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Wetting of molybdenum grain boundaries by nickel: effect of the boundary structure and energy
Acta mater. 48 (2000) 3303±3310 www.elsevier.com/locate/actamat
WETTING OF MOLYBDENUM GRAIN BOUNDARIES BY NICKEL: EFFECT OF THE BOUNDARY STRUCTURE AND ENERGY J. M. PEÂNISSON 1{ and T. VYSTAVEL 1,
2
1
DeÂpartement de Recherche Fondamentale sur la MatieÁre CondenseÂe/SP2M/ME CEA±Grenoble, 17 Av. des Martyrs, 38054 Grenoble Cedex 9, France and 2Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Praha 8, Czech Republic (Received 20 April 2000; accepted 19 May 2000) AbstractÐThe structural eect of the penetration of nickel along symmetrical [101] tilt grain boundaries (GBs) in two dierent molybdenum bicrystals is investigated. The selection of GBs S 3f121g and S 11f323g is governed by their dierent energy so that a dierent penetration behaviour is expected. The temperature of treatment is 13508C, i.e. above the eutectic temperature. The analysis of the Mo±Ni phase formed on the surface of the bicrystal, the concentration pro®le along the GB and the identi®cation of the nanophases present at the GB is performed by using several experimental techniques from microscopic to nanoscopic scales. Important dierences in the penetration of nickel are found for the two investigated GBs. 7 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. ReÂsumeÂÐLes changements structuraux induits dans des bicristaux de ¯exion symmeÂtrique dans du molybdeÁne par la peÂneÂtration de nickel sont eÂtudieÂs par microscopie eÂlectronique aÁ haute reÂsolution et imagerie chimique ®ltreÂe en eÂnergie. Deux types de bicristaux S 3f121g et S 11f323g ont eÂte choisis aÁ cause de leurs eÂnergies treÁs dieÂrentes. Les eÂchantillons sont traiteÂs aÁ 13508C c'est aÁ dire au dessus du point eutectique du diagramme de phase. Les deux joints reÂveÃlent des comportements dieÂrents qui sont analyseÂs. 7 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. Keywords: Grain boundaries; Metals; Segregation; TEM; EELS
1. INTRODUCTION
When a polycrystalline metal is in contact with a liquid metallic phase, the liquid can penetrate along the grain boundaries (GBs), leading to the wetting phenomenon which can be used in powder metallurgy to lower the temperature of the sintering process and to obtain the densi®cation of the ®nal product [1]. A detailed presentation of the wetting of GBs in metals and ceramics has been given by Clarke [2]. The wetting of GBs depends on several parameters, the most important being the chemical nature of the components, the temperature, the pressure and the grain boundary structure and energy. The wetting can be considered as a ®rstorder phase transformation so that a critical wetting temperature can be de®ned below which the boundary is not completely wet. This critical temperature depends also on the energy of the boundary. The
{ To whom all correspondence should be addressed.
energetic conditions in which the wetting of a boundary occurs have been studied in detail by several authors [3±5]. Wetting has been widely studied in dierent materials and using several experimental techniques. Except for the case of ceramic systems in which a thin layer of amorphous phase has been observed by high resolution electron microscopy (HREM) [6], most of the experimental results concerning metallic systems have been obtained using optical microscopy, scanning electron microscopy (SEM) or conventional transmission electron microscopy (TEM) so that a detailed description is missing in these materials. In this study, bicrystals with particular crystallographic characteristics have been grown. They have been observed before and after the heat treatment in the presence of nickel. The use of bicrystals allows the determination of the intrinsic structure of the dierent boundaries before any treatment and its comparison to treated specimens that have exactly the same characteristics. According to the calculations made by Wolf [7], the energy of h011i
1359-6454/00/$20.00 7 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 1 5 7 - 9
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symmetrical tilt boundaries in b.c.c. metals present deep minima at special coincidence orientation such as S 3f121gy 50:488 and S 11f323gy 70:538: These two particular orientation relationships have been chosen in order to demonstrate the eect of the GB energy on the wetting. The structure of pure boundaries has been already determined at an atomic scale using conventional TEM and HREM and the results published elsewhere [8±10]. 2. EXPERIMENTAL PROCEDURE
2.1. Specimen preparation Two dierent molybdenum bicrystals containing symmetrical [101] tilt grain boundaries: S 3f121g and S 11f323g were prepared by electron-beam ¯oating zone melting [11]. Slices with [101]A,B surface normal orientation and 1 mm thickness were cut by spark erosion. The slices were mechanically thinned down to a thickness of 0.1 mm. Discs 3 mm in diameter were prepared by mechanical grinding. Before grinding, the samples were slightly etched (approx. 5±10 s) in a mixture of concentrated acids, H3PO4, CH3COOH and HNO3 in the proportion 2:1:1 to reveal the grain boundary in order to facilitate its positioning at the centre of the discs. The thermal treatments were performed, in general, at 13508C, under a pressure of 10ÿ3 Pa during 30 min. Only one S 11f323g specimen was also treated at 13808C. The 3 mm bicrystalline disc was covered by a 3 mm thick polycrystalline nickel foil; then a polycrystalline molybdenum disc was placed on top of the nickel foil. This composite sandwich was placed between two Al2O3 cover supports. Figure 1 shows schematically the composition of the experimental assembly. Two dierent types of samples were prepared:
layer. The thickness of this removed layer was 50 mm. The thin foils for TEM observations were prepared by electrochemical polishing in an electrolyte made of sulphuric acid in methanol (1:7) at a temperature of ÿ208C: If the GB did not cross the hole, it was enlarged by ion milling until the boundary could be observed. In the thin parts used for HREM, the observation was performed at a depth of 50 mm. In some cases, a plasma cleaning procedure [12] was used to clean the surface of the thin foil in an argon and oxygen plasma using a South Bay Technology PC150 instrument.
2.2. Transmission electron microscopy The high resolution observations were made in a JEOL 4000EX microscope at a voltage of 400 kV. In the [110] orientation of the bicrystals, the imaging conditions are very stringent because two dierent sets of atomic planes must be simultaneously imaged: the {110} d 0:22 nm and the {200} d 0:157 nm: These two spacings are quite dierent and one of them is smaller than the point resolution limit of the microscope. In these conditions, only a narrow range of specimen thickness and of defocus can be used to obtain the projected image of the [110] Mo structure [9]. A JEOL 3010 ®tted with a Gatan Imaging Filter was used for the chemical mapping and EELS experiments. The nickel maps were obtained using either the two- or three-window technique [13] on the Ni L23 855 eV edge. The two-window technique (also called ratio map) consists of dividing the ionization edge image by a pre-edge image [14]. The
. Cross-sections were used for TEM, SEM, X-ray microanalysis and optical measurements. The sandwich was cut using a wire saw in a direction perpendicular to the GB, then mechanically and chemically polished. TEM samples were thinned by ion milling in a Gatan PIPS instrument. . Edge-on TEM samples of the structure of thermally treated GBs were prepared at a de®ned depth from the initial bicrystal surface. Both, polycrystalline molybdenum cover disc and nickel foil, were ®rst removed by mechanical grinding. The bicrystal itself was also mechanically polished in order to remove the perturbed surface
Fig. 1. Experimental assembly used for the thermal treatment of the bicrystals in the presence of nickel.
Fig. 2. Optical micrograph of a cross-section of the S 3f121g grain boundary annealed at 13508C.
PEÂNISSON and VYSTAVEL: WETTING
advantage of this method is to obtain chemical maps with minimum noise and without diraction eects. However, these maps cannot be used for a quantitative composition determination. In all the cases, a 20 eV energy slit was used. 3. EXPERIMENTAL RESULTS
3.1. General view on annealed samples After the thermal treatment the molybdenum bicrystalline disc, the nickel polycrystalline foil and the molybdenum polycrystalline disc formed a welded sandwich structure. An optical micrograph of a cross-section of the sandwich is shown in Fig. 2. The nickel foil has been transformed into a phase of a thickness of about 8 mm. The composition of this layer was determined by X-ray microanalysis (WDX). The average composition of this layer is 3521 wt% of Ni. The phase formed during annealing corresponds to the intermetallic phase MoNi-d [15]. The analysis of HREM images and diraction patterns of this phase [9] and their comparison with simulated ones based on the data of Shoemaker con®rmed the X-ray microanalysis results, nevertheless some small dierences in the interplanar distances have been found which are attributed to a slightly dierent stoichiometry. Figure 2 clearly shows also that important changes occurred in the polycrystalline molybdenum disc. From the MoNi layer, a slab of approximately 50 mm thick is recrystallized. The size of the grains in this zone is higher than in the non-transformed part. The concentration of Ni in the polycrystalline disc measured by X-ray microanalysis shows also a dierence: in the non-transformed part, the concentration is 0.5 wt% of Ni while in the recrystallized part it is 0.9 wt% of Ni.
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locations is in good agreement with that obtained by atomistic simulations [16, 17]. In particular, there is a rigid body translation parallel to the GB plane, t 0:045 nm and an expansion e 0:03 nm: An optical micrograph of the cross-section of the sandwich after a treatment at 13508C is shown in Fig. 2. No tendency to the migration of the GB can be observed. X-ray microanalysis did not show any change in the nickel concentration (which stayed at an average of 0.1 wt%) in the bicrystal part of the sandwich. Conventional TEM observations at a depth of 50 mm did not reveal any change in the dislocation distribution with respect to the pure GB [9]. At the opposite, HREM observations showed important contrast changes at the secondary GB dislocation cores. Figure 3(a) shows the image of the core of an edge dislocation with Burgers vector a a 121A 121 B 3 3 in the pure boundary, while Fig. 3(b) shows the same type of dislocation in a specimen treated at 13508C. Both images are taken using similar experimental conditions (defocus ÿ73 nm, thickness around 10 nm). Chemical imaging of the boundary using the nickel peak L23 (855 eV) is shown on Fig. 4. This picture is taken in the jump ratio mode. Along the boundary, regions of higher intensity can be distinguished. They correspond to a local enrichment in nickel and their positions and spacings are the same as those of the secondary dislocation cores. No nickel can be detected at the boundary itself.
3.2. S 3101f121g As the study of the structure of the pure bicrystal has already been published [8±10], only the main results will be recalled here. The general orientation relationship does not correspond exactly to the coincidence position. The analysis by diraction showed that there is a 1:620:18 additional tilt angle and a 0:420:18 twist component. These two additional misorientations are compensated by a complex distribution of secondary dislocations that have been studied in detail by conventional and high resolution electron microscopy. The tilt component is compensated by pure edge and mixed dislocations with Burgers vectors 1=3h112i and dislocation lines parallel to the [101] tilt axis of the bicrystal. The twist component is compensated by an array of pure screw dislocations with Burgers vectors 1=6h111i and by the screw component of the mixed 1=3h112i dislocations. The structure of the areas of the boundary located between the dis-
Fig. 3. HREM of an edge dislocation with Burgers vector B : (a) non-treated sample; (b) A a=3121 b a=3121 sample treated at 13508C. The contrast dierence in the treated specimen is attributed to the presence of segregated Ni at the dislocation core.
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3.3. S 11101f323g The bicrystal does not correspond to the exact coincidence relationship (54.338 rotation around [101]); the tilt and twist components are 3:920:18 and 0:620:18, respectively. They are compensated by a complex distribution of secondary dislocations which has been studied in detail as well as the atomic structure of the boundary [9] which is in agreement with the calculated one [18]. Two specimens prepared from this bicrystal have been treated at two dierent temperatures: 1350 and 13808C. After treatment at 13508C, the optical micrograph of the cross-section of the sandwich shows that the boundary is slightly curved in a region close to the surface in contact with the nickel, as shown in Fig. 5. This is a clear indication of the migration of the boundary. The thickness of this migration zone is about 20 mm. HRTEM observations have been
Fig. 4. (a) Jump ratio image of the S 3f121g grain boundary treated at 13508C. The L23 nickel absorption edge at 855 eV is used. Energy slit width 20 eV. (b) Integrated intensity pro®le along the boundary. The distance between the intensity peaks corresponds to the distance between two secondary dislocations.
made at a depth 50 mm from the nickel surface, i.e. outside the GB migration zone. Two dierent morphologies were found. In some places where the grain boundary plane changes, small precipitates are present as shown in Fig. 6(a). Their composition and structure are the same as that observed for the MoNi phase that appears in the central zone of the sandwich. The electron energy loss spectra [Fig. 6(b)] show that they have a composition corresponding to the NiMo-d phase. The chemical image using the Ni L23 (855 eV) is shown in Fig. 6(c). Their shape is triangular and one of the interfaces Mo±MoNi is parallel to a set of {110} atomic planes of one of the two Mo crystals as deduced from the analysis of the high resolution image shown in Fig. 6(d). The other interface is more complex and it has not been determined. From the analysis of the HR images and their diffractograms, the mutual orientation relationship has been determined as: 101Mo ==001MoNi : In the symmetric parts of the boundary, HR and chemical mapping reveal the presence at the boundary of a very thin layer of a phase containing Ni and Mo. Figure 7(a) shows the high resolution image and its diractogram. The thickness of the layer is 0:720:1 nm: The presence of the phase is also detected in the diractogram in which it gives rise to an elongated intensity maximum which corresponds to a spacing of 0:220:01 nm: It has been veri®ed that this intensity does not come from any double diraction event so that it is typical of the existence of a crystallized layer at the boundary plane. Its elongated shape is due to the ®nite thickness of the layer. The corresponding Ni map is shown in Fig. 7(b). The presence of nickel is revealed by the higher intensity along the boundary. Although the contrast in the image is very low, if the intensity is averaged along the boundary the
Fig. 5. Optical micrograph (cross-section) of the S 11f323g bicrystal after treatment at 13508C. The arrow indicates the migration zone.
PEÂNISSON and VYSTAVEL: WETTING
presence of nickel becomes much more evident as shown in Fig. 7(c). The spectrum shown in Fig. 7(c) shows also the presence of Ni at the boundary. The low intensity of the L23 Ni absorption edge comes from the large size of the electron probe which includes large parts of the molybdenum crystals. The same type of specimen has also been treated at 13808C. At this higher temperature, in the bicrystal, the migration of the boundary is more pronounced as it can be seen in Fig. 8(a). The polycrystalline Mo foil is completely recrystallized and the nickel foil is almost completely dissolved. Chemical maps show that some symmetric parts of the boundary still contain an enrichment in nickel similar to the one observed at lower temperature but new facets which contain much more nickel are now present as shown on Fig. 8(b). The thickness of the Ni-rich layer in these facets is about 4 nm. A careful determination of the crystallography of the dierent facets shows that they all have common characteristics. All the facets are parallel
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to a set of {110} planes of one of the two molybdenum crystals. This is shown in Fig. 8(b) in which two facets are meeting at an angle of 1258. This angle corresponds to the angle between the {110} planes of the two Mo crystals in the S 11 coincidence orientation. 4. INTERPRETATION OF THE RESULTS AND DISCUSSION
It should be remarked that the experimental observations are made at room temperature so that there is no direct proof of the existence of a liquid phase at the boundary and only the existence in the phase diagram of a liquid domain at the treatment temperature supports this idea. The dierent experimental results show very clearly that, in the conditions used for the thermal treatments, nickel penetrates into the molybdenum grain boundaries. This result has already been obtained by dierent authors either in the molybdenum [19] or in the
Fig. 6. MoNi-d precipitate. (a) Conventional TEM image. (b) Electron energy loss spectrum showing the Ni L23 edge at 855 eV. (c) Chemical image of the precipitate (three-window technique). (d) High resolution image of the same precipitate. On both sides of the precipitate, the (110) planes of each Mo crystal are visible. The upper interface is parallel to the (110) Mo atomic planes.
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Fig. 7. (a) High resolution image of a symmetric portion of the S 11f323g boundary after treatment at 13508C. A thin crystallized layer is present at the boundary core. Intensity streaks are present in the diractogram. They correspond to a 0.2 nm atomic spacing. Their elongated shape comes from the very small thickness of the layer. (b) Ni map taken in the same conditions as Fig. 6(c). (c) Averaged intensity pro®le along the boundary. (d) Electron energy loss spectrum recorded at the same place as (a). The low intensity of the Ni L23 edge is due to the large size of the probe which integrates an important contribution of the two Mo crystals.
PEÂNISSON and VYSTAVEL: WETTING
tungsten which has the same b.c.c. structure and similar physical properties [20]. According to the Mo±Ni phase diagram [21], at a temperature of 13508C or higher, a liquid phase is expected to occur at the Mo±Ni interface. The grain boundary is then in contact with a liquid. As shown by Straumal et al. [5], the wetting of a grain boundary occurs if the expression: gGB > 2gSL is satis®ed. gGB and gSL are the free energy of the grain boundary and that of one of the two solid±liquid interfaces, respectively. The grain boundary energies as well as their atomic structures have been calculated by several authors and using dierent interatomic interaction potentials [7, 17, 22±25]. A com-
Fig. 8. (a) Optical micrograph of the cross-section after a treatment at 13808C. The migration zone (arrow) is more developed. (b) Ni map taken in the migration zone. Two facets are meeting at a 1258 angle. This angle corresponds to the angle between the (110) planes of the two molybdenum crystals in the S 11 coincidence orientation.
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mon characteristic of all these calculations is to show that the coherent twin S 3101f121g has always the lowest energyÐabout 500 mJ/m2. The S 11101f323g boundary has a higher energy (1400 mJ/m2). The two boundaries lie in a wellde®ned local minimum on the energy vs tilt angle curve. After a treatment at 13508C, the experimental results showed clearly that the coherent twin S 3101f121g is not wet by the liquid phase. However, the secondary dislocations contain a nickel enrichment localized at the core. This enrichment gives rise to a change in the contrast of the high resolution images and it is also detected in the nickel maps. A similar behaviour has been found by Charai et al. [26] in the study of sulphur segregation in a germanium bicrystal. These results indicate that, in these particular conditions, there is no wetting of the boundary. The core enrichment is due to a fast diusion of nickel atoms along the secondary dislocation lines. At the same temperature, the S 11101f323g boundary has a dierent behaviour. The boundary in the symmetrical position contains a thin layer of a Mo±Ni phase which can be considered as MoNid phase [15] although, due to its very limited extension, no chemical analysis could be made. The thickness of the layer (0.7 nm) measured from the high resolution images, is of the same order as the atomic parameter of the phase and it has clearly a crystalline structure. This observation diers from the case of ceramic materials in which an amorphous layer has been found at the boundaries [6]. The thickness determined from the nickel map is much larger: about 3 nm as measured from the value at half maximum of the averaged intensity pro®le of Fig. 7(c). This apparent discrepancy can be, at least partly, explained by the lower resolution of the chemical imaging technique (as compared with the structural high resolution imaging) [27]. However, the possibility of a gradient of the nickel concentration outside the crystallized layer cannot be completely ruled out. This problem will be solved in the future by using a very small (0.5 nm) probe which will be scanned across the boundary; the nickel concentration pro®le being deduced from the analysis of the electron energy loss spectra collected at the dierent locations. The small precipitates observed at places where the GB plane changes seem to indicate that the precipitation is favoured by internal strains that are generally present at these locations. Their structure as well as their composition is similar to that of the central layer of MoNi of the sandwich and corresponds to the MoNi-d phase observed along the [100] axis. An important feature is the existence of a large facet parallel to a {101} plane of one of the Mo crystals. In the b.c.c. structure the {101} planes are compact and they have the lowest energy [28]. The surface
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energy is then the factor governing the migration of the boundary. After a treatment at 13808C, the most important change is the migration of the boundary. This migration leads to the formation of facets with dierent orientations. As in the case of the precipitates, the {101} atomic planes are favoured and the facets follow alternatively the {101} planes of each molybdenum crystal as shown in Fig. 8(b). As it has been shown, the condition for wetting is given by: gGB > 2gSL : The energy of the solid±liquid interface is not known but an estimation has been made by Rabkin et al. [19]. If, as proposed by these authors, a value of 500 mJ/m2 can be admitted, then the wetting condition is satis®ed for the S 11f323g boundary E 1440 mJ=m 2 so that the boundary is wet by Ni, while this condition is not satis®ed for the S 3f121g boundary E 500 mJ=m 2 and the boundary is not wet. 5. SUMMARY AND CONCLUSION
The use of bicrystals to study the wetting behaviour of tilt grain boundaries in b.c.c. molybdenum has allowed a detailed comparison of the structure of the boundaries before and after heat treatment in the presence of nickel. The two selected boundaries: S 3 and 11 have dierent structures and energies. S 3 has a low energy while S 11 has a rather large one. It has been shown that their behaviour is completely dierent. While S 3 does not contain nickel after the treatment, a thin layer of crystalline MoNi-d is found at the S 11 boundary as well as precipitates of the same phase are formed at places where the boundary plane is altered. Moreover, at higher temperature, the S 11 boundary migrates and facets parallel to a {110} plane of one of the two crystals which are associated with a high concentration of nickel. This behaviour is in good agreement with the current thermodynamic theories of the wetting of grain boundaries. AcknowledgementsÐThe authors are grateful to P. Bartuska for his help in SEM X-ray microanalysis. Financial support by contracts GA AV CR No. A1010916, GA CR No. 202/99/1665 and French±Czech co-operation contract Barrande ref. 98-032 are also acknowledged.
REFERENCES 1. Hwang, N. M., Park, Y. J., Kim, D. Y. and Yoon, D. Y., Scripta mater., 2000, 42, 421. 2. Clarke, D. R., Mater. Sci. Forum, 1999, 294±296, 1. 3. Shvindlerman, L. S., Lojkowski, W., Rabkin, E. I. and Straumal, B. B., Colloque Physique, 1990, 51(C1), 629. 4. Rabkin, E. and Gust, W., Trans. Indian Inst. Metals (India), 1997, 50, 619. 5. Straumal, B. and Gust, W., Mater. Sci. Forum, 1996, 207±209, 59. 6. Clarke, D. R. and Thomas, G., J. Am. Ceram. Soc., 1977, 60, 491. 7. Wolf, D., Phil. Mag. A, 1990, 62, 447. 8. Vystavel, T., PeÂnisson, J. M. and Gemperle, A., Phil. Mag. A, in press. 9. Vystavel, T., Ph.D. thesis, Universite J. FourierGrenoble 1, 1999. 10. Vystavel, T., PeÂnisson, J. M. and Gemperle, A., Multiscale Phenomena in Plasticity. Nato Summer School, Ouranopolis, Greece, 1999. 11. Glebowski, V. and Semenov, V., Int. J. Refractory Metals Hard Mater., 1994, 12, 295. 12. Zalusec, N. J., Kestel, B. J. and Henriks, D., Microsc. Microanalysis, 1997, 3(2), 983. 13. Hofer, F., Grogger, W., Kothleitner, G. and Warbicher, P., Ultramicroscopy, 1997, 67, 83. 14. Krivanek, O. L., Gubbens, A. J., Kundmann, M. K. and Carpenter, G. C., Proceedings of the 51st EMSA Meeting, 1993, p. 586. 15. Shoemaker, C. B. and Shoemaker, D. P., Acta crystallogr., 1963, 16, 997. 16. Marinopoulos, A. G., Vitek, V. and Carlsson, A. E., Phil. Mag. A, 1995, 72, 1311. 17. Pontikis, V., Private communication, 1999. 18. Paidar, V., Private communication, 1999. 19. Rabkin, E., Weygand, D., Straumal, B., Semenov, V., Gust, W. and Brechet, Y., Phil. Mag. Lett., 1996, 73, 187. 20. Glebowsky, V., Straumal, B., Semenov, V., Sursaeva, V. and Gust, W., Grain boundary wetting in the W± Ni system, 13th International Plansee Seminar, 1993. 21. Massalski, T. B. (ed.), Binary Alloy Phase Diagrams, 2nd edn. ASM International, Ohio, 1990. 22. Vitek, V., Smith, D. A. and Pond, R. C., Phil. Mag. A, 1980, 41, 649. 23. Paidar, V., Colloque Physique, 1990, 51(C1), 299. 24. Papon, A. M., Simon, J. P., Guyot, P. and DesjonqueÁres, M. C., Phil. Mag. B, 1979, 40, 159. 25. Bacia, M., Morillo, J., PeÂnisson, J. M. and Pontikis, V., Phil. Mag. A, 1997, 76, 945. 26. Charai, A., Fares, L., Alfonso, C., Roussel, L. and RouvieÁre, J. L., Surf. Rev. Lett., 1998, 5, 43. 27. Krivanek, O. L., Kundmann, M. J. and Kimoto, K., J. Microsc., 1995, 180, 277. 28. Wolf, D., Phil. Mag. A, 1991, 63, 337.
WETTING OF MOLYBDENUM GRAIN BOUNDARIES BY NICKEL: EFFECT OF THE BOUNDARY STRUCTURE AND ENERGY J. M. PEÂNISSON 1{ and T. VYSTAVEL 1,
2
1
DeÂpartement de Recherche Fondamentale sur la MatieÁre CondenseÂe/SP2M/ME CEA±Grenoble, 17 Av. des Martyrs, 38054 Grenoble Cedex 9, France and 2Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Praha 8, Czech Republic (Received 20 April 2000; accepted 19 May 2000) AbstractÐThe structural eect of the penetration of nickel along symmetrical [101] tilt grain boundaries (GBs) in two dierent molybdenum bicrystals is investigated. The selection of GBs S 3f121g and S 11f323g is governed by their dierent energy so that a dierent penetration behaviour is expected. The temperature of treatment is 13508C, i.e. above the eutectic temperature. The analysis of the Mo±Ni phase formed on the surface of the bicrystal, the concentration pro®le along the GB and the identi®cation of the nanophases present at the GB is performed by using several experimental techniques from microscopic to nanoscopic scales. Important dierences in the penetration of nickel are found for the two investigated GBs. 7 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. ReÂsumeÂÐLes changements structuraux induits dans des bicristaux de ¯exion symmeÂtrique dans du molybdeÁne par la peÂneÂtration de nickel sont eÂtudieÂs par microscopie eÂlectronique aÁ haute reÂsolution et imagerie chimique ®ltreÂe en eÂnergie. Deux types de bicristaux S 3f121g et S 11f323g ont eÂte choisis aÁ cause de leurs eÂnergies treÁs dieÂrentes. Les eÂchantillons sont traiteÂs aÁ 13508C c'est aÁ dire au dessus du point eutectique du diagramme de phase. Les deux joints reÂveÃlent des comportements dieÂrents qui sont analyseÂs. 7 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. Keywords: Grain boundaries; Metals; Segregation; TEM; EELS
1. INTRODUCTION
When a polycrystalline metal is in contact with a liquid metallic phase, the liquid can penetrate along the grain boundaries (GBs), leading to the wetting phenomenon which can be used in powder metallurgy to lower the temperature of the sintering process and to obtain the densi®cation of the ®nal product [1]. A detailed presentation of the wetting of GBs in metals and ceramics has been given by Clarke [2]. The wetting of GBs depends on several parameters, the most important being the chemical nature of the components, the temperature, the pressure and the grain boundary structure and energy. The wetting can be considered as a ®rstorder phase transformation so that a critical wetting temperature can be de®ned below which the boundary is not completely wet. This critical temperature depends also on the energy of the boundary. The
{ To whom all correspondence should be addressed.
energetic conditions in which the wetting of a boundary occurs have been studied in detail by several authors [3±5]. Wetting has been widely studied in dierent materials and using several experimental techniques. Except for the case of ceramic systems in which a thin layer of amorphous phase has been observed by high resolution electron microscopy (HREM) [6], most of the experimental results concerning metallic systems have been obtained using optical microscopy, scanning electron microscopy (SEM) or conventional transmission electron microscopy (TEM) so that a detailed description is missing in these materials. In this study, bicrystals with particular crystallographic characteristics have been grown. They have been observed before and after the heat treatment in the presence of nickel. The use of bicrystals allows the determination of the intrinsic structure of the dierent boundaries before any treatment and its comparison to treated specimens that have exactly the same characteristics. According to the calculations made by Wolf [7], the energy of h011i
1359-6454/00/$20.00 7 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 1 5 7 - 9
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symmetrical tilt boundaries in b.c.c. metals present deep minima at special coincidence orientation such as S 3f121gy 50:488 and S 11f323gy 70:538: These two particular orientation relationships have been chosen in order to demonstrate the eect of the GB energy on the wetting. The structure of pure boundaries has been already determined at an atomic scale using conventional TEM and HREM and the results published elsewhere [8±10]. 2. EXPERIMENTAL PROCEDURE
2.1. Specimen preparation Two dierent molybdenum bicrystals containing symmetrical [101] tilt grain boundaries: S 3f121g and S 11f323g were prepared by electron-beam ¯oating zone melting [11]. Slices with [101]A,B surface normal orientation and 1 mm thickness were cut by spark erosion. The slices were mechanically thinned down to a thickness of 0.1 mm. Discs 3 mm in diameter were prepared by mechanical grinding. Before grinding, the samples were slightly etched (approx. 5±10 s) in a mixture of concentrated acids, H3PO4, CH3COOH and HNO3 in the proportion 2:1:1 to reveal the grain boundary in order to facilitate its positioning at the centre of the discs. The thermal treatments were performed, in general, at 13508C, under a pressure of 10ÿ3 Pa during 30 min. Only one S 11f323g specimen was also treated at 13808C. The 3 mm bicrystalline disc was covered by a 3 mm thick polycrystalline nickel foil; then a polycrystalline molybdenum disc was placed on top of the nickel foil. This composite sandwich was placed between two Al2O3 cover supports. Figure 1 shows schematically the composition of the experimental assembly. Two dierent types of samples were prepared:
layer. The thickness of this removed layer was 50 mm. The thin foils for TEM observations were prepared by electrochemical polishing in an electrolyte made of sulphuric acid in methanol (1:7) at a temperature of ÿ208C: If the GB did not cross the hole, it was enlarged by ion milling until the boundary could be observed. In the thin parts used for HREM, the observation was performed at a depth of 50 mm. In some cases, a plasma cleaning procedure [12] was used to clean the surface of the thin foil in an argon and oxygen plasma using a South Bay Technology PC150 instrument.
2.2. Transmission electron microscopy The high resolution observations were made in a JEOL 4000EX microscope at a voltage of 400 kV. In the [110] orientation of the bicrystals, the imaging conditions are very stringent because two dierent sets of atomic planes must be simultaneously imaged: the {110} d 0:22 nm and the {200} d 0:157 nm: These two spacings are quite dierent and one of them is smaller than the point resolution limit of the microscope. In these conditions, only a narrow range of specimen thickness and of defocus can be used to obtain the projected image of the [110] Mo structure [9]. A JEOL 3010 ®tted with a Gatan Imaging Filter was used for the chemical mapping and EELS experiments. The nickel maps were obtained using either the two- or three-window technique [13] on the Ni L23 855 eV edge. The two-window technique (also called ratio map) consists of dividing the ionization edge image by a pre-edge image [14]. The
. Cross-sections were used for TEM, SEM, X-ray microanalysis and optical measurements. The sandwich was cut using a wire saw in a direction perpendicular to the GB, then mechanically and chemically polished. TEM samples were thinned by ion milling in a Gatan PIPS instrument. . Edge-on TEM samples of the structure of thermally treated GBs were prepared at a de®ned depth from the initial bicrystal surface. Both, polycrystalline molybdenum cover disc and nickel foil, were ®rst removed by mechanical grinding. The bicrystal itself was also mechanically polished in order to remove the perturbed surface
Fig. 1. Experimental assembly used for the thermal treatment of the bicrystals in the presence of nickel.
Fig. 2. Optical micrograph of a cross-section of the S 3f121g grain boundary annealed at 13508C.
PEÂNISSON and VYSTAVEL: WETTING
advantage of this method is to obtain chemical maps with minimum noise and without diraction eects. However, these maps cannot be used for a quantitative composition determination. In all the cases, a 20 eV energy slit was used. 3. EXPERIMENTAL RESULTS
3.1. General view on annealed samples After the thermal treatment the molybdenum bicrystalline disc, the nickel polycrystalline foil and the molybdenum polycrystalline disc formed a welded sandwich structure. An optical micrograph of a cross-section of the sandwich is shown in Fig. 2. The nickel foil has been transformed into a phase of a thickness of about 8 mm. The composition of this layer was determined by X-ray microanalysis (WDX). The average composition of this layer is 3521 wt% of Ni. The phase formed during annealing corresponds to the intermetallic phase MoNi-d [15]. The analysis of HREM images and diraction patterns of this phase [9] and their comparison with simulated ones based on the data of Shoemaker con®rmed the X-ray microanalysis results, nevertheless some small dierences in the interplanar distances have been found which are attributed to a slightly dierent stoichiometry. Figure 2 clearly shows also that important changes occurred in the polycrystalline molybdenum disc. From the MoNi layer, a slab of approximately 50 mm thick is recrystallized. The size of the grains in this zone is higher than in the non-transformed part. The concentration of Ni in the polycrystalline disc measured by X-ray microanalysis shows also a dierence: in the non-transformed part, the concentration is 0.5 wt% of Ni while in the recrystallized part it is 0.9 wt% of Ni.
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locations is in good agreement with that obtained by atomistic simulations [16, 17]. In particular, there is a rigid body translation parallel to the GB plane, t 0:045 nm and an expansion e 0:03 nm: An optical micrograph of the cross-section of the sandwich after a treatment at 13508C is shown in Fig. 2. No tendency to the migration of the GB can be observed. X-ray microanalysis did not show any change in the nickel concentration (which stayed at an average of 0.1 wt%) in the bicrystal part of the sandwich. Conventional TEM observations at a depth of 50 mm did not reveal any change in the dislocation distribution with respect to the pure GB [9]. At the opposite, HREM observations showed important contrast changes at the secondary GB dislocation cores. Figure 3(a) shows the image of the core of an edge dislocation with Burgers vector a a 121A 121 B 3 3 in the pure boundary, while Fig. 3(b) shows the same type of dislocation in a specimen treated at 13508C. Both images are taken using similar experimental conditions (defocus ÿ73 nm, thickness around 10 nm). Chemical imaging of the boundary using the nickel peak L23 (855 eV) is shown on Fig. 4. This picture is taken in the jump ratio mode. Along the boundary, regions of higher intensity can be distinguished. They correspond to a local enrichment in nickel and their positions and spacings are the same as those of the secondary dislocation cores. No nickel can be detected at the boundary itself.
3.2. S 3101f121g As the study of the structure of the pure bicrystal has already been published [8±10], only the main results will be recalled here. The general orientation relationship does not correspond exactly to the coincidence position. The analysis by diraction showed that there is a 1:620:18 additional tilt angle and a 0:420:18 twist component. These two additional misorientations are compensated by a complex distribution of secondary dislocations that have been studied in detail by conventional and high resolution electron microscopy. The tilt component is compensated by pure edge and mixed dislocations with Burgers vectors 1=3h112i and dislocation lines parallel to the [101] tilt axis of the bicrystal. The twist component is compensated by an array of pure screw dislocations with Burgers vectors 1=6h111i and by the screw component of the mixed 1=3h112i dislocations. The structure of the areas of the boundary located between the dis-
Fig. 3. HREM of an edge dislocation with Burgers vector B : (a) non-treated sample; (b) A a=3121 b a=3121 sample treated at 13508C. The contrast dierence in the treated specimen is attributed to the presence of segregated Ni at the dislocation core.
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3.3. S 11101f323g The bicrystal does not correspond to the exact coincidence relationship (54.338 rotation around [101]); the tilt and twist components are 3:920:18 and 0:620:18, respectively. They are compensated by a complex distribution of secondary dislocations which has been studied in detail as well as the atomic structure of the boundary [9] which is in agreement with the calculated one [18]. Two specimens prepared from this bicrystal have been treated at two dierent temperatures: 1350 and 13808C. After treatment at 13508C, the optical micrograph of the cross-section of the sandwich shows that the boundary is slightly curved in a region close to the surface in contact with the nickel, as shown in Fig. 5. This is a clear indication of the migration of the boundary. The thickness of this migration zone is about 20 mm. HRTEM observations have been
Fig. 4. (a) Jump ratio image of the S 3f121g grain boundary treated at 13508C. The L23 nickel absorption edge at 855 eV is used. Energy slit width 20 eV. (b) Integrated intensity pro®le along the boundary. The distance between the intensity peaks corresponds to the distance between two secondary dislocations.
made at a depth 50 mm from the nickel surface, i.e. outside the GB migration zone. Two dierent morphologies were found. In some places where the grain boundary plane changes, small precipitates are present as shown in Fig. 6(a). Their composition and structure are the same as that observed for the MoNi phase that appears in the central zone of the sandwich. The electron energy loss spectra [Fig. 6(b)] show that they have a composition corresponding to the NiMo-d phase. The chemical image using the Ni L23 (855 eV) is shown in Fig. 6(c). Their shape is triangular and one of the interfaces Mo±MoNi is parallel to a set of {110} atomic planes of one of the two Mo crystals as deduced from the analysis of the high resolution image shown in Fig. 6(d). The other interface is more complex and it has not been determined. From the analysis of the HR images and their diffractograms, the mutual orientation relationship has been determined as: 101Mo ==001MoNi : In the symmetric parts of the boundary, HR and chemical mapping reveal the presence at the boundary of a very thin layer of a phase containing Ni and Mo. Figure 7(a) shows the high resolution image and its diractogram. The thickness of the layer is 0:720:1 nm: The presence of the phase is also detected in the diractogram in which it gives rise to an elongated intensity maximum which corresponds to a spacing of 0:220:01 nm: It has been veri®ed that this intensity does not come from any double diraction event so that it is typical of the existence of a crystallized layer at the boundary plane. Its elongated shape is due to the ®nite thickness of the layer. The corresponding Ni map is shown in Fig. 7(b). The presence of nickel is revealed by the higher intensity along the boundary. Although the contrast in the image is very low, if the intensity is averaged along the boundary the
Fig. 5. Optical micrograph (cross-section) of the S 11f323g bicrystal after treatment at 13508C. The arrow indicates the migration zone.
PEÂNISSON and VYSTAVEL: WETTING
presence of nickel becomes much more evident as shown in Fig. 7(c). The spectrum shown in Fig. 7(c) shows also the presence of Ni at the boundary. The low intensity of the L23 Ni absorption edge comes from the large size of the electron probe which includes large parts of the molybdenum crystals. The same type of specimen has also been treated at 13808C. At this higher temperature, in the bicrystal, the migration of the boundary is more pronounced as it can be seen in Fig. 8(a). The polycrystalline Mo foil is completely recrystallized and the nickel foil is almost completely dissolved. Chemical maps show that some symmetric parts of the boundary still contain an enrichment in nickel similar to the one observed at lower temperature but new facets which contain much more nickel are now present as shown on Fig. 8(b). The thickness of the Ni-rich layer in these facets is about 4 nm. A careful determination of the crystallography of the dierent facets shows that they all have common characteristics. All the facets are parallel
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to a set of {110} planes of one of the two molybdenum crystals. This is shown in Fig. 8(b) in which two facets are meeting at an angle of 1258. This angle corresponds to the angle between the {110} planes of the two Mo crystals in the S 11 coincidence orientation. 4. INTERPRETATION OF THE RESULTS AND DISCUSSION
It should be remarked that the experimental observations are made at room temperature so that there is no direct proof of the existence of a liquid phase at the boundary and only the existence in the phase diagram of a liquid domain at the treatment temperature supports this idea. The dierent experimental results show very clearly that, in the conditions used for the thermal treatments, nickel penetrates into the molybdenum grain boundaries. This result has already been obtained by dierent authors either in the molybdenum [19] or in the
Fig. 6. MoNi-d precipitate. (a) Conventional TEM image. (b) Electron energy loss spectrum showing the Ni L23 edge at 855 eV. (c) Chemical image of the precipitate (three-window technique). (d) High resolution image of the same precipitate. On both sides of the precipitate, the (110) planes of each Mo crystal are visible. The upper interface is parallel to the (110) Mo atomic planes.
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Fig. 7. (a) High resolution image of a symmetric portion of the S 11f323g boundary after treatment at 13508C. A thin crystallized layer is present at the boundary core. Intensity streaks are present in the diractogram. They correspond to a 0.2 nm atomic spacing. Their elongated shape comes from the very small thickness of the layer. (b) Ni map taken in the same conditions as Fig. 6(c). (c) Averaged intensity pro®le along the boundary. (d) Electron energy loss spectrum recorded at the same place as (a). The low intensity of the Ni L23 edge is due to the large size of the probe which integrates an important contribution of the two Mo crystals.
PEÂNISSON and VYSTAVEL: WETTING
tungsten which has the same b.c.c. structure and similar physical properties [20]. According to the Mo±Ni phase diagram [21], at a temperature of 13508C or higher, a liquid phase is expected to occur at the Mo±Ni interface. The grain boundary is then in contact with a liquid. As shown by Straumal et al. [5], the wetting of a grain boundary occurs if the expression: gGB > 2gSL is satis®ed. gGB and gSL are the free energy of the grain boundary and that of one of the two solid±liquid interfaces, respectively. The grain boundary energies as well as their atomic structures have been calculated by several authors and using dierent interatomic interaction potentials [7, 17, 22±25]. A com-
Fig. 8. (a) Optical micrograph of the cross-section after a treatment at 13808C. The migration zone (arrow) is more developed. (b) Ni map taken in the migration zone. Two facets are meeting at a 1258 angle. This angle corresponds to the angle between the (110) planes of the two molybdenum crystals in the S 11 coincidence orientation.
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mon characteristic of all these calculations is to show that the coherent twin S 3101f121g has always the lowest energyÐabout 500 mJ/m2. The S 11101f323g boundary has a higher energy (1400 mJ/m2). The two boundaries lie in a wellde®ned local minimum on the energy vs tilt angle curve. After a treatment at 13508C, the experimental results showed clearly that the coherent twin S 3101f121g is not wet by the liquid phase. However, the secondary dislocations contain a nickel enrichment localized at the core. This enrichment gives rise to a change in the contrast of the high resolution images and it is also detected in the nickel maps. A similar behaviour has been found by Charai et al. [26] in the study of sulphur segregation in a germanium bicrystal. These results indicate that, in these particular conditions, there is no wetting of the boundary. The core enrichment is due to a fast diusion of nickel atoms along the secondary dislocation lines. At the same temperature, the S 11101f323g boundary has a dierent behaviour. The boundary in the symmetrical position contains a thin layer of a Mo±Ni phase which can be considered as MoNid phase [15] although, due to its very limited extension, no chemical analysis could be made. The thickness of the layer (0.7 nm) measured from the high resolution images, is of the same order as the atomic parameter of the phase and it has clearly a crystalline structure. This observation diers from the case of ceramic materials in which an amorphous layer has been found at the boundaries [6]. The thickness determined from the nickel map is much larger: about 3 nm as measured from the value at half maximum of the averaged intensity pro®le of Fig. 7(c). This apparent discrepancy can be, at least partly, explained by the lower resolution of the chemical imaging technique (as compared with the structural high resolution imaging) [27]. However, the possibility of a gradient of the nickel concentration outside the crystallized layer cannot be completely ruled out. This problem will be solved in the future by using a very small (0.5 nm) probe which will be scanned across the boundary; the nickel concentration pro®le being deduced from the analysis of the electron energy loss spectra collected at the dierent locations. The small precipitates observed at places where the GB plane changes seem to indicate that the precipitation is favoured by internal strains that are generally present at these locations. Their structure as well as their composition is similar to that of the central layer of MoNi of the sandwich and corresponds to the MoNi-d phase observed along the [100] axis. An important feature is the existence of a large facet parallel to a {101} plane of one of the Mo crystals. In the b.c.c. structure the {101} planes are compact and they have the lowest energy [28]. The surface
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PEÂNISSON and VYSTAVEL: WETTING
energy is then the factor governing the migration of the boundary. After a treatment at 13808C, the most important change is the migration of the boundary. This migration leads to the formation of facets with dierent orientations. As in the case of the precipitates, the {101} atomic planes are favoured and the facets follow alternatively the {101} planes of each molybdenum crystal as shown in Fig. 8(b). As it has been shown, the condition for wetting is given by: gGB > 2gSL : The energy of the solid±liquid interface is not known but an estimation has been made by Rabkin et al. [19]. If, as proposed by these authors, a value of 500 mJ/m2 can be admitted, then the wetting condition is satis®ed for the S 11f323g boundary E 1440 mJ=m 2 so that the boundary is wet by Ni, while this condition is not satis®ed for the S 3f121g boundary E 500 mJ=m 2 and the boundary is not wet. 5. SUMMARY AND CONCLUSION
The use of bicrystals to study the wetting behaviour of tilt grain boundaries in b.c.c. molybdenum has allowed a detailed comparison of the structure of the boundaries before and after heat treatment in the presence of nickel. The two selected boundaries: S 3 and 11 have dierent structures and energies. S 3 has a low energy while S 11 has a rather large one. It has been shown that their behaviour is completely dierent. While S 3 does not contain nickel after the treatment, a thin layer of crystalline MoNi-d is found at the S 11 boundary as well as precipitates of the same phase are formed at places where the boundary plane is altered. Moreover, at higher temperature, the S 11 boundary migrates and facets parallel to a {110} plane of one of the two crystals which are associated with a high concentration of nickel. This behaviour is in good agreement with the current thermodynamic theories of the wetting of grain boundaries. AcknowledgementsÐThe authors are grateful to P. Bartuska for his help in SEM X-ray microanalysis. Financial support by contracts GA AV CR No. A1010916, GA CR No. 202/99/1665 and French±Czech co-operation contract Barrande ref. 98-032 are also acknowledged.
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