The formation diamond-cubic eta (η) phase in type 316 stainless steel exposed to thermal aging or irradiation environments

Scripta METALLURGICA

Vol. 13, pp. 621-626, 1979 Printed in the U.S.A.

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TH~ FORMATION OF DIAMOND-CUBIC ETA (~) PHASE IN TYPE 316 STAINLESS STEEL EXPOSED TO THERMAL AGING OR IRRADIATION ENVIRONMENTS ~ P. J. Maziasz Metals and Ceramics Division, Oak Ridge National Laboratory Oak Ridge, Tennessee USA 37830 (Received April 23, 1979) (Revised May 7, 1979) Introduction Precipitation in type 316 stainless steel in thermal aging (1,2), fast reactor (3-8) or mixed spectr~ra reactor (9,10) environments has been examined by many workers. Many properties such as strength, corrosion resistance or swelling resistance can be sensitive either to precipitation directly or to changes in the composition of the matrix induced by the precipitation. After irradiation in the Experimental Breeder Reactor II (EBR-II) or High Flux Isotope Reactor (HFIR) or after thermal aging, eta phase is found in solution-annealed 316 (I h at I050°C) and 20% cold-worked 316. Based upon data reported in the literature, the widespread appearance and dominance of the diamond-cubic eta phase in type 316 is unexpected. That this phase may exist undetected in other heats of type 316 already investigated, whether thermally aged or irradiated, should at least be considered a distinct possibility because the phase has been found in this heat of type 316 and because it can be difficult to detect. The direct relation between alloy composition, microstructure and properties means that it is important that precipitation behavior be characterized and understood in order to improve elevatedtemperature performance and to support predictions of properties. Experimental The composition (in weight percent) of the 316 stainless steel examined in this work is 18 Cr, 13 Ni, 2.6Mo, 1.9 Mn, 0.8 Si, 0.05 Ti, 0.13 P, 0.016 S, 0.05 N, 0.0005 B, balance Fe, as determined by bulk quantitative chemical methods. Details of the fabrication and descriptions of the irradiation experiments have been published (9,11,12). The final steps in solution-annealed specimen preparation from rod involved a 50% reduction in area by cold swaging and annealing for 1 h at I050°C before machining into buttonhead tensile specimens. The cold-worked specimens underwent a 20% further reduction, and then machining. Specimens of solution-annealed or 20% cold-worked type 316 were irradiated in HFIR at calculated irradiation temperatures of 370~80°C to neutron fluences producing up to 60 dpa and 4100 at. ppm He. Control specimens of annealed and 20% cold-worked type 316 were thermally aged for I0 000 h at 600 and 650°C, and the 20% cold-worked material was also aged for i000 h at 750°C. Some sheet stock of the same heat of material was cross-rolled to a 20% reduction in thickness, fabricated into sheet tensile specimens, and examined after irradiation in EBR-II at 500°C to neutron fluences producing 9 dpa. Recent measurements of temperature in HFIR by Grossbeck (13) indicates that the actual irradiation temperature could be up to 75°C higher than calculated. For continuity with previous work, however, the calculated temperatures will be used to identify specimens. Further temperature measurements in HFIR are in progress. TEM disks were thinned using a two-step thinning procedure developed by DuBose and Stiegler (14). All specimens were examined using either a JEM i00 C (120 kV) AEM or a Hitachi I000 (I MV) HVEM. The crystal structures of the precipitate phases observed were determined by tilting individual precipitate particles to several low-order zone axes and comparing the diffraction data with literature data (15). Chemical analysis of individual precipitate particles extracted from thermally aged material on carbon replicas and identified by electron diffraction was performed using energy dispersive x-ray analysis in CT~. Research sponsored by the Office of Fusion Energy, U.S. Department of Energy, under contract No. W-7405-eng-26 with the Union Carbide Corporation.

621 0036-9748/79/070621-06502.00/0

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Results and Discussion The precipitate phases observed in solution-annealed and 20% cold-worked type 316 stainless steel arepresented in Table I. A diamond cubic phase with a 0 ~ 1.07 nm is observed in both annealed and 20% cold-worked type 316 after exposure to either thermal aging or }{FIR irradiation, along with the phases normally reported by others (i,2) (see Table i). Eta phase formed in 20% cold-worked 516 irradiated in HFIR at 380°C is undesirable because large cavities that reduce the swelling resistance of the material can be attached to the precipitates. This phase is also observed in 20% cold-worked type 316 after irradiation in EBR-II (see Table i). It comprises a dominant or significant portion of the precipitation observed in all of the samples in which it is found. Tables 1 and 2 indicate that the diamond cubic phase is referred to as eta phase. Examples of eta phase formed im 20% cold-worked type 316 during thermal aging for i0 000 h at 600°C, }{FIR irradiation at 380°C (calculated) to 48 dpa and 3500 at. ppm He and EBR-II irradiation at 500°C to ~9 dpa, respectively, are shown in Figs. l(a-c), 2(a-c) and 2(d-f). Kenik TABLE i. Precipitate phases in type 316 stainless steel after exposure to various environments a Exposure Conditions

Temperature

Phases Solution-Annealed 316

Thermally aged, 10 000 h HFIR i r r a d i a t e d , 42 dpa, 3000 a t . ppmHe

600°C 650°C 500°C b

M23C6, g.b.,m eta and M23C6, g.b., m; Laves, m; sigma, g.b.

Thermally aged, i000 h Thermally aged, 10 000 h

750°C

(20% Rx) (Deformed region): M23C6, Laves, chi, (Rx region): M23C6 (?), Laves, chi, sigma M23C6, g.b., m; eta and Laves, m. Massive sigma phase particl~s (<5% Rx) ~ M23C6, g.b., m; eta and Laves, m. Massive sigma phase particles

Eta and Laves, M; M23C6, g.b., m 20% Cold-Worked 316

600°C 650°C

EBR-II irradiated, 9 dpa HFIR irradiated, 1.5 dpa, 30 at. ppm He

500oCb 580-600°C b

M23C6, g.b.; eta, m M23C6, Laves and sigma, m (10-15% Rx) (Deformed regions): M23C6, Laves, sigma (Rx regions): eta, Laves, and sigma

HFIR irradiated, 3.3 dpa, 85 at. p p m H e HFIR irradiated, 48 dpa, 3300 at. p p m H e

380°C b

Eta, g.b., m

HFIR irradiated, 54 dpa, 3800 at. ppm He

460°Cb

Eta, g.b., m; Laves, m

HFIR irradiated, 43 dpa, 3000 at. ppm He HFIR irradiated, 57 dpa, 3900 at. ppm He

550°Cb

Eta and Laves, m; sigma, g.b.

610°C b

(100% Rx) M23C6, m; massive sigma and M23C6

600°C b

(100% Rx) M23C6, m; massive sigma and chi

HFIR irradiated

ag.b. = grain boundary; m = matrix; Rx = recrystallized. bCalculated irradiation temperatures. cphases were similar in recrystallized and deformed regions.

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TABLE 2. Chemical co~uposition (weight percent) of precipitate particles extracted from thermally aged type 316 on a carbon replica and analyzed by A194

Element

Solution-Annealed 316, I0 000 h at 600°C at 650°C

M23C6a Si V Ti Cr Fe Ni

M23C6

Eta

20% Cold-Worked 316, I0 000 h at 650°C Eta M23C6

0.I ± 0.I 17.0 ± 2.5 ~ 1.0 ± 1.0 65.0 ± 8.0

0.5 ± 0.5 18.0 ± 4.0 ~ 1.0 ± 1.0 61.0 ± 8.0

6.0 ± 2.0 30.0 ± 6.0 1.0 ± 1.0 ~ 30.0 ± 9.0

0.5 ± 0.5 ll.0 ± 3.0 ~ 0.2 ± 0.2 73.0 ± 2.5

~

0.5±0.5

~

~

15.0 ± 2.0 2.0 ± 1.0

14.0 ± 3.0 5.0 ± 3.0

13.0 ± 4.0 21.0 ± 6.0

11.0 ± 4.0 5.0 ± 2.5

aThe m a x ~ error in cotmting statistics is 10% of each reported value. variations reported here represent particle-to-particle variations.

7.0 ± 2.0 25.0 ± 1.0 2.0 ± 0.5 29.0 ± 4.0 ii.0 ± ii.0 26.0 ± 7.0

The statistical

0

FIG. i 20% cold-worked type 316 stainless steel, thermally aged i0 000 h at 600°C. (a), (b), and (c) are the bright-field, dark-field and diffraction pattern from eta phase, respectively. The [001] eta zone clearly indicates the missing (200) reflections characteristic of a diamond cubic structure. The particle contains a twin boundary; hence the nomenclature qT for twin. In a twin orientation relation the (001) and (221) planes are parallel for the twinned relative to the untwinned crystal. The diffraction pattern indicates that two separate diffraction patterns are superimposed, one from each half of the precipitate particle. has also observed a similar phase in titanium-modified 316 after ion irradiation (16). Each figure shows diffraction on (001) on which the missing (200) reflections are a clear indication of the diamond cubic structure. However, presence of the (200) reflections is not straightforward proof of an fcc structure. Double diffraction of the matrix, when the precipitate maintains a cube-on-cube orientation relation, inclusion of eta and Mz3C6 in the selector aperture, and possible ordering of the atoms in eta phase can result in non-zero intensity of

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FIG. 2 (a), (b), and (c) are the bright-field, dark-field and diffraction pattern, respectively, from eta phase formed in 20% cold-worked 316 irradiated in HFIR at 380°C to a neutron fluence producing 48 dpa and 3300 at. ppm He. (d), (e), and (f) are the bright-field, dark-field and diffraction pattern, respectively, from eta phase formed in 20% cold-worked 316 irradiated in EBR-II at 500°C to a neutron fluence producing ~9 dpa. Note the cube-on-cube relationship with the matrix in (c) that is clearly not fulfilled in (f).

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(200) that can confuse identification in-foil. In general, eta is morphologically indistinguishable from M23C 6 and has the same orientation relation with respect to the matrix. In annealed 316, both phases are oriented cube-on-cube with respect to the matrix. In 20% cold-worked 316, they can appear either cube-on=cube or twinned, but the twinned orientation is difficult to ascertain. The precipitates appear to nucleate on deformation bands that are parallel to (lll) planes and are twinned with respect to the matrix. The precipitate can then appear twinned with respect to the matrix material while still being cube-on-cube with respect to the deformation band. The composition of this phase for several thermally aged 316 samples is given in Table 2 with the composition of M23C 6 given for comparison. In general, the diamond cubic phase is considerably higher in Si, Mo, and Ni and significantly lower in Cr relative to Mz3C 6 . It can be appreciated that identification of the phases in 316 can be difficult and that proper characterization and separation from other phases requires the application of analytical electron microscopy. A diamond cubic phase, as reported in this work, is not observed in any of the literature data on 316 (1,10). A phase similar in crystal structure, lattice parameter and chemistry, however, has been observed in other steels and referred to as "H-phase," (17,18). Hughes (17) first observed "H-phase" together with M23C 6 in a 12 Cr-4 Ni-3 ~n-3 Si-1 V-0.03 C steel after thermal aging. X-ray diffraction identified "H-phase" as diamond cubic (space group Fd3m) with a 0 = 1.07 nm and chemical analysis showed it high in silicon. Tither and Clark (18) also obsdrved 'W-phase" in 18 Cr-lS Ni-4.5 Si-1 Mn-0.03 C steel after thermal aging. Their published x-ray data clearly indicated the crystal structure was diamond cubic, but they referred to it as face-centered cubic. It is not strictly incorrect, because both structures are based on the same face-centered cubic Bravais lattice. It can, however, be confusing because they each have a different atom basis associated with the Bravais lattice that gives rise to different diffraction behavior (19). The structure factor for diamond cubic (Fd3m) predicts systematic absences of (200), (420), reflections that are present for fcc (15). It is better to distinguish between the two structures for identification purposes. Tither and Clark found "H-phase" to contain (in atomic percent) ll Si, 8 Fe, 44 Cr, 37 Ni, but did not analyze for carbon. Hughes did not directly analyze for carbon either, but indicated that if the carbon in his mixture of M23C 6 and "H-phase" were accounted for by the M23C6, then "H=phase" should be a MSi x compound with x > 1. Tither and Clark asstlned an M23Si 6 compound from their own data and from Hughes' indication that "H-phase" is a silicide. However, neither has clearly proven that "H-phase" contains no carbon and hence is a pure silicide. The possibility that it is a carbide phase containing silicon needs also to be considered. "H-phase" reported above should not be confused with a class of carbon- or nitrogen-containing compounds having a close-packed hexagonal structure (c/a = 4.9), also termed "H-phase" (20). Weiss and Stickler observed '%~6C' in thermally aged 316L, but not in 316. This corrects an earlier statement by Maziasz (10), and makes suspect the observations of '~46C" by the same author (10) as being separate from the diamond cubic phase described above. '~46C" is observed in other austenitic steels (21-23) and in niobit~n- or titanit~n-stabilized steels (24,25) after thermal aging. It is referred to as eta (n) carbide by Goldschmidt (26) and Stadelmaier (20) and is based upon a diamond cubic structure and unit cell determined from x-ray data by Westgren (27). A review of eta phase by Stadelmaier (20) indicates that Westgren's structure can encompass M3M~X, M6M6"X (X is C, N, or O) or MzM3"SiX (with X sites vacant and Si on metal sites) type phases with lattice parameters ranging from 1.07 to 1.22 nm depending upon phase composition. The stoichiometry need not be strictly maintained. The composition and stoichiometry depend, of course, upon the matrix chemistry (20-23,26-28). Both "H-phase" and '946C" can fit into the general classification of eta phase, which would be a safe designation for purposes of this and other work on steel until carbon (and/or nitrogen) can be directly measured to further specify stoichiometry and composition. A small problem, however, arises with this nomenclature because several investigators have referred to Laves phase as eta phase (1, 29,30). However, the predominant portion of the literature data refer to the AB 2 hexagonal intermetallic compound class first defined by F. Laves (31) as simply Laves phase (2-5, 7,10, 15, 24-26, 32-35). In light of the fact that Laves and the diamond cubic eta phase are formed together in 316 (10,16), the need for clarification of the classification seems justified. The appearance of eta phase in 316 can be seen to have several implications. The undetected presence of eta phase similar to that reported above should at least be considered a possibility in 316 previously characterized as having only M23C6 type carbides (1-7). In fact, compositional data from 316 specimens irradiated in EBR-II have been published that seem to indicate the presence of some eta phase (5). M23C 6 has been implicated in possible embrittlement of grain boundaries and welds, in reducing corrosion resistance, and playing a role in

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void or cavity nucleation in reactor irradiations. The presence of eta phase, with its different composition and crystal structure, may be partially responsible for some of the behavior assigned to M23C 6. Silicon and nickel are implicated as having a significant effect on void formation in fast reactor, and their segregation during irradiation is undesirable. After thermal aging, however, M23C 6 contains very little silicon or nickel and eta is substantially enriched in silicon and nickel. Nickel- and silicun-rich phases normally are only reported after irradiation (1,5,8). The formation of eta phase thermally seems to indicate that removal of one or both of these elements is involved in achieving a more stable austenite, and that this potential for silicon and/or nickel removal needs to be considered in addition to any other forced removal such as irradiation-induced solute segregation in determining which phases form after irradiation. Phase instability can have serious consequences in many different exposure environments, and solutions to the problem at least require adequate phase identification and characterization. Acknowledgments Thanks to B. Cox for excellent sample preparation, N. J. Zaluzec for invaluable assistance and guidance with the quantitative chemical analysis, and E. A. Kenik, J. M. Leitnaker, and J. O. Stiegler for fruitful and productive discussions. (i) (2) (3) (4) (5) (6) (7) (8) (9) (i0) (ii) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) ~23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35)

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