Cyclic oxidation of aluminide coatings on Ti3Al+Nb

Scripta METALLURGICA et MATERIALIA

Vol. 24, pp. 1291-1296, 1990 Printed in the U.S.A.

Pergamon Press plc

CYCLIC OXIDATION OF ALUMINIDE COATINGS ON Ti3AI+Nb James L. SmlaIek, Michael A. Gedwill, and Pamela K. Brindley National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohlo 44135

(Received February 26, 1990) (Revised May 2, 1990) Introductlon There Is currently a strong Interest In tltanlum alumlnldes as potential high temperature aerospace alloys because of their hlgh strength-to-weight ratios. One such alumlnlde, TI3AI+Nb, o~ (ordered ~2, disordered B), Is especially attractive because of Its low denslty and reasonable duct111ty (2 to 5 percent) at room temperature (1). Thls alloy Is also a potential matrix for hlgh temperature fiber reinforced composites (2,3). However the oxldation resistance of this material is Inadequate due to formation of fast-growlng nonprotective TIO2 and Nb205 scale phases (4,5). Oxldatlon resistant coatlngs based on the TIAI3 phase have been successfully produced on TI (6-9), TI3AI (5), and TIAI a11oys (10) by conventlonal pack aIumlnlzing. Currently, this is the only pack coating phase on tltanlum alloys which forms protectlve A1203 scales devoid of any TIO2, at least up to 1000 °C (5). Thls phase Is clearly superlor to blnary TIAI (5,11) and quite protectlve as a coating on TI3A1+Nb In 1000 °C/40 h cyclic oxidation (5). However, the presence of transverse cracks In TIAI3 coatlngs (5,6,8,9) may have an adverse effect on long term cyclic behavior. Thls note describes our efforts to optlmlze a pack alumlnizing process for protecting TI3AI+Nb In cycllc oxldatlon. I t Is part of an overall coating development study for fiber reinforced TI3AI+Nb composltes. E~perlmental Procedure Plates of TI-14A1-21Nb (wt %) were prepared by the powder cloth technique. This entalls layup of prealloyed powder sheets which are vacuum hot pressed to f u l l density (2). Test coupons 1.3 mm by 1.3. mm by 1.3. mm were sectloned from these plates. Pack coatlng was performed in argon-flushed InconeI 600 retorts f111ed wlth a 1200 g mix of preblended -lO0 mesh alumlna, -270/+325 mesh alumlnum powder, and halide salt activators. Cyclic oxidation was performed at 9B2 °C for 200 l-hour cycles and characterized by XRD, SEM/EDS, and weight change. Results and Discussion A summary of the pack aluminizing parameters and coating results is given in Table l(a) for 800 °C packs and in Table I(b) for 1038 °C packs. Pack temperature dld not produce any obvious differences In pack behavior. The primary coating phase was TIAl3 at both 800 and 1038 °C. (Weak secondary phases of AIN+AI203 occurred for coating L because of an interruptlon In the Ar cover gas flow). Other pack parameters were however Quite Important in determining the level of aluminum pickup. Higher levels of aluminum source material and longer coating times both resulted in greater coating weights. NaF was also a more potent activator compared to NaCl. Thls Is due to the higher s t a b i l i t y and vapor pressures of Air 3 compared to AICI 3, as determined by Ref. 12. The 982 °C cyclic oxidation welght-change data Is shown In Fig. 1(a) for 800 °C packs and In Flg. 1(b) for I038 °C packs. A11 the coatings show a conslderable degree of protectlon. The 200 h welght galn ranged from about 0.5 to 2.5 mg/cm2. Overall these curves showed

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TABLE [. - SummaryoF Pack Aluminizing of Ti3AI Alloys (a) BOO "C packs Owidation (200 I-hr cycles at 982 "C)

Coating Coating

Source. wt % I% AI 41% AI 4)% AI 10~ AI 41% (AI-2SCr) 41% (AI-12Si)

Activator.

Hours

wt

Neight gain, mg/cm2

2% CrCl3 1% NaF I% NaF I% NaF I% NaF i% NaF

16 25 4 25 25 25

2.53 36.51 13.77 24.51 31.52 27.26

I I NaF I% NaCI I% NaCi I% NaCi I% NaF

25 25 25 B B

" 22,82 IB.51 II.32 B.7S 16.42

Phases

TIAI 3 TiAI 3 TiAI 3 TiAI 3 TiAI 3 TiAI~. (AIN, AT203)

Weight change, mglcm2

N e a r - s u r f a cphases e Scale

O.gO 2.62 .$7 1.84 2.52 1.92

AI203, TiO2 A1203, Tio2b A1203, TiO2b AI203, Tio2b AI203, Tio2b AI203. Tio2b

1.82 1.OO .63 .49 .76 a-4.16 2.01

AI203, TiO2b AI203, TiO2b A1203 A1203, TiO2b Ai203 TiO2, AI203 AI203, Tio2b

Coating TIAI3, TIAI3, TiAI3, TiAI3, TiAl3,

TiA] TiAI TiAI TiAI TiAI TiAI

(b) IO3B "C packs M N

P Q R

Bare TI3AI+Nb Bulk TIAI 3

10% AI IO% AI 2% AI 2% AI 2~ AI

TIAI 3 TiAI 3 TiAI 3 TIAI 3 TiAI 3

TiAI3, TiA1 TiAI3, HAl TIA] TiAI TiAI~, TIAI

Ti3Al , ~iAl, T~2Nb TiAI 3

aloo h data. bHeak.

positive slopes, an Indication of mlnlmal scale spallatlon. Vlsual observation and low magnification optical microscopy revealed only a minute amount of spa11Ing, usually In the f i r s t 20 h of testing. In comparison, the uncoated substrate lost 4 mg/cm~ after just 100 h and spa111ng was quite apparent (Fig. 1(a)). Bulk TIA] 3, on the other hand, showed a rapid weight galn In the f i r s t 100 h, followed by extremely slow scale growth kinetics (Fig. l ( b ) ) . The near-surface phases after oxidation are summarized In Table I. All the coatings formed scales composed primarily of =-AI203, with lesser amounts of TIO2. The coating phases remained primarily TIAI3 wlth some TIA]. One exceptlon was coating B which produced a stronger TIO2 pattern and contained no reslduaI oxidation resistant TIAI3 phase. Also, thls coatlng was the only one that visually exhibited any appreciable scale spaI1atlon throughout the test. The wlde variation in pack parameters maln]y resulted In variation of aIumlnum pickup. Thls in turn was found to have an effect on ox~datlon, as shown In Fig. 2. The relatlonshlp between oxidation welght and coating weight Is reasonably descrlbed by a slngIe line, despite the wlde variety of packs studied. This figure Implies that thicker coatings produce greater weight gains In oxidation. Also, an optimum In performance appears to be located between about B to 16 mg/cm2. Thls coating weight is equivalent to a coating thickness of about 40 to 70 ~m (I.5 to 3 mils), I f the coatings are assumed to be TIAI3 wlth a density of 3.355 gm/cm3. Figure 2 also Indicates that some of the pack-coated TI3AI systems were more oxidation resistant than bulk TIAI3. An addltlonal comparlson of oxldatlon resistance Is shown In Fig. 3. Thls plot relates the oxldatlon weight change of the last 100 h to the I n l t l a l coatlng weights. Behavior similar to that In Fig. 2 Is obtained, except that the bulk TIA]3 Kinetics now look comparable to that of the better coatlngs. The excessive I n l t l a l weight gain of bulk TIAI 3 (Fig. 1(b)) may be due to oxygen uptake by solutlon In the alloy or by Faster growing mixed TIO2+AI203 scales. The thinnest coating (B) possesses a 11mlted l i f e t i m e because I t degraded to the lessprotective TtAi and TtA12 phases. However I t ls not Immediately clear why the thicker coatlngs should result in larger weight gains (Fig. 2) and higher rates (Fig. 3). The surface morphology of these coatlngs provides a clue whlch may ultimately explain thls weight galn correlation. Optical microscopy has shown the coatings to be craze-cracked, starting from about five cycles. Thls was due to the brittleness of the TIAI3 phase and i t s larger thermal expansion, 15xlO-O/°C (13), than the TI3AI substrate, lOxIO-6/°C (3) at 700 °C. These tensile cracks eventually became decorated wlth yellow-orange oxide ridges which were clearly delineated from the grey scale background. Thus, addltlonal paths were formed for tltanlum outward migration and oxygen Inward dlffuslon whlch resulted in elevated weight gains.

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SEM/EDS studies helped to substantiate the above findings. The extremely thin coating B, (wlth the most TiO2), was basically covered wlth hlghIy faceted T l - r i c h crystals. The typical morphology of these crystals is shown in Fig. 4 and was very similar to that produced on bulk TIAI ( l l ) . The EDS spectra produced a relative Ti intensity greater than 0.95 for these areas. AI-rIch areas corresponding to AI203 were sparse. The pattern of coating cracks previously mentioned is shown in Fig. 5 for coating 0, which had the largest AI pickup and largest oxidation weight galn. Unlike coating D, the other coatings t y p l c a l l y contalned many cracks decorated by overgrowths of the large faceted TIO2 crystals. Frequently these overgrowths were In turn cracked, presumably due to subsequent cycling. An example of a cracked overgrowth found on coating H is shown in Fig. 6. EDS analysis of these crystals again produced relatlve TI intensities of 0.95 or greater, as found for coating B. The major f l a t surface areas were Al203 and produced relative AI Intensities of 0.95 or greater. No Nb x-ray intensity was observed for these coatings, unllke the substantlal Nb levels reported previously ( 5 ) . Coating P, one of the better thln coatlngs, contained a network of very fine cracks which are barely discernible In Fig. 7. This Is in contrast to the d i s t i n c t network found for the thick coatings, e.g., coating D In Fig. 5. Only a few small TiO2 crystals were found on coating P. Unfortunately, a simple correlation of oxidation weight gain with the number of cracks or TIO2 crystal overgrowths could not be found. I f the overgrowths accounted for the excess weight galn, then coating D, wlth minimal TIO2 crystals, should have low weight gain, not the hlghest. TIO2 overgrowths, therefore, do not t o t a l l y explain the increased ~xidatlon weight gain, although some contribution is expected. The maximum crack width of a particular coating appears to be more consistently correlated wlth excess weight gain. We speculate that the thicker coatings present a situation producing larger cracks which penetrate to the substrate. A cross-sectlon of coating D shows wlde thru-cracks f i l l e d with mixed oxides. Similar networks of yellow TiO2 overgrowth ridges had been reported previously, but were not discussed in relation to coating cracks (8). Here :he effect of overgrowths on excessive weight gain was more apparent. Summary A serles of pack alumlnlde coatlngs have been produced on TI3Al+Nb and characterized after 200 hr of cycllc oxidation at 982 °C. Manyof the features were conslstent with prior studies, e.g., large faceted TIO2 crystals on TIAI and on less-protectlve coatings, with AI203 formation, multiple coating cracks, and excellent cycllc oxidation reslstance up to IOOO °C for TiAI 3 coatings. The princlpal new findlng Is the decreased protection offered by thicker coatings, believed to be caused by the severity of the coatlng cracks. These presumably result in oxygen ingress and TIO2 overgrowths. An optimum coating thickness of 40 to 70 pm (8-16 mg/cm~) is indicated by this screening study. Conclusions I. Pack aIumlnlzlng of TI3AI+Nb can successfully produce an oxidatlon resistant TiAI 3 coating which forms primarlly ~-Al203 scales. 2. These coatings offer substantial Improvements over the uncoated matrix material in 982 °C cyclic oxidation tests. 3. Coating cracks contribute to degradation of thicker coatings. Acknowledgements The technical assistance of Donald Humphrey, Patrlcla Kraft and Ralph GarIick is g r a t e f u l l y acknowledged.

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References

I. H.A. Llps~tt, Mat. Res. Soc. Symp. Proc., 39, 351 (1984). 2. J.N. Pickens, R.D. Noebe, G.K. Watson, P.K. Brindley, and S.L. Draper, NASA TM-IO2060, 1989 3. P.K. Brlndley, P.A. Bartolotta, and S.J. KI1ma, NASA TM-I00956, (1988). 4. A.H. Kahvecl and G. Welsch, Third Int. Conf. Environ. Degrad. Eng. Mater., Penn. State Univ., 47, (1987). 5. J. Subrahmanyam, J. Mat. Scl., 23, 1906 (1988). 6. R. S t r e I f f and S. Polze, Int. Cong. Met. Corros., I I , 1093 (1981). 7. M. KabbaJ, A. Galerle, and M. CalIIet, J. Less Common Met., 108, 1 (1985). 8. J. Subrahmanyam, and J. Annapura, Oxid. Met., 26, 275 (1986). 9. I.G. Wright, R.A. Wood, and M.S. Seltzer, NASA CR-134681, 1974. 10. H. Mabuchl, T. Asal, and Y. Nakayama, Scripta Metal., 23, 685 (1989). 11. Y. UmakoshI, M. Yamaguchl, T. Sakagaml, and T. Yamane, J. Mat. Sci., 24, (1989). 12. S.R. Levine and R.M. Caves, J. E1ectrochem. Soc., 121, 1051 (1974). 13. D. Mlkkola, J. NIc, private communlcatlon, Jan. (1990).

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FIGURE 2. EFFECTOF INi$iAL coAr[~G ~:TG~T O~ ;~IDATIC~ WE[GHTAFTER200 i - ~ CYCLES.

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(b) I 0 ~ °C ALUMINIZ[NB, FIGURE 1. - ~2 °C (1800 °F) CYCLICOXIDATIONOF PACK ALUMINIZED Ti3AI÷Nb CO~ARED TO BARESUBSTRATEAND EJ,JLK TIA; 3 ALLOYS,

FIGURE 3. - EFFECTOF INITIAL COATING~[r~,T ON O~i~[ION ',~E]GHTOVER Tt'~ LAST 100 Hg.

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1296

OXIDATION

FIGURE 4. - LARGE FACETED TiO2 CRYSTALS FOf~PI~D~ COATING (COATING B, 2.53 mg/cm 2} AFTER 200

THINNEST

AT 982 °C,

FIGURE 6. - C ~ C K E O TiO2 OVER~OI~/THS AT PREEXISTING COATING

CRACKS (COATING H, 2~l.S1 mg/crn 2) AFTER 200 HR AT 982 OK.

OF COATINGS

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FIGURE S. - oLsrlNCT CRACK N E ~ K IN TNJCK.EST CO~,T~% (COATiNG D, JG.51 mg/crn 2) AFTER 200 ~R AT 982 OC.

FIGURE 7. - NETO4~I~K OF FINE CRACKS F ~ O

ON THIN C~TING

(COATING P. 11.]2 mg/cm 2) AFTER 200 HR AT ~ 2 ~C.