Neutron-Induced Spinodal-Like Decomposition of Fe ...

Frank A. Garner,^ Howard R. Brager,^ and Jack M. McCarthy^

Neutron-Induced Spinodal-Like Decomposition of Fe-Ni and Fe-Ni-Cr Alloys REFERENCE: Garner, F. A., Brager, H. R., McCarthy, J. M., "Neutron-Induced SpinodalLike Decomposition of Fe-Ni and Fe-Ni-Cr Alloys," Radiation-Induced Changes in Microstructure: 13th International Symposium {Part I), ASTM STP955, F. A. Garner, N. H. Packan, and A. S. Kumar, Eds., American Society for Testing and Materials, Philadelphia, 1987, pp. 775-787. ABSTRACT: Spinodal decomposition is not known to occur in the iron-rich regime of Fe-Ni and Fe-Ni-Cr fee systems. During neutron irradiations conducted in the range 450 to 600°C, however, a spinodal-like separation occurs in the invar compositional range. Ion irradiation studies show a similar behavior and indicate that the separation process occurs as least as high as 725°C. The spinodal-like process is characterized by compositional microoscillations between —25 and —50% nickel with temperature-dependent wavelengths on the order of hundreds of nanometers. These oscillations exhibit a dependence on crystallographic orientation and cause significant hardening. Comparison of these oscillations with those produced in the Santa Catharina meteorite (Fe-35Ni) leads to the conclusion that Fe-Ni invar alloys decompose very slowly (many thousands of years) in the absence of radiation. During irradiation, the spinodallike decomposition and its subsequent coarsening are accelerated by enhanced diffusion and at lower irradiation temperatures by segregation of nickel via the inverse Kirkendall effect at radiation-produced sinks. KEYWORDS: Fe-Ni alloys, Fe-Ni-Cr alloys, invar, neutron irradiation, compositional oscillations, voids, swelling, spinodal decomposition, spinodal hardening, inverse Kirkendall, FeNi meteorites

In several earlier papers it was shown that Fe-35.5Ni-7.5Cr (in weight percent) decomposed in a spinodal-like manner when irradiated in the Experimental Breeder Reactor II (EBRII) at ~ 1 X lO"*" dpa/s and temperatures of 550 and 593°C to exposures of 12 and 38 dpa, respectively [1,2]. The decomposition was characterized not by separate phases but by large and apparently periodic oscillations in composition. The wavelengths of these oscillations were on the order of hundreds of nanometers. These compositional oscillations were determined by using energy dispersive X-ray (EDX) analysis, since they could not be imaged using electron diffraction or absorption contrast. The local nickel levels were observed to vary from —25 to —50%. The iron and chromium behaved as a separate but single species, oscillating out of phase with the nickel profile. Thus the composition appeared to oscillate between the stoichiometric limits of FcsNi and FeNi, with chromium substituting for iron. At 593°C, the oscillation did not appear to be associated with any currently existing microstructural features, which existed at very low densities. This suggests that either the decomposition occurred in a "sinkless" manner or that pre-existing sinks, such as Frank loops, had caused nickel segregation that was then ' Fellow scientists and associate engineer, respectively, Westinghouse Hanford Company, Hanford Engineering Development Laboratory, Richland, WA 99352. Copyright by Downloaded/printed Copyright® 1987 bGarner y ASTM Frank

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RADIATION INDUCED CHANGES IN MICROSTRUCTURE

maintained after the loops unfaulted. This latter possibility was not thought to be likely since there were very few dislocations found that would represent the consequence of loops unfaulting. At 550°C and 12 dpa the sink density was high enough that it may have played some role in the decomposition. Another feature of these and other neutron irradiated specimens was that the decomposition was accompanied by a densification that could be as large as 1%, as shown in Fig. 1. It was this densification that first provided a clue that some sort of compositional segregation was in progress. Some densification was observed for all alloys near the invar composition range. This densification is eventually obscured, however, as radiation-induced voids nucleate and grow. In addition, the slowly developing (as evidenced by the large displacement dose required to achieve the densification) microstructural record of spinodal decomposition is progressively erased by the relatively rapid segregation of nickel at void surfaces. Note in Fig. 1 that the net densification of the alloy reaches its largest values for specimens that exhibit the slowest rate of void swelling [3]. The selection of specimens for examination of spinodal-like development is therefore limited to the narrow window between the onset of densification and the onset of significant swelling. When one considers that several years of irradiation are required to produce these specimens and that reactor schedules dictate the minimum dpa increments available, this constraint makes it difficult to produce optimum specimens for investigation of the spinodal-like decomposition. Another approach that somewhat circumvents this constraint involves the use of ionbombarded specimens produced at a much higher displacement rate, ~2 x 10"^ dpa/s. Based on a proposed correlation between the spinodal-Uke decomposition and the onset of swelling in the invar regime [4,5], a series of 5-MeV Ni+ ion-irradiated Fe-Ni and Fe-Cr-

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FIG. 1—Swelling and densification observed in two Fe-Ni-Cr alloys after irradiation in EBR11 at eight different temperatures. In EBR-II a neutronfluenceof 1.0 x lO^n cm^^ (E > 0.1 MeV) produces approximately 5 dpa in these alloys. Copyright by ASTM Int'l (all rights reserved); Tue Nov 3 18:42:38 EST 2015 Downloaded/printed by Frank Garner (Radiation effects Consulting) pursuant to License Agreement. No further reproductions authorized.

GARNER ET AL. ON SPINODAL-LIKE DECOMPOSITION

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Ni alloys were examined. It was shown that a similar decomposition with comparable wavelengths also developed at —35% nickel at this higher displacement rate [6].^ The decomposition also occurs in the absence of chromium and was found at irradiation temperatures of 625, 675, and 725°C, although the interpretation at the higher irradiation temperature was comphcated by the concurrent and progressive alteration of the matrix composition [7]. This alteration is due to the influence of the inverse Kirkendall effect, which increases strongly with temperature, and also to the large gradients in displacement rate that are characteristic of ion irradiation experiments. The spinodal-like decomposition was also found to occur at the 45% nickel level in these ion studies and once again the decomposition occurred whether or not chromium was present. Significant hardening has been predicted to result from thermally induced spinodal decomposition [8] and has been verified experimentally [9,10]. In another neutron irradiation experiment conducted at 450°C, it was shown in post-irradiation tension tests that a spinodalUke hardening developed, ranging from a very small contribution at 25% nickel to a very large contribution at 45% nickel, as shown in Fig. 2 [11]. At 45% nickel the spinodal component of the hardening was larger than that associated with the voids, Frank loops, and dislocations. At 35% nickel, the hardening was independent of the three chromium levels employed, which were 7.5, 15, and 22%. In this hardening study, however, the existence and wavelength of the compositional oscillations could not be determining using the EDX technique, since at this temperature the wavelengths are comparable to or smaller than the thickness of the thin foil and the effective diameter of the volume sampled by the electron beam. Thus, only the average composition of the alloy can be measured at 450°C. From these various studies, we conclude that a spinodal-like decomposition occurs in the invar compositional regime over the 450 to 725°C range and perhaps over an even larger range of temperatures. The scale of these microoscillations is large compared to that of typical thermally induced spinodals and appears to be somewhat sensitive to temperature and nickel content, but relatively insensitive to displacement rate and chromium level. To this point, however, we have not adequately demonstrated the periodic nature and crystallographic relationship of the decomposition process, primarily because the wavelengths are so large. The research described in this paper is directed toward demonstrating these features in neutron-irradiated specimens. Another paper addresses a similar effort using ion-bombarded specimens.^ It is also shown in this paper that a remarkably similar decom-

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FIG. 2—Comparison of measured yield strength changes and microstructurally based predictions in five Fe-Ni-Cr alloys after irradiation at 450°C to 12.5 dpa in EBR-Il [11]. • Dodd et al., in this publication, pp. 788-804.

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RADIATION INDUCED CHANGES IN MICROSTRUCTURE

position occurred in the absence of radiation, developing in a very slowly cooled meteorite whose composition was essentially that of invar, Fe-35Ni. Experimental Details This research used conventional techniques of specimen preparation, electron microscopy, and EDX analysis, all described in earlier papers [1,2]. The specimens were irradiated under static sodium in the form of microscopy disks, 3 mm in diameter and 0.25 mm thick, in either the Experimental Breeder Reactor II (EBR-II) or the Fast Flux Test Facility (FFTF) fast reactors. Results: Crystallographic Dependence of the Decomposition Process One must first determine whether the decomposition process results in merely random fluctuations in composition or whether they are regular and periodic in nature. Second, if they are regular, one must determine whether these oscillations exhibit the same dependence on crystallographic direction as predicted for spinodally decomposed fee alloys, in which the longest wavelength is predicted to be along (100) directions [8]. Rehn and Okamoto of Argonne National Laboratory have recently suggested in a letter to the authors that a method exists that can be used to discriminate between random fluctuations and truly periodic behavior. Citing an article by Kac [12], they note that EDX sampling measurements can be treated with number theory. If random numbers are plotted at equidistant intervals, the average distance between apparent peaks in the data will approach three of these distance intervals as the series of numbers grows larger. Therefore if one uses a different EDX sampling interval for compositional measurements at random locations, the apparent period of the compositional oscillation should appear to change for a specimen with random fluctuations. While the impact of this suggestion is still being assessed with respect to questions of foil thickness, probe diameter, and so forth, the crystallographic dependence continues to be investigated using Unear composition traces. In previous studies most of the compositional traces were taken along random and usually unidentified crystallographic vectors, reflecting the difficulty associated with such determinations when using the single tilt stage normally used for our EDX analyses. In this study, however, the specimen was progressively rotated in the holder until a suitably thin area was positioned with several degrees of a (100) foil normal such that EDX measurements were feasible along a series of parallel (100) lines. This specimen was derived from the 593°C, 38-dpa irradiation of Fe-35.5Ni-7.5Cr previously reported [1]. Using random direction traces the period was earlier estimated to be on the order of 300 to 400 nm [1,2]. Figure 3 shows in a region where the foil thickness was on the order of 75 ± 20 nm that the period is very large, so much so that only one minimum in nickel was found in the trace. The period is therefore at least 1200 nm along a given (100) trace. Figure 4 shows a comparable (100) trace in a thicker (120 ± 25 nm) portion of the same area, with the period again appearing to be at least 1000 nm. In both traces the iron and chromium profiles are mirror images of that of nickel. Although the data presented in Figs. 3 and 4 confirm that the period along (100) directions is longer than that previously observed on random-direction traces, the trace procedure chosen does not guarantee that a given (100) trace will go through the maxima defined along perpendicular traces. Figure 5 presents the results of a time-consuming compositional mapping of a fairly large region of crystal volume, all contained in one grain. The area chosen was near that mapped in Figs. 3 and 4; it was near the foil edge and roughly 100 nm thick. The mapping was done along (100) directions, one of which was conveniently parallel to Copyright by ASTM Int'l (all rights reserved); Tue Nov 3 18:42:38 EST 2015 Downloaded/printed by Frank Garner (Radiation effects Consulting) pursuant to License Agreement. No further reproductions authorized.

GARNER ET AL. ON SPINODAL-LIKE DECOMPOSITION

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