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GRAIN BOUNDARY STRUCTURE IN Ni3Al
R. Mackenzie, M. Vaudin, S. Sass
To cite this version:
R. Mackenzie, M. Vaudin, S. Sass. GRAIN BOUNDARY STRUCTURE IN Ni3Al. Journal de
Physique Colloques, 1988, 49 (C5), pp.C5-227-C5-232. �10.1051/jphyscol:1988524�. �jpa-00228021�
Colloque C5, suppl6ment au nolO, Tome 49, octobre 1988
GRAIN BOUNDARY STRUCTURE IN Ni,Al
R.A.D. MacKENZIE, M.D. VAUDIN'') and S . L . SASS
Department of Materials Science and Engineering, Cornell University, Ithaca, NY. U.S.A.
Abstract
-
The influence of boron segregation and non-stoichiometry on grain boundary structure in Ni3M was studied by transmission and scanning electron microscopy techniques. Small angle twist and tilt boundaries were produced by hot pressing misoriented single crystals of both doped and undoped material. Dislocation structures were observed in both bicrystal and polycrystal grain boundaries. In most cases the grain boundary dislocations were found to have the expected a<100> Burgers vector, however in one case dislocations with Burgers vector a/2<110> have been observed. Using a SEM diffraction technique the frequency of occurrence of grain boundary types was examined and found to be unchanged by the addition of boron.I. Introduction
=3A1 is a potentially useful high temperature alloy. In its pure form, however, the polycrystalline alloy is highly prone to intergranular failure. It has been found that the addition of small amounts of boron which segregates to grain boundaries [I] and changes in stoichiometry [21 have the effect of increasing the alloy ductility dramatically. The nickel aluminides have received a great deal of research attention recently, but relatively little of this has been directed a t experimentally investigating the structure of grain boundaries. This paper reports the initial results of a study using a variety of electron microscope techniques to investigate the boundary structure and type in both pure Ni3Al (24 atomic % Al) and boron doped Ni3Al (24 atomic % Al, 0.2 atomic % B). The goal of this work is to understand the roles of boron segregation and stoichiometry in improving the mechanical properties of the nickel aluminides. Sickahs and Sass [3,4] observed that solute segregation causes a change in the dislocation structure of small angle [001] twist boundaries in Fe-Au alloys. The present study is directed towards looking for similar changes in boundary structure caused by boron segregation. Watanabe [5] has shown that grain boundaries with low
C
values h v e high resistance to fracture (whereC
is the inverse of the fraction of atoms in coi1:cidence between the two misoriented crystals which meet a t the grain boundary). In order to determine if the improved fracture resistance of boron-doped Ni3Al is related to the types of grain boundaries which are present, we have also examined the distribution of boundary types in doped and undoped Ni3AI.11. Exverimental Techniaue
The results reported here have been obtained from polycrystalline and bicrystal samples. The bicrystals are produced by polishing a single crystal of NigAl (either doped or pure) so that a flat slice with large faces parallel to (001) is produced. From this slice
(*ow at: Ceramics Division, National Bureau of Standards, Gaithersburg, MD 20899, U.S.A.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988524
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two pieces are cut, and then hot pressed together (after a small misorientation to produce the desired twist angle). A variety of hot pressing conditions were used, with a typical condition being a load of -50 MPa a t a temperature of 1000°C. The pressing was performed in a 4% hydrogen-argon atmosphere for approximately 18 hours.
Transmission electron microscope (TEM) specimens were prepared from these bicrystals to permit plan view observation of the grain boundary. The final preparation step used a sulphuric acid
-
methanol electropolishing solution to produce thin specimens.The polycrystalline specimens were observed using both TEM and electron backscatter patterns (EBSP) which are obtained in a modified scanning electron microscope [61. In both cases the specimens were cut from bulk material, then prepared using
electropolishing techniques.
111. l e
A. Pure Ni3Al
Several bicrystals of this material have been produced, in each case containing a grain boundary with a mixed tilt-twist character. Weak beam images of these boundaries are shown in Figures l(a) and (b). Figure l(a) shows a dislocation spacing of 6.5 nrn with a misorientation angle of 3.2O. Figure l(b) shows a spacing of 8.0 nm, with a misorientation angle of 2.5'. The boundaries in the bicrystals show one set of dislocation lines parallel to, and a second widely spaced and less periodic set of lines perpendicular to, the tilt axis.
The dislocation arrays in both boundaries have Burgers vectors of a<100>-type. In each case there is a significant amount of heterogeneous background contrast, which may be due to the presence of small particles of nickel aluminate (NiA1204).
B. Boron doped Ni3Al
Very few bicrystals of this material have been successfully produced. One example is shown in Figure 2. This weak beam image shows a square dislocation array with a spacing of 3.7 nm. This is a pure [001] twist boundary with a misorientation angle of 3.7".
The angle and the observed dislocation spacing give a dislocation Burgers vector of a/2<110>.
The small angle boundaries in Figure 3 were observed in boron doped polycrystals and show one dimensionally periodic dislocation arrays, typical of tilt boundaries, but with an additional periodicity superimposed, which suggests the occurrence of faceting. Figure 3(a) shows a dislocation periodicity of 2.8 nm, with a facet periodicity of 27 nm. Figure 3 6 ) shows a dislocation periodicity of 1.4 nm, with a facet periodicity of 11 nm; another fainter periodicity perpendicular to these is present with a spacing of 9 nm. Measurement of the misorientation angles and dislocation spacings indicates Burgers vectors of a<100>- type. Faceting is frequently observed in Ni3Al containing boron, while observations on a limited number of small angle tilt boundaries in undoped Ni3Al show no evidence of faceting.
misorientation angle 3.2
3
dislocation Burgers vector a<100>, (b) spacing 8.0 nm, misorientation angle 2.50, dislocation Burgers vector a<100>.Figure 2. Weak beam image of pure twist bicrystal boundary i n boron doped Ni3A1, dislocation spacing 3.7 nm, misorientation angle 3.7",islocation Burgers vector a /2<1 lo>.
IV.
Electron Backscatter Diffraction StudyThe EBSP investigation permitted the distribution of boundary types to be studied in the pure and boron-doped polycrystals. The samples had been produced by different techniques, but were given final heat treatments to ensure that the grain sizes were similar, and that there was no processing related texture present. Grain boundary misorientations were measured from 393 boundaries in Ni3Al+B (24/at.% Al) and 363 in pure Ni3AI (24 at.% Al). Each boundary was classified, using a scheme based on that of Brandon 171, as a low angle boundary (having a misorientation of less than 7.5"), a high
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Figure 3. Bright field images i n boron doped Ni3Al (a) showing dislocation spacing of 2.8 nm, and facet periodicity of 27 n m (b) showing dislocation spacing of 1.4 n m and facet periodicity of Y 1 nm, and also a second dislocation spacing of 9 nm.
(a) boron doped 80
(b)
pure 80l o w a n g l e
b o u n d a r i e s r a n d o m l o w a n g l e
b o u n d a r i e s b o u n d a r i e s random
b o u n d a r i e s Figure 4. Grain boundary type distribution for (a) boron doped Ni3Al and (b) pure Ni3Al.
coincidence boundary (with
C
values of 3 to 19) or a high angle 'random' boundary. The frequency of occurrence of the different classes, expressed as percentages, is shown in Figure 4(a) for Ni3Al+B, and in Figure 4(b) for Ni3AI. It can be seen there is aconsiderable similarity between Figures 4(a) and 4(b). In each case there are a significant number of C3 boundaries; 20% in the boron doped sample and 25% in the pure sample.
Very small percentages of boundaries with
C
between 5 and 19 were present in bothpercentages of high angle random boundaries were recorded from each specimen; 72% in the doped specimen and 68% in the pure sample. In both materials very few low angle boundaries were observed. Since the difiaction pattern will change its appearance only very slightly when crossing a low angle boundary, this technique may very well
underestimate the number of these boundaries present.
V. Discussion
In Ni3Al, both with and without boron, the dislocation structures of the small angle grain boundaries have, in every case, been observed to be very uniform (see Figures 1,2 and 3). This uniformity is particularly clear in the polycrystal boundaries where the images are of significantly higher quality than in the bicrystal boundaries. This observation suggests that the boron distribution in these small angle boundaries is also uniform, at least on the scale of the spacing of the dislocations. The presence of boron at the grain boundaries has been confirmed by atom probe studies [81 performed on the same polycrystalline material (specification IC-15, prepared by Oak Ridge National Laboratory) used in the present investigation. The atom probe studies also showed a variation in boron distribution within the grain boundaries on a 15 nm scale. The dislocation spacings observed in the present work were in the range 1.4 nm to 8.0 nm, and thus might have been expected to show a variation in structure on the scale of the boron variation. This has not been seen in the present investigation of polycrystalline boundaries where bright field imaging has been used to observe the interface dislocations.
In Ni3A1 containing boron the small angle tilt boundaries in polycrystalline Ni3Al appear faceted, while the few boundaries observed in Ni3Al without boron are planar.
Rellick, McMahon, Jr., Marcus and Palmberg [9] in Fe-Te and Donald
[lo]
in Cu-Bi reported a characteristic grain boundary morphology in the presence of solute segregation, in the form of faceting. Thus the observations on boron doped Ni3A.l may fit into a general pattern of faceting in the presence of solute segregation. More work is needed in order to confirm this possibility.In most of the boundaries examined the dislocations were found to have the Burgers vector of a<100>, which is the shortest perfect dislocation Burgers vector, expected in an L12 structure. In the case of the twist boundary in boron doped Ni3AI dislocations with a Burgers vectors of a/2<110> were found (Figure 2). There are two possible explanations for this, (i) the a/2c110> dislocations are partial dislocations, and the expected anti-phase boundary contrast is not visible, and (ii) the Ni3A1 disorders in the vicinity of the interface with the local structure being disordered f.c.c., so that the observed dislocations have the expected f.c.c. Burgers vector. The possibility of compositional disordering at grain boundaries in Ni3AI has been suggested to occur based on theoretical Ell] and experimental [I21 considerations.
The EBSP results indicate that in this system there is no relationship between the frequency of occurrence of grain boundary types and the ductility of the polycrystalline material. Farkas, Lewus and Ranganathan (1987, unpublished) investigated the frequency of occurrence of grain boundary types in boron doped Ni-rich and boron doped
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stoichiometric Ni3AI. The work of Farkas et a1 indicates that the intrinsic brittleness of Ni3AI is related to a larger percentage of high C grain boundaries (Z = 37,41,43 and 51) being present than in ductile Ni3Al. The present work showed no evidence for difYerences between the frequency of occurrence of these high C boundaries. In addition, Farkas et a1 observed a considerable number of low angle boundaries, while the present work detected a very small number of these boundaries. As pointed out in section
IV,
the EBSPtechnique may underestimate the number of low angle boundaries. The origin of the disagreement between the results of the present work and that of Farkas et al, with respect to the frequency of occurrence of high
C
boundaries is not clear at present.VI. Conclusions
The electron backscatter diffraction study indicates that the distribution of grain boundary types is the same in Ni3Al with and without boron. The small angle tilt
boundaries in Ni3AI containing boron are observed to be faceted, while similar boundaries in pure Ni3A1 are planar. The observation of dislocation Burgers vectors of the type a/2<110> in Ni3Al containing boron suggests either the presence of anti-phase boundaries or disordering in the vicinity of the interface. Further work is necessary before the relationship between changes in grain boundary structure due to boron segregation and the improvement in the ductility of Ni3M can be established.
The authors wish to thank C.T. Liu of the Oak Ridge National Laboratory for supplying the Ni3Al polycrystalline material, D. McMaster and F. Schmidt of the Ames Laboratory for supplying the Ni3Al single crystals, D.J. Dingley for access to the EBSP equipment at the University of Bristol, England and J. Liu for access to the EBSP equipment a t the ALCOA Technical Center, Pa. This work was supported by the United States Department of Energy under grant number DE-FG02-85ER45211.
- 8
1. K. Aoki and O.Izumi, Nippon Kinzaku Gakkaishi,
a
(1979) 1190 2. C. T. Liu, C. L. White and J. A. Horton, Acta Met.,33
(1984) 213 3. K. Sickafus and S. L. Sass, Scripta Met.,U
165 (1984)4. K. E. Sickafus and S. L. Sass, Acta Met.,
a
69 (1987) 5. T. Watanabe, Res Mechanics,11
(1984) 476. D. J. Dingley, Scanning Electron Microscopy (1984) II/569 7. D. G. Brandon, Acta Met.,
14
(1966) 14798. D. D. Sieloff, S. S. Brenner and M. G . Burke, Proc. MRS Symp., fl(1987) 87 9. J. R. Rellick, C. J. McMahon, Jr., H. L. Marcus and P. W. Palmberg,
Met. Trans., 2 (1971) 1492 10. A. M. Donald, Phil. Mag.,
34,
(1976) 118511. S. M. Foiles, Proc. MRS Symp.,
81,
(1987) 5112. J. A. HorLon and M. K. Miller, Acta Met.
