Aluminide coatings on TiAl compound

Scripta

METALLURGICA

Vol. 23, pp. 685-689, 1989 Printed in the U.S.A.

ALUMINIDE Hiroshi

Mabuchi,

COATINGS Tatsuya

ON TiAI Asai

Pergamon Press plc All rights reserved

COMPOUND

and Yutaka

Nakayama

D e p a r t m e n t of M e t a l l u r g i c a l Engineering, College Engineering, U n i v e r s i t y of O s a k a P r e f e c t u r e Sakai 591, Japan

of

(Received S e p t e m b e r 22, 1988) (Revised February 13, 1989) Introduction

The compound TiAI, having an Llo-type structure, is very d e s i r a b l e in ad v a n c e d structural applications due to light-weight, superior elevated temperature strength and creep p r o p e r t i e s and improved oxidation resistance versus c o n v e n t i o n a l t i t a n i u m alloys. In air, however, it was shown that the TiAI alloy o x i d i z e s at a more rapid rate at t e m p e r a t u r e s above 1223 K (l), therefore the m a t e r i a l is not p r e s e n t l y used. C o a t i n g s of the TiAI c o m p o u n d are therefore e s s e n t i a l to high t e m p e r a t u r e o x i d a t i o n resistance. Among many coating processes to improve oxidation resistance, pack c e m e n t a t i o n is a r e l a t i v e l y simple p r o c e s s and remains a widely used technique for gas turbine components. Particularly, aluminide coatings are the best d o c u m e n t e d pack p r o c e s s e s used to p r o d u c e h i g h - t e m p e r a t u r e p r o t e c t i v e coatings for nickel-based superalloys(2). The purpose of this study is to apply a l u m i n i d e c o a t i n g s to the TiAI c o m p o u n d by pack c e m e n t a t i o n techniques, and to e s t a b l i s h the coating phase w h i c h forms. Experimental

Procedure

The a l l o y was p r e p a r e d by n o n - c o n s u m a b l e electrode arc-melting under an argon atmosphere, using sponge t i t a n i u m ( 9 9 . 8 % ) and high p u r i t y a l u m i n u m ( 9 9 . 9 9 % ) . The c o m p o s i t i o n of the a l l o y was T i - 3 8 . S m a s s % A l ( 5 2 . 6 a t % A l ) . The alloy button was then h o m o g e n i z e d at 1373K in v a c u u m for three days, r e s u l t i n g in a s i n g l e - p h a s e TiAI compound. C o u p o n s of a TiAI a l l o y of suitable size for various m e t a l l o g r a p h i c and analytical experiments were c o a t e d by a typical pack c e m e n t a t i o n procedure; co a t i n g s were formed by immersing the coupons in a m i x t u r e c o n s i s t i n g t y p i c a l l y of 2 5 % A I ( 2 0 0 mesh powder), 1.5%NH~Cl(activator) and 7 3 . 5 % A l t O s ( 1 5 0 mesh, inert filler), and then h e a t i n g in an a l u m i n a boat in a l a b o r a t o r y furnace fitted for ar g o n f l u s h i n g during the coating f o r m a t i o n cycle. In these processes, coating t h i c k n e s s is a f u n c t i o n of pack composition, time and t e m p e r a t u r e of the heating cycle. These variables were adjusted in the study to yield coatings to thickness suitable for the a n a l y t i c a l t e c h n i q u e s used to d e t e r m i n e structure and composition. Thus, the coating c o n d i t i o n selected for this study was a t e m p e r a t u r e of 973K and d u r a t i o n of 10h, using the pack c e m e n t a t i o n p r o c e d u r e d e s c r i b e d above. Subsequently, to study the c o m p o s i t i o n and s t r u c t u r e changes w h i c h occur during the lifetime of the coating-substrata alloy system, specimens were an n e a l e d for 4h at 1323 K in an inert a t m o s p h e r e ( a r g o n ) . S p e c i m e n s were sectioned, m o u n t e d in epoxy, p o l i s h e d down to 0.06 ~ m AltOs paste and etched with Kroll's reagent(HF+HNOs) for examination using conventional metallographic and e l e c t r o n microprobe analysis techniques. In 685 0 0 3 6 - 9 7 4 8 / 8 9 $3.00 + .00 C o p y r i g h t (c) 1989 P e r g a m o n Press

plc

686

C O A T I N GS

addition, standard X-ray diffraction i d e n t i f i c a t i o n in the coating. Results

ON TiAI

Vol.

techniques

were

employed

for

23, No.

S

structure

and D i s c u s s i o n

In pack c e m e n t a t i o n processes, the a c t i v a t o r reacts at the aluminum source to form aluminum-halides. The transported towards the substrate and react with the d i f f u s i o n r e a c t i o n o c c u r s with the s u b s e q u e n t f o r m a t i o n of These thermodynamics and kinetics of pack cementation d e s c r i b e d by several i n v e s t i g a t o r s ( 2 - 5 ) in the terms of a l u m i n i d e c o a t i n g s on n i c k e l - b a s e d superalloys.

high t e m p e r a t u r e with aluminum-halides are substrate alloy. A an a l u m i n i d e coating. processes have been the Ni-AI system for

Figure i shows the m i c r o s t r u c t u r e of a s p e c i m e n c o a t e d by a typical pack cementation. The c o m p o s i t i o n p r o f i l e of the same s p e c i m e n as m e a s u r e d by the e l e c t r o n m i c r o p r o b e t e c h n i q u e is shown in Fig.2. These o b s e r v a t i o n s have shown unambiguously that only one coating layer develops, and that the chemical c o m p o s i t i o n of the coating is indeed TiAI~ within the e x p e r i m e n t a l e r r o r of 1at%. Also, microprobe analysis indicates that the layer c o n s i s t s almost entirely of TiAls; no o t h e r intermetallic compounds show up and no solid s o l u t i o n of A1 in the s u b s t r a t e a l lo y arises. These p h e n o m e n a are similar to those in the study by v a n - L o o and Rieck(6); d i f f u s i o n - c o u p l e e x p e r i m e n t s in the Ti-AI system, in w h i c h a l u m i n u m is one of the starting m a t e r i a l s have shown that a l u m i n u m is the only d i f f u s i n g c o m p o n e n t and that only one phase TiAls develops.

FIG.I M i c r o s t r u c t u r e of the a l u m i n i d e coating on TiAI compound, p a c k e d for 10h at 973K.

80 T0

~60

Ti

i00

~so

90 80 <

N 40

v0 H 30

-~

60~

,,.20

so N

u 10

40 ~

H

3o ~ O 20 ~ O

l O r~

------T 140

I

T

I

i00

80

60

DISTANCE

FROM

120

SURFACE

- -

T - - i - 40

20

( ~m )

0

o

FIG.2 C o m p o s i t i o n profile by electron m i c r o p r o b e a n a l y s i s for the a l u m i n i d e coating on TiAI compound, p a c k e d for 10h at 973K.

Vol.

23,

No.

5

COATINGS

ON T i A I

687

FIG.3 TiAI 3

Microstructure and phase ident i t i e s of the a l u m i n i d e c o a t i n g on TiAI compound, p a c k e d for 10h at 9 7 3 K and d i f f u s i o n a n n e a l e d for 4h at 1323K.

TiA12

TiAI

8O - -

TiAI

~ I TiAI2 ~ - TiAI 3

70 60

Ti

r I I

f

f

% so

I I

z 40 o H ~

zo

~ o u

2o

~Lf

r i l

lO

i

I

DISTANCE

8'0

6'0

i

i00 - 90 ~

-80 < -70~ - 60 ~ -50 z -40 ~ -30 ~ -20~ O --i0 u 0

FIG.

4

Composition profile by electron microprobe analysis for the aluminide coating on TiAI c o m p o u n d , p a c k e d for 10h at 973K and diffusion-annealed f o r 4h a t 1323K.

FROM SURFACE ( ~m )

In a d d i t i o n , by X - r a y d i f f r a c t i o n a n a l y s i s of the s u r f a c e in the p l a n e of the coating, the c o a t i n g w a s i d e n t i f i e d as a T i A I 3 c o m p o u n d w i t h the s a m e l a t t i c e p a r a m e t e r s as t h o s e g i v e n in the l i t e r a t u r e (6) for the D 0 2 ~ - t y p e s t r u c t u r e . Coatings were apparently excellent in a d h e s i o n to the s u b s t r a t e and, for example, w e r e not p e e l e d o f f e v e n w h e n t h e y w e r e q u e n c h e d in w a t e r f r o m 1 2 2 3 K to r o o m t e m p e r a t u r e . T h e r e f o r e , if the T i A l s h a s e x c e l l e n t o x i d a t i o n r e s i s t a n c e , it should make an effective coating for the high-temperature structural material. Yamaguchi et a l . ( 7 ) s t a t e d t h a t the T i A I 3 , b e c a u s e of its g r e a t e r a l u m i n u m c o n t e n t , has a l o w e r d e n s i t y a n d p r o b a b l y b e t t e r o x i d a t i o n r e s i s t a n c e t h a n TiAI. Figures 3 a n d 4 s h o w the m i c r o s t r u c t u r e a n d the c o m p o s i t i o n p r o f i l e of a specimen packed and diffusion-annealed. Phase identities obtained from the results of the various analytical techniques are also presented in t h e s e figures. The results have shown that an a d d i t i o n a l layer develops on the original coating(TiAls)-substrate(TiAl) i n t e r f a c e . T h i s l a y e r w a s f o u n d to be of a stoichiometric composition as T i A I ~ f r o m the m i c r o p r o b e a n a l y s i s . T h e r e a f t e r , X-ray diffraction analyses were made f r o m the s u r f a c e of the w e l l - d e v e l o p e d diffusion l a y e r a f t e r c a r e f u l r e m o v a l of the a m o u n t of o u t e r s u r f a c e l a y e r by grinding and electropolishing. O u r d a t a f r o m an X - r a y d i f f r a c t i o n p a t t e r n are l i s t e d in T a b l e i, t o g e t h e r w i t h the c o r r e s p o n d i n g r e f l e c t i o n s and the sin a 8 values and relative intensities of T i A I ~ , calculated for the C u K ~ - r a d i a t i o n u s i n g the l a t t i c e p a r a m e t e r s . T h e d a t a in T a b l e 1 s h o w g o o d a g r e e m e n t b e t w e e n

688

COATINGS

TABLE

ON

TiAI

Vol.

1

Observed and calculated reflections and estimated for "TiAlm" ( tetragonal, HfGam-type, a=0.3971nm,

intensities c=2.432nm).

Reflection h k I

Observed sin 2 8

Calculated intensity

0 0 4 0 1 1 0 1 3 0 1 5 0 0 8 1 1 2 0 1 7 1 1 6 0:I 9 0 0 12 0'2 0 0 1 11 0 2 4 1110 1 2 1 1 2 3 0 1 13 1 2 5 0 2 8 1 2 7 0016 0 1 15 1 2 9 1 1 14 *0 2 12 *2 2 0 1 2 11 2 2 4 0117 0 3 1 0 3 3 1 2 13 0 3 5 2 2 8 1 3 2 0 3 7 "1 1 18 0119 0020 0216 "1 3 6 1215 0 3 9 *2 2 12 0311 1310 1 2 17

0.0161 0. 0384 O. 0 6 2 5 0.0641 O. 0 7 9 3 O. O867 0.1115 0.1191 0.1445 0.1505 0.1596

O. 1889 0.2075 0.2143 0.2373 0.2639 0.2723 0.2948 O. 3008 0.3100

0.3575

0.4004

0.4123 0.4219 0.4454

0.4800

Calculated sin ~ 8

Observed intensity

0.0161 0.0387 0.0467 0.0628 0.0643 0.0794 0.0869 0.1116 0.1191 0.1447 0.1508 0.1593 0 1668 0 1759 0 1895 0 1975 0 2075 0 2136 0 2151 0 2377 0 2572 0 2638 0 2698 0 2723 0 2950 0.3010 0.3100 0.3176 0.3281 0.3402 0.3483 0.3583 0.3643 0.3658 0.3809 0.3884 0.4003 0.4004 0.4019 0.4080 0.4124 0.4145 0.4206 0.4455 0.4608 0.4774 0.4788

v s i n d i c a t e s v e r y s t r o n g ; s, s t r o n g ; m, vw, v e r y w e a k . * For these reflections, CuK~-radiation

vw

0.1 7.4 1.5 0.6 2.7 2.9 4.1 100 3.4 14.1 33.5 0.2 >0.1 0.9 1.1 0.4 0.9 0.1 1.3 1.3 0.4 2.1 1.7 I.I 16.0 9.6 0.3 >0.1 0.6 0.1 0.1 0.5 >0.1 0.4 0.2 0.2 7.3 0.1 0.1 0.7 21.6 1.7 0.3 8.2 0.I 0.3 0.6

m

vw w vw m vs w m s vw

vw w vw vw w vw s m

w

vw

m

m

vw w

vw medium; was

w,

used.

weak;

and

25,

No.

5

Vol.

23, No.

5

COATINGS

ON TiAI

689

o b s e r v e d and c a l c u l a t e d intensities, and t h e r e f o r e the TiAI~ structure was the same s t r u c t u r e as g i v e n in the l i t e r a t u r e ( 8 ) for the phase "HfGa~-type" space group D~I4,/amd, containing 24 atoms/cell. Thus, s i g n i f i c a n t changes of the coating occur as a result of interdiffusion in the o r i g i n a l coating - a s u b s t r a t a a l l o y system; short time d i f f u s i o n - a n n e a l i n g c o n v e r t s the matrix of the c o a t i n g from a TiAls phase to a TiAlm phase. Since the m e c h a n i c a l and ch e m i c a l properties of the TiAI2 compound have not been confirmed, further i n v e s t i g a t i o n s on this c o m p o u n d are necessary. Conclusion For the typical c o a t i n g c o n d i t i o n s used (pack c e m e n t a t i o n with an a l u m i n u m activity), the c o a t i n g was formed by inward d i f f u s i o n of a l u m i n u m and a well a d h e r e n t layer of TiAI3 was formed; no other i n t e r m e t a l l i c c o m p o u n d s a p p e a r e d and no solid solution of A1 in TiAI was detected. Subsequent short time diffusion-annealing caused faster interdiffusion in the original coatings u b s t r a t e a l l o y system, r e s u l t i n g in the f o r m a t i o n of an a d d i t i o n a l d i f f u s i o n layer. This layer was identified as a TiAI2 compound by X - r a y d i f f r a c t i o n analyses. Consequently, the effectiveness of the coating may he seriously a f f e c t e d by the TiAI~ p h a s e w h i c h d e v e l o p e d during its lifetime. Acknowledgements This work was s u p p o r t e d by the Light Metals E d u c a t i o n a l Foundation. The a u t h o r s w o u l d like to thank Dr. K. Morii for his useful advice and discussion. In addition, the a u t h o r s are very g r a t e f u l to the e x p e r i m e n t a l a s s i s t a n c e of Mr. T. Shinya. References I.

N.S.Choudhury, H . C . G r a h a m and J.W.Hinze, in " P r o p e r t i e s of High T e m p e r a t u r e A l l o y s with E m p h a s i s on E n v i r o n m e n t a l Effects", E l e c t r o c h e m i c a l S o c i e t y Inc., p.688(1976). R.M~vrel, C.Duret and R.Pichoir, Mat. Sci. Technology, 2,201(1986). S.R.Levine and R.M.Caves, J. Electrochem. Soc., 121,1052(1974). D.C.Tu and L.L.Seigle, Thin Solid Films, 95(1),47(1982). B.Nciri and L.Vandenhulcke, J . L e s s Common M e t . , 95,55(1983). F.J.J. v a n Loo a n d G . D . R i e c k , Acta Met., 21,61(1973). M.Yamaguchi, Y,Umakoshi and T.Yamane, Phil. Mag. A, 5 5 , 3 0 1 ( 1 9 8 7 ) . M . P o t z s c h k e and K.Schubert, Z. Metallk., 53,548(1962).