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aluminum-lithium alloy matrix composite
Volume 9, number 1 I
OXIDATION-SENSITIVE LOW-ENERGY AUGER PEAKS IN AN ALUMINUM OXIDE FIBER/ALUMINUM-LITHIUM J. HOMENY
July 1990
MATERIALS LETTERS
ALLOY MATRIX COMPOSITE
and M.M. BUCKLEY
Department of Materials Science and Engineering-Ceramics Urbana, IL 61801, USA
Division, University of Illinois,
Received 11 April 1990
Auger electron spectroscopy was performed on a continuous aluminum oxide fiber reinforced aluminum-lithium alloy. Area surveys of metal matrix regions yielded two low-energy aluminum peaks (38.0 and 54.5 eV) associated with the main AILMMline (67.0 eV). These two low-energy peaks were seen to increase linearly with respect to increased oxidation of the metal matrix.
1. Introduction The attempt to optimize the mechanical properties of metal matrix composites has drawn the attention of a great many researchers over the past two decades. During this time period significant advancements in the fabrication techniques of these composites have led to major improvements, although to date, the high cost of fabrication is the limiting aspect in their use in the automotive and aerospace industries. Nevertheless, certain composites, specifically aluminum oxide fiber/aluminum metal matrix composites have been shown to exhibit specific strength and stiffness values equal to or greater than that of higher-density alloys. In order to obtain optimal mechanical properties, however, it is necessary to understand and to control the reactions that take place during fabrication. Therefore, Auger electron spectroscopy of an aluminum oxide fiber/aluminum-lithium alloy matrix composite was performed to identify these reactions. Specilically, this paper reports on one aspect of the fabrication process, that is the degree of oxidation of the metal matrix that takes place during the casting process.
2. Experimental
procedure
reinforced aluminum-lithium alloy. The lithium content was 2.5 weight percent and was added to promote wetting between the a-aluminum oxide fibers and the metal matrix [ 11. The filaments (designated FP), as well as the composite (designated FP/Al), were fabricated by E.I. Du Pont de Nemours and Company [ 2,3]. The FP/Al composite was formed via liquid metal infiltration [ 41. The physical system implemented to perform the Auger electron spectroscopy was a Perkin-Elmer Phi 660 scanning Auger microprobe (SAM) which consisted of four main parts; ( 1) a coaxially mounted electron gun operated at 10 kV for optimum image resolution, (2) a single-stage, cylindrical-mirror electron energy analyzer (CMA), (3 ) an argon ion gun operated at 3 kV, and (4) a computer system for data collection, storage, and manipulation. Axial specimens of dimensions 0.25 x 0.25 x 1.65 cm were machined from composite billets and mounted in an in-situ fracture stage. A thin notch was placed on the tensile surface of the specimen to aid in the fracture process. Auger electron spectroscopy analyses were performed on in-situ fracture surfaces of metal matrix regions found between the ceramic fibers (see fig. 1). All analyses were performed under high vacuum ( 5 x 1OW9to 5 x 1O- lo Torr ) immediately after specimen fracture in order to eliminate contamination during specimen transport or analysis.
The metal matrix composite examined was a 55 volume percent continuous u-aluminum oxide fiber 0167-577x/90/$
03.50 0 Elsevier Science Publishers B.V. (North-Holland)
443
Volume 9, number
MATERIALS
11
LETTERS
July 1990
Fig. 1. Scanning electron microscope micrograph of a cross section of the FP/Al composite. (Aluminum num-lithium alloy matrix region, where the Auger electron spectroscopy analysis was performed.)
3. Results and discussion
A typical low-energy Auger spectrum, obtained from an area survey of the aluminum-lithium matrix, is shown in fig. 2. The standard Auger peak for aluminum (AILMM)is seen at 67.5 eV and was observed to shift between 66.5 and 68.0 eV. Associated with this main Al peak are the two peaks of interest located at 54.5 and 38.0 eV, which were observed to shift between 53.0 and 55.0 eV and 37.5 and 39.0
i9.0 :
:
22.5
: 30.0:
: 37.5:
: 45.0:
oxide fibers border
the alumi-
eV, respectively. Suleman and Pattinson [ 51 claim that the 54.5 eV peak can be attributed to an energy loss associated with volume plasmon excitations of the AILMMpeak transition. The peak at 38.0 eV, however, has little explanation behind its origin. Suleman and Pattinson [ 5 ] claim that this peak may be due to metal oxidation. Jenkins and Chung [6] performed research on aluminum and found that heavily oxidized samples yielded three peaks which were not evident in pristine aluminum surfaces.
: 52.5:
: 60.0!
! 67.5
75.0
E2.5
90
0
KINETIC ENFRGY. e"
Fig. 2. Typical low-energy
444
Auger electron
spectroscopy
spectrum
( 15-90 eV) taken from metal matrix regions of the FP/Al
composite.
Volume 9, number
11
MATERIALS
These lines were seen at 39.0, 41.3, and 54.0 eV and in some cases were as prominent as the AILMM line. They postulated that the appearance of these peaks may be an indication as to the degree of metal oxidation or changes in valence band structure due to surface oxidation. There exist several explanations behind the shift of Auger peaks about some central value. Grant [ 71 claims that a peak shift occurs whenever atoms transfer charge from one to the other. Kowalczyk et al. [ 81 identified relaxation effects as contributing to variation in peak position. Grant and Haas [ 91 state that surface absorbed oxygen can produce significant shifts in metals and that the shift becomes greater with increased oxygen absorption. The findings of this investigation tend to substantiate the premise that oxidation plays a key role in Auger line shifts. The low-energy peaks tended to shift to lower energy values as the oxygen peak intensity increased. Although it should be noted that the oxygen, which was always present in the metal matrix, was undoubtedly associated with the processing of the composite and not surface absorption during analysis. Fig. 3 shows a plot of both low-energy peak inten-
LETTERS
July 1990
sities versus oxygen peak intensity. The metal matrix oxidation is believed to have originated from a composite casting process carried out in an air atmosphere. As can be seen there exists a strong linear relationship between the line intensities and the amount of oxidation in the metal matrix. These tindings substantiate the hypothesis that both the 38.0 and 54.5 eV peaks are related to the degree of oxidation of the metal matrix. The 47.3 eV peak, seen by Suleman and Pattinson [ 51 at 46.5 eV, was never documented in over 40 area surveys of the metal matrix. Suleman and Pattinson [ 51, in correlating their work to Powell and Swan [ lo], claim that this peak may be the result of either a combination of surface and bulk losses or multiples of the bulk loss. It is believed that this peak is specific to the oxidized surfaces of pure aluminum and that the small alkali addition altered the low-energy aluminum spectrum such that this peak was not seen. The final point of interest is that the lithium peak, typically seen at 43.0 eV, was never observed in the Auger spectra. Tsangarakis et al. [ 111 showed by analysis of the metal matrix after casting that the lithium content decreased from 2.5 to 2.2 weight percent. This small percentage is near the resolution limit of Auger electron spectroscopy. It has also been shown that the lithium tends to segregate towards the fiber surface and actively participates in the formation of an interphase region [ 121. Combining these facts with the high mobility of lithium, it is possible that the element is being masked by either the 38.0 or 54.5 eV peaks or it is unresolvable in these trace amounts.
4. Conclusions
Oxygen
Peak
Intensity
(kcmmls)
Fig. 3. Plot of 38.0 and 54.5 eV peak intensities versus oxygen peak intensity. (The units represent a scaling factor or the number of thousands of counts detected by the computer per second, kc/s, multiplied by the height of the peak in millimeters, mm.)
Auger electron spectroscopy was successfully utilized to investigate the degree of metal matrix oxidation in an aluminum oxide tiber/aluminum-lithium alloy matrix composite by examining the lowenergy peaks associated with aluminum. It was found that both the 38.0 and 54.5 eV peak intensities increased linearly as a function of increased metal oxidation. In many cases the two low-energy peaks were more pronounced than the main AILMM peak. The high degree of metal matrix oxidation is believed to 445
Volume 9, number 11
MATERIAL
be due to a casting procedure carried out in an air atmosphere. Acknowledgement The authors wish to thank the Chrysler Motors Corporation for their continued support under the Chrysler Challenge Fund. The Auger analysis was conducted in the Center for Microanalysis of Materials, University of Illinois, which is supported by the U.S. Department of Energy. References [ 1 ] F. Delannay, L. Froyen and A. Deruyttere, J. Mater. Sci. 22 (1987) 1.
446
LETTERS
July 1990
[2] A.K. DhinIqa, U.S. Patent #3,828,939 (1974). [ 31 P.G. Riewald, W.H. Krueger and A.K. Dhingra, U.S. Patent #4,012,204 (1977). [4] K.M. Prewo, Interim Technical Report R-77912245-3, United Technologies Research Center, East Hartford, CT (1977). [ 51M. Suleman and E.B. Pattinson, J. Phys. F 1 ( 1971) L21. [ 61 L.H. Jenkins and M.F. Chung, Surface Sci. 28 ( 197 1) 409. (71 J.T. Grant, Appl, Surface Sci. 13 (1982) 35. [ 81S.P. Kowalczyk, L. Ley, F.R. McFeely, R.A. Pollak and D.A. Shirley, Phys. Rev. B 9 ( 1974) 38 1. [ 9 ] J.T. Grant and T.W. Haas, Surface Sci. 26 ( 1971) 669. [lo] C.J. Powell and J.B. Swan, Phys. Rev. 115 ( 1959) 869. f 111 N. Tsangarakis, J.M. Slepetz and J. Nunes, ASTM STP 864, American Society for Testing and Materials, Philadelphia, PA (1985) 131. [ 121 I.W. Hall andV. Barrailler, Metall. Trans. A 17 ( 1986) 1075.
OXIDATION-SENSITIVE LOW-ENERGY AUGER PEAKS IN AN ALUMINUM OXIDE FIBER/ALUMINUM-LITHIUM J. HOMENY
July 1990
MATERIALS LETTERS
ALLOY MATRIX COMPOSITE
and M.M. BUCKLEY
Department of Materials Science and Engineering-Ceramics Urbana, IL 61801, USA
Division, University of Illinois,
Received 11 April 1990
Auger electron spectroscopy was performed on a continuous aluminum oxide fiber reinforced aluminum-lithium alloy. Area surveys of metal matrix regions yielded two low-energy aluminum peaks (38.0 and 54.5 eV) associated with the main AILMMline (67.0 eV). These two low-energy peaks were seen to increase linearly with respect to increased oxidation of the metal matrix.
1. Introduction The attempt to optimize the mechanical properties of metal matrix composites has drawn the attention of a great many researchers over the past two decades. During this time period significant advancements in the fabrication techniques of these composites have led to major improvements, although to date, the high cost of fabrication is the limiting aspect in their use in the automotive and aerospace industries. Nevertheless, certain composites, specifically aluminum oxide fiber/aluminum metal matrix composites have been shown to exhibit specific strength and stiffness values equal to or greater than that of higher-density alloys. In order to obtain optimal mechanical properties, however, it is necessary to understand and to control the reactions that take place during fabrication. Therefore, Auger electron spectroscopy of an aluminum oxide fiber/aluminum-lithium alloy matrix composite was performed to identify these reactions. Specilically, this paper reports on one aspect of the fabrication process, that is the degree of oxidation of the metal matrix that takes place during the casting process.
2. Experimental
procedure
reinforced aluminum-lithium alloy. The lithium content was 2.5 weight percent and was added to promote wetting between the a-aluminum oxide fibers and the metal matrix [ 11. The filaments (designated FP), as well as the composite (designated FP/Al), were fabricated by E.I. Du Pont de Nemours and Company [ 2,3]. The FP/Al composite was formed via liquid metal infiltration [ 41. The physical system implemented to perform the Auger electron spectroscopy was a Perkin-Elmer Phi 660 scanning Auger microprobe (SAM) which consisted of four main parts; ( 1) a coaxially mounted electron gun operated at 10 kV for optimum image resolution, (2) a single-stage, cylindrical-mirror electron energy analyzer (CMA), (3 ) an argon ion gun operated at 3 kV, and (4) a computer system for data collection, storage, and manipulation. Axial specimens of dimensions 0.25 x 0.25 x 1.65 cm were machined from composite billets and mounted in an in-situ fracture stage. A thin notch was placed on the tensile surface of the specimen to aid in the fracture process. Auger electron spectroscopy analyses were performed on in-situ fracture surfaces of metal matrix regions found between the ceramic fibers (see fig. 1). All analyses were performed under high vacuum ( 5 x 1OW9to 5 x 1O- lo Torr ) immediately after specimen fracture in order to eliminate contamination during specimen transport or analysis.
The metal matrix composite examined was a 55 volume percent continuous u-aluminum oxide fiber 0167-577x/90/$
03.50 0 Elsevier Science Publishers B.V. (North-Holland)
443
Volume 9, number
MATERIALS
11
LETTERS
July 1990
Fig. 1. Scanning electron microscope micrograph of a cross section of the FP/Al composite. (Aluminum num-lithium alloy matrix region, where the Auger electron spectroscopy analysis was performed.)
3. Results and discussion
A typical low-energy Auger spectrum, obtained from an area survey of the aluminum-lithium matrix, is shown in fig. 2. The standard Auger peak for aluminum (AILMM)is seen at 67.5 eV and was observed to shift between 66.5 and 68.0 eV. Associated with this main Al peak are the two peaks of interest located at 54.5 and 38.0 eV, which were observed to shift between 53.0 and 55.0 eV and 37.5 and 39.0
i9.0 :
:
22.5
: 30.0:
: 37.5:
: 45.0:
oxide fibers border
the alumi-
eV, respectively. Suleman and Pattinson [ 51 claim that the 54.5 eV peak can be attributed to an energy loss associated with volume plasmon excitations of the AILMMpeak transition. The peak at 38.0 eV, however, has little explanation behind its origin. Suleman and Pattinson [ 5 ] claim that this peak may be due to metal oxidation. Jenkins and Chung [6] performed research on aluminum and found that heavily oxidized samples yielded three peaks which were not evident in pristine aluminum surfaces.
: 52.5:
: 60.0!
! 67.5
75.0
E2.5
90
0
KINETIC ENFRGY. e"
Fig. 2. Typical low-energy
444
Auger electron
spectroscopy
spectrum
( 15-90 eV) taken from metal matrix regions of the FP/Al
composite.
Volume 9, number
11
MATERIALS
These lines were seen at 39.0, 41.3, and 54.0 eV and in some cases were as prominent as the AILMM line. They postulated that the appearance of these peaks may be an indication as to the degree of metal oxidation or changes in valence band structure due to surface oxidation. There exist several explanations behind the shift of Auger peaks about some central value. Grant [ 71 claims that a peak shift occurs whenever atoms transfer charge from one to the other. Kowalczyk et al. [ 81 identified relaxation effects as contributing to variation in peak position. Grant and Haas [ 91 state that surface absorbed oxygen can produce significant shifts in metals and that the shift becomes greater with increased oxygen absorption. The findings of this investigation tend to substantiate the premise that oxidation plays a key role in Auger line shifts. The low-energy peaks tended to shift to lower energy values as the oxygen peak intensity increased. Although it should be noted that the oxygen, which was always present in the metal matrix, was undoubtedly associated with the processing of the composite and not surface absorption during analysis. Fig. 3 shows a plot of both low-energy peak inten-
LETTERS
July 1990
sities versus oxygen peak intensity. The metal matrix oxidation is believed to have originated from a composite casting process carried out in an air atmosphere. As can be seen there exists a strong linear relationship between the line intensities and the amount of oxidation in the metal matrix. These tindings substantiate the hypothesis that both the 38.0 and 54.5 eV peaks are related to the degree of oxidation of the metal matrix. The 47.3 eV peak, seen by Suleman and Pattinson [ 51 at 46.5 eV, was never documented in over 40 area surveys of the metal matrix. Suleman and Pattinson [ 51, in correlating their work to Powell and Swan [ lo], claim that this peak may be the result of either a combination of surface and bulk losses or multiples of the bulk loss. It is believed that this peak is specific to the oxidized surfaces of pure aluminum and that the small alkali addition altered the low-energy aluminum spectrum such that this peak was not seen. The final point of interest is that the lithium peak, typically seen at 43.0 eV, was never observed in the Auger spectra. Tsangarakis et al. [ 111 showed by analysis of the metal matrix after casting that the lithium content decreased from 2.5 to 2.2 weight percent. This small percentage is near the resolution limit of Auger electron spectroscopy. It has also been shown that the lithium tends to segregate towards the fiber surface and actively participates in the formation of an interphase region [ 121. Combining these facts with the high mobility of lithium, it is possible that the element is being masked by either the 38.0 or 54.5 eV peaks or it is unresolvable in these trace amounts.
4. Conclusions
Oxygen
Peak
Intensity
(kcmmls)
Fig. 3. Plot of 38.0 and 54.5 eV peak intensities versus oxygen peak intensity. (The units represent a scaling factor or the number of thousands of counts detected by the computer per second, kc/s, multiplied by the height of the peak in millimeters, mm.)
Auger electron spectroscopy was successfully utilized to investigate the degree of metal matrix oxidation in an aluminum oxide tiber/aluminum-lithium alloy matrix composite by examining the lowenergy peaks associated with aluminum. It was found that both the 38.0 and 54.5 eV peak intensities increased linearly as a function of increased metal oxidation. In many cases the two low-energy peaks were more pronounced than the main AILMM peak. The high degree of metal matrix oxidation is believed to 445
Volume 9, number 11
MATERIAL
be due to a casting procedure carried out in an air atmosphere. Acknowledgement The authors wish to thank the Chrysler Motors Corporation for their continued support under the Chrysler Challenge Fund. The Auger analysis was conducted in the Center for Microanalysis of Materials, University of Illinois, which is supported by the U.S. Department of Energy. References [ 1 ] F. Delannay, L. Froyen and A. Deruyttere, J. Mater. Sci. 22 (1987) 1.
446
LETTERS
July 1990
[2] A.K. DhinIqa, U.S. Patent #3,828,939 (1974). [ 31 P.G. Riewald, W.H. Krueger and A.K. Dhingra, U.S. Patent #4,012,204 (1977). [4] K.M. Prewo, Interim Technical Report R-77912245-3, United Technologies Research Center, East Hartford, CT (1977). [ 51M. Suleman and E.B. Pattinson, J. Phys. F 1 ( 1971) L21. [ 61 L.H. Jenkins and M.F. Chung, Surface Sci. 28 ( 197 1) 409. (71 J.T. Grant, Appl, Surface Sci. 13 (1982) 35. [ 81S.P. Kowalczyk, L. Ley, F.R. McFeely, R.A. Pollak and D.A. Shirley, Phys. Rev. B 9 ( 1974) 38 1. [ 9 ] J.T. Grant and T.W. Haas, Surface Sci. 26 ( 1971) 669. [lo] C.J. Powell and J.B. Swan, Phys. Rev. 115 ( 1959) 869. f 111 N. Tsangarakis, J.M. Slepetz and J. Nunes, ASTM STP 864, American Society for Testing and Materials, Philadelphia, PA (1985) 131. [ 121 I.W. Hall andV. Barrailler, Metall. Trans. A 17 ( 1986) 1075.