Formation of alumina on TiAl alloys

Scripta

METALLURGICA

Vol. 21, pp. 1 5 0 5 - 1 5 1 0 , 1987 P r i n t e d in the U.S.A.

P e r g a m o n J o u r n a l s , Ltd. All rights r e s e r v e d

FORMATION OF ALUMINA ON Ti-AI ALLOYS R.A. Perkins and K.T. Chiang Research and Development Division Lockheed Missiles & Space Company, Inc. Palo Alto CA 94304 and G.H, Meier Department of Materials Science and Engineering University of Pittsburgh Pittsburgh PA 15261 ( R e c e i v e d J u l y 20, 1987) ( R e v i s e d A u g u s t 31, 1987)

Introduction Titanium aluminlde (TIAI) is a candidate for advanced aircraft engines and light-welght, hightemperature structures. It has good resistance to oxidation in oxygen and shows alumina forming kinetics up to 950"C. In air, however, the alloy oxidizes at a more rapid rate and exlblts titania forming kinetics in both air and oxygen at temperatures above 950oc (I). Iron and nickel-base alloys with AI, on the other hand, readily form alumina vlth 50 at.% or less A1 at temperatures to 1100°C in both air and oxygen. It is postulated that the inability to form external alumina scales on TiA1 above 950°C results from the high oxygen solubility (NO (s) ), high oxygen diffusivity (DO ) and low aluminum diffusivity (DAI) in the alloy at high temperatures coupled with the rapid growth of transient Ti-rich oxides. Nagner (2) has developed a limitlng criterion for the transition from internal oxide precipitation to external scale formation which states a crltical solute atom fraction (N crit) in an alloy must be exceeded so that the outward flux of solute is rapid enough for continuous layer formation. Based on this criterion, the following relation can be used to approximate the critical alumlnum content: crit

NA1

{q* "

-

No(S )

DO Vm

-

~I12

i%xf

where No(S) is the oxygen solubility (atom fraction) in the alloy; D n and Dal are the diffusivities of O and A1 in the alloy, V m and Vox are the molar volumes~of th~alloy and oxide, and g* is the critical volume fraction of oxide formed by internal oxidation. According to the theory, when the aluminum content (NAI(O)) is < NAI crit, the A1 should oxidize internally and rapid scaling will ensue. If NAI (°) > NAI crit a thin AI2O 3 scale should form externally to separate the alloy from the oxidizing environment resulting in slow scaling rates. The ability of Ti-AI alloys to form an external alumina scale will be enhanced by: (1) Increase DAI in the alloy- Such an increase may be accomplished by retaining a crystal structure in which DAI is high. A maximum of 34 wt.% A1 is soluble in BCC ~-Ti at 1470"C. The diffusivity of A1 in ~-Ti is about two orders of magnitude higher than that in any other phase. The diffusivity of A1 in B--TI also increases vlth increasing A1 content (3). (2) Decrease N0(s) and D O in the alloy- The effect of alloying on N0(s) in BCC Ti has not been studied extensively but, by analogy with other BCC metals such as Nb and Ta, elements which increase the electron to atom ratio (e/a) should decrease the solubility of interstitials such as O and N (4). Elements such as Cr, Mo, N, Mn, Re, and Ru may serve this function. Based on this premise, Goldberg and Rapp (5) demonstrated the utility of Re in reducing the rate of internal oxidation of Nb-Zr alloys. The oxygen ingress may also be decreased by alloying with elements which can act as "traps" for oxygen (4). Those elements which interact more strongly with oxygen than does Ti (e.g. Zr and Hf) or those with smaller atomic radii (e.g. Cr, V, and Mn) are expected to produce this effect.

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(3) Decrease or eliminate transient oxidation- The presence of transient Ti-rich oxides will require higher A1 contents than those calculated from Eqn. 1 in order to form external alumina (6). Therefore, elements which decrease the oxidation rates of binary Ti alloys would be expected to decrease the transient oxidation of Al-containing alloys. Limited data are available for BCC alloys. Coddet and co-workers have found that Cr additions to Ti increase the oxidation rate at low concentrations but decrease it at high concentrations (7). Addition of V accelerated oxidation under all conditions studied (8). This paper describes the results of a study of A1203 formation on Ti-AI alloys which are predominantly @-phase with emphasis on the effects of alloying additions of Cr and V on the transition from internal to external oxidation of AI. The relative importance of changes in DAI, DO, and N0(s) in promoting external AI203 formation is discussed. Experimental Procedure The alloys were tungsten-arc melted under an argon atmosphere, to form 20 gram buttons. Each button was melted four times and drop-cast into a copper chill with a diameter of 1.5 cm and height of 1.5 cm. gelght loss during melting was less than 0.2Z. Compositions as reported are nominal wt.Z added. Oxidation specimens were cut from the ingots as 3 mm thick disks, polished through 600 grit SiC paper, and ultrasonically cleaned in acetone prior to exposure. Most oxidation experiments were performed in air in a horizontal globar furnace at 1400°C. Selected alloys were also tested in air and oxygen at 800 and II00°C. Tests in 1 atm. 02 at 800 and 1100°C were conducted with a continuously reading Cahn mlcrobalance. The mlcrobalance experiments were initiated by raising a heated furnace around the specimen chamber which allowed the specimen temperature to reach 800"C in less than 4 minutes and 1100°C in less than 8 minutes. The globar experiments were initiated by sliding the specimen into the heated furnace so that 1400°C was attained in 5 minutes. The oxidized specimens were examined by conventional optical metallography and scanning electron microscopy (SEM) with energy dispersive x-ray analysis (EDX) and x-ray diffraction (XRD). Results and Discussion I.

O x i d a t i o n B e h a v i o r o f Binary T i - A I A l l o y s

The first set of experiments was designed to examine the effects of increasing DAI by testing binary Ti-AI @-phase alloys at 1400°C. Binary Ti-AI alloys with from 8 to 30% A1 were exposed in air at 1400°C for I hour and examined for oxide scale and internal oxidation. Large weight gains were found for all alloys, as well as for a semi-commarcial TIA1 alloy (Ti-33Al-1V) which is just beyond the llmlt of A1 solubility in ~-Ti at 1400"C (31.5%). All alloys formed thick external Ti-oxide scales, which spalled on cooling, with internal oxidation of A1. Figure 1 shows the internal oxidation zone of Ti- 22130AI alloys. The internal Al-oxide precipitates formed to a depth of about 80 ~m. It is apparent from these results that retaining a BCC structure to increase DAI is not sufficient to reduce NA1 crit for external alumina formation on binary Ti-AI alloys to 0.43 (30 wt.%). This behavior is consistent with the model (Eqn.1) which predicts values of NAI crlt in excess of unity when solubility and diffusivity parameters for 0 and AI typical of @-Ti-Al alloys are used to solve the equation for NAI crit. It.

Oxidation Behavior of Ti-AI-Cr-V Alloys in Air at 1400°C

The study of binary beta Ti-AI alloys indicates that a simultaneous decrease in No(S) and D O in addition to an increase in DAI is required for external alumina formation. This hypothesis was tested by alloying vanadium and chromium into a Ti-3OAI base alloy. The selection of chromium and vanadium to affect No(S) and D o is based on three factors. Firstly, these elements increase the electron-to-atom ratio of the alloy and should decrease the oxygen solubility No(S). Secondly, both are substitutional solutes, with smaller atomic radii than Ti, that may act as oxygen traps to reduce oxygen dlffusivity. Thirdly, both elements are beta stabillzers that may act to retain the @-phase, as needed to increase A1 diffusivity, over a wide range of temperatures. A. T i - A I - V A l l o y s - A d d i t i o n s o f 5-30% V were made to a Ti-30Z A1 b a s e . Samples were e x p o s e d t o a i r f o r l h a t 1400°C. Three d i f f e r e n t t y p e s o f b e h a v i o r were o b s e r v e d . I n r e g i o n I (O-15~V), T i - o x i d e s c a l e s were formed and t h e r a t e o f o x i d a t i o n i n c r e a s e d w i t h i n c r e a s e d vanadium c o n t e n t as r e p o r t e d for o t h e r Ti-base a l l o y s (8). In r e g i o n I I (20-25% V), a c o n t i n u o u s a l u m i n a s c a l e formed u n d e r a t r a n s i e n t s c a l e c o n s i s t i n g o f o x i d e s o f Ti and A1 w i t h a c o r r e s p o n d i n g d e c r e a s e in the oxidation rate. The s p e c i m e n s t y p i c a l o f r e g i o n I e x h i b i t e d p r o f u s e i n t e r n a l o x i d a t i o n o f A1. The t r a n s i t i o n between r e g i o n s I and I I o c c u r r e d a t V c o n c e n t r a t i o n s between 15 and 20%. I n c r e a s i n g t h e V c o n t e n t to 30% r e s u l t e d i n t h e f o r m a t i o n o f c o n t i n u o u s , e x t e r n a l a l u m i n a w i t h v i r t u a l l y no t r a n s i e n t o x i d a t i o n ( r e g i o n I I I ) . The t r a n s i t i o n between r e g i o n s I I and I I I

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occurred between 25 and 30% V. The alumina scale formed on a Ti-30AI-30V alloy in air at 1400°C is shown in Fig. 2A. B. Ti-AI-Cr Alloys- Additions of 4-20% Cr were made to a Ti-30% A1 base. Samples were exposed to air for lh at 1400°C. The rate of oxidation decreased continuously with increasing Cr content. The same three regions of behavior were observed; however, the transitions from one region to the next required less Cr than V. The transition from region I to II occurred between 8 and 12% Cr while that between regions II and III occurred between 12 and 16% Cr. Alumina was formed on all alloys containing 16% or more Cr. The alumina scale formed on a Ti30AI-16Cr alloy is shown in Fig. 2B. The fact that approximately half as much Cr as V is required to produce external alumina is believed to result from Cr having a larger e/a and a smaller atomic radius than vanadium. As such, it should be more effective in reducing both the solubility and diffusivity of oxygen in titanium. C. Ti-AI-Cr-V Alloys- A series of alloys with chromium additions from 4-12 wt.%, and vanadium additions from 5-15'wt.% were prepared in a Ti-3OAI base alloy and tested in air for lh at 1400°C. An oxide map summarizing the 1 hour oxidation behavior of these as well as the ternary Ti-AI-Cr and Ti-AI-V alloys is shown in Fig. 3A. The lh weight gains and the nature of the oxide scales formed are shown on the map. The oxide map is divided into the same three regions as described for the ternary alloys. Combined additions of Cr and V produced the same transitions from internal to external oxidation of A1 as observed for the ternary alloys. The effects of Cr and V essentially are additive with V being half as effective as Cr as demonstrated in Figure 3A. A Ti-3OAI-8Cr alloy undergoes internal oxidation of A1 whereas Ti30AI-6Cr-IOV formed alumina with an external [ransient oxide similar to that formed on a Ti30AI-12Cr alloy. The experiments on single additions at 1400°C indicated about 1.8 times as much V as Cr is required to produce the transition. Therefore, if one considers a "Cr Equivalent" as %Cr + %V/1.8, the 10V-6Cr alloy has the equivalent of 12%Cr. The effect of A1 content from 21-30 wt.% (32-44 at.Z) on the oxidation behavior of the Ti-AI12Cr-15V alloys in air at 1400 O C was evaluated. All alloys formed thin adherent alumina scales with correspondingly low weight gains of 0.7-1.9 mgcm -2 in lh. The behavior varied between Type II and Type III with random areas of single layer alumina and two layer alumina + transient oxide on all alloys. None of the alloys were oxidized internally, indicating the NAlCrit for this system at 1400aC is <0.32B (<21 wt.%). Binary Ti-AI alloys with NAI(O)=0.34 0.43 (22-30 wt.%) were oxidized internally under the same conditions (Fig.l). Since all are beta at 1400°C, the formation of alumina by the addition of Cr and V must result from the effect of these elements on the solubility and/or diffusivity of oxygen. III- Effect of Oxidation Temperature The slowest oxidation kinetics at 1400°C were found for a Ti-30AI-12Cr-15V alloy which formed a thin, single layer external scale of =-alumina (Fig. 4). The parabolic rate constant (Table I) is comparable to that of NiAI (9) as extrapolated to 1400°C. However, this alloy does not form single layer alumina in air at 1100°C, and oxidizes more rapidly than NiAI at lower temperatures. The oxidation behavior of Ti-3OAI alloys modified with Cr and V was studied at 1100 and 800°C and the results are summarized in Figs. 3B and 5 and in Table 1. TABLE I Oxidation Kinetics for Ti-AI-Cr-V Alloys Alloy*

800°C 02__

Parabolic Rate Constant (kp-m~2cm-4min-1) ** 1100°C 1400°C 02__ 02__ Air

Ti-3OA1-8Cr-10V 2.0 Ti-30AI-12Cr-15V 3.2xi0 -2 2.5x10 -5 2.7xi0 -3 NiAI (ref. 9) 3.8xi0 -8.** 3.8xi0 -5 3.0xlO -3.** TiAI (ref. i) 3.3xi0 -I * All Compositions in wt.Z ** Calculated from the slope of ~m/A vs tl/2 following Pieraggi (ref. 10) *** Extrapolated from data in 1000-1200°C range A. Oxidation at 1100°C - Chromium and V are less effective in promoting alumina formation at 1100°C. As shown in the oxide map (Fig. 3B), higher Cr and V additions to a Ti-3OAI alloy base are required for the transition between regions I and II, and none of the alloys fall into region III. Figure 5 shows that the transition from internal to external oxidation is similar to that observed at 1400°C, but occurs at a higher Cr + V content. The ZCr + %V/I.8 Cr equivalent appears valid at IIO0°C for low V/Cr ratios, but does not hold for Ti-AI-V alloys

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w i t h o u t Cr b e c a u s e of r a p i d f o r m a t i o n of V - r i c h t r a n s i e n t o x i d e s . Ternary Tt-30AI-V alloys e x h i b i t o n l y r e g i o n I b e h a v i o r a t a l l V l e v e l s to 35 wt.% ( F i g . 3B). The Ti-3OAI-12Cr-15V a l l o y o x i d i z e d a t e s s e n t i a l l y t h e same r a t e as NiA1 i n 02 a t 1100"C b u t p r o d u c e d somewhat larger weight gains in air. The difference may be the result of small variations in composition from specimen to specimen or to the effect of N 2, since all the alloy constituents are nitride formers. If the Cr and V contents of the alloy are reduced slightly to 8 and 10%, respectively, the kp in oxygen is 5 orders of magnitude greater than that for NiAI (Table I). The reason for this difference is apparent from the specimen cross~sections (Fig. 5). The alloy containing 12Cr-15V has formed a continuous alumina film under small amounts of transient oxide while the 8Cr-10V alloy exhibits p~ofuse internal oxidation. A thick Ti-rich scale spalled from this specimen on cooling. B. Oxidation at 800°C - As temperature was reduced further to 800"C the oxidation rates exceeded those for alumina formation by orders of magnitude (Table 1). Alloys containing V oxidized faster than binary Ti~3OAI. The 12-hour weight gain in air for Ti-30AI-IOV was 8.5 mg/cm 2 as compared to 0.3 mg/cm z for Ti-3OAI. None of the alloys appear to have formed single layer alumina. Aluminum was not oxidize~ Internally and the difference in rate resulted from a difference in thickness of the external scales. Alloys with vanadium had very thick, two-phase scales. The scale formed on Ti-AI has not been analyzed to date. IV- Effect of Microstructure The difficulty in forming alumina on Ti-AI-Cr-V alloys as the temperature is reduced is governed in part by microstructure. Alloy structure was determined by optical metallography a n d XRD as a function of annealing temperature and rate of cooling. Alloys containing 30ZAI with or without Cr or V are single phase beta at 1400°C but transform on cooling below 1250°C. The structure at 1100aC is 2-phase beta + TiAI while at 800°C, a third phase, which may be Ti3Al,.precipitates from the beta phase. Additions of Cr and/or V increase both the amount and stability of the beta phase at temperatures below 1250°C. Since the diffusivity of A1 is highest in beta, the ability to form alumina is decreased as the amount of retained beta is decreased. Thus, as temperature is reduced, the concentration of Cr and/or V must be increased to retain beta as a major phase, in order to form alumina as an external scale. Summary External alumina scales can be formed on Ti-A1 alloys with 21-30 wt.g A1 in air at 1100-1400°C by increasing DAI through retention of a BCC (t3-Ti) structure as a major phase and simultaneously decreasing No(S) and D o in the beta phase through alloying additions of Cr and V. Chromium is more effective than V which has a "Cr equivalency" of ZV/1.8. An alloy of Ti30Al-12Cr-15V forms a single layer alumina scale in air at 1400"C with a parabolic rate constant the same as that for NiAI. The rate constant at 1100°C in oxygen also is the same as that of NiAI but in air at 1100°C the rate is somewhat greater. In either air or oxygen at 800°C, the rate constant is several orders of magnitude greater than that of NiAI as a result of increased transient oxidation. This is a result in part of reduced amounts of retained beta phase at lower temperatures. Increased amounts of Cr and/or V must be added to stabilize beta to lower temperatures in order to form protective alumina. Additions of V are detrimental in alloys which do not contain sufficient beta because of the accelerating effect of V on the formation of transient oxides. Acknowledgement This work was supported by the Lockheed Independent Research and Development program. References 1. N. S. Choudhury, H. C. Graham, J. N. Hinze, in "Properties of High Temperature Alloys with Emphasis on Environmental Effects," Electrochemical Society Inc., p. 668 (1976). 2. C. Wagner, Z. Elektrochem., 63, 772 (1959). 3. K. Hirano and Y. lijima, in T'~Diffusion in Solids: Recent Developments", AIHE, 1984, p.141. 4. R. A. Perkins, K. T. Chiang, and G. H. Meier, "Effect of Alloying, Rapid Solidification, and Surface Kinetics on the High Temperature Environmental Resistance of Niobium", AFOSR Contract F49620-86-C-0018, LMSC-F195926, Lockheed Missiles & Space Co., June, 1987. 5. R. A. Rapp and G. N. Goldberg, Trans. AIME, 236, 1619 (1966). 6. F. Gesmundo and F. Viani, Oxid. Metals, 25, 2-~ (1986). 7. A. M. Chase and C. Coddet, Oxid. Metals,-~l, 205 (1984). 8. K. Ramoul, C. Coddet, and G. Beranger, J.-'Less Comm. Metals, 98, 221 (1984). 9. F. S. Pettit, Trans. AIME, 239, 129 (1967). IO.B. Pieraggi, Oxid. Metals, ~ , 177 (1987)

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ALUMINA ON Ti-AI

A- 22% A1

20~m

1509

B- 30% A1

25Wm

FIG.I. Internal Oxidation of A1 in TI-AI Alloys in Air, lh-1400°C

A- 30% V

B- 16X Cr

13um

FIG.2. External Alumina Scale Formation on Ti-3OAI Alloys by Addition of V and Cr, Ih-1400°C in Air. I~OO'C, AIR. ! h (Ti-30 wt t k l } ~+* 5 10

."15

I. 0 - ~ICH OXIDE I I . Q . TI-RICH O X l ~ OVER ALUMINA I I I . • - ALUMINA SCALE

I 1 N ° C . AIR, | h |~-M m t kl)

5 I0

10

10

Is~

c~ 2 0 25

20 25

30

30

3:.5

35

A- lh-1400°C, Air

B- 3 h - l l O 0 ° C ,

Air

FIG.3. Oxide Maps for (Ti-3OAI)-Cr-V Alloys in Air at Ii00 and 1400°C.

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A- lh-1400"C, 1 Thermal Cycle B- 32h-1400°C, 5 Thermal Cycles FIG.4. Adherent Alumina Scales Formed on TI-30AI-12Cr-15V Alloys in Air at 1400"C.

13Um

A- 8Cr-IOV, 6h-llO0"C B- 12Cr-15V, 170h-llO0°C FIG.5. Effect of Cr+V on Alumina Formation in OxyEen at 1100"C.

13um