Effect of Oxygen Exposure on Auger Spectra

Robert Sherman? Southwest Research Institute, San Antonio, TX 78284, USA

The effect of residual vacuum contamination on Auger spectra, and subsequently on quantitative analysis, is well known. These effects become especially obvious when measuring the extent of phosphorus grain boundary segregation in the many alloys that require various methods of in situ fracture. To gain insight into the effects of vacuum contamination on the phosphorus Auger signal, we exposed a grain facet to controlled oxygen dosages and monitored the decreasing phosphorus-to-iron PPHR as a function of the increasing oxygen-to-iron PPHR. This communication reports the results of this experiment and suggests ways to compensate for vacuum contamination.

INTRODUCTION

One of the major early accomplishments of Auger elec- tron spectroscopy (AES) was the establishment of the relationship between the segregation of trace elements (P, S, Sn, and Sb) to grain boundaries and temper embrittlement (TE).1-3 In recent years, intergranular stress corrosion cracking (IGSCC) of turbine rotors and discs has also been related to the intergranular chemistry of the alloys under certain environmental condition^.^-^ The relationship between the material's susceptibility to TE or IGSCC and the extent of grain boundary phos- phorus segregation has been determined by use of in-situ fracture methods (usually impact fracture at low tem- peratures). The importance of in-situ fracture was demonstrated by Seah7 who observed a decrease in the peak-to-peak height of segregated tin signal by a factor of 25 due to air exposure. Generally, for materials that are embrittled, in-situ intergranular fracture occurs. However, for low strength or de-embrittled alloys, other techniques, often involving hydrogen charging, are used to obtain in-situ intergranular In those inst- ances, the alloys are charged with hydrogen, stressed, and then fractured to yield several intergranular facets.

Problems with residual vacuum contamination can arise with samples that are hydrogen charged. The cause of increased contamination, in comparison to samples fractured by impact at low temperatures, is twofold. First, concern over possible hydrogen evolution from the sample disallows system bakeout. This yields a higher than normal base pressure. The second source of contamination is the increased water vapor background caused by hydrogen interaction with hot filaments or with vacuum system walls." Therefore, due to problems associated with hydrogen outgassing, a higher than nor- mal base pressure occurs. The increased residuals adsorb on the sample's surface, possibly reducing the amount of detected phosphorus and leading to an underestima- tion of the extent of solute segregation.

As part of an extensive program involving the investi- gation of IGSCC of low pressure turbine discs, the extent of grain boundary phosphorus segregation was deter-

t Present address: The BOC Group, Inc., Group Technical Center, 100 Mountain Ave, Murray Hill, NJ 07974, USA

mined in six different discs (all nominally 3.5 NiCrMoV and with yield strengths ranging from 104 to 161 ksi). It was very important to rank the materials according to their phosphorus grain boundary content to establish the importance of each parameter (segregated phos- phorus, yield strength, environment, etc.) in determining the overall susceptibility to IGSCC." Three of the six disc steels required hydrogen charging to expose IG facets. For these three discs, the Auger spectra indicated large oxygen peaks, and a question arose about a poss- ible reduction of the phosphorus to iron peak-to-peak height ratio (PPHR). This reduction of the detected phosphorus Auger signal due to an oxygen overlayer is caused by the small mean free path of the Auger electron. In general, it was necessary to determine the functional relationship between the increasing oxygen to iron PPHR and the decreasing phosphorus to iron PPHR.

To determine this relationship, a sample of one of the discs that displayed in-situ intergranular fracture was fractured within the vacuum system. After fracture, the sample was exposed to controlled oxygen exposures. Increases in the oxygen PPHR were monitored along with the subsequent decreases in the phosphorus PPHR. From these experiments, there appeared to be three stages to the decrease of the phosphorus PPHR. This communication reports the results of the effect of con- trolled oxygen exposures on Auger spectra of a low alloy disc steel.

EXPERIMENTAL

The material, as mentioned earlier, was nominally a 3.5 NiCrMoV disc steel with a bulk phosphorus content of 0.008 wt%. This steel displayed IG fracture upon impact at low temperatures. A Model 595 Auger Micro- probe (Physical Electronics), operated at 5 keV and 0.2 FA, was used. Background pressure was below 2.6 x

Pa (2 x 10-'Otorr). The oxygen source was one of about 99.95 purity. Auger spectra did not indicate the presence of any other gaseous impurities adsorbing on the sample's surface. The experiments were performed in a dynamic mode. Oxygen flowed continuously into the ion pumped vacuum system for preset time intervals. This technique provided for controlled dosage experi-

CCC-0142-242 1 /85/OO07-0232 $01 .SO

232 SURFACE AND INTERFACE ANALYSIS, VOL. 7, NO. 5, 1985 @ Wiley Heyden Ltd, 1985

EFFECT OF OXYGEN EXPOSURE ON AUGER SPECTRA

OXYGEN IIXPOSURE - LANGMUIRS

. *.. . i . . .

,- .2 .3 4 .5 6 .7

OXYGEN /IRON

Figure 1. Plot of the relative phosphorus PPHR versus t h e oxygen PPHR (also the oxygen exposure-top scale).

ments that simulated residual vacuum exposure. The reported dosages, in Langmuirs (1 Langmuir is equal to an exposure of torr for 1 s), were obtained under these conditions.

RESULTS AND DISCUSSION

The dependence of the: phosphorus PPHR as a function of the oxygen PPHR is shown in Fig. 1. As the oxygen signal increased, the corresponding phosphorus signal displayed a decrease l>y about a factor of two over the entire range of oxygen exposures. As seen in Fig. 1, the decrease in the phosphorus PPHR can be divided into three separate regions. For low oxygen coverages (below about oxygen PPHR of about 0.25), the phosphorus PPHR was approximately constant (or showed only a slight decrease) about an average value of 0.075. This average value is represented in Fig. 1 by a relative PPHR of 1.0, and all measured phosphorus PPHR were normal- ized by this average value.

The second region., from an oxygen PPHR of about 0.25 to about 0.60, displayed a strong decrease in the phosphorus PPHR. The relative phosphorus signal decreased from an initial average value of 1.0 to about 0.5. Above this oxygen exposure, the third region in the phosphorus PPHR showed little to no decrease over the remaining oxygen dosages. The third region, above an oxygen PPHR of 0.55, was clearly evident when the phosphorus PPHR was plotted versus the oxygen exposure on a linear scale.

The above results indicate that simulated vacuum exposures can lead lo reductions of a measured segre- gated phosphorus signal. Though a problem (the reduc-

tion of the measured segregated phosphorus signal due to oxygen exposure) is indicated, the data can assist the user in estimating the amount of time available before a significant reduction in the detected phosphorus signal occurs. The present experiments showed that below a total oxygen exposure corresponding to about 1 Lang- muir, no significant reductions in the phosphorus PPHR occurred. By knowing the residual vacuum before frac- ture, and assuming, at the worst, that the residual vacuum consists only of oxygen, or oxygen compounds, then the time to obtain a 1 L exposure can be calculated. This time represents a lower time limit before any sig- nificant peak reduction occurs. For example, if the back- ground pressure is 1.3 x lo-' Pa (1 x torr), then a minimum time of about 17 min. after fracture is available before the phosphorus PPHR starts to show a reduction.

Another example of the above calculation can be applied in a situation where the vacuum of 1.3 x lo-' Pa (1 x lo-'' torr) is available. In this case, the time estimate would indicate a minimum time of a little less than three hours, which would provide more than enough time to sample many facets. However under these conditions a greater time would be available, since the oxygen partial pressure is lower than the actual pressure.

A second possible use of the data in Fig. 1 is to estimate the grain boundary phosphorus PPHR of an alloy in the absence of oxygen contamination when the only data available is taken after significant contamination has occurred. By measuring the oxygen PPHR, the corre- sponding relative phosphorus PPHR can be obtained from Fig. 1. Then, with the relative phosphorus PPHR and the measured phosphorus PPHR, the phosphorus PPHR can be obtained as if no contamination were present.

In the above-mentioned study of IGSCC of turbine discs, the phosphorus PPHR was determined from six different discs. The low strength materials required hydrogen charging (except one that was intentionally tempered embrittled) for facets to be exposed. In each of these samples, significant oxygen PPHRs were observed, which were, at the most about 0.3. For this oxygen PPHR and the data from Fig. 1, only a small relative decrease in the phosphorus PPHR was deduced, indicating that no major reductions in the measured phosphorus PPHR occurred. The data in Fig. 1 was acquired for only one alloy but we believe that it may be applicable to other alloys and segregants.

Acknowledgements

Support was provided through the Electric Power Research Institute (EPRI) RP 1398-5 and the Internal Research Program of Southwest Research Program of Southwest Research Institute. Helpful dis- cussions with Mr. A. McMinn in regard to the overall EPRI program are acknowledged. The BOC Group Technical Center assisted in preparation and publication costs.

REFERENCES

1. H. L. Marcus and P. W. Palmberg, Trans. TMS-AIME. 245, 1664

2. R. A. Mulford, C. J. McMahon, D. P. Pope and H. C. Feng, Met.

3. A. Joshi and D. F. Stein, ASTM STP499, p. 59, ASTM, Philadel-

4. J. D. Atkinson, A. M. Adams, M. A. M. Kahn and P. J . Worthing- ton, Report No. RD/ L/ N169/79, Central Electricity Research Laboratory, Leatherhead, England (1980).

5. K. L. Molornik, C. L. Briant, and C. J. McMahon, Corrosion, 35, 331 (1979).

6. N. Eandyopadhyay and C. L. Eriant, Met. Trans. 14A. 2005 (1983).

(1969).

Trans., 7A, 1 183 (1 9'76).

phia (1972).

SURFACE AND INTERFACE ANALYSIS, VOL. 7, NO. 5, 1985 233

R. SHERMAN

7. M. P. Seah. Surf Sci.. 53, 168 (1975). 8. A. Joshi and L. E. Davis, J. Vac. Sci. & Tech., 14, 1310 (1977). 9. B. C. Edwards, H. E. Bishop, J. C. Riviere, and B. L. Eyre, Acta

and Environment on the Stress Corrosion Susceptibility of Tur- bine Disc Steels, in Electric Power Research Institute Final Report RP 1398-5, Vol. 1, in press.

Met., 24, 957 (1976). 10. L. C. Beavis, J. Vac. Sci., Tech., 10, 386 (1973). 11. A. McMinn and F. F. Lyle, The Influence of Material Properties Received 5 February 1985; accepted 25 March 1985

234 SURFACE AND INTERFACE ANALYSIS, VOL. 7, NO. 5, 1985