Vol. 73 No. 4 April 1996 Section 1 Page 155

PHILOSOPHICAL MAGAZINE LETTERS, 1996, VOL. 73, NO. 4, 155± 162

High-resolution electron microscopy analysis of the structure of copper precipitates in a martensitic stainless steel of type PH 15± 5 By H. R. HABIBI-BAJGUIRANI and M. L. JENKINS Oxford Centre for Advanced Materials and Composites, Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, England [Received 21 November 1995 and accepted 2 January 1996]

ABSTRACT

The copper-rich precipitates of size 4± 8 nm which are found in a PH 15± 5 martensitic stainless steel aged at 480 C for 2 h are found to have the same twinned 9R structure as copper-rich precipitates of similar size in thermally aged and in electron-irradiated model Fe-Cu and Fe-Cu± Ni alloys. Larger precipitates produced by ageing for the same time at 500 C appear to have transformed to a distorted fcc structure without first untwinning by a mechanism also found in the model alloys, which suggests that the precipitation sequences in the steel and the model alloys are essentially similar.

1. INTRODUCTION Stainless steels which have been subjected to precipitation hardening (PH) are increasingly important structural materials in a variety of industrial applications. They are basically maraging steels with the addition of chromium and fall into three classes, namely austenitic, semiaustenitic and martensitic, depending on the concentration of nickel and manganese. The austenitic grades usually contain 10 wt% (Ni+ Mn) and the martensitic grades usually contain 4± 7 wt% Ni. Elements added to increase strength through precipitation hardening are copper, molybdenum, aluminium, titanium and niobium. A characteristic of PH stainless steels which has limited their use in certain applications is that they embrittle when maintained at temperatures as low as 532 C and the rate of embrittlement increases markedly with increasing ageing temperature (Clarke 1969). Martensitic stainless alloys exhibit higher stress corrosion resistance and fracture toughness than do austenitic or semiaustenitic PH stainless steels and also have good formability and weldability which makes them candidates for use in critical applications despite this propensity to embrittlement. They are widely used in the aeronautic and nuclear industries. Among the martensitic grades of PH stainless steels, 17± 4 and 15± 5 are strengthened by precipitation of copper in the martensitic matrix (Rack and Kalish 1974, Ozbaysal and Inal 1990). These alloys contain only a small amount of carbon (approximately 0 05 wt%) in order to avoid the formation of carbides or carbonitrides. The effect of the ageing treatment on microstructures of PH 15± 5 stainless steel has been studied in detail by Habibi-Bajguirani, Servant and Cizeron (1993a). The precipitation sequence in this alloy, which contains approximately 3 wt%Cu, appears complex. Various morphologies and sizes of copper-rich precipitates were observed, depending on ageing conditions. Their degree of coherency evolved from high at small sizes to partial at large sizes. X-ray microanalysis on extraction replicas 0950± 0839/96 $12 00

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showed that the chemical composition of the precipitates in the first stage of the structural hardening occurring in the temperature range 500± 600 C was essentially pure copper (Habibi-Bajguirani, Molins, Strudel and Servant 1996, HabibiBajguirani, Servant and Lyon 1993b). This result was subsequently confirmed by anomalous small-angle X-ray scattering at three absorption edges (Cr, K, Fe K and Cu K) (Habibi-Bajguirani, Servant and Lyon 1994). In the early stages of ageing, the copper-rich precipitates showed either double-lobe or striated contrast in diffractioncontrast images and they gave streaking on diffraction patterns. When the precipitates grew, they become partially incoherent and appeared as rods. At this stage the precipitates could be shown to have a fcc structure, with a Kurdjumov and Sachs orientation relationship to the bcc martensite matrix. There are numerous reports in the literature on the early stage of copper-based precipitation phenomena in an iron matrix in model Fe± Cu alloys using various methods, including transmission electron miscroscopy (TEM), MoÈssbauer spectroscopy and extended X-ray absorption fine structure (for example Hornbogen and Glenn (1960), Goodman, Brenner and Low (1973), Phythian et al. (1990) and Pizzani et al. (1990)). These studies have confirmed that copper-rich precipitates first form with a coherent bcc structure, while large overaged precipitates are fcc with a Kurdjumov and Sachs orientation relationship to the bcc ferrite matrix. More recently, Othen, Jenkins, Smith and Phythian (1991) and Othen, Jenkins and Smith (1994) have studied, by TEM and high-resolution electron microscopy (HREM), model aged Fe± 1 30wt% Cu and Fe± 1 28wt%Cu± 1 43 wt%Ni alloys. They showed that copper-rich precipitates in these materials do not undergo a direct bcc fcc transformation, but the coherent bcc precipitates first transform to a twinned 9R structure at a size of about 4 nm. They then grow with the 9R structure, with the addition of further twinned segments, to a size of the order of 15± 20nm before transforming to fcc, probably via an intermediate 3R structure. 9R precipitates have also been found in an Fe± 1 5 wt%Cu alloy irradiated at 295 C with 2 5 MeV electrons to a dose of 3 1 1023 m 2, but these transformed to 3R or fcc at a smaller size of about 8 nm (Hardouin Duparc, Doole, Jenkins and Barbu 1995). At low resolutions, the enriched copper precipitates in PH 15± 5 show a striking resemblance to those seen in the model alloys, showing, for example, banded structures consistent with twinning (for example Habibi-Bajguirani et al. (1993a)). However, the structure of the smaller precipitates could not be elucidated in detail, although it was possible to show that this was not simple fcc. The aim of the present work was to examine more closely the structure of precipitates in a PH 15± 5 martensitic stainless steel by HREM, and in particular to see whether these too have intermediate 9R and 3R structures at small precipitate sizes. 2. EXPERIMENTAL DETAILS The martensitic stainless steel referred to here as PH 15± 5 corresponds to ARMCO PH 15± 5, namely AFNOR E CNU 15± 05. It was shaped as a circular bar having a diameter of 87 mm. The nominal composition is given in the table. The alloy was solution-treated for 1h at 1050 C under an argon atmosphere followed by a water quench. Austenitization under argon was necessary to prevent the decarburization of the small samples (10mm 10 mm 15 mm) employed. The solutiontreated samples were then aged under an argon atmosphere for a duration of 2 h at different temperatures. TEM specimens were obtained by punching discs of diameter 3 mm from sheets of the aged samples and then mechanically grinding to a

Copper precipitates in stainless steel PH 15± 5

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Chemical composition of the PH 15± 5 stainless steel. Element Amount (wt%) Amount (at%)

C Cr 0 041 14 8 0 18 15 8

Ni 49 46

Cu 31 28

Mn Nb Si Co P S 0 75 0 3 0 28 0 08 0 02 20ppm 0 75 0 18 0 55 0 075 0 035 20ppm

Fe 76 75

thickness of 100 m. Finally the specimens were electropolished with a twin-jet Struers Tenupol-2, using a polishing solution consisting of 75 cm3 of methanol, 175 cm3 of ethyleneglycolmonobutyl ether and 100 cm3 of perchloric acid at 17 C. TEM and HREM were performed at room temperature on a JEOL 200EX operating at 200kV and a JEOL 4000EX operating at 400kV respectively. All HREM images were obtained with the beam direction closely parallel to a 111 direction in the bcc martensite matrix. This is the optimum foil orientation for imaging the characteristic `herring-bone’ fringe pattern within multiply twinned 9R precipitates.

3. RESULTS 3.1. Material in as-quenched condition The microstructure of the as-quenched PH 15± 5 alloy consisted of lath martensite with a high dislocation density, similar to that published earlier (see, for example, ® g. 3 of Habibi-Bajguirani et al. (1993a)). This martensite had a bcc structure with lattice parameter abcc 0 2878 nm, determined by X-ray diffraction. 3.2. Specimens aged for 2 h at 480 C In these specimens, precipitates were observed with sizes between 4 and 8 nm. Many exhibited the characteristic herring-bone fringe pattern which allows the immediate identification of a twinned 9R structure (fig. 1) from the following features. First, the fringes generally have spacings of about three times the expected (001) 9R close-packed plane spacing (i.e. about 6 nm). The spacings are more regular in fig. 1(b) than in fig. 1(a). Second, the angle between fringes in adjacent twin segments was measured at about 129 . This is the case for both precipitates shown in fig. 1. Third, the interfaces between adjacent twin segments lie parallel to a set of matrix {110} fringes. These observations are consistent with a 9R structure in the expected orientation relationship with the bcc matrix, namely 011

bcc

114

9R

111 bcc

110 9R

This relationship predicts that the angle between (011) bcc and (001) 9R is 64 49 . The close-packed (001) 9R planes give rise to the herring-bone fringes, so that the angle between fringes in adjacent twin-related segments would be expected to be double this, as found experimentally. The 9R structure can be regarded as a close-packed structure with a stacking-fault every third plane, giving a stacking sequence ABC/ BCA/CAB/A. . . consisting of three blocks of three planes with a nine-layer repeat. The three-layer periodicity of the herring-bone fringes is a consequence of this. Generally only two twin-related segments were present in precipitates of size 5 nm or less (e.g. fig. 1 (a)) whilst, in larger precipitates, further twinning had often taken place, so that these contained three or more twin segments. In fig.

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H. R. Habibi-Bajguirani and M. L. Jenkins Fig. 1

(a)

(b) Precipitates in PH 15± 5 stainless steel aged for 2 h at 480 C. They show the herring-bone fringes characteristic of a twinned 9R structure. The fringes subtend an angle of approximately 129 . In (a) two twin segments are seen, and in (b) three.


159

1 (b) a third segment is just discernible. This suggests that the maximum thickness of each 9R twin segment is about 3 nm. In some regions, the three-layer periodicity of the fringes was replaced by fringes with spacings of four or two layers (indicated by c and h respectively in fig. 1). The irregular spacings are particularly evident in the precipitate shown in fig. 1 (a). Following Othen et al. (1994), we identify the four- or two-layer spacings arising from cubic or hexagonal stacking faults respectively. A few precipitates at this ageing condition were larger than 8 nm, and these generally presented an ellipsoid shape and displayed different contrast features. Such precipitates appeared similar to those seen in specimens aged at 500 C which will now be described. 3.3. Specimens aged for 2 h at 500 C In specimens aged for 2 h at 500 C, most precipitates had an ellipsoid shape with an average size of about 9 nm. Smaller precipitates (6± 9 nm) were generally more spherical in shape. Figure 2 (a) shows a high-resolution electron micrograph of a more spherical precipitate (diameter, about 9 nm). The characteristic zigzag herringbone fringe structure is not seen, although the precipitate does appear twinned. The angle between the fringes is, however, close to 120 . Other precipitates, such as that shown in fig. 2(b), show regions where the fringe angle is about 129 and other regions where the fringe angle is close to 120 . 4. DISCUSSION Although the industrial alloy PH 15± 5 contains large quantities of chromium and nickel as well as other alloying elements it is clear these had little or no effect on either the structure of the smaller copper precipitates or the types of fault within them. In particular it has been confirmed that the precipitates first transform from bcc to an intermediate 9R structure. We shall now briefly consider the subsequent transformation at larger precipitate sizes. In previous studies by Othen et al. (1994) and Hardouin Duparc et al. (1995), many of the 9R precipitates were thought to transform to a 3R (a distorted fcc structure) after first untwinning. Certainly many untwinned precipitates were seen, and it could be shown that these did not have a perfect fcc structure. Hardouin Duparc et al. (1995) also showed one example where a precipitate had partially transformed to fcc without first untwinning, although this precipitate still contained bands thought to be of 3R structure. The larger precipitates in the present experiments are not identical with either of the types described in previous work; they seem twinned, but the fringes in some or all regions subtend an angle of 120 rather than 129 . The precipitate shown in fig. 2(a) shows a strong resemblance to that shown previously in fig. 4 of Othen et al. (1994), which is reproduced here for convenience as fig. 3. This has prompted us to reappraise this earlier figure. It can be seen that three-layer herring-bone fringes are resolved clearly, but the angle that they subtend is not 129 but approximately 122 . One set of fringes lies closely parallel to a set of matrix {110} planes whilst the second set lies about 2 away from a second set of matrix {110} planes. This micrograph suggests that the 9R close-packed planes in each twin segment are rotating to align with matrix {110} planes. This is occurring without the precipitate first untwinning, and before the removal of the regular stacking faults. Figure 2(a) also shows three-layer fringe modulations, suggesting that the regular stacking faults are still present, but with both sets of fringes aligned

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H. R. Habibi-Bajguirani and M. L. Jenkins Fig. 2

(a)

(b) Precipitates in PH 15± 5 stainless steel aged for 2 h at 500 C. The precipitates appear twinned but do not show typical 9R fringes. In (a) the angle subtended by the fringes visible within the precipitate is 120 . Similar fringes are visible in (b) but in some regions the angle is closer to 129 .


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Fig. 3

A precipitate in Fe± Cu aged for 100h at 550 , taken from the work of Othen et al. (1994). Herring-bone fringes are present, but they subtend an angle of only 122 . One set of fringes lies parallel to a set of matrix {110}.

parallel to matrix {110} planes; so the structures in these two cases may be essentially the same. Although the transformation appears to occur here without the precipitates untwinning, the mechanism is not identical with that reported by Hardouin Duparc et al. (1995). In our case a subsequent relaxation to fcc would result in an irrational twin plane, whilst in the case analysed by Hardouin Duparc et al. (1995) the twin plane is {111}. This required that the close-packed 9R planes rotate about 6 in the opposite sense to that seen in figs 2 and 3. This mechanism may be favoured if the precipitate is unconstrained by the matrix, as was the case, since it results in a twin plane of lower energy. Another point of note is that partially transformed precipitates seem very common in all the materials. Clearly, the details of the various possible mechanisms for transforming to fcc and the circumstances when each occurs

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require further elucidation, and this will be the subject of a future paper. It seems likely, however, that the martensite stainless steel PH 15± 5 shows a precipitation sequence very similar to or identical to those of simple model alloys. 5. CONCLUSIONS (1) Copper-rich precipitates in a 15± 5 martensitic stainless steel transform from a coherent bcc structure to a twinned 9R structure at a size of about 4nm and grow with this structure to a size of about 8 nm. (2) 9R precipitates larger than 8 nm are not seen. A possible new transformation mechanism has been identified, whereby precipitates transform without first untwinning. This mechanism is seen both in the 15± 5 steel and in an Fe± Cu model alloy. ACKNOWLEDGMENTS We thank our colleagues Dr P. J. Othen and Dr G. D. W. Smith for their agreement to reproduce fig. 3. H. H.-B. also thanks Dr J. W. Martin for his interest, and Professor B. Cantor for the provision of laboratory facilities. REFERENCES

CLARKE, W. C., JR, 1969, Trans. AIME, 245, 2135. GOODMAN, S. R., BRENNER, S. S., and LOW, J. R., 1973, Metall. Trans., 4, 2363. HABIBI-BAJGUIRANI, H. R., MOLINS, R., STRUDEL, J. L., and SERVANT, C., 1996, Acta metall. mater. (to be published). HABIBI-BAJGUIRANI, H. R., SERVANT, C., and CIZERON, G., 1993a, Acta metall. mater., 64, 1613. HABIBI-BAJGUIRANI, H. R., SERVANT, C., and LYON, O., 1994, Nanostruct. Mater., 4, 833; 1993b, J. Phys. Paris, IV, 3, C8± 299. HARDOUIN DUPARC, H. A., DOOLE, R. C., JENKINS, M. L., and BARBU, A., 1955, Phil. Mag. L ett., 71, 325. HORNBOGEN, E., and GLENN, R. C., 1960, Trans. metall. Soc. AIME, 218, 1064. OTHEN, P. J., JENKINS, M. L., and SMITH, G. D. W., 1994, Phil. Mag. A, 70, 1. OTHEN, P. J., JENKINS, M. L., SMITH, G. D. W., and PHYTHIAN, W. J., 1991, Phil. Mag. L ett., 64, 383. OZBAYSAL, K., and INAL, O. T., 1990, Mater. Sci. Eng., A130, 205. PHYTHIAN, W. J., FOREMAN, A. J. E., ENGLISH, C. A., BUSWELL, J. T., HETHERINGTON, M., ROBERTS, K. J., and PIZZINI, S., 1990, Proceedings of the 15th International Symposium on the Effects of Radiation in Materials, ASTM Special Technical Publication No. 1127 (Philadelphia, Pennsylvania: American Society for Testing and Materials), p. 131. PIZZINI, S., ROBERTS, K. J., PHYTHIAN, W. J., ENGLISH, C. A., and GREAVES, G. N., 1990, Phil. Mag. L ett., 61, 223. RACK, H. J., and KALISH, D., 1974, Metall. Trans., 5, 1595.