Chemical states of oxygen segregated intergranular fracture surfaces of molybdenum

42

CHEMICAL FRACTURE

Applied Surface Science 26 (1986) 42-50 North-Holland, Amsterdam

STATES OF OXYGEN SEGREGATED SURFACES OF MOLYBDENUM

INTERGRANULAR

Masaoki OKU and Shigeru SUZUKI Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan

and Hiroaki KURISHITA

and Hideo YOSHINAGA

Graduate School of Engineering Science, Department Kjwhu University. Fukuoka, Japan

Received 15 November

of Materials Science and Technology,

1985; accepted for publication

8 February 1986

The Auger and electron energy loss spectra (EELS) of a grain boundary fracture plane of bicrystal molybdenum (32 wt.ppm oxygen) are compared with the spectra of pure and oxidized molybdenum. The Auger spectrum of the fracture surface contains molybdenum and oxygen peaks, and the MO M4,sNN line coincides with that of the pure metal. The interfacial Auger transition peak is observed on the low energy side of the MO Na,sW Auger peak. Both AES and EELS spectra of the fracture plane are different from those of the oxidized molybdenum. These results show that the segregated oxygen is bound to the grain boundary fracture plane as if it were adsorbed.

1. Introduction

Molybdenum has a body-centered-cubic structure, and its mechanical properties are altered by the presence of impurity elements such as oxygen and carbon. For example, it has been observed that the addition of carbon improves the ductility of molybdenum and that the addition of oxygen enhances the brittleness [l-5]. The effect of impurities on the brittleness of molybdenum must be strictly related to the change of the chemical state of the grain boundaries of molybdenum by segregated impurities. Some AES studies have revealed that there exists a relationship between the amount of impurity at the grain boundary fracture plane and the mechanical properties [3,5,6]. However, fine structures of AES and EELS have not been reported. The aim of the present study is to investigate the chemical state of molybdenum grain boundaries at which oxygen is segregated. 0169-4332/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

M. Oku et al. / Oxygen segregated fracture swfaces of molybdenum

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2. Experimental Three kinds of molybdenum sample were used. The first is an electronbeam-melted pure polycrystalline molybdenum sample supplied by Daido Steel Company, Ltd. The second is a molybdenum bicrystal with a symmetrical tilt boundary in which the rotation axis is in (110) direction and the rotation angle is 33”. The bicrystal is made from electron-beam-melted molybdenum by induction floating-zone melting in an argon atmosphere [4], and heated at 2320 K for 5 h in wet hydrogen. The interstitial impurity contents of the as-grown bicrystal were 5 wt.ppm oxygen, 90 wt.ppm carbon, and 7 wt.ppm nitrogen. The hydrogen treatment increased the oxygen content to 30-50 wt.ppm and reduced the concentrations of carbon and nitrogen. The third sample is molybdenum oxidized at 773 K for 20 min in air. The combined method of AES and EELS was reported previously [7]. Briefly, both techniques were performed using a PHI Model 590 instrument, and the primary electron energy was 3 keV for AES and 200 eV for EELS.

3. Results Fracture of the pure polycrystalline and of the bicrystal molybdenum samples gives the transgranular and intergranular fracture planes, respectively. Fig. 1 shows broad Auger spectra for the cleaved plane of the pure molybdenum (a), the grain boundary fracture plane of the bicrystal (b) and the surface of the oxidized molybdenum (c). The cleaved plane of the transgranular fracture molybdenum shows only molybdenum Auger peaks. At the fracture surface of the bicrystal, molybdenum and oxygen Auger peaks are observed. The intensity ratio of OKLL to MoM,,,N,,Y depends on the heat treatment and the misorientation of the bicrystals. A typical spectrum is shown in fig. lb. Other specimens gave spectral features for the molybdenum and oxygen Auger and EELS spectra which were similar to that of sample (b). The spectra of the oxidized molybdenum exhibited small carbon peaks in addition to the molybdenum and oxygen peaks. MoMNN Auger spectra for the MO-O system are shown in fig. 2, in which the transition modes are indicated. The cleaved surface of pure molybdenum and the intergranular fracture plane give almost the same spectra with respect to position and relative peak intensity, as shown in (a) and (b), where the reproducibility of the kinetic energy in this region was i-0.4 eV. They are different from that for the oxidized surface (c). The intensity ratio of M,,,N,N,,Y for (c) is about twice that of molybdenum metal. Moreover, the M,,,W has a shoulder. Lin and Lichtman have reported the MoMW spectra for the MO-O system [8]. Their M*,N,,Y spectra have double peaks for MOO,, MoOZ and oxidized MO. They assigned this double structure to Mo6+

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M. Oku er al. / Oxygen segregated fracture sutfaces of nwlybabum

(b)

C

0

200 400 600 800 Kinetic energy ( eV)

1000

Fig. 1. Broad Auger spectra of the molybdenum-oxygen system. The notations of the samples are the same in figs. 1 through 5: (a) transgranular fracture plane of pure molybdenum; (b) grain boundary fracture plane of a molybdenum bicrystal with a bulk oxygen concentration of 32 wt.ppm; (c) surface of molybdenum oxidized at 773 K for 20 min in air. The primary electron energy is 3 keV, and the modulation voltage is 3 V.

and Mo4’. However, the p resent M,,,W spectrum has only one peak for the oxidized MO even with a smaller modulation voltage than that of Lin and Lichtman. Fig. 3 shows MoNW Auger spectra in the second derivative mode. The error in kinetic energy of the MoN,N,Y line for the transgranular fracture plane of pure molybdenum and for the intergrannlar fracture plane of the bicrystal is f 1 eV. However, the positions of the MoN,,,W peak at 26.5 eV and the extra peak at 19.8 eV are determined with an experimental error of -+0.2 and f 0.5 eV, respectively. The oxidized surface gives a spectrum which is different from the other spectra as shown in (c), where the experimental error is f0.5 eV. Fig. 4 shows the first derivative OKW Auger spectra of the fracture surface of the bicrystal and of the oxidized molybdenum. Both spectra have

M. Oku et al. / Oxygen segregatedfracture

50

100 Kinetic

Fig. 2. Molybdenum the kinetic energy respectively.

150 energy

200 (eV)

surfaces

ofmo+bdenum

45

; 10

MNN Auger spectra of the molybdenum-oxygen of the primary electron and the modulation

system. In figs. 2 through 4, voltage are 3 keV and 1 V,

similar features in the KL,L, and KL,L2,3 Auger transitions. The energy difference of 3.6 eV between the maximum positive and negative excursion of the KLz,L,,, transition for the grain boundary is smaller than that for the oxidized molybdenum (4.3 eV), where the error in the difference was k 0.2 eV. The same energy difference of 3.6 eV was also obtained from an oxygen segregated fracture surface of a polycrystalline molybdenum sample which was contaminated by heating at 823 K in a quartz tube after evacuation to about 1 Pa. Therefore, the energy difference is characteristic of oxygen at a grain boundary. For the grain boundary fracture plane of the bicrystal, the peak at 521 eV is clearly separated from the main peak. On the other hand, in the case of the oxidized surface, the line is too broad to determine the position of the identical transition. Fig. 5 shows the EELS spectra with a primary electron energy of 200 eV. The loss energies above 10 eV for the transgranular fracture plane and for the

M. Oku et al. / Oxygen segregated fracture surfaces of molybdenum

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20

Kinetic

30

40

Fig. 3. Second derivative molybdenum

’

460

480

500

Kinetic

energy

Fig. 4. Oxygen KW

50

1 CV 1

energy

’

NW

Auger spectra of the molybdenum-oxygen

.

520

540

( eV 1

Auger spectra of the molybdenum-oxygen

system.

spectra.

M. Oku et at. / Oxygen segregatedfracture

IO

20

Loss energy

30 (

40

surfaces of molybabum

47

IO

eV 1

Fig. 5. Second derivative electron energy loss spectra of the molybdenum-oxygen system. The kinetic energy of the primary electron is 200 eV, and the modulation voltage is 1 V.

grain boundary of the bicrystal coincide. The spectra exhibit the same energy loss peaks at 10.5 eV, but have different energy loss peaks near 6 eV. Although the peaks of 6 and 10 eV were also observed in the spectrum of the oxidized molybdenum, they have different peak widths compared to those of (a) and (b). Firment and Ferretti investigated the EELS spectrum of stoichiometric MOO, with a primary electron energy of 130 eV [9]. The present loss energies are in close agreement with their result.

4. Discussion It is concluded from figs. 2 through 5 that the grain boundary of a bicrystal has a different chemical state from that of the molybdenum surface oxidized at 773 K in air, where EELS indicates that the latter specimen has MOO, in the top oxide layer. Thus the chemical state of the grain boundary of the bicrystal

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M. Oku et al. / Oxygen segregated fracture

surfaces of molybdenum

will be discussed in terms of the initial oxidation process of clean molybdenum. In the literature concerning adsorption of oxygen on molybdenum [lo-161, the chemical effect in the MoM,,,NN Auger spectrum due to adsorption is reported by Hooker et al. [lo]. A clean Mo(100) surface was exposed to 1 X lop7 Torr of oxygen for 3 min at room temperature. For M,,,N,N,,, and M,,,N2,3N2,3 the points midway between the maximum positive and negative excursions of those peaks shifted by -0.3 + 0.2 eV. However, owing to the error of kO.4 eV in the kinetic energy in this report, such a chemical shift between the pure molybdenum and the grain boundary fracture plane of the bicrystal could not be detected. If there is a chemical shift, it probably will be small. Then it is concluded that the oxide is not formed at the grain boundary fracture surface. Nozoye et al. found that the full width at half maximum of the OKW Auger peak in the N(E) mode depends on the chemical state of the oxygen-molybdenum bond [16]. The FWHM of the adsorbed oxygen is smaller than that of the oxide on the metal. This arises from the broadening of the 02p level in forming the oxide. The energy separation between the maximum positive and negative excursion of the OKL,,,L,,, for the grain boundary fracture plane is smaller than that of the oxide on the metal. Thus it is concluded that the oxygen is bound to the grain boundary fracture plane as if it were adsorbed. A similar phenomenon is observed at the intergranular fracture plane of iron-sulfur alloy at which sulfur is segregated [17]. The peak width of the SL,sW for the sulfur atom is smaller than for the iron sulfides. The peak at 521 ‘eV is due to the interatomic Auger transition. It is clearly separated from the intra-atomic transition peak, but the corresponding transition at (c) is broad. The clear separation of the interatomic Auger transition from the intra-atomic transition is observed for sulfur at the intergranular fracture plane of the iron-sulfur alloy. Evidence for the state of the adsorbed-like oxygen at the grain boundary fracture plane of the bicrystal is observed in fig. 3b. When titanium and iron are exposed to oxygen or nitrogen, interfacial Auger peaks are observed [18]. The peaks have lower kinetic energies than the metal M, JW Auger peaks. The peak at 19.8 eV of the grain boundary fracture plane ‘is separated from the N2,3W line by about 6 eV. The separation is explained by the binding energy of 0 2p for the adsorbed oxygen on molybdenum metal. The EELS of the clean Mo(100) surface have been reported by Ballue et al. [19]. Although the loss energies do not coincide with the present loss energies, the peaks at 6.3 and 10.6 eV are described as the interband transition and surface plasmon loss peaks, respectively. The loss energy of the interband transition in the grain boundary fracture plane is different from that of pure molybdenum. This indicates that the adsorbed-like oxygen changes the density of states of the valence or conduction band. To summarize, the results in figs. 2 through 5 indicate that oxygen segre-

M. Oku et al. / Oxygen segregatedfracture surfaces of molybdenum

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gated at the grain boundary fracture plane of the bicrystal does not form any oxide, but is bound as if it were adsorbed. The oxygen causes a small change of outer valence electrons. The change may affect the strength of the cohesion of grain boundaries of molybdenum, although the details have not yet been discussed. Now, we estimate the amount of oxygen at the grain boundary fracture plane of the bicrystal. The fine structures of the molybdenum and oxygen Auger peaks vary with the chemical state. For simplicity, however, assume that the intensities of these peaks are not influenced by the chemical change. Neglect the effect of carbon at the oxide on the intensity ratio, M~M4,5NZ,~V)/O(KL2,3L2,3) in fig. lc, and assume that the oxygen at the grain boundary of the bicrystal is segregated at the top layer. Based on these assumptions, the amount of the oxygen is estimated to be about 0.6 monolayers by a quantitative method [20]. This indicates that the bonding between molybdenum atoms at grain boundaries is considerably weakened, and the bonding between molybdenum atoms and oxygen atoms is strengthened by the segregation of oxygen. It is concluded that the change of the number of bonding species at grain boundaries may be one of the causes of the change of the cohesion as well as of the change of the valence. In order to clarify the relationship between such a change of bonding of each atom and the brittleness of grain boundaries of molybdenum, further investigation of the effect of oxygen on the outer valence electrons of the grain boundaries is desirable.

5. Conclusion

AES and EELS were measured for the cleaved surface of molybdenum, the oxygen segregated grain boundary of the molybdenum bicrystal and the surface of oxidized molybdenum. The results show that segregated oxygen does not form any compound at the grain boundaries of molybdenum, but causes a small change of valence electrons. It is suggested that the segregation of oxygen considerably weakens the bonding between molybdenum atoms at grain boundaries, and can affect the cohesion of grain boundaries in molybdenum.

References [l] [2] [3] [4] [5] [6]

R.E. Olds and G.W. Rengstoff, J. Metals 8 (1956) 150. K. Tsuya and N. Aritomi, J. Less-Common Metals 15 (1966) 245. A. Kumar and B.L. Eyre, Proc. Roy. Sot. (London) A370 (1980) 431. H. Kurishita and H. Yoshinaga, Bull. Japan Inst. Metals 22 (1983) 138. T. Noda, T. Kainuma and M. Okada, J. Japan Inst. Metals 48 (1984) 25. S. Morozumi, T. Komuro and S. Suzuki, J. Japan Inst. Metals 47 (1983) 710.

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M. Oku et al. / Oxygen segregated fracture surfaces of molybdenum

[7] M. Oku, S. Suzuki, K. Abiko, H. Kimura and K. Hirokawa, J. Electron Spectrosc. 34 (1984) 55. [8] T.T. Lin and D. Lichtman, J. Vacuum Sci. Technol. 15 (1978) 1689. [9] L.E. Firment and A. Ferretti, Surface Sci. 129 (1983) 155. [lo] M.P. Hooker, J.T. Grant and T.W. Haas, J. Vacuum Sci. Technol. 12 (1975) 325. [ll] E. Bauer and H. Poppa, Surface Sci. 88 (1979) 31. [12] H.M. Kennet and A.E. Lee, Surface Sci. 48 (1975) 591. [13] T.W. Haas and T.W. Jackson, J. Chem. Phys. 44 (1966) 2921. [14] A. Benmnghoven, 0. Ganschow and L. Wiedmann, J. Vacuum Sci. Technol. 15 (1978) 506. [15] R. Riwan, C. Guillot and J. Paigne, Surface Sci. 47 (1975) 183. [16] H. Nozoye, Y. Matsumoto, T. Or&hi, T. Kondow and K. Tamaru, J. Phys. C8 (1975) 4131. [17] M. Oku, S. Suzuki, K. Abiko, H. Kimura and K. Hirokawa, J. Electron Spectrosc., to be published. [18] H.D. Shih, K. Olegg and F. Jona, Surface Sci. 54 (1976) 355. [19] Y. Ballu, J. Lecante and H. Rousseau, Phys. Rev. B14 (1976) 3201. [20] K. Hirokawa, S. Suzuki, K. Abiko, H. Kimura and M. Oku, J. Electron Spectrosc. 24 (1981) 243.