The oxidation behaviour of cobalt-base alloys containing dispersed oxides formed by internal oxidation

Corrosion Science, 1977, Vol. 17, pp. 879 to 891. Pergamon Press. Printed in Great Britain

THE OXIDATION BEHAVIOUR OF COBALT-BASE ALLOYS CONTAINING DISPERSED OXIDES FORMED BY INTERNAL OXIDATION* D. P. WHITTLE, M. E. EL-DAHSHAN and J. STRINGER Department of Metallurgy and Materials Science, University of Liverpool, Liverpool L69 3BX, U.K. Abstract--Co-Cr alloys containing a dispersed oxide phase have been produced by internally oxidizing alloys to which I wt 7ooof a reactive element--Hf, Ti or Zr--has been added. Internal oxidation was carried out in a sealed quartz capsule containing a 50/50 powder mixture of Cr-Cr2Oa or X-40-Cr, O~. The alloys produced in this way show all the beneficial characteristics demonstrated by similar alloys made by other techniques: (a) a continuous protective Cr20~ scale is established at a chromium level of 10~o, considerably below that required (approximately 257o) in the absence of a dispersoid; (b) a reduction in the growth rate of the Cr.~O3compared to particle-free alloys, particularly at high temperatures, and (c) greatly improved adhesion of the protective scale to the substrate. The beneficial effects appear to be independent of the composition of the dispersoid, and also its distribution. Oxidation of the Co-Cr-I HI', Zr or Ti alloys without pre-treatment produces scales characteristic of the chromium content of a corresponding binary alloy, indicating that some internal oxidation treatment is necessary and it is not sufficient to rely on the internal oxides formed during normal oxidation. INTRODUCTION

THE EFFECTS of a finely distributed stable oxide such as ThO2, AlcOa, CeO2, Y2Oa or other rare earth metal oxides, on the oxidation behaviour at temperatures above 800°C of alloys based on N i - C r , C o - C r or F e - C r alloys have been well documented. 1-~ Similar dispersions in pure chromium also modify the oxidation rate, 8,9 The following effects summarize the role of the dispersed oxide: (1) On alloys, a continuous, protective Cr203 scale is established at chromium levels considerably below those required in the absence of a dispersoid; (2) A reduction in the growth rate of the Cr2Oa compared to particle-free alloys, particularly at high temperatures; (3) I m p r o v e m e n t in the adhesion o f the protective scale to the substrate; and (4) An apparent alteration in the mode o f growth o f the scale: in dispersion-free alloys the scale-forming reaction is at the scale/atmosphere interface, but at the scale/ alloy interface with alloys containing a dispersion. The effects appear to be independent of the matrix and of the particular stable oxide used. A number of models have been proposed to explain the mechanism by which the dispersed oxides modify the oxidation behaviour of these alloys, the two most relevant being due to Giggins and Pettit ~ and Stringer e t al. 5 Giggins and Pettit attribute the reduction in oxidation rate to a blocking effect by the dispersoid in the inner layer of the scale. Initially the Cr~Os scale grows by normal cation diffusion o f c h r o m i u m outwards resulting in an accumulation o f the dispersoid at the alloy surface. *Manuscript received 17 September 1976; in revised form 16 November 1976. 879

880

D.P. WHITTLE,M. E. EL-DAHSHANand J. STRINGER

Eventually, the outward movement of chromium is so restricted that dissociation at the inner interface of the scale occurs, and the released oxygen can now pass around the inert oxide particles and form a new, inner layer of Cr2Oa incorporating the dispersoid. Eventually, the outer scale layer volatilizes off at high temperature and the scale continues to grow at the alloy/oxide interface, thus improving its adhesion to the substrate. Pegging of the oxide scale by the oxide dispersion is also considered to contribute to the improved scale adhesion. Elegant as this model is, it does not readily explain the formation of a Cr~Oa scale at lower chromium levels. Stringer e t a l . 5 suggest that the dispersoid particles act as nucleation sites for the initially-formed oxides, increasing the number of oxide nuclei and hence accelerating the approach to the steady state scaling condition, which with all the alloys studied is Cr2Oa formation. The reduction in oxide grain size is supposed to reduce the number of short circuit diffusion paths for chromium--dislocations, etc. and increase those for oxygen transport--oxide grain boundaries. Thus, the scale forming reaction is shifted to the alloy/scale interface, eliminating any problem with vacancy condensation. As indicated earlier, the improved oxidation resistance afforded by the presence of the stable oxide dispersion in the alloy seems to be independent of the alloy base and also of the composition of the dispersoid. There have not been sufficient systematic investigations to establish whether the size and distribution of the particles were critical. Many of the earlier alloys were prepared by traditional powder preparation methods, such as ball-milling, co-precipitation or colloidal mixing. Improvement in the mechanical properties was the main criterion and c a . 2 vol. ~ of the oxide phase comprising particles c a . 10 ~tm in dia. and 100 txm apart appeared suitable. Similar distributions were used later in developing oxidation resistant alloys, although chiefly •by a mechanical alloying technique. The optimum particle configuration for oxidation resistance has not been studied, partly stemming from the ditficulties and expense involved in producing alloys containing a dispersed oxide phase, particularly with a specific oxide distribution. The present investigation examines the feasibility of producing a suitable stable oxide dispersion within an alloy by controlled internal oxidation. Co-Cr alloys have been used as the base system because of the rapid oxidation rates of dispersion-free alloys containing < 25 ~ Cr, compared with Fe-Cr and Ni-Cr alloys.1° Control of the initial, internal oxidation conditions allows the dispersion morphology and distribution to be altered. Three reactive element additions were selected: Hf, Ti and Zr; they all form oxides more stable than Cr203 and so should be oxidized by a Cr/Cr20 a powder mixture. They would also probably be suitable for Al2Os-forming alloys. A 1 addition was used since this should give c a . 2 vol. ~ internal oxide. Eventually it is hoped to extend the method to more complex alloys where the pre-treatment conditions may well be more stringent. In order to calculate the approximate duration of the internal oxidation treatments required to produce a given depth of dispersed oxide, the kinetics of internal oxidation of binary Co-1 Hf, Co-1 Ti and Co-1 Zr were measured. The precipitated oxides serve as markers for the inward diffusion of oxygen, and at sufficiently small concentrations of the solute element, the effect of the oxide dispersion on the diffusion of oxygen is assumed to be negligible.

The oxidation behaviour of cobalt-base alloys

881

EXPERIMENTAL METHOD The nominal compositions of the alloys studied are shown in Table I. TABLE 1.

Cr Hf Zr Ti

10 1 ---

10 -1 --

NOMINAL COMPOSITION OF ALLOYS

I0 --1

15 I --

15 -I

- -

- -

15 --1

20 1 --

20 -1

- -

- -

20 --1

balance is cobalt in each case The alloys were prepared from high purity elements by induction melting and casting under a vacuum of approximately 10-~ T. The alloy ingots were cleaned by machining off the outer surface layers, and then homogenized by annealing in evacuated, sealed quartz tubes for 24 h at 1100°C. Coupons approximately 8 × 8 × 1 mm were cut from the centre of the homogenized blocks and polished to 600 grit. The internal oxidation pre-treatment was carried out by sealing the samples into one arm of a dumb-bell shaped quartz tube, the other arm containing a mixture of equal weights of chromium and Cr2Os powders. The coupons were not in contact with the powder. At one stage, a pack of powdered X-40 and Cr~O3 was used in place of the Cr/Cr~Oa mixture in order to eliminate any possible transfer of chromium to the alloys. However, chromium transfer did not seem to be a problem and no difference in the internal oxidation or subsequent oxidation behaviour was observed. Internal oxidation was performed at 1100, 1200 and 1300°C for times up to 350 h. The surfaces of the samples after internal oxidation were cleaned by a light abrasion on 600 grade SiC paper. Subsequent oxidation of the prepared samples was carried out in a horizontal tube furnace in flowing oxygen at 1 atm pressure, or in air, in the temperature range 1000-1200°C. Two commercial alloys--Haynes 188 (22~oCr, 2 2 ~ Ni, 1 4 ~ W, 1 . 5 ~ Fe, 0 . 8 0 ~ Mn, 0 . 4 0 ~ Si, 0.01 ~ C, 0 . 0 8 ~ La, bal. Co) and IN 586 (24 ~ Cr, 9.3 ~ Mo,0.04 ~Ce, bal. Ni)--were oxidized under the same conditions. For the kinetic experiments, similar sized coupons which had been given identical surface preparation and internal oxidation pre-treatment, were used. The samples, after intermediate cleaning, were oxidized in a pyrex helix thermobalance, the increase in weight being followed continuously. The oxidation runs were commenced by lowering the clean, dry samples into the hot furnace. Internal oxide penetration measurements were made on similar types of samples. After internal oxidation in the temperature range I000-1300°C, the coupons were metallographically sectioned normal to the flat surface of the sample and the depth of internal oxidation measured in an optical microscope. The average of a large number of determinations around each section was used in thefinal calculation. EXPERIMENTAL

RESULTS

AND DISCUSSION

Rate of internal oxidation Samples of cobalt containing 1 wt ~ Hf, Zr or Ti were internally oxidized for v a r i o u s t i m e s in t h e t e m p e r a t u r e r a n g e 1 0 0 0 - 1 3 0 0 ° C u s i n g a C r / C r ~ O a p o w d e r m i x t u r e as t h e o x y g e n s o u r c e as i n d i c a t e d i n t h e p r e v i o u s s e c t i o n . T h e i n t e r n a l o x i d a t i o n f r o n t w a s well d e l i n e a t e d , a n d its d e p t h b e l o w t h e s u r f a c e w a s m e a s u r e d i n a n u m b e r o f locations around the section; individual measurements did not vary by more than ca. 10 ~o. B e c a u s e t h e i n t e r n a l o x i d a t i o n p r o c e s s is d i f f u s i o n - c o n t r o l l e d , t h e t i m e s tx a n d t2, r e q u i r e d t o p r o d u c e a g i v e n d e p t h o f i n t e r n a l o x i d a t i o n a t t e m p e r a t u r e s 7"1 a n d T~ respectively are related through I n t2 _

tl

Q (T1--T2), R Tx T2

w h e r e Q is t h e a c t i v a t i o n e n e r g y f o r o x y g e n d i f f u s i o n i n t h e a l l o y , a n d R is t h e g a s constant. The mean activation energy measured for the Co-1 Hf, Co-1 Zr and Co-1 T i a l l o y s w a s 216 k J / m o l e , w h i c h c o m p a r e s well w i t h o t h e r m e a s u r e m e n t s in c o b a l t

D. P. WHITTLE,M. E. EL-DAHSHANand J. STRINGER

882

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FIG. 1. Depth of internal oxidation of cobalt-base alloys in Cr/Cr2Os packs as a function of time and temperature. using an internal oxidation technique with Co-0.08 and Co-0.55 Si alloys of around 231 k J/mole. n Figure 1 enables the depth of internal oxide for a particular pre-treatment (time and temperature) to be estimated. This is based on the measurements with the binary alloys in the temperature range 1000-1300°C; it has been extrapolated to lower temperatures using the equation given above. The additional presence of chromium in the ternary alloys may well alter the inward diffusion of oxygen, but binary alloys were used for the penetration measurements since these produced a better delineation of the internal oxidation front. Figure 1 is only intended to give an order of magnitude value.

Internal oxide morphology The microstructure of Co-10 Cr-1 Zr after internal oxidation for 86 h at 1300°C by Cr/Cr~Os mixture is shown in Fig. 2. The internal oxide particles are fairly large (of the order of several microns), particularly in the grain boundaries; however, smaller particles, only just visible at high magnification under the optical microscope, are randomly distributed throughout the alloy grains. There is some evidence to suggest that zirconium tends to segregate towards the alloy grain boundaries thus promoting

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FIG. 7.

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FIG. 8.

Cross-section of Co-10 Cr-I Ti internally oxidized by Cr/Cr203 for 260 h at 1200°C followed by oxidation at I I00°C for 330 h.

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The oxidation behaviour of cobalt-base alloys

887

the uneven distribution of the internal oxide; however, sufficient zirconium remains in the alloy grains to develop an oxide there. Hafnium additions behave similarly, producing again a relatively coarse grain boundary network of internal oxide, together with a fine distribution throughout the grains. The depth of penetration of the internal oxide front for a given treatment varies with alloy composition, decreasing with increasing alloy chromium content. According to Fig. 1 it should take approximately 50 h at 1300°C to completely internally oxidize the 1 mm thick samples, whereas in practice because of the higher chromium content in the alloy it takes slightly longer. The titanium-containing alloy shows somewhat different morphology after internal oxidation treatment: Figs. 3(a) and (b) show typical cross-sections after internal oxidation for 86 h at 1340°C and 200 h at 1200°C respectively. Again the individual oxide particles are of the order of several microns, but they are completely randomly distributed throughout the grains. The particle size decreases away from the alloy surface as is to be expected, although there is an apparent change in the subscale midway through the section. An oxide-free band is present at the surface of the alloy, and initially it was thought that the surface layers might be enriched in chromium which had been vapour deposited from the Cr/Cr~O3 pack. However, microprobe analysis indicates that the surface composition is identical to that of the alloy. With other alloys and different treatments there was no outer, dispersion-free layer as shown in Fig. 3(b) for a Co-10 Cr-I Ti alloy oxidized for 200 h at 1200°C. Oxidation

The isothermal kinetics of Co-15 Cr alloys containing the dispersed oxides formed by internal oxidation and dispersion-free alloys at 1000 and I I00°C are shown in Fig. 4. The most striking feature is the enormous effect the dispersed oxide phase produces in the Co-15 Cr alloy, reducing the overall oxidation rate by several orders of magnitude. Furthermore, the beneficial effect is long lasting, and there are no sudden increases in weight gain even at l l00°C in exposure times up to 300 h. Figure 5 compares Co-10 ~ Cr and Co-I 5 o / C r alloys containing the dispersed oxide oxidized at 1000c'C and there is little difference. Figure 6 shows a cross-section of Co-20 Cr-1 Hf oxidized for 44 h at 1200°C without any prior oxidation treatment. The surface scale is typical of that formed on a binary Co-Cr alloy which does not form Cr208: an outer layer of CoO and an inner layer consisting largely of the spinel CoCr204. There is a little internal oxidation of the hafnium, and some of the internal oxide precipitates have been incorporated into the inner scale layer. Similar scale morphologies are produced on the zirconium- and titanium-containing alloys which have not been pre-treated. The alloys containing 10 and 15~ Cr also behave in a similar manner although they oxidize at a correspondingly faster rate, befitting their lower chromium contents. There is a marginal decrease in oxidation rate with the ternary alloys in comparison with the corresponding binaries, but it is very slight. Figure 7 shows a cross-section of Co-10 Cr-1 Hf which has been internally oxidized oxidized by the Cr/Cr~O3 powder for 120 h at 1100°C before being oxidized for 100 h at 1000°C. In comparison to Fig. 6, the contrast is striking. The scale is thin, apparently single-layered, and is relatively adherent to the alloy; it is mainly Cr203. Clearly, the

888

D . P . WHITYLE, M. E. EL-DAHSHAN and J. STRINGER

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presence of the dispersed internal oxide has aided the establishment of a CrzOa scale at a substantially lower alloy chromium content than is possible without the internal oxidation pretreatment. Indeed, the general appearance of the scale compares very favourably with that shown by Wright xs for mechanically alloyed Co-13~o Cr-3 vol. ~o Y~O3 oxidized under similar conditions (compare his Fig. 29d). The CrsOa surface scale is not always established immediately, and in some areas of the sample cross-section, traces of CoO formed during the initial transient stage of oxidation are

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found outside the Cr2Oa scale. The other alloys after pre-treatment also develop Cr203 protective scales. In some cases Cr2Oa is formed as an internal precipitate in addition to the surface scale implying a depletion of chromium in the alloy near the surface. This is shown in Fig. 8 for a Co-10 Cr-I Ti alloy internally oxidized for 260 h at 1200°C and oxidized for 330 h at I I00°C. Even when internal oxidation has been carried out at lower

TABLE 2.

Alloy

O X I D A T I O N OF COBALT-BASE ALLOYS

Pre-treatment

Co-10 Co-10 Co-10 Co-10

Cr Cr-I Ti Cr-I Zr Cr-I Hf

Co-10 Co-10 Co-10 Co--I 5 Co--I 5 Co-15 Co-15

Cr-I Cr-I Cr-I Cr Cr-I Cr-1 Cr-1

None

Ti Zr Hf

Internally oxidized at 1200°C for 260 h

Ti Zr Hf

None

Time (h)

Temp (°C) Weight gain (mg/cm 2)

48

1000

100

1000

48

I000

Co-15 Cr-I Ti Co-15 Cr-1 Zr Co-15 Cr-1 Hf

Internally oxidized at 1200~C for 200 h

330

1100

Haynes 188 IN 568

None

330

1100

16 15 12 11 1.8 1.8 1.4 7.6 6.8 6.8 6.4 1.2 2.0 1.6 1.0 1.1

890

D . P . WHI'I-rLE, M. E. EL=DAItSHAN and J. STRINGER

temperatures where the oxide precipitates are much coarser, there is still no difficulty in establishing a CraG3 protective scale. Two commercial alloys, Haynes 188 and IN 568, were oxidized under similar conditions for comparison. Table 2 compares the weight gains. Cr~.Oais also formed on the commercial alloys together with some internal oxide which might well play a significant role in the oxidation resistance of these alloys. In order to determine whether the dispersed internal oxide had any influence on the growth rate of CoO, the Co-I Hf, Zr or Ti alloys were also given an internal oxidation pretreatment and then oxidized. Figures 9(a)-(c) compare the behaviour after 5 h oxidation at 1200°C of Co-1 Hf without any pretreatment, after a prior internal oxidation by Cr/Cr203 for 146 h at 1100°C, and pure cobalt respectively. The scale in all cases is CoO, although the hafnium oxide precipitates collect in the inncr layer of the two binary alloys, this being a little more pronounced in the sample which had been pre-internally oxidized. Clearly, however, the scale thicknesses are approximately the same in each case, and the internal oxide particles exhibit no blocking effect. CONCLUSIONS

One of the original aims of the present work was to establish the feasibility of producing stable oxide dispersions by controlled internal oxidation, which were capable of improving the oxidation resistance of Cr=O3-forming Co-Cr alloys. Using a Cr/Cr~O3 powder mixture, which produces an oxygen activity below that required to form Cr203 on the alloys, appears to be eminently suitable and the dispersed oxide containing alloys produced show all the beneficial characteristics demonstrated by similar alloys made by other techniques. The composition of the dispersed oxide does not seem important, nor does its distribution and size. However, it is thought that the relatively coarse oxide particles in the alloy grain boundaries play little role, and it is the finely distributed particles in the matrix which are important. Reducing the active metal content of the original alloys may well eliminate segregation into the grain boundaries and thus the coarse oxide. The size of the internal oxide particles decreased away from the alloy surface as might be expected. However, certain pre-oxidation times appeared to produce 'dispersion-free' zones near to the alloy surface, and to a lesser extent midway through the internal oxide zone. Neither seemed to affect the subsequent oxidation behaviour. The lower the chromium content of the alloy the greater was the depth and fineness of the internal oxide particles for a given pre-oxidation time and temperature. Alloys with as little as 10~ Cr have been induced to form Cr20~ by the presence of the dispersed oxide, in comparison with over 25 ~o Cr being required in the absence of the inert oxide. However, it is anticipated that this represents the lower limit, and the behaviour of alloys with less than 10~ Cr would be expected to revert to that of a low chromium content, binary alloy. Relatively long pre-treatment times have been used in the present study producing complete penetration of the internal oxide through the l mm thick samples. However, the rates of oxide penetration measured suggest that the duration of the pre-treatments could probably be considerably shortened. Furthermore, it may not be necessary to convert all the active element to oxide during the pre-treatment: a zone near to the

The oxidation behaviour of cobalt-base alloys

891

alh~y surface may be sufficient and further work is necessary to determine the o p t i m u m thickness of this band. As the results indicate, none of the additions had any significant effect unless the alloys were given the internal oxidation t r e a t m e n t : it is not sufficient to rely on internal oxides formed during subsequent oxidation. A l t h o u g h the surface scale/alloy interface generally develops an irregular m o r p h ology a n d the scale is retained on the alloy surface on cooling from the reaction temperature, both observations pointing towards good scale/metal adhesion, the behaviour under thermal cycling conditions requires detailed examination. Acknowledgement--The authors gratefully acknowledge the Ministry of Defence for support of this

work.

I. 2. 3. 4. 5.

6. 7. 8.

9. 10. I I. 12.

REFERENCES G. R. WALLWORKand A, Z. HED, Oxid. Metals3, 229 (1971). C. S. GIGGINSand F. S. PETTIT,Met. Trans. 3, 1071 (1971). H. H. DAVIS, H. C. GRAHAMand I. A. KVERNES,O.vid. Metals 3, 431 (1971). M. S. SELTZER,B. A. WILCOXand R. |. JAFFEE,Met. Trans. 3, 2390 (1972). J. STRINGER, B. A. W|LCOX and R. I. JAEFEE, Oxid. Metals 5, I l (1972). J. S'rRINGERand I. G. WRIGHT,ibid., 59. I. G. WRIGHTand B. A. WILCOX,Met. Trans. 5, 953 (1974). J. STRINGER, A. Z. HED, G. R. WALLWORK and B. A. WILCOX,Corros, Sci. 12, 625 (1972). |. G. WRIGHTand J. STRINGER,Melallogr. 6, 65 (|973). G. C. WooD, I. G. WRIGHT,T. HODGKIESSand D. P. WHITTLE, Werkstoffe und Korros. 21,900 (1970). P. J. GRUNDYand P. J. NOLAN,J. Mat. Sci. 7, 1086 (1972). 1. G. WRIGHT, B. A. WILCOXand R. I. JAFFEE,Final Rept. on Naval Air Systems Command Contract No. WOO 19-72-C-0190, January 0973).