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DIRECT OBSERVATION OF BORON
SEGREGATION TO LINE AND PLANAR DEFECTS IN Ni3Al
M. Miller, J. Horton
To cite this version:
M. Miller, J. Horton. DIRECT OBSERVATION OF BORON SEGREGATION TO LINE AND PLANAR DEFECTS IN Ni3Al. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-379-C6-384.
�10.1051/jphyscol:1987662�. �jpa-00226870�
DIRECT OBSERVATION O F BORON SEGREGATION TO LINE AND PLANAR DEFECTS IN Ni3AI
M.K. Miller and J.A. Horton
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
ABSTRACT
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The atom probe field-ion microscope has been used to characterize the distribution of boron in nickel aluminides near the stoichiometric Ni,Al composition. Boron has been found to segregate to all line and planar defects and a thin boron-enriched phase was observed on high angle grain boundaries in substoichiometric (<25 at. % Al) materials. Boron clustering was detected in the stoichiometric and superstoichiometric materials.INTRODUCTION
Polycrystalline NisAl is inherently brittle and fractures intergranularly. However, small additions of boron to substoichiometric alloys (<25 at. % Al) can result in room temperature ductilities of up to 50%.111 One of the important parameters in fully understanding the role of boron in preventing intergranular failure is to determine its segregation behavior to all internal defects. Auger electron spectroscopy has shown boron segregation of up to 10 at. % to intergranular fracture surfaces and was unable to detect the presence of any embrittling a g e n t ~ . l ~ ~ ~ I
The atom probe field-ion microscope (APFIM) is well suited to the analysis of boundaries and other internal line and planar defects. In contrast to Auger electron spectroscopy the atom probe field-ion microscope can analyze boundaries that do not fail intergranularly. In addition the atom probe has a substantially higher lateral spatial resolution so that variations in segregation along the boundary can be investigated. The atom probe also exhibits excellent light element sensitivity.
This investigation exploits the unique capability of the atom probe field-ion microscope to resolve individual boron atoms because of their imaging characteristics. This bright imagin feature for individual boron atoms has been observed in several materials such as metallic glassesh, and boron containing nickel aluminides.161 This true atomic resolution capability permits the direct observation of boron segregation to line and planar defects.
Previous atom probe analyses of nickel aluminides of similar compositions to those used in this study have revealed boron segregation to twin boundaries in conventionally cast materialw, and to grain boundaries and antiphase boundaries (APB) in rapidly solidified material.[718] In addition, a thin boron containing grain boundary phase has been observed on some high angle In this paper, boron segregation to other microstructural features is explored, together with examination of stoichiometric and superstoichiometric materials.
EXPERIMENTAL
The materials used in this investigation had nominal compositions of 24, 25 and 26 at. % A1 doped with either 0.24 or 0.48 at. % B. Field-ion specimens were electropolished from wires produced directly from the liquid phase by a pendant drop melt extraction technique[g] (PDME) and conventionally cast material by standard procedures. The rapid solidification technique produced wires with a cross section varying from 40 to 80 pm and a grain size of between 1 and 4 pm. The conventionally cast material was subjected to a multi-step homogenizing heat treatment in vacuum consisting of 5 h at 1200°C, 12 h at 1 100°C, 5 h at 1000°C furnace cool to room temperature, 5 h at 800°C, 24 h at 700°C, 24 h at 600°C, 24 h at 500°C and a final furnace cool to room temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987662
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In order to investigate a number of line and planar defects the 24 at. % A1 material was subjected to a series of different annealing treatments and plastic deformation, as summarized in Table 1. The grain size of the heat treated materials, as measured by optical microscopy, is also included in Table 1.
All atom probe analyses were performed using the ORNL instrument.['0] The field-ion micrographs were recorded using neon as the image gas at a specimen temperature of 70K. Atom probe analyses were performed with a specimen temperature of 50K and a minimum pulse fraction of 20%. Prior to atom probe analysis, all FIM specimens were examined in a Philips EM430T transmission electron microscope to select specimens containing features of interest and to assist in characterizing the type and orientation relationship of boundaries. Compositions are quoted in atomic percent.
TABLE 1. Summary of Heat Treatments for 24.0 at. % A1 Alloy
Heat Treatment Grain Size, pm Microstructure
Rapidly solidified starting material 1 to 4 APBs
1 h a t 700°C 1 to 4 APBs
I h a t 800°C 1 to 4 SISFs, No APBs
cold worked, 9 min a t 1000°C 6 to 14
cold worked, 30 m b at 1000°C 10 to 30 Dislocation Cell Walls 30 min at 100O0C, 5 h a t 800°C 10 to 30
I h a t 1050°C 100 to 250
RESULTS AND DISCUSSION
The rapidly solidified wires were examined i n a scanning electron microscope after tensile tests to determine the fracture mode and estimate the ductility, Fig. I. A reduction in area of greater than 50% was measured in the 24% A1 alloy, whereas, only limited ductility was observed in the alloys with 25 and 26% Al. Intergranular fracture surfaces were evident in the 25 and 26% A1 materials. A similar reduction in area (>50%) was observed in the 24% A1 that was annealed for 1 h a t 1050°C indicating that the annealing treatments did not greatly influence the fracture process.
Boron was found to segregate to all high angle grain boundaries examined to date in both the annealed and rapidly solidified PDME materials with 24% A1 as indicated by the bright spot decoration.
These bright spots have been shown to be due to single boron atoms using the single atom analysis capability of the atom probe.Es] However, the coverage was found to vary both along a given boundary, as shown in the pair of field-ion micrographs of a C27 boundary in material annealed 9 min at 1000°C, Fig. 2, and from one boundary to another. A slightly lower level of boron coverage was observed in the 25 and 26% A1 materials than in the 24% Al material, Fig. 3. This lower level of segregation in specimens with a higher aluminum content is in agreement with Auger spectroscopy A thin brightly-imaging boron-enriched phase has also been observed on a high proportion of the high angle grain boundaries in the 24% A1 material. An example of this phase in material annealed 1 h at 700°C is shown in Fig. 4. This phase has not yet been observed in either the 25 or 26% A1 alloys. However, it should be noted that fewer boundaries have been examined in these materials.
A variety of other line and plane defects have also shown boron segregation in the 24% A1 material. An example of boron segregation to a dislocation is shown in Fig. 2(b). In the case of dislocations the boron was generally observed near but rarely a t the core of the dislocation. Similar segregation behavior has been observed in iron-carbon alloys by Chang et al. using the imaging atom A superlattice dislocation in material annealed 9 min at 1000°C is shown in Fig. 5. The separation of the dislocations was measured to be 5.5 nm and the antiphase boundary (APB) connecting the dislocations was on the (100) plane. While this APB did not reveal significant boron segregation, previous studies have reported high boron coverage on some APBS.[']
Several low angle boundaries or dislocation cell walls were observed in 24% Al material cold-worked and annealed 30 min at 1000°C, Fig. 6. While the boron was evident as bright spots in field-ion micrographs taken at best image voltage (BIV), Fig. 6(a), the location of the boundary was more easily discernible during field evaporation sequences as a definite bright line in the image, Fig. 6(b). This contrast was due to the smearing together of the brightly-imaging boron atoms as they imaged and subsequently evaporated.
Field-ion micrographs together with the corresponding TEM micrographs of a stacking fault are shown in Fig. 7. The stacking faults were also best discerned in field-ion micrographs taken during field evaporation. TEM characterization indicated that these stacking faults were of the superlattice
lower than to the stacking fault. In low angle boundaries, antiphase boundaries, and superlattice intrinsic stacking faults, boron was found to be confined to the precise plane of the defect and no second phase was detected.
Atom probe chemical analysis of the matrix showed that over 80% of the boron remained in solution. Random area analysis revealed that the boron formed small clusters in the 25 and 26% A1 alloys but not in the 24% A1 material, Fig. 8(a). These clusters were also visible in field-ion micrographs, Fig. 8(b). This cluster formation is in agreement with positron annihilation studies.['2]
However, the size of the cluster was considerably smaller than inferred by the less direct technique of positron annihilation. The largest cluster detected consisted of only 7 boron atoms. Approximately 15% of the boron in the matrix was found in the clusters. Carbon was also detected in some of these clusters. These clusters would reduce the amount of boron that is available to segregate to the boundaries and precipitate in the grain boundary phase. In addition, the presence of clusters might increase the flow stress of the matrix by increased pinning of dislocations and thereby make the grain boundaries more sensitive to failure.
The degree of order of the L1, ordered matrix was measured from atom probe composition profiles[13] to be 0.99 in the conventionally cast 26% A1 materials. This value is similar to that previously reported for the PDME 24% A1 al1oy.[~1 It is therefore improbable that mechanisms based on deviations from prefect order are likely to explain the change in fracture properties between the materials.
It has been suggested that boron additions to Ni,AI soften the grain boundaries and make them more accommodating to slip, instead of failing intergran~larl~.['~] The presence of the grain boundary phase supports this argument since the grain boundary phase might allow dislocations to more easily reorient themselves before reemission into the next grain.
CONCLUSIONS
The atom probe field-ion microscope has revealed that boron segregates to most types of line and planar defects in NisAl. These defects included dislocations, superlattice intrinsic stacking faults, antiphase boundaries, low angle boundaries, and grain boundaries. The level of boron segregation was found to vary both along an individual boundary and also from one boundary to another. A thin boron-containing phase was also observed on high angle grain boundaries. The boron was found to form small clusters in the stoichiometric and superstoichiometric materials but not in the substoichiometric material although the majority of boron was found to be randomly distributed in the matrix in all cases.
Acknowledgments
The authors would like to thank Dr. R. Maringer of Battelle Columbus Laboratory for the preparation of the rapidly solidified wires, and K.F. Russell for her technical assistance. Research sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
REFERENCES
I. C.T. Liu, C.L. White, and J.A. Horton, Acta Metall., (1985) 213.
2. C.L. White, R.A. Padgett, C.T. Liu, and S.M. Yalisove, Scripta Metall.,
18
(1984) 1417.3. T. Takasugi, E.P. George, D.P. Pope, and 0. Izumi, Scripta Metall.,
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(1985) 551.4. R. Grune, J. Piller, M. Oehring and R. Wagner, Proc. 29th Int. Field Emission Symposium, 1982, Gothenburgh, ed. H-0. Andren and H. Norden, p. 533, Almqvist and Wiksell, Stockholm, 1982.
5. J.A. Horton and M.K. Miller, Acta Metall.,
2
(1987) 133.6. M.K. Miller and J.A. Horton, Scripta Metall., &(1986) 789. I 7. J.A. Horton and M.K. Miller, J. de Physique, (1986) 209.
8. M.K. Miller and J.A. Horton, J. de Physique,
a
(1986) 263.9. R.E. Maringer and C.E. Mobley, Wire Journal, Jan 1970, 70.
10. M.K. Miller, J. de Physique,
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(1986) 499.11. L. Chang, S.J. Barnard, and G.D.W. Smith, Proc. 30th Int. Field Emission Symposium, 1983, Portland, p.97.
12. A. Dasgupta, L.C. Smedskjaer, D.G. Legnini and R.W. Seigel, Mat. Let. 3 (1985) 457.
13. M.K. Miller, J. Microscopy, 1987, in press.
14. E.M. Schulson, T.P. Weihs, I. Baker, H.J. Frost and J.A. Horton, Acta Metall., (1986) 1395.
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Fig. 1. Scanning electron micrographs of the fracture .surfaces in 24, 25 and 26 at. % A1 alloys containing 0.24 at. % B showing the change in fracture mode from ductile to intergranular.
K
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Fig. 2. A E27 grain boundary fn the 24% Al material aged 9 min at 1000°C. Note variation in boron segregation from one region to another on the same boundary. Boron was also observed at dislocation.
Fig. 3. Grain boundaries in rapidly solidified PDME a) 25% Al and b) 26% Al alloys containing 0.24 at. % B showing a lower boron coverage than the 24% A1 material.
boundaries. Example shown was aged 1 h at showed a separation of 5.5 nm.
7OO0C.
Fig. 6. Low angle boundaries or dislocation cell walls were observed in the 24% A1 alloy that was cold worked and annealed for 30 min at 1000°C. The location of the boundary was more evident in micrographs taken during field evaporation (b) than in field-ion micrographs taken at best image voltage (a). Boron was observed at these low angle boundaries.
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aged I h at WO0C. Note boron w& found t o segregate to the SISF rather than the nearby grain boundary (b-b).
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Fig. 8. Small boron clusters were observed in random area chemical analysis (a) and also in the field- ion micrographs (b) in the 25 and 26% A1 alloys. Example shown is from conventionally cast material.
