- Email: [email protected]
Intergranular corrosion of a curved ∑=5 grain boundary in an FeSi alloy bicrystal
Scripta METALLURGICA et MATERIALIA
Vol. 30, pp. 283-286, 1994 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
INTERGRANLrLAR CORROSION OF A CURVED E=5 GRAIN BOUNDARY IN AN Fe-Si ALLOYBICRYSTAL P. Lej~ek and V. Paidar Institute of Physics, Academy of Sciences Na Slovance 2, 180 40 Prague 8, Czech Republic
(Received July 30, 1993) (Revised October 19, 1993)
Introduction Intergranular corrosion plays an important role in the cohesion and thus, in the use of polycrystalline metals and alloys. Due to the differencesin structure and energy, the environmental attack of grain boundaries can differ in individual cases. The orientation dependence of intergranular corrosion has already been studied: however, attention was mainly paid to symmetrical grain boundaries. Minima of the penetration depth or the width of the etched boundary groove corresponding to the minima of the measured boundary energy were found at certain [100] tiltgrain boundaries in aluminum (1,2), Cr-Ni-austenitic stainlesssteel(3), niobium (4), and copper and a-Cu-Al alloy (5). Low values of the etch groove depth were also found at the low-~ grain boundaries in a polycrystalline Fe-Ni-Cr alloy (6). It was surprising to find that the maxima o£ penetration depth were reported for the low energy {013}, {012} and {023} symmetrical tilt grain boundaries in an austenitic stainless steel containing silicon (7). With an exception of Ogura's work (6) the sets of bicrystals containing a single symmetrical grain boundary were etched and the results were then compared. In the present work the corrosion behavior of the 36.9°[100] symmetrical and asymmetrical tilt grain boundaries in one sample were studied on a smile curved grain boundasy.
Experimental The 36.9°[100] tilt bicrystal (diaa'neter of 13 ram, length of 50 ram) with a {013} symmetrical grain boundary parallel to the axis was grown by the floating zone technique in an Fe-6at%Si alloy containing 0.03at%P, 0.009at%C, 0.008at%N and 0.003at%O (8). The samples, approximately 2 mm thick, were cut from the bicrystal perpendicular to the common [100] axis. Thus, the boundary plane was perpendicular to the sample surface. The angle between the boundary and both side surfaces was approximately 30°. The geometry of the sample (fig. 1) was similar to that of the sample used for the study of grain boundary migration by the modified reversedcapillary technique (9,10). The sample was annealed for 1800 s at 1400 K in a hydrogenized atmosphere. During this treatment both ends of the boundary, in the sample, migrated to reduce the boundary energy, thus decreasing the boundary area. After the sample was annealed, the boundary became S-shaped so that nearly right angles between the boundary and both side surfaces were established. To ensure that the boundary stops in a desired position, an obstacle can be introduced to the boundary migration at that position (e.g. to cut a groove on the side surface by spark erosion). The orientation of the curved boundary, in certain parts, changes continuously from the original {013} symmetrical boundary through a sequence of asymmetrical 36.9°[100] tiltgrain boundary into the other symmetrical {012} grain boundary etc.
283 0956-716X/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press
Ltd.
284
INTERGRANULAR
CORROSION
Vol.
30, No.
3
The sample was chemically polished for 200 s in the mixture of HF(40%) + H202(30%) + H20 (8 + 100 + 17) and etched for 120 s in the 2% solution of H N O 3 in methanol. During this procedure, the microstructure of the sample has been visualized. Individual dislocations and subboundaries were apparent at the (100) surface and a groove is etched at the curved boundary. Due to changing orientation of the boundary, geometrical parameters of the groove (i.e. its width and depth) change along the curved boundary. The orientation of the groove and the corresponding width were determined by means of an optical microscope (Opton Stereomicroscope) equipped with the Nomarski interferencecontrast (fig.2). Results and Discussion The etched groove at the boundary exhibits a V-shaped Nomarski interference contrast enabling easy measurement of the groove width (fig. 2). It was shown by Yamashita et al. on Cu and a-Cu-A1 alloy that the width and depth of the etched groove exhibit qualitatively the same dependence on the boundary orientation (5). Therefore, the width of the etched groove at the grain boundary can be well used for the characterization of corrosion behavior of differently oriented parts of the boundary. The orientation of the boundary determined by means of the goniometer stage of the microscope, is represented by the deviation angle ~ between its tangent and the symmetrical {013} grain boundary in the remaining straight non-migrated rest part. The width was measured parallel to the actual boundary normal as it is depicted by arrows in fig. 2. In general, the width of the etched groove depends on the etching time, and variations in etching solution etc. Therefore, it is very difficultto keep the same etching conditions for a series of different samples, particularly for low etching times. In the case of the single sample with the curved boundary this disadvantage is avoided since all parts of the sample are equally exposed to etching. Thus, the relative differences of the geometrical parameters of the etched groove at differentlyoriented parts of the boundary reflectthe anisotropy of the corrosion behavior. Characteristic orientation dependence of the relative width to of the etched groove (with respect to its largest value to,.)at the 36.9°[100] tiltgrain boundary is represented by the deviation angle ~b of the boundary plane [tom the symmetrical {013} boundary orientation as shown in fig. 3. It is clearly seen that anisotropy of the corrosion behavior of grain boundaries does exist. The small widths were measured at both symmetrical {013} and {012} grain boundaries, i.e. ~b = 0°(013} and 45°{013}, respectively. In axldition,two other minima of the width can be distinguished at the orientation dependence corresponding to the deviations ~b = 18.0° and ~b = 26.5° from the {013} symmetrical grain boundary. Assuming the boundary remains perpendicular to the surface in the whole sample, as proved on side surfaces, the former minimum is very near to the (001)/(034-)asymmetrical tiltgrain boundary (~b = 18.4°{013}) and the other corresponds perfectly to the (0£7)/(011) one (~b = 26.6°{013}). The a~ymmetrical grain boundaries seem to play an important role in the behavior of polycrystalline materials as they are relatively more frequent than with the symmetrical ones. It was shown that the asymmetrical grain boundaries form stable low-energy facets (11). A m o n g the low-energy asymmetrical grain boundaries, those composed at least by one dense low-index plane are expected to be favorable (11). In the set of the 36.9°[100] tiltgrain boundaries, the 18.4°{013}, (001)/(034-) and 26.6°{013}, (017)/(011-) ones could be candidates of such behavior. The (001)/(03~) tiltgrain boundary was observed to be the stable configuration in NiO with its energy comparable with the symmetrical boundaries (12). In addition, the straight (001)/(034-) asymmetrical facets were observed in gold albeit not the (0i7)/(011-')ones (13). In contrast, the calculations of the structure of some 36.9°[100] asymmetrical grain boundaries suggest that structures of both the (001)/(034-) and the (017)/(011-) asymmetrical grain boundaries can be composed from the {013} and {012} symmetrical segments (14). Two cusps existing at the orientation dependence of the width of etched groove at the (0T7)/(011-) and (001)/(034-) grain boundaries in the b.c.c. Fe-Si alloy suggest that these boundaries poszess low energy. This finding corresponds very well to the orientation dependence of the segregation enthalpies of carbon, phosphorus and siliconin c~-iron: the minima of their absolute values were found not only at the (013} and {012} symmetrical grain boundaries, which are generally considered as special, but even at the (001)/(034-) and (017)/(011-) asymmetrical boundaries (fig. 4) (15,16). Nevertheless, the found corrosion behavior seems to reflect the changes of the boundary energy along a curved 36.9°[100] grain boundary segment rather than changes of solute segregation since large segregation effects cannot be expected as a result of annealing of the sample at 1400 K (16). Analogously to symmetrical
Vol.
30, No.
3
INTERGRANULAR
CORROSION
285
grain boundaries (17), the structure of the asymmetrical grain boundary can be very simply char~terized by the value of effective interplanar spacing (18), dell = ~[d(hl, kr, lr) + d(h2, ks,/2)]- As it is seen in figs. 3 and 4, all minima of absolute values of segregation enthalpies and widths of etched boundary grooves correspond to the boundaries which possess high values of the effective interplanar spacing. Because of their exceptional behavior, the (001)/(034-) and (017)/(011--) asymmetrical grain boundaries can be considered as special as in the {013} and {012} symmetrical boundaries. Conclusion The study of corrosion behavior of a curved 36.9°[100] tilt grain boundary in a bicrystal of an Fe-6at%Si Mloy revealed low propensity of the {012} and {013} symmetrical and the (001)/(034-) and (0i-r)/(0iD asymmetrical grain boundaries to chemical attack. The orientation dependence of the width of the grain boundary corrosion groove is qualitatively identical to the orientation dependences of absolute values of enthalpies of Si, P and C segregation in c~-Fe. The anisotropy of corrosion behavior arises as a result of differences in structure and energetics of differently oriented boundary parts. It can be well correlated to the effective interplanas spa~ing of the corresponding grain boundaries. Acknowledgment This research was supported by the Grant Agency of the Academy of Sciences of the Ozech Republic under Contracts Nos. 19096 and 11089. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
O.P. Arora and M. Metzger, Trans. Met. Soc. AIMB 236, 1205 (1966). G. Hasson, J.-Y. Boos, I. Herbeuval, M. Biscondi and C. Goux, Surface Sci. 31,115 (1972). L. Beannier, M. l~omment and C. Vignaud, J. Electroanal. Chem. 119, 125 (1981). X.R. Quian and Y.T. Chou, Phil. Mug. 45, 1075 (1982). M. Yamashita, T. Mimaki, S. Hashimoto and S. Miura, Phil. Mug. A63, 695 (1991). T. Ogura, T. Watanabe, S. Karashima and T. Masumoto, Acta metall. 35, 1807 (1987). J. Stolam and J. LeOoze, J. Phys. France 51, 01-641 (1990). S. Kade~kov~., P. Toula and J. Ad~.mek, J. Crystal Growth 83, 410 (1987). P. Lej~ek, S. Kade~kov~ and V. Paidar, in Annealing Processes- Recovery, R.ecrystalhzation and Grain Growth, edited by N. Hansen, D. Juul Jensen, T. Leffers and B. Ralph (Proc. 7th Kis¢ Int. Symp. Metall. Mater. Sci. Roskilde 1986), p. 437. 10. P. Lej~ek, V. PMdar, J. Adhmek and S. Kade~kov,~, Interface Sci., in press. 11. K.L. Merkle, Ultramicroscopy 37, 130 (1991). 12. K.L. Merkle, J. Phys. France 51, C1-251 (1990). 13. F. Cosandey, S.-W. Cahn and P. Stadelman, J. Phys. Prance 51, O1-109 (1990). 14. A. Brokman, P.D. Bristowe and t~.W. Ballufti, Scripta metall. 15,201 (1981). 15. S. Hofmann and P. Lej~ek, Scripta metall, mater. 25, 2259 (1991). 16. P. Lej~ek, J. Ad~.mek and S. Hofmann, Surface Sci. 264, 449 (1992). 17. V. Paidar, Acta metall. 35, 2035 (1987). 18. S. Yip and D. Wolf, Mater. Sci. Forum 46, 77 (1989).
286
INTERGRANULAR CORROSION
'f/
Vol.
1.0 •
o
co
30,
No.
o° °o
o
o
0.5 F[< ;. }
15
~;~omptry ~)f the sample with curved grain boundary G B
30
~'(013)
5
IwtweP:l two adjacent grains h and B after annealing. l)ash~,d line depicts original position of as-grown planar
l)~).ndary.
FIG. 3
Orientation dependence of the width w of the etched 36.9°[100] grain boundary normalized by the maximum value of the width w,,.
60
C
(a)
~4S
P
Z 30
0
*
i
,
0.4 I
(b) 1
0
IS
30
,.(0~3)
45
FIG. 2
FIG. 4
Microstructure of etched groove at curved grain boundary. The arrows indicate the actual widths of the etched groove. Tlle changes of the etched groove width at differently oriented parts are apparent. Optical microscopy with Nomarski interference contrast,
Orientation dependence of(a) the segregation enthalpies of C, P and Si in 36.9°[100] tilt bicrystals of a-Fe and (b) corresponding values of effective interplanar spacing dell normalized by the lattice pard,meter a for a b.c.c. lattice.
3
Vol. 30, pp. 283-286, 1994 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
INTERGRANLrLAR CORROSION OF A CURVED E=5 GRAIN BOUNDARY IN AN Fe-Si ALLOYBICRYSTAL P. Lej~ek and V. Paidar Institute of Physics, Academy of Sciences Na Slovance 2, 180 40 Prague 8, Czech Republic
(Received July 30, 1993) (Revised October 19, 1993)
Introduction Intergranular corrosion plays an important role in the cohesion and thus, in the use of polycrystalline metals and alloys. Due to the differencesin structure and energy, the environmental attack of grain boundaries can differ in individual cases. The orientation dependence of intergranular corrosion has already been studied: however, attention was mainly paid to symmetrical grain boundaries. Minima of the penetration depth or the width of the etched boundary groove corresponding to the minima of the measured boundary energy were found at certain [100] tiltgrain boundaries in aluminum (1,2), Cr-Ni-austenitic stainlesssteel(3), niobium (4), and copper and a-Cu-Al alloy (5). Low values of the etch groove depth were also found at the low-~ grain boundaries in a polycrystalline Fe-Ni-Cr alloy (6). It was surprising to find that the maxima o£ penetration depth were reported for the low energy {013}, {012} and {023} symmetrical tilt grain boundaries in an austenitic stainless steel containing silicon (7). With an exception of Ogura's work (6) the sets of bicrystals containing a single symmetrical grain boundary were etched and the results were then compared. In the present work the corrosion behavior of the 36.9°[100] symmetrical and asymmetrical tilt grain boundaries in one sample were studied on a smile curved grain boundasy.
Experimental The 36.9°[100] tilt bicrystal (diaa'neter of 13 ram, length of 50 ram) with a {013} symmetrical grain boundary parallel to the axis was grown by the floating zone technique in an Fe-6at%Si alloy containing 0.03at%P, 0.009at%C, 0.008at%N and 0.003at%O (8). The samples, approximately 2 mm thick, were cut from the bicrystal perpendicular to the common [100] axis. Thus, the boundary plane was perpendicular to the sample surface. The angle between the boundary and both side surfaces was approximately 30°. The geometry of the sample (fig. 1) was similar to that of the sample used for the study of grain boundary migration by the modified reversedcapillary technique (9,10). The sample was annealed for 1800 s at 1400 K in a hydrogenized atmosphere. During this treatment both ends of the boundary, in the sample, migrated to reduce the boundary energy, thus decreasing the boundary area. After the sample was annealed, the boundary became S-shaped so that nearly right angles between the boundary and both side surfaces were established. To ensure that the boundary stops in a desired position, an obstacle can be introduced to the boundary migration at that position (e.g. to cut a groove on the side surface by spark erosion). The orientation of the curved boundary, in certain parts, changes continuously from the original {013} symmetrical boundary through a sequence of asymmetrical 36.9°[100] tiltgrain boundary into the other symmetrical {012} grain boundary etc.
283 0956-716X/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press
Ltd.
284
INTERGRANULAR
CORROSION
Vol.
30, No.
3
The sample was chemically polished for 200 s in the mixture of HF(40%) + H202(30%) + H20 (8 + 100 + 17) and etched for 120 s in the 2% solution of H N O 3 in methanol. During this procedure, the microstructure of the sample has been visualized. Individual dislocations and subboundaries were apparent at the (100) surface and a groove is etched at the curved boundary. Due to changing orientation of the boundary, geometrical parameters of the groove (i.e. its width and depth) change along the curved boundary. The orientation of the groove and the corresponding width were determined by means of an optical microscope (Opton Stereomicroscope) equipped with the Nomarski interferencecontrast (fig.2). Results and Discussion The etched groove at the boundary exhibits a V-shaped Nomarski interference contrast enabling easy measurement of the groove width (fig. 2). It was shown by Yamashita et al. on Cu and a-Cu-A1 alloy that the width and depth of the etched groove exhibit qualitatively the same dependence on the boundary orientation (5). Therefore, the width of the etched groove at the grain boundary can be well used for the characterization of corrosion behavior of differently oriented parts of the boundary. The orientation of the boundary determined by means of the goniometer stage of the microscope, is represented by the deviation angle ~ between its tangent and the symmetrical {013} grain boundary in the remaining straight non-migrated rest part. The width was measured parallel to the actual boundary normal as it is depicted by arrows in fig. 2. In general, the width of the etched groove depends on the etching time, and variations in etching solution etc. Therefore, it is very difficultto keep the same etching conditions for a series of different samples, particularly for low etching times. In the case of the single sample with the curved boundary this disadvantage is avoided since all parts of the sample are equally exposed to etching. Thus, the relative differences of the geometrical parameters of the etched groove at differentlyoriented parts of the boundary reflectthe anisotropy of the corrosion behavior. Characteristic orientation dependence of the relative width to of the etched groove (with respect to its largest value to,.)at the 36.9°[100] tiltgrain boundary is represented by the deviation angle ~b of the boundary plane [tom the symmetrical {013} boundary orientation as shown in fig. 3. It is clearly seen that anisotropy of the corrosion behavior of grain boundaries does exist. The small widths were measured at both symmetrical {013} and {012} grain boundaries, i.e. ~b = 0°(013} and 45°{013}, respectively. In axldition,two other minima of the width can be distinguished at the orientation dependence corresponding to the deviations ~b = 18.0° and ~b = 26.5° from the {013} symmetrical grain boundary. Assuming the boundary remains perpendicular to the surface in the whole sample, as proved on side surfaces, the former minimum is very near to the (001)/(034-)asymmetrical tiltgrain boundary (~b = 18.4°{013}) and the other corresponds perfectly to the (0£7)/(011) one (~b = 26.6°{013}). The a~ymmetrical grain boundaries seem to play an important role in the behavior of polycrystalline materials as they are relatively more frequent than with the symmetrical ones. It was shown that the asymmetrical grain boundaries form stable low-energy facets (11). A m o n g the low-energy asymmetrical grain boundaries, those composed at least by one dense low-index plane are expected to be favorable (11). In the set of the 36.9°[100] tiltgrain boundaries, the 18.4°{013}, (001)/(034-) and 26.6°{013}, (017)/(011-) ones could be candidates of such behavior. The (001)/(03~) tiltgrain boundary was observed to be the stable configuration in NiO with its energy comparable with the symmetrical boundaries (12). In addition, the straight (001)/(034-) asymmetrical facets were observed in gold albeit not the (0i7)/(011-')ones (13). In contrast, the calculations of the structure of some 36.9°[100] asymmetrical grain boundaries suggest that structures of both the (001)/(034-) and the (017)/(011-) asymmetrical grain boundaries can be composed from the {013} and {012} symmetrical segments (14). Two cusps existing at the orientation dependence of the width of etched groove at the (0T7)/(011-) and (001)/(034-) grain boundaries in the b.c.c. Fe-Si alloy suggest that these boundaries poszess low energy. This finding corresponds very well to the orientation dependence of the segregation enthalpies of carbon, phosphorus and siliconin c~-iron: the minima of their absolute values were found not only at the (013} and {012} symmetrical grain boundaries, which are generally considered as special, but even at the (001)/(034-) and (017)/(011-) asymmetrical boundaries (fig. 4) (15,16). Nevertheless, the found corrosion behavior seems to reflect the changes of the boundary energy along a curved 36.9°[100] grain boundary segment rather than changes of solute segregation since large segregation effects cannot be expected as a result of annealing of the sample at 1400 K (16). Analogously to symmetrical
Vol.
30, No.
3
INTERGRANULAR
CORROSION
285
grain boundaries (17), the structure of the asymmetrical grain boundary can be very simply char~terized by the value of effective interplanar spacing (18), dell = ~[d(hl, kr, lr) + d(h2, ks,/2)]- As it is seen in figs. 3 and 4, all minima of absolute values of segregation enthalpies and widths of etched boundary grooves correspond to the boundaries which possess high values of the effective interplanar spacing. Because of their exceptional behavior, the (001)/(034-) and (017)/(011--) asymmetrical grain boundaries can be considered as special as in the {013} and {012} symmetrical boundaries. Conclusion The study of corrosion behavior of a curved 36.9°[100] tilt grain boundary in a bicrystal of an Fe-6at%Si Mloy revealed low propensity of the {012} and {013} symmetrical and the (001)/(034-) and (0i-r)/(0iD asymmetrical grain boundaries to chemical attack. The orientation dependence of the width of the grain boundary corrosion groove is qualitatively identical to the orientation dependences of absolute values of enthalpies of Si, P and C segregation in c~-Fe. The anisotropy of corrosion behavior arises as a result of differences in structure and energetics of differently oriented boundary parts. It can be well correlated to the effective interplanas spa~ing of the corresponding grain boundaries. Acknowledgment This research was supported by the Grant Agency of the Academy of Sciences of the Ozech Republic under Contracts Nos. 19096 and 11089. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
O.P. Arora and M. Metzger, Trans. Met. Soc. AIMB 236, 1205 (1966). G. Hasson, J.-Y. Boos, I. Herbeuval, M. Biscondi and C. Goux, Surface Sci. 31,115 (1972). L. Beannier, M. l~omment and C. Vignaud, J. Electroanal. Chem. 119, 125 (1981). X.R. Quian and Y.T. Chou, Phil. Mug. 45, 1075 (1982). M. Yamashita, T. Mimaki, S. Hashimoto and S. Miura, Phil. Mug. A63, 695 (1991). T. Ogura, T. Watanabe, S. Karashima and T. Masumoto, Acta metall. 35, 1807 (1987). J. Stolam and J. LeOoze, J. Phys. France 51, 01-641 (1990). S. Kade~kov~., P. Toula and J. Ad~.mek, J. Crystal Growth 83, 410 (1987). P. Lej~ek, S. Kade~kov~ and V. Paidar, in Annealing Processes- Recovery, R.ecrystalhzation and Grain Growth, edited by N. Hansen, D. Juul Jensen, T. Leffers and B. Ralph (Proc. 7th Kis¢ Int. Symp. Metall. Mater. Sci. Roskilde 1986), p. 437. 10. P. Lej~ek, V. PMdar, J. Adhmek and S. Kade~kov,~, Interface Sci., in press. 11. K.L. Merkle, Ultramicroscopy 37, 130 (1991). 12. K.L. Merkle, J. Phys. France 51, C1-251 (1990). 13. F. Cosandey, S.-W. Cahn and P. Stadelman, J. Phys. Prance 51, O1-109 (1990). 14. A. Brokman, P.D. Bristowe and t~.W. Ballufti, Scripta metall. 15,201 (1981). 15. S. Hofmann and P. Lej~ek, Scripta metall, mater. 25, 2259 (1991). 16. P. Lej~ek, J. Ad~.mek and S. Hofmann, Surface Sci. 264, 449 (1992). 17. V. Paidar, Acta metall. 35, 2035 (1987). 18. S. Yip and D. Wolf, Mater. Sci. Forum 46, 77 (1989).
286
INTERGRANULAR CORROSION
'f/
Vol.
1.0 •
o
co
30,
No.
o° °o
o
o
0.5 F[< ;. }
15
~;~omptry ~)f the sample with curved grain boundary G B
30
~'(013)
5
IwtweP:l two adjacent grains h and B after annealing. l)ash~,d line depicts original position of as-grown planar
l)~).ndary.
FIG. 3
Orientation dependence of the width w of the etched 36.9°[100] grain boundary normalized by the maximum value of the width w,,.
60
C
(a)
~4S
P
Z 30
0
*
i
,
0.4 I
(b) 1
0
IS
30
,.(0~3)
45
FIG. 2
FIG. 4
Microstructure of etched groove at curved grain boundary. The arrows indicate the actual widths of the etched groove. Tlle changes of the etched groove width at differently oriented parts are apparent. Optical microscopy with Nomarski interference contrast,
Orientation dependence of(a) the segregation enthalpies of C, P and Si in 36.9°[100] tilt bicrystals of a-Fe and (b) corresponding values of effective interplanar spacing dell normalized by the lattice pard,meter a for a b.c.c. lattice.
3