observed Binomial LBM Frequency Composition (%) Frequency (h) Observed Binomial LBM model Composition (%) 18

19 Hardness (Kg/mm 2 ) Annealing Time (hours) 19

20 A 2 λ Hardness (Kg/mm 2 ) 20