Effect of Ordering on Susceptibility to Hydrogen Embrittlement of a Ni-Base Superalloy KAORI MIYATA and MASAAKI IGARASHI The effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy (alloy C-276) has been investigated by means of tensile tests in air and with hydrogen-charging in 1N-H2SO4 solution. The annealed specimen has exhibited intergranular fracture by hydrogencharging, resulting in a marked reduction in tensile elongation and ultimate tensile strength. The mode of fracture was changed by aging at 773 K, and the transgranular fracture has been found to be dominant in the aged specimens. The susceptibility to hydrogen embrittlement, as identified by the test method used in this study, seems to be reduced by short-term aging, though it turns out to be increased again by further aging. The fractured boundaries have been characterized using electron channeling pattern (ECP) analysis of adjacent grains. It is found that the misorientation of grain boundaries plays an important role in fracture, and 2~3 boundaries, twin boundaries in a face-centered cubic (fcc) lattice, are most likely to fracture in the aged specimens. Transmission electron microscopy (TEM) observation has shown that a short-range ordering reaction from a disordered fcc lattice into an ordered Ni2(Cr, Mo) (Pt2Mo type) superlattice takes place by aging, and hence, superdislocation triplets with APB (antiphase boundary) become predominant when deformed. It is also seen that in the aged specimens, deformation twinning is another mode of deformation, and this leads to the transgranular fracture at twin boundaries by hydrogen-charging. These results suggest that a change in the mode of deformation after aging plays a major role in fracture due to hydrogen embrittlement as a consequence of the heterogeneous interaction between slip dislocations and twin boundaries.

I.

INTRODUCTION

IN

severe corrosive environments, such as deep sourgas and oil fields, geothermal energy recovery processes, and coal liquefaction processes, Ni-base superalloys such as HASTELLOY* C-276 alloy and SM2550"* have often been used because of their excel*HASTELLOY is a trademark of Stoody Deloro Stellite, Inc., Industry, CA. **SM2550 is a trademark of Sumitomo Metal Industries, Ltd. Japan.

lent corrosion resistance and high strength in such environments. [~j However, it is considered that these alloys, in particular, alloys with high Ni content, occasionally exhibit intergranular fracture due to hydrogen embrittlement during long-term aging in a coldworked condition, f21 The effect of long-term aging on hydrogen embrittlement of the HASTELLOY C-276 alloy has been extensively investigated by Asphahani and Tawancy t3j and later by o t h e r s . [4'5'6] Berkowitz and Kane f41 have suggested that in such an alloy, aging causes segregation of phosphorus to grain boundaries, and the segregated P atoms act as a poison for hydrogen combination reaction, resulting in intergranular fracture even at low applied stress. Formation of the NizCr-type ordered phase during aging has also been found to enhance hydrogen embrittlement which changes the slip mode from wavy to planar and increases internal stress concentrations at dislocatio n

pileups, f2,3J Hinotani et al. tSj have shown that in Ni-Cr binary alloys, the mode of fracture due to hydrogen embrittlement by cathodic charging changes from the intergranular to the transgranular type with increasing aging time. They have suggested that the ordering reaction during aging promotes the coplanar slip and deformation twin, and as a result, the stress concentration to grain boundaries varies to a great extent, which plays an important role in deciding the mode of fracture. Borowiecka and Fiore t61 have proposed that surface crack formation due to hydride formation and subsequent decomposition causes hydrogen embrittlement in alloy C-276. The studies previously described have pointed out several important features of hydrogen embrittlement observed in Ni-base superalloys. However, there is still no established picture attained for the origin of the hydrogen embrittlement in these alloys. The aim of this study therefore is to clarify the mechanism of intergranular fracture due to hydrogen embrittlement in a Ni-base superalloy (alloy C-276), with a particular emphasis on formation of ordered phase (Ni2(Cr, Mo)) during aging. The present article describes the effect of changes in the deformation mode by ordering on hydrogen embrittlement and specifies the fractured boundaries in terms of a E value based on the coincidence site lattice (CSL) theory. The observed resuits will be discussed by comparison with the deformation model in the ordered phase and the prediction of interaction between the slip and boundaries. II.

KAORI MIYATA and MASAAKI IGARASHI, Researchers, are with the Iron and Steel Research Laboratories, Research and Development Division, Sumitomo Metal Industries, Ltd., 1-8 Fusocho, Amagasaki, 660 Japan. Manuscript submitted May 13, 1991. METALLURGICAL TRANSACTIONS A

EXPERIMENTAL PROCEDURES

A. Materials

The material used in this study was vacuum induction melted and processed by hot-forging and hot-rolling it VOLUME 23A, MARCH 1992--953

into 8-ram-thick plates. The chemical composition of the material is given in Table I. The plates were solutiontreated for 1 hour at 1423 K and subsequently quenched in water. Some of these plates were then aged for from 0 to 1000 hours at 773 K.

B. Tensile Tests with Hydrogen Charging Susceptibility to hydrogen embrittlement was assessed in terms of a decrease in elongation and ultimate tensile strength due to cathodic hydrogen charging. The tensile specimens with 2.54-mm diameter and 25.4-mm gage length were machined from the plates parallel to the rolling direction. The annealed specimens were identified to be single-phase austenite with average grain diameters of about 100 /zm. Each specimen was hydrogenprecharged for 3 hours and subsequently deformed in tension with hydrogen-charging at room temperature in an electrolyte consisting of 1N-H2SO4 solution with 1.4 k g / m 3 of thiourea as a poison. A couple of the tensile tests were carried out to optimize the charging condition, with current density in the range between 10 and 300 A / m 2. These tests were performed using an Instron-t),pe tensile machine at an initial strain rate of 3 • 10-~/s. The fracture surfaces of the specimens after tensile tests were examined using scanning electron microscopy (SEM) in JEOL840.

C. Transmission Electron Microscopy Observations of Deformation Structures Thin foils cut from the specimen deformed 5 pct in tension were prepared by a conventional twin-jet polishing technique using a 10 pct perchloric acid-90 pct acetic acid bath at 288 K. The foils were examined metallographically in a transmission electron microscope (JEM200CX) operated at 200 kV.

D. Characterization of Fractured Boundaries Characterization of the fractured boundaries was conducted on the basis of the C S L I8-12] t h e o r y which specifies the misorientation relationship between two adjacent grains by means of electron channeling pattern (ECP) analysis using SEM. The specimens for the ECP analysis were prepared to remove surface damage due to machining by mechanical polishing using an alumina compound (0.3/xm) and then electropolishing in an electrolytic solution consisting of 10 pct perchloric acid in acetic acid with a platinum cathode at 288 K. To determine the orientation of each grain, ECP micrographs were taken from individual grains using a JEOL840 scanning electron microscope equipped with an ECP unit. It was operated at an accelerating voltage of 25 KV, with a working distance of 15 mm and a beam

Table I.

Ni Balance

Cr 15.7

locking angle of ---2 deg. A rotational relationship between two adjacent grains was then determined by 24 possible sets of rotational axis and angle, tl~ The calculated results were compared with the CSL relationship, and thus, the E value of the grain boundary was determined. After the characterization of 150-grain boundaries for each specimen, the tensile test with hydrogen-charging was performed in the same condition as described previously. The ~ values of the fractured boundaries after the tests were determined using SEM. III.

EXPERIMENTAL RESULTS

A. Effects of Aging Scanning electron micrographs of the fracture surfaces due to hydrogen-charging are shown in Figure 1. It is seen that hydrogen cracks have initiated at the specimen surface and propagated toward the center, but the central region has ruptured in a ductile manner. The fracture mode has predominantly changed from an intergranular to a transgranular type by aging. In the case of the solution-treated specimen, intergranular fracture has mainly been observed as shown in Figure l(a). However, after 100 hours aging, transgranular fracture becomes predominant, as shown in Figure l(b). Further aging more than 1000 hours has resulted in numerous step-like cracks and crossing of slip traces (Figure 1(c)) which seem to occur by separation of specific crystallographic planes in grains. Figure 2 shows the deformation structure observed in the 1000 hour aged specimen after 5 pct deformation in tension. Numerous microtwins have been observed, even at the first stage of deformation, which have not been observed at all in the specimens aged for 100 hours. Superlattice diffraction spots have been observed after aging for 1000 hours at the positions corresponding to the NiECr-type ordered phase shown in Figure 3. However, these superlattice diffraction spots still have been accompanied with diffuse scattering, which is identified by the streaks along the (110) direction. This diffuse scattering of the superlattice diffraction has also been identified with a very weak intensity in the specimens aged for 100 hours. It is thus considered that in the present alloy, the ordering rate is much slower than that of NiECr-binary alloy, t5,71 Figure 4 shows the dislocation configuration observed in the 1000 hour aged specimen after 5 pct deformation in tension. The dislocation triplets have extensively been observed over all of the specimen, as shown in Figure 4(a) (g = [111] m, where g is the reciprocal lattice vector of the diffracting planes). Fringe contrast between the dislocation lines, which shows the existence of superdislocations separated by antiphase boundaries

Chemical Compositions of Alloys Used (Weight Percent)

Mo 15.7

W

Co

Fe

Mn

C

3.4

1.0

7.1

0.8

0.002

P and S < 0.001.

954--VOLUME 23A, MARCH 1992

METALLURGICAL TRANSACTIONS A

(APB), is also visible when imaged with g -- [002] m (Figure 4(b)). In the other area, dissociation of individual partials into two partials is identified when imaged with g = [202] m, which is not visible when imaged with g -- [220] m- Using the vanishing criterion of the dislocation images g. b = 0 or -+ 1/3, Burgers vectors, b, of individual partials are given by b = 1/21101] m and each one is expected to dissociate into two partials described by the following reactions: 1/21101] ~ = 1/61211] ~ + CSF + 1/6[1i2] and 1/21101] m = 1/612il] m + CSF + 1/61112] m where CSF denotes complex stacking fault. The tensile properties of solution-treated and aged specimens are shown in Figure 5. It is seen that in the case of uncharged specimens, the 0.2 pct proof stress and the ultimate tensile strength (UTS) increase with increasing aging time while the elongation decreases with increasing aging time. Hydrogen charging, however, has reduced both the UTS and elongation. The annealed specimen has exhibited a marked reduction in tensile elongation as well as UTS by hydrogen-charging. The UTS and elongation have increased with increasing aging time up to 100 hours but have decreased again by further aging. B. Characterization of the Fractured Boundaries Figure 6 shows typical examples of the characterization of hydrogen cracks for each specimen, and Figure 7 shows frequency of coincidence boundaries (high-angle boundaries with E3 to E51) initiating hydrogen cracks as a function of E value. The number of grain boundaries characterized in s value is 150 in total for each specimen. In this figure, R denotes random boundaries and also the coincidence boundaries with s > 51. The frequency of transgranular fracture indicates the number of grains in which the transgranular fracture has been observed out of 100 grains. In the case of a solution-treated specimen, there is no specific rotational relationship identified which is favorable to hydrogen cracks, as shown in Figure 7. However, hydrogen cracks have been observed along grain boundaries labeled E49, having a specific crystallographic orientation relationship between the grain boundary and the slip lines shown in Figure 6. This suggests that hydrogen-cracking takes place at specific boundaries which are determined by the crystallographic orientation relationship between boundaries and slip activated by the applied stress. In the aged specimen, on the other hand, the ratio of transgranular to intergranular fracture has increased significantly except for s boundaries which correspond to annealing twin boundaries in a face-centered cubic (fcc) lattice. As shown in Figure 6, cracks seem to initiate along twin boundaries and the slip plane in the aged specimens. IV. Fig. 1 - - S c a n n i n g electron micrographs of the fracture surfaces of the specimens after tensile tests with cathodic charging: (a) solution-treated, (b) aged for 100 h, and (c) aged for 1000 h. METALLURGICAL TRANSACTIONS A

DISCUSSION

The present study has shown that in alloy C-276, the susceptibility to hydrogen embrittlement and the mode VOLUME 23A, MARCH 1992--955

Fig. 2 - - T r a n s m i s s i o n electron micrographs taken from the specimen aged for 1000 h at 773 K and deformed 5 pct in tension: (a) bright-field image, (b) dark-field image, and (c) electron diffraction pattern.

of fracture have changed to a great extent by aging at 773 K, in which ordering reaction from a disordered fcc into the Ni2Cr-type ordered phase takes place. It is considered that aging changes the deformation structure and hence changes the mode of fracture as a consequence of change in the interaction between specific boundaries,

such as grain boundaries and twin boundaries, and slip activated by the applied stress. During aging, ordered orthorhombic superlattice isomorphous with the Ni2Cr type has been formed in the present alloys. The ordered phase has the following crystallographic relationship with a disordered fcc matrix, tTl as shown in Figure 8: [ lO0]ort h = [110Ira

[OlO]o,,h = [O01]m [O01]o,,h = [110]m

Fig. 3 - - E l e c t r o n diffraction pattern taken from the 1000 hour aged specimen corresponding to (001) m orientation. 956--VOLUME 23A, MARCH 1992

In the present study, however, only diffuse scattering from the Ni2(Cr, Mo) ordered phase has been observed. This shows that in the present alloy, ordering reaction takes place in a short range even after aging for 1000 hours at 773 K, though a Ni-Cr binary alloy with NizCr stoichiometric composition shows a fully ordered phase after aging for 100 hours at the same temperature, t5,71 The susceptibility to hydrogen embrittlement seems to METALLURGICAL TRANSACTIONS A

Fig. 4--Bright-field electron micrographs showing dislocation configurations taken from the 5 pct deformed specimen aged for 1000 h at 773 K. Images recorded with (a) g = [ i l l ] and beam direction, B, near [110], (b) g = [002] (b = 1/2[110]), (c) g = [220] and B near [111], and (d) g = [202] (b = 1/21101]).

be most enhanced in the solution-treated specimen. It seems to be reduced by short-term aging (100 hours), though it turns out to be increased again by further aging (1000 hours). The mode of fracture dominantly changes corresponding to the susceptibility of hydrogen embrittlement. In the case of the present alloy, phosphorus or sulfur content, which acts as a poison for hydrogen embrittlement, is kept low enough. Therefore, the segregation of impurity atoms could not be the major factor of intergranular fracture due to hydrogen embrittlement. However, transmission electron microscopy (TEM) observation shows that the mode of deformation as well as the mode of fracture has been changed during aging at 773 K. So the formation of the ordered phase, which would change dislocation configuration or the interaction between slip and interfaces, is considered to be an important factor in hydrogen-cracking, though the ordering reaction is not fully completed. The annealed specimen which is disordered has exhibited intergranular fracture by hydrogen-charging, resuiting in a marked reduction in tensile elongation and UTS. This result indicates that stress concentration to grain boundaries, which is caused by the interaction between slip and grain boundaries, promotes intergranular METALLURGICAL TRANSACTIONS A

cracking because of its low boundary cohesive strength before aging. Transgranular fracture has been dominant in the aged specimens, and the susceptibility to hydrogen embrittlement is reduced by short-term aging. Hinotani has suggested that grain boundaries are strengthened by segregation of carbon during aging.tS.13l This model supports the results that the intergranular fracture is suppressed by short-term aging in this study. After further aging, hydrogen cracks have been observed along the same specific crystallographic planes in grains, in particular, in ~3 boundaries (twin boundary in an fcc lattice). Deformation twins are induced predominantly with an increasing degree of order, and the deformation twin boundaries seem to fracture as well. It is explained that the formation of the Ni2(Cr, Mo) ordered phase has changed the deformation mode from slip to twinning and has promoted the stress concentration to twin boundaries. As a result, the embrittlement at twin boundaries instead of the embrittlement along grain boundaries becomes dominant. As shown in Figure 4, superdislocation triplets with antiphase boundaries have been observed in specimens aged for 1000 hours. This indicates that the deformation VOLUME 23A, MARCH 1992--957

0

9

in air H-charged

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Fig. 5 - - T e n s i l e properties of solution-treated and aged specimens in air and with cathodic charging (1 N H2SO4-thiourea-900 m V calomel electrode (SCE); ~ = 3 * 10-4/s).

structures in the present specimens after aging are the same type as those observed in the Ni2Cr ordered phase, u4JS~ Figure 9 shows the deformation model of the Ni2Cr ordered phase originally proposed by Amelinckx. [14] The four equivalent families of (111) planes of a disordered fcc lattice now belong to either type (I) or type (II) slip systems in each variant. There are six orientation variants corresponding to the six possible orientations of [ l l 0 ] with respect to the cubic axis. Furthermore, there are three translation variants. The glide paths in the Type (I) glide planes are indicated in Figure 8. The paths "a" through "d" lead to superdislocations which consist of three partial dislocations with the same Burgers vector as a perfect glide dislocation in a disordered fcc lattice. On the other hand, glide parallel to the orientation of [100] (paths "a" through "e") takes place by a single dislocation. The glide paths in the type (II) glide planes are indicated in Figure 9. 9 5 8 - - VOLUME 23A, MARCH 1992

vs

saturated

The three close-packed directions all lead to similar symmetrical threefold superdislocations. In the present study, dissociation of individual partials into two partials with complex stacking fault has been observed for the first time. It is explained that dissocia'tion into partials is due to a low stacking fault energy caused by substitution of a part of Cr atoms by Mo atoms in the Ni2Cr ordered phase. As shown in Figure 6, hydrogen cracks always occur along grain boundaries having a specific crystallographic orientation relationship with slip lines. It seems that the interaction between slip dislocations and interfaces plays an important role in hydrogen-cracking. Considering dislocation reaction at grain boundaries, hydrogen cracks may be nucleated by formation of sessile dislocations and dislocation piling up at grain boundaries, in which hydrogen atoms seem to be trapped. In the case of twin boundaries, slip-twin interactions are proposed by METALLURGICAL TRANSACTIONS A

Fig. 7--Frequency of fractured coincidence boundaries as a function of X value: (a) solution-treated, (b) aged for 100 h, and (c) aged for 1000 h. The shaded area designates the number of boundaries initiating hydrogen cracks.

Remy, t~61as shown in Figure 10. Figure 10(a) shows that an incident dislocation in the matrix propagates into the twin, leaving twin boundary dislocations at its boundary. On the other hand, Figure 10(b) shows a sessile configuration formed by the splitting of a Frank partial into a Shockley partial in the matrix and a stair-rod dislocation at the boundary.

Fig. 6--Scanning electron micrographs showing hydrogen crack formation in the 10 pct deformed specimens with cathodic charging: (a) solution-treated, (b) aged for 100 h at 773 K, and (c) aged for 1000 h at 773 K. METALLURGICAL TRANSACTIONS A

Fig. 8--Ordered structure of Ni~Cr. Broken lines indicate the original fcc lattice, t71 VOLUME 23A, MARCH 1992--959

Type(I)

Type(II)

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Parent

[115](111)

Twin

61[ii~] Parent ~ 2 1 1 1 0 ]

p

f. !

The present results do not enable one to predict the reaction which will be favorable in each aged specimen. However, these dislocation reactions at twin boundaries seem to depend on the mode of deformation, as described previously. Moreover, the reaction seems to be more complicated in the ordered phase because of its anisotropic deformation mode, formation of deformation twin, and hence difficulty of twin-twin interaction. Further investigation into the dislocation reactions at grain boundaries and twin boundaries in order to get a better understanding of the mechanism of hydrogencracking is needed.

(a)

[115](111) Twin

V.

Z~>

(b) Fig. 1 0 - - S e q u e n c e showing the interaction of a 1/21110] dislocation with twin boundaries. An incident matrix dislocation dissociates into (a) a perfect dislocation in the twin and residual twinning dislocations or (b) a Frank partial (sessile dislocation) and residual twinning dislocations. 960--VOLUME 23A, MARCH 1992

CONCLUSIONS

The effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy (alloy C-276) has been examined using tensile tests with hydrogen-charging, characterization of fractured boundaries based on the CSL theory and TEM observation of deformation structure. The formation of the NiE(Cr, Mo) ordered phase during aging at 773 K has been confirmed by using TEM in the present alloy. The annealed specimen which is disordered has exhibited intergranular fracture by hydrogencharging. The mode of fracture has been changed, and METALLURGICAL TRANSACTIONS A

the transgranular fracture has been found to be dominant by ordering. The characterization of fractured boundaries has shown that E3 boundaries, twin boundaries in a fcc lattice, are most likely to fracture in the ordered specimens. Ordering reaction has also changed the mode of deformation. In the aged specimens, superdislocation triplets with APB and deformation twinning become predominant when deformed in tension. It is found that the interaction between slips and grain boundaries plays an important role in hydrogen embrittlement.

ACKNOWLEDGMENTS

The authors would like to thank T. Kyogoku, Director and General Manager, Iron and Steel Research Laboratories, Sumitomo Metal Industries, Ltd., for permission to publish this article. Helpful discussions with Dr. Y. Okada, Manager, Steel Materials Research Laboratory, and Professor Y. Ohmori, Faculty of Engineering, Ehime University, are gratefully acknowledged. The considerable assistance in specimen preparation, test, and examination of K. Murata and Y. Nishioka is also acknowledged.

METALLURGICAL TRANSACTIONS A

REFERENCES 1. P.R. Rhodes: Paper 322, Corrosion "86, NACE, Houston, TX, 1986. 2. A.I. Asphahani: Proc. 2nd Int. Cong. on Hydrogen in Metals, Paris, 1977, Sect. H-2. 3. A.I. Asphahani and H.M. Tawancy: Corrosion and Corrosion Protection, Proc., 1981, vol. 81-8, pp. 154-65. 4. B.J. Berkowitz and R.D. Kane: Corrosion, 1980, vol. 36 (1), pp. 24-29. 5. S. Hinotani, Y. Ohmori, and F. Terasaki: Mater. Sci. Eng., 1985, vol. 74, pp. 119-31. 6. E. Lunarska-Borowiecka and N.F. Fiore: Metall. Trans. A, 1981, vol. 12A, pp. 101-07. 7. M. Hirabayashi, M. Koiwa, K. Tanaka, T. Tadaki, T. Saburi, S. Nenno, and H. Nishiyama: Trans. Jpn. Inst. Met., 1969, vol. 10, pp. 365-71. 8. W. BoUmann: Crystal Defects and Crystalline Interfaces, SpringerVerlag, Berlin, 1970. 9. G.L. Bleris and P. Delavigneue: Acta Cryst., 1981, vol. A37, pp. 779-86. 10. H. Grimmer, W. Bollmann, and D.H. Warrington: Acta Cryst., 1974, vol. A30, pp. 197-207. 11. H. Grimmer: Acta Cryst., 1974, vol. A30, pp. 685-88. 12. M. Dechamps, F. Baribuer, and A. Marrouche: Acta Metall., 1987, vol. 35 (1), pp. 101-07. 13. S. Hinotani, Y. Ohmori, and F. Terasaki: Mater. Sci. Eng., 1985, vol. 74, pp. 119-31. 14. S. Amelinckx: in Dislocations in Solids, vol. 2, F.R.N. Nabbaro, ed., pp. 270-72. 15. L.Ye. Popov and E.V. Kozlov: Fiz. Metal. Metalloved., 1964, vol. 18 (1), pp. 102-06. 16. L. R6my: Metall. Trans. A, 1981, vol. 12A, pp. 387-408.

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