The Effect of Ordering on the Hydrogen Embrittlement Susceptibility of Ni2Cr B. J. B E R K O W I T Z A N D C. M I L L E R Isothermal annealing at 500 ~ for various lengths of time after rapid quenching from 900 ~ results in different degrees of ordering in Ni2Cr. Tensile specimens of disordered and ordered Ni2Cr were subjected to hydrogen embrittling and nonembrittling environments before and during tensile testing. The hydrogen embrittlement susceptibility was determined by reduction-in-area losses (RA loss) after failure. The results of the tensile tests indicated that the disordered alloy and highly ordered alloy were the most susceptible to hydrogen embrittlement. The RA loss in each case was approximately 70 pct. The tests revealed a minimum susceptibility to hydrogen embrittlement between 40 and 50 pct order. The fracture surfaces of the specimens, as examined by scanning electron microscopy, showed that the extent of embrittlement is correlated with the amount of intergranular failure. The mechanisms for failure in Ni2Cr appear to depend upon the extent of aging in the alloy.
IT has recently been found that highly cold worked H A S T E L L O Y C-276 is susceptible to hydrogen embrittlement after aging at temperatures in the range of 200 to 500 ~ It was further shown that the grain boundary segregation of phosphorus is a key factor for the increased susceptibility to hydrogen induced intergranular failure. 2 In contrast, it has been suggested by Asphahani 3 that an ordering reaction, as opposed to segregation, is responsible for the increased susceptibility. It was hypothesized that A2B type order in the form of Ni 2 (Cr, Mo) nucleated during aging of H A S T E L L O Y C-276 and that it was the growth of this ordered phase which caused an increase in the hydrogen embrittlement susceptibility? In order to study this phenomenon as a possible mechanism for increased embrittlement, we investigated the effect of ordering on the hydrogen embrittlement susceptibility of Ni2Cr, a model ordering alloy. Ni2Cr undergoes a first order, order-disorder transformation at the critical temperature 863 K. 4 However, a high degree of long range order is obtained only when the alloy is heated for long periods at temperatures somewhat below the critical temperature. The ordering reaction appears to occur most rapidly at 773 K. Taunt and Ralph 5 have found that samples initially heat treated at 1173 K and quenched ordered more rapidly at 773 K than samples which are initially highly cold worked. They suggested that at 1173 K, short range order fluctuations exist and are frozen in when the alloy is rapidly quenched. At 773 K these fluctuations grow in amplitude and wavelength producing the fine, ordered domains found in ordered Ni2Cr. The need for nucleation at grain boundaries or other defect sites is eliminated and the result is homogeneous growth of the ordered state within the disordered matrix. The ordered structure in Ni2Cr was determined by B. J. BERKOWITZ is Staff Metallurgist, Exxon Research and Engineering Company, Corporate Research Science Laboratories, Linden, NJ 07036. C. MILLER is a Graduate Student at Columbia University, Department of Metallurgy, New York, NY 10027. Manuscript submitted October 18, 1979.
X-ray, 6 neutron, 7 and electron 8 diffraction (and found to be isomorphous with Pt2Mo). In the disordered state, the lattice is face centered cubic (fcc) with a lattice parameter of 3.566~, (after quenching from 1273 K). The ordered structure is an orthorhombic superlattice containing six atoms. The slightly smaller lattice parameter in terms of the original cubic coordinate is 3.562~ (after aging at 460 ~ for 3200 h). 8 Figure 1 illustrates the crystallographic orientations and the relationship between the disordered fcc and the ordered orthorhombic superlattice. The (100)orth and (010)or,h planes are parallel to the (110)fcc and (T10)fcc planes respectively and the (001)o,h plane is parallel to the (001)fc~ plane. The superlattice parameters are described by: a = ao/x/-2, b = 3ao/X/2 and c = a0 .6 The degree of long range order has been determined as a function of aging time by neutron diffraction as shown in Fig. 2. 9 The degree o f order, i.e., the volume percent of ordered phase, was found to decrease sharply when the alloy deviated from stoichiometry. There have been some studies on the influence of ordering on the mechanical properties of Ni2Cr. For example, increases in critical resolved shear stresses 9 and work hardening coefficients ~~in single crystals have been found. However, only modest increases in hardness and yield strength were observed in polycrystalline Ni2Cr until the volume fraction of ordered phase was more than 75 pct. The major part of the observed strengthening has been attributed to the formation of antiphase domain boundaries left behind after the movement of single dislocations. 8 It is not at all clear what the effect of ordering would be on the hydrogen.embrittlement of Ni2Cr. The present authors are unaware of any previous studies addressing this subject. There has, however, been limited work on the hydrogen permeability of ordered Ni2Cr by Goltsov, et al. ~2 After ordering, the hydrogen permeability decreased by three times, which Goltsov attributed to a change in grain orientation upon ordering. The purpose of the present study is to determine the effect of ordering on the hydrogen embrittlement susceptibility of this model ordering alloy.
ISSN 0360-2133 / 80/1111-1877500.75/0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND VOLUME 11A, NOVEMBER 1980--1877 THE METALLURGICAL SOCIETY OF AIME
ORDERED ORTHORHOMBIC STRUCTURE aT
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with a platinum wire as the anode, the specimen as the cathode and an aqueous electrolytic solution of 0.1N H z S O 4 and N a A S O 3 0.25 g/1, The N a A s O 3 was added to poison the hydrogen recombination reaction, enhance the hydrogen absorption rate, and decrease the precharging time to 15 minutes. All specimens were charged at a current density of 103 A / m 2 before and during the test. Tensile specimens of each ordered state fractured in air and in H2SO 4 were selected for SEM analysis. They were cut to 1.27 • 10 -2 m in length which included the fractured face. They were sonically cleaned to remove debris and then mounted for observation.
Fig. 1 - - T h e crystallographic relationship between the disordered fcc and the ordered orthorhombic superlattice of Ni,Cr.
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RESULTS The variation in Vickers hardness with degree of order is shown in Fig. 3. Hardness measurements were made on the longitudinal and transverse sections of the wire. The hardness measurements as a function of aging of this work were consistent with those of previous workers, ~ indicating that the heat treatments produced the proper degree of order. The grain size of the disordered and highly ordered alloys were the same. l lydrogen embrittlement susceptibility is defined by the relative loss in reduction in cross-sectional areas (RA) with and without hydrogen. Thus:
AGING TIME (HOURS)
Fig. 2 The relationship between the degree of long range order and time of isothermal aging at 500 ~ for Ni,Cr?
EXPERIMENTAL PROCEDURE Pure (99.95 pet) 70 wt pct Ni-30 wt pet Cr was purchased in the form of 1.57 x 10 3 m diam drawn wire which was cut into 2.54 x 10 2 m sections and 1.27 x 10 ~m sections. The hmger sections were machined in the centers to produce a 6.35 x 10 3 m gage section with a 1.02 x 10 3 m diam reduced cross section to be used as tensile specimens. All samples were ultrasonically cleaned with acetone and rinsed with methanol, then heat treated at 1173 K in vacuum (1.33 x 10 -4 Pa) for 2 h and water quenched. The ordering treatment which followed consisted of heat treating groups of eight hmg wires and two short wires at 773 K under vacuum (1.33 x l0 -4 Pa) for various periods of time from 2 to 100 h. Each aging period produced a corresponding amount of long range order as shown in Fig. 2. 9 After aging, half of the tensile specimens (to be hydrogen embrittled) were coated with lacquer with only the gage section and one end exposed for electrical contact. This was done to limit hydrogen absorption to the gage section only. One-short wire from each aging treatment was mounted in epoxy with the wire axis perpendicular to the face of the mount. Five microhardness measurements were taken for each specimen with the use of a Shimadzu microhardness tester using a Vickers Diamond Pyramid indentor and a 400g load. The tensile tests were run at a crosshead speed of 4.23 X 10 6 m / s (strain rate of 6.67 X 10-4 s-~) at 298 K in air or in a hydrogen charging environment. The hydrogen charging tests were performed in an electrolytic cell 1 8 7 8 - - V O L U M E 11A, N O V E M B E R 1980
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Figure 4 shows that there is very little effect of ordering on RA when tested in air. However, as shown in Fig. 5, hydrogen charging produces a dramatic effect. The maximum embrittlement susceptibility exists in the disordered and highly ordered samples. They each display a loss in RA of approximately 70 pct. Upon ordering, there is a sharp decrease in embrittlement susceptibility with increasing order to about 40 to 50 pct order followed by an increase in embrittlement thereafter. There is a minimum in susceptibility at 40 to 50 pct order. Scanning electron microscopy (SEM) revealed much about the mode of failure. The air-tested samples typically exhibited transgranular dimpled ductile failure as shown in Fig. 6, indicating substantial ductility. All the air tested samples displayed the same type of ductile
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DEGREE OF ORDER, %
Fig. 3 - - T h e effect of the degree of order on the hardness of Ni2Cr. METALLURGICAL TRANSACTIONS A
Fig. 4---Theeffect of the degreeof order on the ductility of Ni2Cr.
Fig. 5~The effect of the degree of order on the embrittlement susceptibilityof Ni2Cr. Regionsof intergranular (IG) and ductile transgranular failure (TG) are shown. fracture independent of the amount of ordering. The hydrogen charged specimens, on the other hand, varied considerably in failure mode. The disordered specimen showed ductile transgranular failure in the center and intergranular failure along the perimeter as exhibited in Fig. 7. The percentages of intergranular and ductile failure varied in amount and related directly with the degree of embrittlement resulting from hydrogen charging. Figures 8 and 9 show the fracture surfaces of hydrogen charged specimens which was ordered - 4 4 pct and 74 pct respectively. The depth of penetration of brittle failure is highest for disordered and highly ordered specimens. Samples which showed less effect of hydrogen (e.g., 14 to 44 pct order) displayed much less intergranular failure. It appears that quasi-cleavage may be the mode of failure at the extreme perimeter of these samples. Above 44 pct order, the brittle regions are intergranular and increase in area and penetration with increasing order. DISCUSSION In light of the present results, it appears that the presence of ordering cannot be used as a simple and METALLURGICALTRANSACTIONSA
Fig. 6--Fractographs of disordered Ni2Crfractured in air.
exclusive explanation for the increased hydrogen embrittlement susceptibility in H A S T E L L O Y C-276, as suggested by Asphahani. 3 The present findings, taken in conjunction with the results of the study of impurity segregation in Hastelloy C-276, 2 demonstrates the complexity of the mechanism of hydrogen pickup, transport and subsequent embrittlement. It is known that short range order affects the slip mode of materials. Short range order, like a decrease in stacking fault energy, will increase the amount of coplanar slip. It has been suggested that as the amount of coplanar slip is increased, there is increased tendency for preferential chemical attack at the surface where a slip step has formed. 11 This is a good explanation for the increased transgranular stress corrosion cracking susceptibility for low stacking fault materials. Likewise, Thompson has shown that low stacking fault energy will increase planar slip and increase the hydrogen embrittlement susceptibility of 309S stainless steel) 3 Although there is an RA loss due to hydrogen, the fracture mode is still not changed from ductile rupture. As the stacking fault energy decreases, the hydrogen carrying dislocations cannot easily cross slip around small particles. Thompson suggests that VOLUME 11A, NOVEMBER 1980--1879
Fig. 7--Fractographs of disordered Ni,Cr fractured during hydrogen charging.
Fig. 8--Fractographs of Ni2Cr, ordered 44 pct, fractured during hydrogen charging.
hydrogen collects at the surfaces of these small particles resulting in enhanced microvoid nucleation and ductility loss. In addition, in Ti-AI alloys, slip mode has been shown to affect intergranular stress corrosion cracking. 14 In this case, it is possible that hydrogen may be involved. In spite of all these results, it is not clear what effect short or long range order would have on intergranular hydrogen embrittlement susceptibility. The most striking result of this work is that there appears to be two regimes in the susceptibility of NizCr as a function of ordering. As we noted in Fig. 5, the susceptibility to embrittlement decreased to approximately 40 to 50 pct order and then increased thereafter. In fact, it appears that a different curve can be drawn for each regime. As the a m o u n t of order increases to 40 to 50 pct the susceptibility to intergranular hydrogen embrittlement decreases. Beyond this point, further ordering results in an increase in the hydrogen embrittlement susceptibility with failures of the most susceptible samples being intergranular. There are a number of possible explanations for the variation embrittlement behavior in hydrogen producing environments. It is likely that the nucleation and growth of ordered domains will affect dislocation
motion. If dislocation motion is responsible for hydrogen transport, ~5-~6changes in size and distribution of ordered domains could control the deliverance of hydrogen to the grain boundaries. However, another possible contribution to this behavior can be suggested. It is quite possible that isothermal aging at 500 ~ results in the segregation of impurities to grain boundaries which is known to enhance hydrogen embrittlement susceptibility of nickel base alloys. 2 Thus, the increase in intergranular failure with aging may be enhanced by the segregation of impurities to the grain boundaries. Both of these possibilities can be occurring simultaneously or one may dominate for each regime of embrittlement. Variations in SFE may also be contributing. However, although the stacking fault energy increases with increasing order, 1~the effect of this on cross slip is overshadowed by the direct effect of the presence of an ordered phase. The disordered alloy has a measured SFE of ~ 4 0 to 70 e r g s / c m 2 which is rather large to begin with. An increase to 110 e r g s / c m 2 should not significantly increase the amount of cross slip. The relationship between embrittlement susceptibility and isothermal aging is not a simple one as evidenced
1880--VOLUME 11A, NOVEMBER 1980
METALLURGICAL TRANSACTIONSA
SUMMARY 1) The hydrogen embrittlement susceptibility of Ni2Cr is a strong function of the amount of ordering. In the disordered and highly ordered alloy, the embrittlement susceptibility is maximum. At about 40 to 50 pct order the susceptibility is a minimum. 2) The amount of intergranular fracture is directly related to the hydrogen embrittlement susceptibility. 3) It is hypothesized that the amount of order affects dislocation motion which in turn controls the transport of hydrogen to grain boundaries. ACKNOWLEDGMENT We would like to acknowledge R. M. Latanision and R. D. Kane for their helpful suggestions during the course of this work. We would also like to acknowledge the late H. B. Oppenheimer for his technical assistance during the initial stages of this work. REFERENCES
Fig. 9--Fractographs of Ni2Cr, ordered 74 pct, fractured during hydrogen charging.
by the complicated behavior of Ni2Cr. We therefore believe that the behavior Hastelloy C-276 is at least as complex and cannot be explained simply by an ordering reaction. Further work is therefore needed to investigate hydrogen transport by dislocations and impurity segregation in order to understand the interesting relationships between the hydrogen embrittlement susceptibility and isothermal aging.
M E T A L L U R G I C A L TRANSACTIONS A
1. R. D. Kane, M. Watkins, D. F. Jacobs, and G. L. Hancock: Corrosion, 1977, vol. 30, no. 9, p. 309. 2. B. J. Berkowitz and R. D. Kane: Corrosion, 1980, vol. 36, no. 1, pp. 24-29. 3. A. I. Asphahani: Hydrogen in Metals, Proceedings of the Second International Congress, vol. 4, section 3C, pp. 1-10, Pergamon Press, Paris, June 1977. 4. Ye. Z. Vintaykin and G. G. Urushadze: Fiz. Met. Metalloved., 1969, vol. 27, pp. 895-903. 5. R. J. Taunt and B. Ralph: Phys. Status. SolidiA, 1975, vol. 29, pp. 431-42. 6. G. Baer: Z. Metallkd., 1958, vol. 49, p. 614. 7. V. I. Gomankov, D. F. Litvin, A. A. Loshmanov, and B. G. Lyashchenko: Phys. Met. Metallogr., 1962, vol. 14, p. 133. 8. M. Hirabayashi, M. Koiwa, K. Tanaka, T. Tadake, T. Saburi, S] Nenno, and H. Nishyoma: Trans. Jpn. Inst. Met., 1969, vol. 10, pp. 365-71. 9. G. I. Nosova and N. A. Polyakov~t: Fiz. Met. Metalloved., 1971, vol. 32, pp. 825-30. 10. G. I. Nosova, N. A. Polyakova, and E. A. Medvedev: Fiz. Met. Metalloved., 1974, vol. 38, pp. 176-82. 11. D. L. Douglass, G. Thomas, and W. R. Roser: Corrosion, Jan. 1974, vol. 20, pp. 15-28. 12. V. A. Gol'tsov, V. Yu. Kosheleva, and G. E. Kagan: Tr. Ural. Politekh. lnst., 1974, vol. 231, pp. 62-65. 13. A. W. Thompson: Mater. Sci. Eng., 1974, vol. 14, pp. 253-64. 14. C. J. McMahon, Jr. and D. J. Truax: Corrosion, 1973, vol. 29, no. 2, pp. 47-55. 15. J. K. Tien, A. W. Thompson, I. M. Bernstein, and R. J. Richards: Met. Trans. A, 1976, vol. 7A, p. 821. 16. M. Kurkela and R. M. Latanision: Scr. Metall., 1979, vol. 13, pp. 927-32.
VOLUME 1IA, NOVEMBER 1980--1881