Investigation of the segregation behavior of boron in Ni3Al alloys by PTA technique

Acta metall, mater. Vol. 39, No. 4, pp. 523-528, 1991

0956-7151/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press pie

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INVESTIGATION OF THE SEGREGATION BEHAVIOR OF BORON IN Ni3A1 ALLOYS BY PTA TECHNIQUE D O N G L I A N G LIN (T. L. LIN) 1, DA CHEN 1 and HUI LIN 2 IDepartment of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, P.R. China and 2Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. (Received 20 August 1990) Abstract--The particle-tracking autoradiograph (PTA) technique has been used, combined with quantitative statistical analYsisusing the image processing system, to study the effect of such factors as aluminium content of base alloy, the bulk boron concentration and different heat treatments on the grain boundary segregation behaviors of boron in Ni3A1 alloys. Moreover, the mechanical properties of Ni3A1 alloys subjected to different heat treatments and therefore with various quantities of segregated boron have been tested. Their fracture surfaces have also been observed in SEM. The relation between the level of boron segregation and ductility has been inspected. R~sum~--On utilise la technique d'autoradiographie par traceur, combin6e ~i l'analyse quantitative statistique en se servant d'un syst6me de traitement d'image, pour &udier l'effet de facteurs tels que la teneur en aluminium de l'alliage de base, la concentration globale en bore et diff~rents traitements thermiques, sur la s~gr6gation aux joints de grains du bore dans les alliages Ni3AI. De plus, on effectue des essais en vue de tester les propri&6s m6caniques des alliages Ni3AI soumis ~ diff6rents traitements thermiques, pr6sentant par cons6quent diff6rentes quantit6s de bore s6gr6g6. On observe aussi les surfaces de rupture par MEB. On &udie enfin la relation entre le niveau de s~gr~gation de bore et la ductilit6.

Zusammenfassnng--Die autoradiografische Verfolgung von Teilchen, kombiniert mit einer quantitativen statistischen Analyse mittels Bildverarbeitung, wurde benutzt, den Einflul3 von Faktoren wie etwa des Aluminiumgehaltes der Basis legierung, der Borkonzentration und der verschiedenen W/irmebehandlungen, auf das Segregationsverhalten des Bors an Korngrenzen in Ni3A1--Legierungen zu studieren. Dariiberhinaus werden die mechanischen Eigenschaften dieser Legierungen in Abhfingigkeit von verschiedenen W/irmebehandlungen und damit von verschiedenen Segregationsgraden des Bors gemessen. AuBerdem werden die Bruchoberfl/ichen im Rasterelektronenmikroskop analysiert. Der Zusammenhang zwischen Grad der Borsegregation und Duktilit/it wird hergestellt.

1. INTRODUCTION Ordered intermetallic Ni3A1 alloy exhibits many attractive properties for high temperature applications. However, they have high propensity for low ductility and intergranular fracture in polycrystalline forms. Although, a substantial increase in the ductility of polycrystalline Ni3A1 has been achieved by microalloying with small quantities of boron [1], there still exists great uncertainty concerning the mechanism of ductilization and effect of stoichiometry on the ductility. The addition of small amount of boron was found to segregate at grain boundary and induce ductility in Ni-rich Ni3A1 alloys [2]. Because of the fact that the segregation of boron plays a decisive role on the ductility of Ni3A1, so to systematically study the effects of various factors on the segregation state has a significant meaning for understanding the mechanical behaviors of Ni3A1 and the mechanism of boron induced ductility. Choudhury et al. [3] have studied the effect of thermal history on intergranular boron segregation with Auger Electron Spectroscopy (AES) and conAM 39/4~G

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clusion was made that the effect of thermal history is entirely reversible and appears to be consistent with equilibrium segregation. However, AES does not actually look at random boundaries but those can by hydrogen charging be induced to intergranular failure. The particle-tracking autoradiograph (PTA) technique does look at all boundaries and it can be used for an application involving grain boundary [4]. In this work, the PTA has been used to study the effect of such factors as aluminium content, the level of added boron and different heat treatments on the grain boundary segregation behaviors of boron in Ni3A1 alloys. Besides, the effect of segregated boron on the ductility has also been inspected by the tensile testing and SEM observation. 2. EXPERIMENTAL Under the irradiation of the thermal neutron, the fission reaction of the isotope l°B, which is present as about 19.8% in natural boron, takes place 10B + 1n-4~ + VLi.

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D O N G L I A N G L I N (T. L. LIN) et al.:

S E G R E G A T I O N O F B IN Ni3A1 A L L O Y S

Table 1. The composition of Ni~A1 alloys for PTA testing No.

B (ppm wt.%)

AI (at.%)

1 2 3

700 700 1200

24 26 24

3. R E S U L T S

Figures 1-3 show the boron segregation states of the alloys with different composition and subjected to various heat treatments. In the figures, the black dots are corresponding to the etching pits in the detectors (CA films), their densities are directly proportional to

The capture area of this reaction can be used as a characteristic reaction to detect the existence of boron in a material. A solid detector (a cellulose acetate film) is glued with a small amount of acetate onto the polished surface of the specimen (un-etched) which is mounted in the bakelite, then they are baked at 150°C for 5 h. After that the specimen is irradiated by thermal neutrons with the irradiation flux of 6.12 x 10~3 neutron per cm ~, the ]°B(n, ~t)7Li reaction takes place in the surface layer of the specimen where I°B atoms exist, and the fission products shoot into the film causing irradiation damage along their tracks in the detector. The film is removed and etched with a strong alkaline solution (7.5 N NaOH/50°C/10 min), the tracks in the film are etched into etching pits and the pattern which is made of etching pits of different densities can be seen under a microscope. Because the average free path of a fission fragment in the materials is very short, approximately 1 #m, the pattern of etching pits reveals the distribution of boron in the surface layer of the specimen, and the pit density is directly proportional to the boron concentration, so it can be used to study the behaviors of boron in a material. In order to obtain good resolution in the PTA method, it is very important to stick the film closely onto the specimen surface. However it is difficult to stuck the CA film closely onto the polished surface of the specimen, a doublegluing technique for the CA film has been developed to solve this problem [4]. The composition of Ni3A1 alloys for PTA testing is shown in Table 1. For the No. 1 alloy, it has been heat treated with several techniques shown in Table 2. All the specimens were PTA tested and then their pit densities were quantitatively analysed using an image processing system. Besides, the alloy with the composition of 24 at.% A1 + 700 ppmB were held at 700, 900 and 1150°C for 2 h, respectively, then water quenched. After that, tensile tests were carried out a room temperature for their specimens with the gage size of 25 x 7 x 1 were tensile tested at room temperature in the A U T O G R A P H DCS-25 testing machine. Their fracture surfaces were then observed in the SEM. Table 2. Heat treatments for No. 1 alloy A B C D E

1150°C/2h+500°C/1 h + W . Q . 1150°C/2 h + 700°C/1 h + W.Q. 1150°C/2 h + 900°C/1 h + W.Q. II50°C/2h+W.Q. + 1150°C/2h + 900°C/1 h + 700°C/1 h + 500°C/1 h + W. Q. 1150°C/2 h + 900°C/1 h + 700°C/1 h + 500°C/1 h + W.Q. + 1150°C/2 h + W. Q.

Fig. 1. Particle tracking autoradiographs showing the boron distribution in Ni3AI alloys with the composition of (a) 24 at.% A1 + 700 ppmB, (b) 26 at.% A1 + 700 ppmB, (c) 24 at.% + 1200 ppmB.

DONGLIANG LIN (T. L. LIN) et al.:

SEGREGATION OF B IN Ni3A1 ALLOYS

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the boron concentration under the same experimental conditions. The density of black dots in the background region without the grain boundary and that in the region with the grain boundary were measured, respectively. Then the difference between them reflects the level of segregated boron. In order to make the results more accurate and comparable for different measurements, the size of measured

Fig. 3. Particle tracking autoradiographs showing the boron distribution in Ni3AI alloy (24at.% + 700ppmB) after (a) heat treatment D, quenching + step annealing, (b) heat treatment E, step annealing + quenching.

Fig. 2. Particle tracking autoradiographs showing the boron distribution in Ni3A1 alloy (24at.% + 700ppmB) after (a) 500°C quenching, (b) 700°C quenching, (c) 900°C quenching.

zone and the length of grain boundary in it were kept constant. All the measurements were repeated for several times and final results were taken from their statistical mean values. Figure 4 shows the relative comparison of the grain boundary segregation amount of boron between alloys with different composition and subjected to different heat treatments. From Fig. 1 (a) and (b), which are corresponding to alloys with the composition of 24at.% AI + 700 ppmB and 26 at.% A1 + 700 ppmB respectively, it can be seen that in the former alloy the segregation of boron is rather obvious and its etching pits have connected with each other becoming a continuous line at the grain boundary. While in the latter alloy, the level of segregated boron is much lower, only in some parts of the area can be indistinctly seen the indication of boron segregation. Therefore, the segregation of boron is seriously affected by the aluminium concentration in the base alloy. From the statistical analysis of the segregation density using the image processing system, the quantity of segregated B is decreased

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DONGLIANG LIN (T. L. LIN) et al.:

SEGREGATION OF B IN Ni3A1 ALLOYS

7"

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A

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.,"

D

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Fig. 4. The comparison of the relative born segregation amount among alloys with compositions Nos 1-3 (Table 1) and after heat treatments A-E (Table 2). from one to one third while the aluminium concentration is increased from 24 at.% A1 to 26 at.% A1 (see Fig. 4). For the alloy with the composition of 24 at.% A1 + 1200 ppmB, the density of etching pits at the grain boundary is higher than that in the alloy with 700 ppm boron addition, its etching pits at the grain boundary become wider and continuous while with the deepening of background density within the grain. However, there is no linear relationship between the increment of segregated boron and that of boron addition in the base alloy because of the homogeneous residence for some of the boron atoms within the grain. However, there is no linear relationship between the increment of segregated boron and that of boron addition in the base alloy because of the homogeneous residence for some of the boron atoms within the grain. In this work, when the added boron is increased from 700 to 1200ppm with the increment about 70%, the corresponding increment of segregated boron is only 10%. The segregation behavior is strongly affected by temperature. From Fig. 2 it can be seen that the level of boron at the grain boundary decreases with increasing quenching temperature. When the temperature is increased to 900°C, there is only few segregated boron atoms at the boundary. Besides, the decrement rate of segregated boron increases quickly with the increase of temperature, for example, the level of segregated boron at 900°C is only one fifth of that at 500°C. Figure 3 (a) and (b) show the segregation states of the 24 at.% A1 + 700 ppmB alloy being treated by step annealing following quenching and quenching following step annealing respectively. In the former case, its segregation state is similar to that of the alloy quenched at 500°C. While in the latter case, its situation is equivalent to that of the alloy quenched at 1150°C. These results indicate that the effect of thermal history on the boron segregation is reversible.

St.rain Fig. 5. The tensile properties for 24 at.% + 700 ppmB alloy. (a) Initial state (b) after heat treatment A, 500°C quenching, (c) after heat treatment B, 700°C quenching, (d) after heat treatment C, 900°C quenching. The comparison of the tensile property among the 24 at.% AI + 700 ppmB specimens subjected to different heat treatments is shown in Fig. 5. By quenching the specimens at different temperatures, the effect of boron on ductility can be estimated. In this case the only factor influencing the ductility is the amount of boron segregated to the grain boundary. It is shown in Fig. 5 that the plasticity decreases with the increase of quenching temperature, which is in agreement with the changing tendency of the segregated boron level with the variation of quenching temperature. It indicates that the amount of boron segregated to the grain boundary is indeed an intrinsic factor to enhance the ductility of Ni3A1 alloy. Figure 6 shows the fracture surfaces, which are corresponding to the curves in Fig. 5. The fracture

•

;

e:

Fig. 6. Effect of quenching temperature on the fracture mode for 24 at.% + 700ppmB alloy. (a) Initial state, (b) 700°C quenching, (c) 900°C quenching, (d) 1150°C quenching.

DONGLIANG LIN (T. L. LIN)

et al.:

SEGREGATION OF B IN Ni3A1 ALLOYS

mode, which is correspondent to the segregated boron amount at different quenching temperature, gradually from transgranular fracture to intergranular fracture with the quenching temperature varying from room temperature to 1150°C. 4. DISCUSSION Although the solubility of boron in Ni3A1 is not strongly influenced by a variation of aluminium concentration in the vicinity of alloy stoichiometry, its variation will lay an important role on the segregation behavior of boron. The study of Auger spectroscopy indicates that with the increasing of aluminium concentration in the base alloy, its corresponding concentration increment at the grain boundary is much higher [4]. Therefore, the aluminium concentration at the grain boundary will be markedly affected by that in the base alloy, and the effect of bulk aluminium concentration on the segregation behavior of boron takes its role through the corresponding change of grain boundary aluminium concentration. In the other words, the level of segregated boron is closely related to the grain boundary aluminium concentration. Indeed, Ni-rich boundaries are more favorable to the segregation of boron than Al-rich boundaries. The PTA result here is in accord with the Auger spectroscopy analysis. The effect of aluminium concentration on the boron segregation may be resulted from the following reasons. First, the vacancy concentration in NiaA1 has a significant composition dependence and its value is much higher in Al-rich alloys than that in Ni-rich alloys [5, 6]. In this case, when the aluminium concentration is equal to or exceeds the stoichiometry, some of the boron atoms will reside in vacancies, thus the segregated boron atoms at the grain boundary will be reduced. Next, the segregation behavior of boron is affected by the hydrostatic stress at the grain boundary as well. There is a greater propensity for atoms to segregate to the sites associated with larger hydrostatic tensile stress. The segregation energies are smaller in the sites associated with hydrostatic tensile stress than those with hydrostatic compressive stress. It was found [6] that the stress fields of Ni-rich boundaries are more tensile than those of Al-rich boundaries, therefore, there exists a higher tendency for segregation in Ni-rich grain boundaries. Besides, the segregation behavior of boron is also related to the geometrical and physical environment of the grain boundary. All these above factors will jointly govern the segregation energy of boron in Ni3A1. The increase of boron segregation can be obtained in some degree by increasing added boron in the base alloy, but there exists a range, probably being from 500 to 1000ppm, within which the increasing of added boron is most effective to the increasing of segregated boron. Moreover, if the amount of added boron is too high, that is exceeding 3000 ppm, the ductility of Ni3A1 will be decreased due to the appear-

527

ance of second phase particles (Ni20A13B6) instead of the ductility improving with the increasing of added boron [2]. With regard to the width of boron-enriched region at the grain boundary, it is generally considered as being several (about three) atomic layers by the analysis of Auger spectroscopy. However, the other study of field-ion microscopy and atom probe [7] indicates that the boron-enriched region is wider than previous deduced one from Auger spectroscopy. Although, it is unable to obtain the quantitative information in this aspect by PTA technique, from the analysis of both the width of etching pits line and its distribution at the grain boundary, it can be supposed that the width of boron-enriched region will, more probably, not be broader than that determined by the Auger spectroscopy. This is because if the boron-enriched region is wider than several atomic layers, then in the PTA photographs there will not be the appearances of discontinuity for etching pits in the grain boundary region, furthermore, the scattering and overlapping effect of particle will make the etching pits in the grain boundary region become as a continuous line. Besides, by considering the computer simulation results of Ni3A1 grain boundary structures, it can also be found that the grain boundary environment suitable for the segregation, that is the region with loose atomic arrangement, is only several atomic layers. Therefore, the width of boronenriched region is more likely inclined to the result of Auger spectroscope. The segregation of boron at the grain boundary plane is inhomogeneous, that is the segregated boron concentration varies from site to site along the grain boundary plane inferred from the discontinuity of etching pits line appeared in PTA photographs. Therefore, there exists the selectivity of boron segregation that is manifested by the significant variation in segregation energies both from one grain boundary and from one boundary atomic site to another, which has been confirmed by the experiment [7]. The results shown in Fig. 4, which shows the influence of temperature on the boron segregation, is in line with the classical theory of equilibrium grain boundary segregation, that is the boron concentration Cgb at a grain boundary Cg b ~- A C m exp(Q/RT)

where Q is the binding energy between boron atoms at the grain boundary, Cm is the boron concentration in the matrix and A is a constant. The result shown in Fig. 5. indicate that, in accord with the conclusion of Auger spectroscopy study, the effect of thermal history on the segregation is reversible, indicating the equilibrium nature of boron segregation to Ni3AI grain boundary rather than the non-equilibrium segregation caused by the vacancy flux. The effect of quenching temperature on the ductility can also be reflected on the fracture surfaces. The results shown in Fig. 6 are caused by the difference

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DONGLIANG LIN (T. L. LIN) et al.: SEGREGATION OF B IN Ni3AI ALLOYS

of segregated boron amount in alloys quenched at different temperatures. The bulk boron content, aluminium content in matrix and thermal history will simultaneously affect the amount of boron segregated to the grain boundary, and the ductility will be changed. However, the improvement of ductility is not directly proportional to the segregated boron amount [8]. There is a sudden improvement of ductility at the bulk boron content about 0.03 wt%. If the boron addition increases up to 0.1~).2wt°/0, the corresponding increment of ductility is unremarkable. When its content exceeds 0.12wt%, the ductility will not increase, but decrease. Besides, there exists a critical boron concentration to strengthen the grain boundary, its value is about 0.03 wt% for bulk boron concentration or between 7 to 11 at.% for segregated boron amount [9].

4. The higher of the temperature, the less of the segregated boron, and the effect of heat history on the segregation are reversible, therefore, the segregation of boron in Ni3AI is equilibrium in character. 5. The ductility decreases with the increasing of quenching temperature, which is consistent with the variation of segregated boron as the quenching temperature changing, and failure mode changes from transgranular to mixed and then to completely intergranular as the quenching temperature from room temperature to 1150°C. Acknowledgement--This work was supported by National

Natural Science Foundations of China.

REFERENCES

5. CONCLUSION I. The amount of segregated boron is affected distinctly by the aluminium content of matrix, while the aluminium content increases from 24 to 26%, the amount of boron at the grain boundary in latter alloy is only about one third of that in former alloy. 2. The amount of segregated boron increases with the increasing of bulk boron amount in the alloy, but these two increments are out of proportion, and the former is less than the latter. 3. The thickness of segregated region is about several atomic layer, furthermore, inhomogeneity and site selection of the segregation are existed.

I. K. Aoki and O. Tzumi, Nippon Kinzaku Gakkaishi 43, 1190 (1979). 2. C. T. Liu, C. L. White and J. A. Horton, Acta metall. 33, 213 (1985). 3. A. Choudhury, C. L. White and C. R. Brooks, Scripta metall. 20, 1161 (1986). 4. X. L. He and Y. Y. Chu, J. Phys. D., Appl. Phys. 16, 1145 (1983). 5. A. Dasgupta, I. C. Smedskjaer, D. G. Legnini and R. W. Siegel, Mater. Lett. 3, 457 (1985). 6. T. L. Lin and D. Chen, Mater. Res. Soc., Syrup. Proc., Vol. 193 (1990). 7. D. D. Sieloff, S. S. Brenner and M. G. Burke, Mater. Res. Soc. Symp. Proc., Vol. 81, p. 87 (1987). 8. C. T. Lin and C. L. White, Mat. Res. Soc. Symp. Proc., Vol. 39, P. 365 (1985). 9. C. C. Kock, C. L. White, R. A. Padgett and C. T. Liu, Scripta metall. 19, 963 (1985).