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Examination of Oxide Scales in the SEM Using Backscattered Electron Images C. W. PRICE, I. G. WRIGHT, AND G. R. WALLWORK The complementary use of the scanning electron microscope in the backscattered electron mode with the more usual secondary electron mode results in a significant increase in the versatility of the instrument, since regions of different chemical composition (at. no.) can be readily detected, and their morphology examined. The use of this technique to examine complex oxide scales formed on heat-resistant alloys is described, and in particular the location of thoria particles in the scale formed on a Ni-20 wt pct Cr-2.3 wt pct ThO2 alloy. and the examination of the behavior of yttrium during the high-temperature oxidation of a Co-Cr-Ai-Y alloy are discussed. THE scanning electron microscope (SEM) has been used by a number of workers in the characterization of the morphology of oxide scales, but little effort has been made to use the SEM for the examination of metal- lographic cross-sections of the scales. The reason for this is that most SEM examinations are conducted in the secondary electron (SE) mode, because the low- energy secondary electrons which are emitted only from relatively shallow surface layers on a specimen provide the highest resolution and also the greatest amount of surface detail of the various SEM imaging modes. However, SE images are not particularly sen- sitive to chemical composition, and phases of different composition are usually not revealed in SE images. In contrast to this, the intensity of backscattered electrons which are emitted from a specimen increases systemat- ically as the atomic number of the scattering species increases, and phases of different composition which usually cannot be distinguished in SE images are read- ily detected in backscattered electron (BSE) images. Consequently, BSE images have proven to be extremely useful in metallographic examinations. 1 The purpose of this paper is to demonstrate the use of BSE images in the examination of complex oxide scales formed on some nickel- and cobalt-based alloys. EXPERIMENTAL PROCEDURE The compositions of the alloy systems examined are given in Table I. The specimens were 0.02 by 0.025 by 0.0017 m. The techniques for the preparation and oxi- dation of the specimens have been described previ- ously. %a'4 Alloy 1 had been electropolished prior to oxidation while Alloy 2 had been given only a standard metallographic polish. Examinations were conducted both on the surfaces of the oxide scales and on metallographic sections through the centers of the specimens. The metallo- graphic sections were mounted in an epoxy resin to minimize the separation of the scales from the sub- C. W. PRICE is Semor Metallurgist in the Deformation and Fracture Section, and I. G. WRIGHTis Senior Scientist in the High-Temperature Materials and Processes Section of the Metal Science Group, Battelle-- Columbus Laboratories, Colmnbus, Ohio 43201 G. R WALLWORK is Professor in Metallurgy, School of Metallurgy, The University of New South Wales, P. O. Box 1, Kensington, Australia. Mauscnpt submitted March 12, 1973. strate. They were polished by the standard techniques described by Samuels s with precautions being exerted to prevent unnecessary rounding of the edges of the specimens, although edge rounding is less of a problem in the SEM than it is in optical microscopy. Both the surfaces of the scales and the polished metallographic sections were coated with a thin conductive layer of carbon to eliminate charging effects in the SEM. The carbon layer did not significantly affect the contrast of the BSE images. Both SE and BSE images were used in the examina- tion of the surfaces of the scales. As mentioned previ- ously, SE images have a higher resolution than BSE images and also provide more surface detail. However, SE images are not as sensitive to chemical composition as are BSE images, and correlations between SE and BSE images frequently provide more accurate inter- pretations of surface structures than can be obtained from SE images alone. Such correlations proved to be useful in examining the phases that were present in the oxide scales. The metallographic specimens were ex- amined only in the BSE mode since no useful informa- tion was obtained from the SE images from the polished sections. The SEM examinations were conducted in a MAC* *Materml~ Analysl~ Corporation,PaloALto, Cahforma. Modei 700 SEM aianaccelerating potent(ai of 20 kV. - The BSE detector in this instrument is positioned on the bottom surface of the intermediate lens facing the specimen, and the BSE electron flux to the detector is maximized when the specimen surface is normal to the incident electron beam (i.e., a tilt angle of 0 rad from the horizontal). Therefore, a tilt angle of 0 rad was used for the BSE images from the metallographic spec- imens. Since the optimum tilt angle for the formation of SE images is approximately 7r/4 rad, a compromise tilt angle of 0.194 rad was used for the corresponding SE and BSE images from the surfaces of the scales. Table I. Alloy Composition (Wt. Pct) Alloy NI Co Cr AI ThO2 Y I. {TDNI('}* Bal 20 2 3 2 (('o('~AIY) Bal. 10 ~} I *Tladenalne, Fanslecl MelallurgJcal Corporatmn. METALLURGICAL TRANSACTIONS VOLUME 4, OCTOBER 1973 2423

Examination of oxide scales in the SEM using backscattered electron images

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Examination of Oxide Scales in the SEM Using Backscattered Electron Images

C. W. PRICE, I. G. WRIGHT, AND G. R. WALLWORK

The complemen ta ry use of the scanning e lec t ron mic roscope in the backsca t t e r ed e lec t ron mode with the more usual secondary e lec t ron mode r e su l t s in a s igni f icant i nc r ea se in the ve r s a t i l i t y of the i n s t rumen t , s ince reg ions of different chemica l composi t ion (at. no.) can be read i ly detected, and the i r morphology examined. The use of this technique to examine complex oxide sca les fo rmed on h e a t - r e s i s t a n t al loys is descr ibed , and in pa r t i cu l a r the locat ion of thor ia pa r t i c l e s in the sca le formed on a Ni-20 wt pct Cr -2 .3 wt pct ThO2 alloy. and the examinat ion of the behavior of y t t r ium dur ing the h i g h - t e m p e r a t u r e oxidation of a C o - C r - A i - Y al loy a re d i scussed .

T H E scanning e lec t ron mic roscope (SEM) has been used by a number of workers in the cha rac t e r i za t ion of the morphology of oxide sca les , but l i t t le effort has been made to use the SEM for the examina t ion of me ta l - lographic c r o s s - s e c t i o n s of the sca les . The r ea son for this is that most SEM examinat ions a re conducted in the secondary e lec t ron (SE) mode, because the low- energy secondary e lec t rons which a re emi t ted only f rom re l a t ive ly shallow sur face l aye r s on a spec imen provide the highest r e so lu t ion and also the g rea tes t amount of sur face detai l of the va r ious SEM imaging modes. However, SE images a re not p a r t i c u l a r l y s en - si t ive to chemica l composi t ion, and phases of different composi t ion a re usual ly not revea led in SE images . In con t ras t to this , the in tens i ty of backsca t t e r ed e lec t rons which a re emi t ted f rom a spec imen i n c r e a s e s s y s t e m a t - ical ly as the a tomic number of the s ca t t e r ing species i n c r e a s e s , and phases of different composi t ion which usual ly cannot be d is t inguished in SE images a re r e a d - ily detected in backsca t t e red e lec t ron (BSE) images . Consequently, BSE images have p roven to be ex t r eme ly useful in meta l lographic examina t ions . 1 The purpose of this paper is to demons t r a t e the use of BSE images in the examinat ion of complex oxide sca les formed on some n icke l - and coba l t -based al loys.

EXPERIMENTAL PROCEDURE

The composi t ions of the al loy sy s t ems examined a re given in Table I. The spec imens were 0.02 by 0.025 by 0.0017 m. The techniques for the p r epa ra t i on and oxi- dation of the spec imens have been desc r ibed p r e v i - ously. %a'4 Alloy 1 had been e lec t ropol i shed p r io r to oxidation while Alloy 2 had been given only a s tandard meta l lographic polish.

Examinat ions were conducted both on the su r faces of the oxide sca les and on meta l lographic sec t ions through the cen te r s of the spec imens . The me ta l lo - graphic sec t ions were mounted in an epoxy r e s i n to min imize the separa t ion of the sca les f rom the sub-

C. W. PRICE is Semor Metallurgist in the Deformation and Fracture Section, and I. G. WRIGHT is Senior Scientist in the High-Temperature Materials and Processes Section of the Metal Science Group, Battelle-- Columbus Laboratories, Colmnbus, Ohio 43201 G. R WALLWORK is Professor in Metallurgy, School of Metallurgy, The University of New South Wales, P. O. Box 1, Kensington, Australia.

Mauscnpt submitted March 12, 1973.

s t ra te . They were pol ished by the s tandard techniques desc r ibed by Samuels s with p recau t ions being exer ted to p reven t u n n e c e s s a r y rounding of the edges of the spec imens , although edge rounding is l ess of a p rob lem in the SEM than it is in optical microscopy . Both the sur faces of the sca les and the pol ished meta l lographic sec t ions were coated with a thin conductive layer of carbon to e l imina te charging effects in the SEM. The carbon layer did not s igni f icant ly affect the con t ras t of the BSE images .

Both SE and BSE images were used in the examina - tion of the sur faces of the sca les . As ment ioned p r e v i - ously, SE images have a higher r eso lu t ion than BSE images and also provide more sur face detai l . However, SE images a re not as sens i t ive to chemica l composi t ion as a re BSE images , and co r r e l a t i ons between SE and BSE images f requent ly provide more accura te i n t e r - p re ta t ions of sur face s t r u c t u r e s than can be obtained f rom SE images alone. Such co r r e l a t i ons proved to be useful in examin ing the phases that were p r e sen t in the oxide sca les . The meta l lographic spec imens were ex- amined only in the BSE mode s ince no useful i n fo rma- t ion was obtained f rom the SE images f rom the polished sect ions .

The SEM examina t ions were conducted in a MAC*

*Materml~ Analysl~ Corporation, Palo ALto, Cahforma.

Modei 7 0 0 SEM a i a n a c c e l e r a t i n g potent(ai of 20 kV. - The BSE detec tor in this i n s t r u m e n t is posi t ioned on the bottom sur face of the in t e rmed ia t e lens facing the spec imen, and the BSE e lec t ron flux to the detec tor is maximized when the spec imen sur face is n o r m a l to the incident e l ec t ron beam (i.e., a t i l t angle of 0 rad f rom the hor izonta l ) . There fore , a t i l t angle of 0 rad was used for the BSE images f rom the meta l lographic spec- imens . Since the opt imum t i l t angle for the format ion of SE images is approx imate ly 7r/4 rad, a compromise t i l t angle of 0.194 rad was used for the cor responding SE and BSE images f rom the su r faces of the sca les .

Table I. Alloy Composition (Wt. Pct)

Alloy NI Co Cr AI ThO2 Y

I. {TDNI('}* Bal 20 2 3 2 (('o('~AIY) Bal. 10 ~} I

*Tladenalne, Fanslecl MelallurgJcal Corporatmn.

METALLURGICAL TRANSACTIONS VOLUME 4, OCTOBER 1973 2423

and shows a thin a r e a su r rounded by an a r e a of l a r g e r oxide c rys t a l l i t e s . The thin a r ea is shown at higher magnif ica t ion in Figs. 2(a) and 2(b) which are c o r r e - sponding SE and BSE images , r espec t ive ly . The smooth appearance of the scale in the thin a r ea suggested that vapor iza t ion had occur red , and this appeared to be con- f i rmed by the format ion of an oxide deposit in the cooler por t ions of the tube in which the spec imen was oxidized. 4 Spalling may also have been a factor in the fo rmat ion of these thin a reas as evidenced by the observa t ion of c rack ing in the meta l lographic sect ion of this spec imen. Fig. 2(b) a lso shows that an accumula t ion of thor ia p a r - t i c les occu r r ed on the su r faces of the thin scale , and this is shown at a higher magnif ica t ion in Fig. 2(c) which was taken f rom the cen t ra l por t ion of Fig. 2(b). The thor ia pa r t i c l e s appear br igh t in the BSE images

Fig. 1--Surface of oxidized TDNiC showing a smooth area sus- pected to have resulted from vaporization, surrounded by an area of larger oxide crystallites.

This caused a sl ight de t e r io ra t ion of the BSE images which did not appear to be object ionable in the p r e se n t study. The e lec t ron flux to the BSE detector can be op- t imized by posi t ioning the detec tor as close as poss ib le to the spec imen to max imize the solid angle of the e lec - t ron flux that is sampled by the de tec tor . A d is tance of 0.00762 m between the i n t e rmed ia t e lens and the spec i - men was found to be op t imum for the r e su l t s desc r ibed in this paper .

RESULTS

A spec imen of Alloy 1 (TDNiC) which was oxidized for 2.592 • 105 s at 1473 K in 1.33 • 104 N / m 2 oxy- gen lost weight rapid ly dur ing the oxidation. Fig. 1 is typical of the sca le that was formed on this spec imen

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2424 VOLUME 4, OCTOBER 1973

(c) Fig. 2-Corresponding secondary and baekseattered electron images from the smooth area in Fig. 1. (a) Secondary Electron (SE) image. (b) Back-Scattered Electron (BSE) image. (c) BSE image of (b) at higher magnification.

METALLURGICAL TRANSACTIONS

because of the higher a tomic number of thor ium com- pa red to the other e l emen t s in the scale .

Figs. 3(a) and 3(b) a re co r r e spond ing SE and BSE images , r espec t ive ly , f rom the th icker a r ea of the scale . These f igures show what appea r s to be e i ther an al loy gra in boundary over which the oxide is much th inner than e l sewhere 6 or a c rack in the layer of l a rge oxide c r y s t a l l i t e s along which the thor ia pa r t i c l e s a re c l ea r ly v i s ib le .

Since examina t ion of the sur face of a scale provides only l imi ted evidence on the growth pa t t e rn of the scale , meta l lographic c r o s s - s e c t i o n s were examined. Figs. 4(a) and 4(b) a re BSE images f rom a meta l lographic sect ion of the spec imen shown in Figs . 1 through 3; Fig. 4(b) shows a segment of the scale nea r the center of Fig. 4(a) at higher magnif ica t ion. The d i s t r ibu t ion

of thor ia pa r t i c l e s in the scale is c l ea r ly shown in Fig. 4(b). The absence of thor ia pa r t i c l e s in the outer por - t ion of the scale and the p r e sence of poros i ty only in this por t ion of the sca le suggests that the inner por t ion of the scale may have fo rmed by inward diffusion of oxygen and that the outer por t ion of the scale may have fo rmed by outward diffusion of n ickel or ch romium. X- r ay dif f ract ion of s i m i l a r sca les has indicated that the outer l aye r s a re p redominan t ly NiO with NiCr204 in amounts which i nc r e a se d with oxidation exposure t ime, 7 while the inner por t ion of the scale is p redominan t ly Cr203. This appea r s to be supported by the change in con t r a s t of the sca le in Fig. 4(b). The scale appears to be darker toward the ox ide -meta l in te r face and b r igh te r toward the outer sur face . This would be cons i s ten t with both the i nc r ea s ing amount of meta l l i c ions in the ox-

(a) (a)

(b) Fig. 3--Corresponding SE and BSE images of the large oxide crys ta l l i tes in Fig. 1; the BSE image shows a concentrat ion of thoria par t ic les along ei ther an underlying grain boundary or a crack in the scale . (a) SE image. (b) BSE image.

(b) Fig. 4--BSE images f rom a metallographie section through the scale on the TDNiC specimen showing the thor ia par t ic les . (a) Low magnification. (b) High magnification f rom the central p o r - tion of (a).

METALLURGICAL TRANSACTIONS VOLUME 4, OCTOBER 1973 2425

ides in the o rde r Cr~Os: NiCr~O4:NiO (40, 42.9, and 50 at. pct, respec t ive ly) and the higher a tomic number of nickel as compared to ch romium.

Another demons t r a t i on of the use of BSE images in the examina t ion of a meta l lographic sect ion of an oxide sca le is shown in Figs. 5(a) through 5(c). These f ig- u res a re f rom a spec imen of Alloy 2 in the a s - c a s t condit ion that had been oxidized for 5.04 x l0 s s at 1373 K in 1.01 x 105 N/m2 oxygen. Fig. 5(a) shows the d i s t r ibu t ion of the phases in the meta l l i c ma t r ix with a deplet ion of the phases along the oxidized s u r - face. The ma t r ix is the coba l t - r i ch a phase, the dark dendr i t ic phase co r r e sponds to fl'-CoA1, and the b r igh t - appear ing phase assoc ia ted with the fS' is p r e s u m e d to be CosY. The br igh t CosY phase was not detected by optical meta l lography. Remnants of this phase can be seen in the depleted layer along the surface in Fig. 5(a); this is compat ible with the expected s lower diffu- s ion ra tes of y t t r ium in this al loy. Fig. 5(b) is a higher magnif icat ion view of oxide scale near the cen te r of Fig. 5(a), and Fig. 5(c) is f rom the center of Fig. 5(b) at a s t i l l higher magnif ica t ion. The con t ras t had to be at tenuated in Figs . 5(b) and 5(c) to show the detai ls of the scale . The oxide appears to have pene t ra ted p re f - e r en t i a l l y along the y t t r i u m - r i c h phase in seve ra l a r eas such as that shown in Fig. 5(c), and this indi- cated that the y t t r ium content of the al loy was too great . Three d i f ferent shades of con t r a s t can be seen in the scale in Fig. 5(c) which indica tes the p re sence of th ree di f ferent phases . Subsequent X - r a y diffract ion ana lyses conf i rmed that the sca le was composed of a-A12Os, CoAI~O 4 spinel , and CoO with t r a c e s of Cr2Os. 4 F rom the rmodynamic cons ide ra t ions , a-A1203 should be the major phase at the ma t r ix -ox ide in ter face with CoO formed in the in i t ia l s tages at the outer surface . The CoA1204 spinel was expected to form between the AltOs and the CoO as a r e su l t of a r eac t ion between these two oxides. The con t r a s t in Fig. 5(c) is in ag reemen t with this predic t ion . F r o m cons idera t ions of the re la t ive amounts of meta l l i c ions and their r e la t ive a tomic n u m b e r s as before , the da rkes t por t ion of the scale is at the ma t r ix -ox ide in ter face and would cor respond

to A1203, the b r igh tes t por t ion of the scale is at the outer sur face and would co r re spond to CoO and the layer between these is of in te rmedia te con t ras t and would co r re spond to the CoA1204 spinel .

DISC USSION

Optical metallography has long been established as one of the standard tools in investigations concerning oxidation mechanisms. More recently, the SEM has been utilized to study the surfaces of oxidized speci- mens, but the information obtained from most SEM examinations has been unduly limited because of the use of only the SE mode of imaging. The present paper demonstrates the important contribution of the BSE

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2426-VOLUME 4, OCTOBER 1973

Co) Fig. 5--(a) BSE image showing the metallographic structure of the Co-Cr-A1-Y alloy. (b) Same area as (a) at a slightly higher magnification; the contrast was attenuated to show the structure of the scale. (c) Area from the center of (b) showing preferential oxidation along remnants of the secondary phases.

METALLURGICAL TRANSACTIONS

mode of imaging, s ince BSE images a r e cons ide rab ly m o r e s ens i t i ve to c h e m i c a l compos i t ion than a re SE images .

The d i f f e r ences in con t r a s t of BSE images a r e p r i - m a r i l y r e l a t e d to d i f f e r ences in the weighted ave rage of the a tomic number s of the e l e m e n t s that a r e p r e s - ent, although the c r y s t a l l o g r a p h i c s t r u c t u r e and o r i e n - tat ion can affect the con t ras t . 1 Surface r e l i e f may also have s e r i ous e f fec t s on the con t r a s t of BSE images s ince b a c k s c a t t e r e d e l e c t r o n s a r e highly d i r ec t iona l . Although su r f ace r e l i e f is n o r m a l l y not a fac to r in the examina t ion of pol i shed me ta l l og raph i c spec im ens , it mus t be cons ide r ed in the examina t ion of oxide s c a l e s with highly i r r e g u l a r s u r f a c e s . Ambigui t ies in the in- t e r p r e t a t i o n of BSE con t r a s t r e l a t ed to su r f ace r e l i e f may usual ly be r e m o v e d by s t e r e o s c o p i c examina t ion and by c o r r e l a t i o n s with co r r e spond ing SE images .

The use of BSE images has enabled the c l a r i f i c a t i on of impor tan t m e c h a n i s m s of the oxidat ion p r o c e s s e s of the two a l loys d e s c r i b e d in this paper . In the oxidat ion of Alloy 1 at 1473 K, the tho r i a p a r t i c l e s probably ac ted as ine r t m a r k e r s , and the locat ion of the p a r t i c l e s sug- ges ted that the growth of the sca le o c c u r r e d by both cat ion and anion diffusion. Cr2Os, which f o r m e d the inner oxide l aye r in this al loy, is g e n e r a l l y cons ide red to grow by cat ion diffusion. The apparen t inward growth of this oxide which was o b s e r v e d he re may be a d i r ec t e f fec t of the solut ion of tho r ium in the a l loy or oxide phases . In additio~i, the su r f ace s tudies a l so conf i rmed the r e m o v a l of the oxide f r o m the su r f ace by v a p o r i z a - tion, and this appea r s to account for the weight loss of this a l loy when it is oxid ized at 1473 K.

In the study of Alloy 2, (the C o - C r - A l a l loy with an addit ion of y t t r ium) , the use of BSE images p e r m i t t e d the y t t r i u m - r i c h Cosy i n t e r m e t a l l i c to be de tec ted . Al- though the 13'-CoAl phase had been consumed to a depth of some 17 ~zm to f o r m the A1203-containing sca le , much of the Co3Y r e m a i n e d in this reg ion . The a r e a s where the main sca le had pene t ra t ed into the al loy were o b s e r v e d to co inc ide with the oxidat ion of the y t t r ide p a r t i c l e s in s i t u , and probably r e s u l t e d f r o m the m o r e rapid diffusion r a t e of oxygen in the Y203 than in Al203.

The f r equency of o c c u r r e n c e of such oxide incurs ions into the al loy may have led to a m a c r o s c o p i c keying of the sca le to the al loy. Yt t r ium addit ions below the so l - ubil i ty l imi t have a l so been found to im prove the adhe- s ion of AlzO 3 sca les :8 on t h e r m o d y n a m i c grounds, y t t r i - um in solut ion should be oxidized to d i s c r e t e p a r t i c l e s of Y203 ahead of the A1203-internal oxidat ion front , so that the o b s e r v e d i m p r o v e m e n t of sca l e adhesion might be a s soc i a t ed with the i r subsequent incorpora t ion into the main s ca l e . However , no such p a r t i c l e s were ob- s e r v e d in this al loy, even though this SEM-BSE tech- nique al lowed examina t ion of the m e t a l - o x i d e in te r face at a magni f ica t ion at l e a s t two t i m e s g r e a t e r than con- vent ional opt ical m i c r o s c o p y .

ACKNOWLEDGMENTS

This work was conducted under NASA R e s e a r c h Grant NGR 36-002-070. The authors wish to gra te fu l ly acknowledge this support . One of us (G. R. Wallwork) would l ike to thank Bat te l le- .Columbus L abo ra to r i e s for the award of a Bat te l le Inst i tute Fel lowship , during the t enure of which this work was c a r r i e d out. Mr. R. O. Dodds c a r r i e d out a l a rge pa r t of the e x p e r i m e n - tal work, and the X - r a y d i f f rac t ion ana lyses were p e r - f o r m e d by Mr. D. W. Johnson.

RE FE RE N CE S

1. C. W. Price and D. W. Johnson Scannhtg Electron Mwroscopy, p. 145, O. Johan, ed., liT Research Inst., Chicago, lthnols, 1971.

2. P. K. Kofstad and A. Z. Hed. J. Electrochem. Soc.. 1969, vol. 116, pp 224-34 3. G. R. Wallwork and A. Z. Hed J. Oxidation of Metals, 1971, vol 3, pp 229-41. 4. G. R. Wallwork and A. Z. Hed 3". Oxidation of Metals, 1971, vol 3, pp. 243-50 5. L. E. Samuels: Metallographic Polishing by Mechanical Means, Sir Isaac Pitman &

Sons Ltd., Melbourne and London, 1967 6. C. E. Lowell' NASA TMX-67867, June 8, 1971. 7. C. E. Lowell, D. L. Deadmore, S. J. Gnsaffe, and J. L. Drell NASA TND-6290.

April, 1971. 8. J. K. Tlen and F. S. Pettlt Met Trans, 1972, vol. 3, p. 1587.

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