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Effects of CoCrAlY coating on microstructural stability and creep behavior of a nickel-base superalloy
Thin Solid Films, 168 (1989) 207 220 METALLURGICALAND PROTECTIVECOATINGS
207
E F F E C T S OF CoCrAIY C O A T I N G ON M I C R O S T R U C T U R A L STABILITY A N D C R E E P B E H A V I O R OF A N I C K E L - B A S E SUPERALLOY T. K. CHAKI*, A. K. SINGHt AND K. SADANANDA Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 203 75 (U.S.A.)
(Received January 26, 1988; revised May 3, 1988: accepted July 25, 1988)
High temperature creep and microstructural stability of a cast nickel base superalloy Ren6 80, coated with electron beam physically vapor deposited CoCrA1Y alloy and subsequently heat treated, were studied. Creep tests were performed on coated and uncoated Ren6 80 specimens at 1000 °C in air. The analysis of the creep data shows that the creep rupture life of the coated specimens is close to the extreme case in which the coating does not bear any load. As a result of the heat treatment a diffusion zone rich in (Ni,Co)A1 precipitates was formed at the boundary between the coating and the superalloy substrate. Pores of size about 0.5 ~tm were found at the interface between the coating and the diffusion zone. These pores are formed during the deposition of the coating. Yttrium-rich precipitates, about 0.1 ~tm in size, were observed at the interface between the coating and the diffusion zone. Creep cavities of size about 10 lam were formed in the gauge section of the creep-ruptured specimens and they were more prevalent in the diffusion zone. The cracks were initiated in the diffusion zone and propagated into the substrate along grain boundaries.
1. INTRODUCTION Nickel-base superalloy Ren6 80 (registered trademark of General Electric Company) is used as a high temperature blade material ~, especially in marine turbine engines. Nickel-base superalloys are well known for excellent strength 2'3 and creep ~'2'4 6 properties at high temperature. However, corrosion at high temperature in marine environments reduces their lives significantly v. Hot corrosion attack may occur at temperatures in the range 600 1000 °C when operating in combustion or marine environments. The main reactants are O2, SOz, SO3, CO, N 2, H 2 0 and NazSO4. Deposits of metallic sulfates 8'9 (such as Na2SO4 and COSO4) are found on the blades after service exposure and their effect 1° is known to be * Present address: Department of Mechanical Engineering, State University of Buffalo, NY 14260, U.S.A. t Present address: AT & T Bell Laboratories, Holmdel, NJ 07733, U.S.A. 0040-6090/89/$3.50
(c) Elsevier Sequoia/Printed in The Netherlands
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T. K. ('HAKI, A. K. S1NGH, K. S A D A N A N D A
detrimental to the life of the blades. Different types of protective coatings ~ ~s are applied to the blades to reduce environmental degradation. At high operating temperatures, an interdiffusion takes place between the coating and the superalloy substrate, producing microstructural changes at the interface. Degradation or improvement in mehcanical properties due to coating has been reported ta. Smith ~s examined the effect of low pressure plasma-sprayed Co 29Cr 6AI Y coating on IN 738 superalloy from room temperature to 870 C and found no significant effect of the coating and the coating process on the tensile. creep and fatigue properties. Castillo and Willett 1~' studied the influence of chrome aluminide coating on the creep and stress rupture properties of wrought Udimet 520 superalloy at 802 C . They showed that application ofchromium--aluminide coating caused a marked deterioration in rupture strength and ductility. Kang et al. ~ observed that application of low activity aluminide pack cementation treatment on directionally solidified Rend 80 resulted in a dramatic reduction in rupture lives (83'!,; at 760 'C, 62'~,. at 871 C and 36'~, at 982 'C). The degradation in life was less severe for conventionally cast Rend 80, but no degradation in rupture ductility was observed. Kolkman Is studied the effect of Codep B coating (aluminum applied to the surface by low activity pack cementation) on creep and fatigue properties of Rend 80 at 900 'C. The coating reduced the high cycle fatigue life by 62,'i. and creep rupture life by 1 l'!i,. Paskiet et al. I'~ performed fatigue tests on both uncoated and aluminide-coated wrought Udimet 700 over a range of temperatures from ambient to 927 'C. The aluminide coating enhanced the high cycle fatigue limit up to 480 C and above this temperature a slight reduction was observed. Schneider et al. 2° reported a reduction in the high cycle fatigue life in IN 738LC and IN 939 nickel-base alloys at 850 C due to a platinum aluminide coating. Strang and Lang 2~ have summarized data concerning the influence of coatings on mechanical properties of superalloys. They concluded that no loss in creep rupture properties should be seen. provided that a suitable heat treatment is carried out after coating and that the coatings do not have a deleterious effect on low cycle fatigue life above the ductile brittle transition temperature of the coating. The objective of the present work is to study the effect of heat-treated CoCrAIY coating deposited by electron beam physical vapor deposition (EBPVD) on high temperature creep properties of cast Rend 80 superalloy and to characterize the microstructural development at the interface between the coating and the substrate. 2. EXPERIMENTAI. DETAILS
2.1.
Materials
Cylindrical pieces of cast blanks of Rend 80 were obtained from a vendor and from these blanks specimens of gauge length 1.5 x 10 2m and gauge diameter 5.1 x 10 s m were machined. The specimens were given a heat treatment similar to that given to the blades in service. First they were solutionized at 1205 C for 2 h inside a cold wall furnace in a vacuum of 6.6 x 10 "* Pa. They were rapidly cooled in vacuum (in less than 10 rain) to 600 C and then slowly cooled to room temperature. They were subsequently aged at 1095 C for 4h and vacuum cooled to room temperature. Some of the specimens were taken out for coating and the rest were
EFFECTS OF CoCrAIY ON Ni-BASE SUPERALLOY
209
annealed again in vacuum at 1050°C for 2 h. The specimens were then cooled rapidly (in less than 10 rain) to 650 °C and held at 650 °C for 10 rain. The specimens were reheated to 850 °C and were aged at that temperature for 16 h. A CoCrAIY coating was applied by vendor using an EBPVD technique. The surface of the specimens was prepared for coating by degreasing, grit blasting and vapor honing. The substrate temperature was 982 °C and the pressure was about 6.6 x 10- 2 Pa throughout the coating process. The specimens were rotated by a gear arrangement for uniform coverage. The thickness of the coating was about 80 pm and the deposition rate was about 6 pm min 1 The composition of the coating and the substrate superalloy is given in Table I. The coated specimens were vacuum cooled for 5 rain and then quenched in argon gas for 5 min. The specimens were peened by glass beads to an Almen intensity of 15 N. The specimens were heat treated at 1050 °C for 2 h in vacuum. A diffusion bonding between the coating and the substrate was produced by the heat treatment. The specimens were cooled in vacuum to approximately 550 °C and then argon quenched. Finally, the coated specimens were aged in vacuum at 850°C for 16h. The coated and uncoated specimens underwent the same heat treatment. TABLE I THE N O M I N A L COMPOSITION OF THE COATING A N D THE SUPERALLOY SUBSTRATE
Composition (wt.%) Ni Coating Substrate
60.5
Co
Cr
66 9.5
Al
Ti
23
10.5
--
14
3
5
W
Mo
Y
--
0.25
4
4
--
2.2. Creep test andmicrostructure Constant load creep tests were carried out in air for both the coated and the uncoated specimens using a creep frame attached to a furnace. The temperature was maintained at 1000 °C. Five tests were performed for both the coated and uncoated specimens. The elongation was measured by a linear variable-differential transformer. The creep data were analyzed by considering different load-bearing capacities of the coating and the substrate. The initial microstructures of the coated and the uncoated specimens after heat treatment were examined using scanning electron microscopy (SEM). The specimens for SEM were prepared by polishing and etching chemically. A few specimens were observed without etching to make sure that etching did not produce any artifact. The images were observed by backscattered electrons. For comparison purposes, some coated specimens were annealed at 1000°C for 100 h. The creepruptured specimens were cut perpendicular and parallel to the load axis and examined by SEM. Energy-dispersive X-ray (EDX) spectroscopic analysis was performed to investigate the diffusion characteristics of the different elements within the coating-substrate system. This slices were cut perpendicular to the axis of the cylindrical specimens for transmission electron microscopy (TEM). The slices were ground on emery paper and small discs containing the coated area were punched
210
T . K. C H A K I , A. K. S I N G H , K. S A I ) A N A N I ) A
out. The discs were then jet-polished and ion milled to produce suitable specimens with a thin area at the interface between the coating and the substrate. 3. RESULTS
3. I. Creep Figure l shows typical strain t,s. time curves for coated and uncoated specimens creep tested at 1000 ~C. The loads for the coated and uncoated specimens are
~COATED
z I
< t,r-
w er 0
;OATED
0~ 0
25
50 TIME, HOURS
75
100
Fig. 1. Strain rs. time curves for c o a t e d and u n c o a t e d specimens, creep tested at 1000 C and at 138 M Pa. T A B L E 1I CREEP TEST RESULTSFOR COATEDAND UNCOATEDRENI~80 AT ] 0 0 0 ' C AND 138 M P a
Specimen number
1 2 3 4 5
Uncoated specimen
Coated specimen
Rupture li[k,
Creep rate
(h)
(xl0
95 103 91 96 101
5.6 4.9 5.5 4.9 4.6
Ss
i
Rupture l[['e
Creep rate
(h)
(xlO
73 68 67 71 75
7.6 8.4 7.5 8.0 8.5
Ss-l)
EFFECTS OF C o C r A I Y o n Ni-BASE SUPERALLOY
211
3.02 kN and 2.82 kN respectively. It is clear that the coated specimen elongated faster and broke earlier. The rupture lives and minimum creep strains for different specimens are given in Table II. The average rupture lives were 71 + 3 h and 97 + 5 h for coated and uncoated specimens respectively. The average minumum creep strain rates for the coated and uncoated specimens were (8.0_+0.5)x 10 8 s-1 and (5.1 + 0.4) x 10 8 s 1 respectively. The rupture strain (calculated 1 h before rupture) was about the same (about 6.5~o) in both cases. Two extreme cases are considered. First, it is assumed that the coating has the same load-bearing capacity as the substrate, implying that both the coated and the uncoated specimens are subjected to the same stress of 138.0 MPa. However, the experimental data of this work show that the coated specimens fail earlier. In the second case, it is assumed that at high temperatures the coating is too weak to bear any load. In this case, the stress on the coated specimen is calculated by dividing the load with the cross-sectional area of the substrate alone and is found to be 147.8 MPa which is higher than the stress in the first case. If we assume a power law creep, the rupture life t is given by a power of the stress o: t
= Aa-"
(1)
where A is a constant, and the exponent n for Rend 80 is 5.6, obtained by fitting the data in ref. 2. The result of this work has shown that at a stress level of 138.0 MPa the uncoated Ren6 80 has a creep rupture life of 97 h. With the help ofeq. (1), the rupture life of Ren6 80 at a stress of 147.8 MPa is given by 97 × (138.0/147.8) s'6 or 66 h. If the coating does not have any load-bearing capacity, the creep rupture life of the coated specimens should be 66 h which is close to the experimental value of 71 4- 3 h. The actual rupture life, therefore, is close to the extreme case in which the coating can be considered to bear zero load. 3.2. M i c r o s t r u c t u r e s
Figure 2 is an SEM backscattered image of the initial microstructure of the coated specimen before the creep test. The specimen has undergone shot peening and heat treatment after coating. The micrograph corresponded to a section cut perpendicular to the axis of the specimen. It consisted of three regions: the coating, a diffusion zone and the substrate. The coating (region a in Fig. 2) contained almost equal amounts of s-Co solid solution (f.c.c.) and [3-CoAl phase (ordered b.c.c.) 11. Other elements such as chromium, aluminium and yttrium were dissolved in these two phases. Cobalt has a low temperature ~ phase (h.c.p.) z3, but its formation requires prolonged aging 24. It was not a prominent phase in the present coating. The and 13phases in the coating were elongated in the direction of the deposition (i.e. along the radial direction of the cylindrical specimens). The diffusion zone (region b in Fig. 2) contained [3-(Ni,Co)AI precipitates in a-Co,Ni solid solution. The precipates were faceted and elongated in directions different from those in the coating. A diffusion zone about 5 lam thick grew into the superalloy substrate during the heat treatment after the deposition of the coating. The substrate (region c in Fig. 2) consisted of 7"-Ni3AI precipitates 25 (about 0.5 lam in size) in nickel solid solution. The 7' precipitates near the interface between the diffusion zone and the substrate were larger than those in the interior of the substrate. Figure 3 shows pores
212
T. K. C H A K I , A. K+ SINGH+ K. S A D A N A N D A
(a)
(b)
(c) Fig. 2. Scanning electron micrograph of a coated specimen before creep test. Region a shows the coating containing CoAl precipitates (black contrast). The diffusion zone (region b) contains faceted (Ni,Co)AI precipitates. Region c shows the substrate containing 7' precipitates.
Fig. 3. Scanning electron micrograph of a coated specimen, showing pores at the interface between the coating and the diffusion zone. One of the pores is shown by an arrow. o f size a b o u t 0.5 ~tm a t t h e i n t e r f a c e b e t w e e n t h e c o a t i n g a n d t h e d i f f u s i o n z o n e . T h e pores were seen in specimens prepared without any chemical etching. The pores are n o t a r t i f a c t s d u e t o p r e f e r e n t i a l e t c h i n g . A l a r g e n u m b e r o f p o r e s w e r e f o r m e d in t h e
Fig. 4. (a) Transmission electron micrograph of the middle part of the coating. Grain 1 corresponds to 13-CoAl and grain 2 to or-Co solid solution. (b) Transmission electron micrograph at the interface between the coating and the diffusion zone.
{a)
(b)
t..,o
I,o
© ,.<
d~
z.
.< ©
O
©
,.r]
214
"1-. K. ('HAKI, A. K. SINGH, K. SADANANDA
coating during the deposition. Subsequent shot peening and heat treatment closed only some of the pores. Figure 4(a) is a TEM micrograph of the middle part of the coating. The grain sizes of [3-CoAl and s-Co solid solution were the same (about 0.5 ~tm), but the grains were elongated along the deposition. Figure 4(b) shows a T E M image of the interface between the coating and the diffusion zone. The grains in the coating near this interface were smaller than those in the middle part of the coating. ~-(Ni,Co)A1 precipitates in the diffusion zone grew as a single crystal and they were about 8 ~tm long and 3 ~tm wide. There were precipitates, about 0.1 ~tm in diameter, at the interface between the coating and the diffusion zone. EDX analysis of the precipitates reveals that they are rich in yttrium. Figure 5 shows an SEM micrograph of the coated specimen annealed in air at 1000 ~C for 100 h. The size of the diffusion zone increased to 16+ 1 jam while the thickness of the coating remained the same, indicating that the interdiffusion zone grew into the substrate.
Fig. 5. Scanning electron micrograph of a coated specimen, annealed in air at 1000 C for 100 h. It shows a larger diffusion zone compared with the specimens that were not annealed.
Figure 6 shows the microstructure of a creep-ruptured specimen. The specimen ruptured after 71 h at 1000°C and 138MPa. The diffusion zone in the creepruptured specimen was 18 + 1 ~tm. By comparing this size with that of the annealed specimen and assuming that the diffusion distance varies as the square root of time 26, one can conclude that the stress enhanced the size of the diffusion zone. In Fig. 6 it is also seen that a crack was initiated in the diffusion zone and it started to propagate into the substrate along a grain boundary. Creep cavities, about 10 ~tm in size, were seen (Fig. 7) in the gauge section of the creep-ruptured specimens and they were more prevalent in the diffusion zone. Such large cavities were not observed at the interface between the coating and the diffusion zone of unstressed as-coated or
EFFECTS OF C o C r A I Y ON Ni-BASE SUPERALLOY
215
Fig. 6. Scanning electron micrograph of a creep-ruptured specimen. A crack (shown by an arrow) initiates in the diffusion zone and starts to propagate along a grain boundary into the substrate.
Fig. 7. Scanning electron micrograph of a creep-ruptured specimen, showing creep cavitation. One of the cavities is shown by an arrow. a n n e a l e d specimens, n o r was there any c a v i t a t i o n on the s h o u l d e r (low stress region) of the c r e e p - r u p t u r e d specimens. In Fig. 8 it can be seen that a crack was p r o p a g a t i n g a l o n g the interface of the c o a t i n g a n d the diffusion zone of a c r e e p - r u p t u r e d specimen, indicating the weakness of this interface. F i g u r e 9 shows the m i c r o s t r u c t u r e in the interior of a c r e e p - r u p t u r e d specimen. This S E M m i c r o g r a p h c o r r e s p o n d e d to a surface cut parallel to the direction of the a p p l i e d stress. The 7' precipitates were e l o n g a t e d p e r p e n d i c u l a r to the direction of
216
T. K. CHAKI, A. K. SINGH, K. SADANANDA
Fig. 8. Scanning electron micrograph of a creep-ruptured specimen, showing spallation at the interface between the coating and the diffusion zone.
Fig. 9. Scanning electron micrograph of a creep-ruptured specimen, cut parallel to the direcuon of the stress {shown by an arrow in the upper middle part). 7' precipitates are elongated perpendicular to the direction of the stress.
the applied stress. Similar elongation of 7' precipitates under stress has been observed in other superalloys 5'2v. N o such stress dependence of the orientation of CoAl precipitates in the coating or in the diffusion zone was noticed. Figure 9 also shows some a phase needles zs in the substrate right below the diffusion zone. These
EFFECTS OF CoCrA1Y ON Ni-BASE SUPERALLOY
217
tr needles were produced as a result of prolonged aging and were not observed in ascoated specimens. An oxide scale (Fig. 10) of thickness about 2 ~tm was formed on the surface of the coating in the creep-ruptured specimens. The oxide scale was also observed in specimens annealed in air. An EDX spectrum from this region shows that the scale was rich in aluminum and chromium. A similar oxide layer has been reported 29 for FeCrAIY. There was a precipitate-free region beneath the oxide layer. In this layer there was not enough aluminum to form CoAl precipitates. The presence of nickel was observed in this region. Nickel has diffused all the way from the substrate to the surface of the coating during creep at 1000 °C. EDX analysis shows that the diffusion zone was rich in cobalt and chromium.
Fig. 10. Scanningelectron micrograph of a creep-ruptured specimen, showing an oxide scale on the surface of the coating. An arrow in the upper right-hand corner points the scale. 4. DISCUSSION During heat treatment of the coated specimen, cobalt from the coating diffuses into the superalloy substrate which contains v'-NiaA1 precipitates. Addition of cobalt makes NiaAl precipitates unstable 3°. Instead, 13-(Ni,Co)A1 precipitates are formed in the region where cobalt diffuses. Since the concentration of aluminum (Table I) in the coating is higher than that in the substrate, aluminum diffuses into the substrate and a 7'-rich region is formed in the substrate below the diffusion zone. The diffusion of cobalt in 7' ordered phase is difficult and the concentration of cobalt decreases rapidly in the substrate below the diffusion zone. Cobalt solid solution in the coating is f.c.c, in structure and 13-CoAl precipitates in the coating are disordered 31 b.c.c, above 800 °C. C o - N i solid solution in the diffusion zone is also f.c.c.32 in structure. The self-diffusion coefficients of cobalt and nickel are about the same. In fact, bulk self-diffusion data 33 for f.c.c, and h.c.p, metals at high temperature show that they have similar 34 diffusion coefficients with D O~ 5 × 10 5 m 2 s- 1 and
2]8
T. K. CHAKI, A. K. S1NGH, K. SADANANDA
Q / R Tm ~ 18, where D Ois the pre-exponential factor and Q is the activation energy, Tm is the melting point in kelvins and R is the universal gas constant. The concentration of cobalt in the coating and that of nickel in the substrate are about the same (Table I). The diffusion of cobalt and nickel balances 3° each other and no Kirkendall pores have been observed at the interface between the diffusion zone and the substrate. However, the pores formed during processing at the interface between the coating and the diffusion zone might have a deleterious effect on long-term creep properties. As a matter of fact, more creep cavities were observed in the diffusion zone than in the interior of the gauge section. Kang et al. 17 have attributed the reduction in creep rupture life of an aluminidecoated Ren6 80 superalloy to cracking in the coating. CoCrA1Y used in the present study has a ductile-brittle transition temperature 13 at 650"C. The coating is very ductile at the test temperature of 1000 "C and no cracks were seen to initiate from the coating. Since the coating was deposited at a substrate temperature of 982 "C and the test temperature was 1000 '~C there is hardly any stress at the interface, during the test, due to mismatch in the thermal expansion coefficients of the coating and the substrate. The o- phase needles formed at the interface between the diffusion zone and the substrate can have a deleterious effect 35 on the mechanical properties of the superalloy. Castillo and Willett ~6 have observed fracture path associated with needle-like precipitates below the diffusion zone in chrome aluminide-coated Udimet 520, creep tested at 8 0 2 C and 414 MPa. In the present study, the precipitates do not have any effect on the creep life of Ren6 80 superalloy at 1000 C . No cracks or cavities were seen near a precipitates. The role of yttrium-rich precipitates at the interface between the coating and the diffusion zone is not understood at present. Yttrium is added in the coating to improve 36 its oxidation resistance and adhesive properties. No cavities have been found near yttrium-rich precipitates. 5. CON('LUSIONS
(1) The results of creep tests at 1000 C on cast Ren6 80 coated with EBPVD CoCrAIY alloy and subsequently heat treated show that the rupture life is close to the extreme case in which the coating does not bear any load. (2) A diffusion zone consisting of I3-(Ni,Co)A1 precipitates in ~-Co-Ni solid solution is formed at the interface between the coating and the substrate. 100 h of annealing at 1000 C in air produced a diffusion zone of about 16 I,tm thickness. (3) Yttrium-rich precipitates, about 0.1 gm in size, are formed at the interface between the coating and diffusion zone. (4) A protective oxide layer is formed on the surface of the coating in specimens annealed or creep tested in air at 1000 C . ACKNOWLEDGMENT
The present research is supported by the Office of Naval Research.
EFFECTS OF C o C r A 1 Y ON Ni-BASE SUPERALLOY
219
REFERENCES
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G . ( ' h e n , C. Yao, Z. Z h o n g a n d W . Yu, inJ. K. Tien, S.T. Wlodek, t t . G . M o r r o w l l I , M. Gelland G. E. Maurer (eds.), ,S'tqJerallo.v.~ 19,~,'0, American Society l\~r Metals, Metals Park, OH, 1980, pp. 355 364. D.P. Whittle and J. Stringer, Philo.~. "l)'ans. R. Soc. Lomlcm. Set. ,4,295 (1980) 309 329.
207
E F F E C T S OF CoCrAIY C O A T I N G ON M I C R O S T R U C T U R A L STABILITY A N D C R E E P B E H A V I O R OF A N I C K E L - B A S E SUPERALLOY T. K. CHAKI*, A. K. SINGHt AND K. SADANANDA Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 203 75 (U.S.A.)
(Received January 26, 1988; revised May 3, 1988: accepted July 25, 1988)
High temperature creep and microstructural stability of a cast nickel base superalloy Ren6 80, coated with electron beam physically vapor deposited CoCrA1Y alloy and subsequently heat treated, were studied. Creep tests were performed on coated and uncoated Ren6 80 specimens at 1000 °C in air. The analysis of the creep data shows that the creep rupture life of the coated specimens is close to the extreme case in which the coating does not bear any load. As a result of the heat treatment a diffusion zone rich in (Ni,Co)A1 precipitates was formed at the boundary between the coating and the superalloy substrate. Pores of size about 0.5 ~tm were found at the interface between the coating and the diffusion zone. These pores are formed during the deposition of the coating. Yttrium-rich precipitates, about 0.1 ~tm in size, were observed at the interface between the coating and the diffusion zone. Creep cavities of size about 10 lam were formed in the gauge section of the creep-ruptured specimens and they were more prevalent in the diffusion zone. The cracks were initiated in the diffusion zone and propagated into the substrate along grain boundaries.
1. INTRODUCTION Nickel-base superalloy Ren6 80 (registered trademark of General Electric Company) is used as a high temperature blade material ~, especially in marine turbine engines. Nickel-base superalloys are well known for excellent strength 2'3 and creep ~'2'4 6 properties at high temperature. However, corrosion at high temperature in marine environments reduces their lives significantly v. Hot corrosion attack may occur at temperatures in the range 600 1000 °C when operating in combustion or marine environments. The main reactants are O2, SOz, SO3, CO, N 2, H 2 0 and NazSO4. Deposits of metallic sulfates 8'9 (such as Na2SO4 and COSO4) are found on the blades after service exposure and their effect 1° is known to be * Present address: Department of Mechanical Engineering, State University of Buffalo, NY 14260, U.S.A. t Present address: AT & T Bell Laboratories, Holmdel, NJ 07733, U.S.A. 0040-6090/89/$3.50
(c) Elsevier Sequoia/Printed in The Netherlands
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T. K. ('HAKI, A. K. S1NGH, K. S A D A N A N D A
detrimental to the life of the blades. Different types of protective coatings ~ ~s are applied to the blades to reduce environmental degradation. At high operating temperatures, an interdiffusion takes place between the coating and the superalloy substrate, producing microstructural changes at the interface. Degradation or improvement in mehcanical properties due to coating has been reported ta. Smith ~s examined the effect of low pressure plasma-sprayed Co 29Cr 6AI Y coating on IN 738 superalloy from room temperature to 870 C and found no significant effect of the coating and the coating process on the tensile. creep and fatigue properties. Castillo and Willett 1~' studied the influence of chrome aluminide coating on the creep and stress rupture properties of wrought Udimet 520 superalloy at 802 C . They showed that application ofchromium--aluminide coating caused a marked deterioration in rupture strength and ductility. Kang et al. ~ observed that application of low activity aluminide pack cementation treatment on directionally solidified Rend 80 resulted in a dramatic reduction in rupture lives (83'!,; at 760 'C, 62'~,. at 871 C and 36'~, at 982 'C). The degradation in life was less severe for conventionally cast Rend 80, but no degradation in rupture ductility was observed. Kolkman Is studied the effect of Codep B coating (aluminum applied to the surface by low activity pack cementation) on creep and fatigue properties of Rend 80 at 900 'C. The coating reduced the high cycle fatigue life by 62,'i. and creep rupture life by 1 l'!i,. Paskiet et al. I'~ performed fatigue tests on both uncoated and aluminide-coated wrought Udimet 700 over a range of temperatures from ambient to 927 'C. The aluminide coating enhanced the high cycle fatigue limit up to 480 C and above this temperature a slight reduction was observed. Schneider et al. 2° reported a reduction in the high cycle fatigue life in IN 738LC and IN 939 nickel-base alloys at 850 C due to a platinum aluminide coating. Strang and Lang 2~ have summarized data concerning the influence of coatings on mechanical properties of superalloys. They concluded that no loss in creep rupture properties should be seen. provided that a suitable heat treatment is carried out after coating and that the coatings do not have a deleterious effect on low cycle fatigue life above the ductile brittle transition temperature of the coating. The objective of the present work is to study the effect of heat-treated CoCrAIY coating deposited by electron beam physical vapor deposition (EBPVD) on high temperature creep properties of cast Rend 80 superalloy and to characterize the microstructural development at the interface between the coating and the substrate. 2. EXPERIMENTAI. DETAILS
2.1.
Materials
Cylindrical pieces of cast blanks of Rend 80 were obtained from a vendor and from these blanks specimens of gauge length 1.5 x 10 2m and gauge diameter 5.1 x 10 s m were machined. The specimens were given a heat treatment similar to that given to the blades in service. First they were solutionized at 1205 C for 2 h inside a cold wall furnace in a vacuum of 6.6 x 10 "* Pa. They were rapidly cooled in vacuum (in less than 10 rain) to 600 C and then slowly cooled to room temperature. They were subsequently aged at 1095 C for 4h and vacuum cooled to room temperature. Some of the specimens were taken out for coating and the rest were
EFFECTS OF CoCrAIY ON Ni-BASE SUPERALLOY
209
annealed again in vacuum at 1050°C for 2 h. The specimens were then cooled rapidly (in less than 10 rain) to 650 °C and held at 650 °C for 10 rain. The specimens were reheated to 850 °C and were aged at that temperature for 16 h. A CoCrAIY coating was applied by vendor using an EBPVD technique. The surface of the specimens was prepared for coating by degreasing, grit blasting and vapor honing. The substrate temperature was 982 °C and the pressure was about 6.6 x 10- 2 Pa throughout the coating process. The specimens were rotated by a gear arrangement for uniform coverage. The thickness of the coating was about 80 pm and the deposition rate was about 6 pm min 1 The composition of the coating and the substrate superalloy is given in Table I. The coated specimens were vacuum cooled for 5 rain and then quenched in argon gas for 5 min. The specimens were peened by glass beads to an Almen intensity of 15 N. The specimens were heat treated at 1050 °C for 2 h in vacuum. A diffusion bonding between the coating and the substrate was produced by the heat treatment. The specimens were cooled in vacuum to approximately 550 °C and then argon quenched. Finally, the coated specimens were aged in vacuum at 850°C for 16h. The coated and uncoated specimens underwent the same heat treatment. TABLE I THE N O M I N A L COMPOSITION OF THE COATING A N D THE SUPERALLOY SUBSTRATE
Composition (wt.%) Ni Coating Substrate
60.5
Co
Cr
66 9.5
Al
Ti
23
10.5
--
14
3
5
W
Mo
Y
--
0.25
4
4
--
2.2. Creep test andmicrostructure Constant load creep tests were carried out in air for both the coated and the uncoated specimens using a creep frame attached to a furnace. The temperature was maintained at 1000 °C. Five tests were performed for both the coated and uncoated specimens. The elongation was measured by a linear variable-differential transformer. The creep data were analyzed by considering different load-bearing capacities of the coating and the substrate. The initial microstructures of the coated and the uncoated specimens after heat treatment were examined using scanning electron microscopy (SEM). The specimens for SEM were prepared by polishing and etching chemically. A few specimens were observed without etching to make sure that etching did not produce any artifact. The images were observed by backscattered electrons. For comparison purposes, some coated specimens were annealed at 1000°C for 100 h. The creepruptured specimens were cut perpendicular and parallel to the load axis and examined by SEM. Energy-dispersive X-ray (EDX) spectroscopic analysis was performed to investigate the diffusion characteristics of the different elements within the coating-substrate system. This slices were cut perpendicular to the axis of the cylindrical specimens for transmission electron microscopy (TEM). The slices were ground on emery paper and small discs containing the coated area were punched
210
T . K. C H A K I , A. K. S I N G H , K. S A I ) A N A N I ) A
out. The discs were then jet-polished and ion milled to produce suitable specimens with a thin area at the interface between the coating and the substrate. 3. RESULTS
3. I. Creep Figure l shows typical strain t,s. time curves for coated and uncoated specimens creep tested at 1000 ~C. The loads for the coated and uncoated specimens are
~COATED
z I
< t,r-
w er 0
;OATED
0~ 0
25
50 TIME, HOURS
75
100
Fig. 1. Strain rs. time curves for c o a t e d and u n c o a t e d specimens, creep tested at 1000 C and at 138 M Pa. T A B L E 1I CREEP TEST RESULTSFOR COATEDAND UNCOATEDRENI~80 AT ] 0 0 0 ' C AND 138 M P a
Specimen number
1 2 3 4 5
Uncoated specimen
Coated specimen
Rupture li[k,
Creep rate
(h)
(xl0
95 103 91 96 101
5.6 4.9 5.5 4.9 4.6
Ss
i
Rupture l[['e
Creep rate
(h)
(xlO
73 68 67 71 75
7.6 8.4 7.5 8.0 8.5
Ss-l)
EFFECTS OF C o C r A I Y o n Ni-BASE SUPERALLOY
211
3.02 kN and 2.82 kN respectively. It is clear that the coated specimen elongated faster and broke earlier. The rupture lives and minimum creep strains for different specimens are given in Table II. The average rupture lives were 71 + 3 h and 97 + 5 h for coated and uncoated specimens respectively. The average minumum creep strain rates for the coated and uncoated specimens were (8.0_+0.5)x 10 8 s-1 and (5.1 + 0.4) x 10 8 s 1 respectively. The rupture strain (calculated 1 h before rupture) was about the same (about 6.5~o) in both cases. Two extreme cases are considered. First, it is assumed that the coating has the same load-bearing capacity as the substrate, implying that both the coated and the uncoated specimens are subjected to the same stress of 138.0 MPa. However, the experimental data of this work show that the coated specimens fail earlier. In the second case, it is assumed that at high temperatures the coating is too weak to bear any load. In this case, the stress on the coated specimen is calculated by dividing the load with the cross-sectional area of the substrate alone and is found to be 147.8 MPa which is higher than the stress in the first case. If we assume a power law creep, the rupture life t is given by a power of the stress o: t
= Aa-"
(1)
where A is a constant, and the exponent n for Rend 80 is 5.6, obtained by fitting the data in ref. 2. The result of this work has shown that at a stress level of 138.0 MPa the uncoated Ren6 80 has a creep rupture life of 97 h. With the help ofeq. (1), the rupture life of Ren6 80 at a stress of 147.8 MPa is given by 97 × (138.0/147.8) s'6 or 66 h. If the coating does not have any load-bearing capacity, the creep rupture life of the coated specimens should be 66 h which is close to the experimental value of 71 4- 3 h. The actual rupture life, therefore, is close to the extreme case in which the coating can be considered to bear zero load. 3.2. M i c r o s t r u c t u r e s
Figure 2 is an SEM backscattered image of the initial microstructure of the coated specimen before the creep test. The specimen has undergone shot peening and heat treatment after coating. The micrograph corresponded to a section cut perpendicular to the axis of the specimen. It consisted of three regions: the coating, a diffusion zone and the substrate. The coating (region a in Fig. 2) contained almost equal amounts of s-Co solid solution (f.c.c.) and [3-CoAl phase (ordered b.c.c.) 11. Other elements such as chromium, aluminium and yttrium were dissolved in these two phases. Cobalt has a low temperature ~ phase (h.c.p.) z3, but its formation requires prolonged aging 24. It was not a prominent phase in the present coating. The and 13phases in the coating were elongated in the direction of the deposition (i.e. along the radial direction of the cylindrical specimens). The diffusion zone (region b in Fig. 2) contained [3-(Ni,Co)AI precipitates in a-Co,Ni solid solution. The precipates were faceted and elongated in directions different from those in the coating. A diffusion zone about 5 lam thick grew into the superalloy substrate during the heat treatment after the deposition of the coating. The substrate (region c in Fig. 2) consisted of 7"-Ni3AI precipitates 25 (about 0.5 lam in size) in nickel solid solution. The 7' precipitates near the interface between the diffusion zone and the substrate were larger than those in the interior of the substrate. Figure 3 shows pores
212
T. K. C H A K I , A. K+ SINGH+ K. S A D A N A N D A
(a)
(b)
(c) Fig. 2. Scanning electron micrograph of a coated specimen before creep test. Region a shows the coating containing CoAl precipitates (black contrast). The diffusion zone (region b) contains faceted (Ni,Co)AI precipitates. Region c shows the substrate containing 7' precipitates.
Fig. 3. Scanning electron micrograph of a coated specimen, showing pores at the interface between the coating and the diffusion zone. One of the pores is shown by an arrow. o f size a b o u t 0.5 ~tm a t t h e i n t e r f a c e b e t w e e n t h e c o a t i n g a n d t h e d i f f u s i o n z o n e . T h e pores were seen in specimens prepared without any chemical etching. The pores are n o t a r t i f a c t s d u e t o p r e f e r e n t i a l e t c h i n g . A l a r g e n u m b e r o f p o r e s w e r e f o r m e d in t h e
Fig. 4. (a) Transmission electron micrograph of the middle part of the coating. Grain 1 corresponds to 13-CoAl and grain 2 to or-Co solid solution. (b) Transmission electron micrograph at the interface between the coating and the diffusion zone.
{a)
(b)
t..,o
I,o
© ,.<
d~
z.
.< ©
O
©
,.r]
214
"1-. K. ('HAKI, A. K. SINGH, K. SADANANDA
coating during the deposition. Subsequent shot peening and heat treatment closed only some of the pores. Figure 4(a) is a TEM micrograph of the middle part of the coating. The grain sizes of [3-CoAl and s-Co solid solution were the same (about 0.5 ~tm), but the grains were elongated along the deposition. Figure 4(b) shows a T E M image of the interface between the coating and the diffusion zone. The grains in the coating near this interface were smaller than those in the middle part of the coating. ~-(Ni,Co)A1 precipitates in the diffusion zone grew as a single crystal and they were about 8 ~tm long and 3 ~tm wide. There were precipitates, about 0.1 ~tm in diameter, at the interface between the coating and the diffusion zone. EDX analysis of the precipitates reveals that they are rich in yttrium. Figure 5 shows an SEM micrograph of the coated specimen annealed in air at 1000 ~C for 100 h. The size of the diffusion zone increased to 16+ 1 jam while the thickness of the coating remained the same, indicating that the interdiffusion zone grew into the substrate.
Fig. 5. Scanning electron micrograph of a coated specimen, annealed in air at 1000 C for 100 h. It shows a larger diffusion zone compared with the specimens that were not annealed.
Figure 6 shows the microstructure of a creep-ruptured specimen. The specimen ruptured after 71 h at 1000°C and 138MPa. The diffusion zone in the creepruptured specimen was 18 + 1 ~tm. By comparing this size with that of the annealed specimen and assuming that the diffusion distance varies as the square root of time 26, one can conclude that the stress enhanced the size of the diffusion zone. In Fig. 6 it is also seen that a crack was initiated in the diffusion zone and it started to propagate into the substrate along a grain boundary. Creep cavities, about 10 ~tm in size, were seen (Fig. 7) in the gauge section of the creep-ruptured specimens and they were more prevalent in the diffusion zone. Such large cavities were not observed at the interface between the coating and the diffusion zone of unstressed as-coated or
EFFECTS OF C o C r A I Y ON Ni-BASE SUPERALLOY
215
Fig. 6. Scanning electron micrograph of a creep-ruptured specimen. A crack (shown by an arrow) initiates in the diffusion zone and starts to propagate along a grain boundary into the substrate.
Fig. 7. Scanning electron micrograph of a creep-ruptured specimen, showing creep cavitation. One of the cavities is shown by an arrow. a n n e a l e d specimens, n o r was there any c a v i t a t i o n on the s h o u l d e r (low stress region) of the c r e e p - r u p t u r e d specimens. In Fig. 8 it can be seen that a crack was p r o p a g a t i n g a l o n g the interface of the c o a t i n g a n d the diffusion zone of a c r e e p - r u p t u r e d specimen, indicating the weakness of this interface. F i g u r e 9 shows the m i c r o s t r u c t u r e in the interior of a c r e e p - r u p t u r e d specimen. This S E M m i c r o g r a p h c o r r e s p o n d e d to a surface cut parallel to the direction of the a p p l i e d stress. The 7' precipitates were e l o n g a t e d p e r p e n d i c u l a r to the direction of
216
T. K. CHAKI, A. K. SINGH, K. SADANANDA
Fig. 8. Scanning electron micrograph of a creep-ruptured specimen, showing spallation at the interface between the coating and the diffusion zone.
Fig. 9. Scanning electron micrograph of a creep-ruptured specimen, cut parallel to the direcuon of the stress {shown by an arrow in the upper middle part). 7' precipitates are elongated perpendicular to the direction of the stress.
the applied stress. Similar elongation of 7' precipitates under stress has been observed in other superalloys 5'2v. N o such stress dependence of the orientation of CoAl precipitates in the coating or in the diffusion zone was noticed. Figure 9 also shows some a phase needles zs in the substrate right below the diffusion zone. These
EFFECTS OF CoCrA1Y ON Ni-BASE SUPERALLOY
217
tr needles were produced as a result of prolonged aging and were not observed in ascoated specimens. An oxide scale (Fig. 10) of thickness about 2 ~tm was formed on the surface of the coating in the creep-ruptured specimens. The oxide scale was also observed in specimens annealed in air. An EDX spectrum from this region shows that the scale was rich in aluminum and chromium. A similar oxide layer has been reported 29 for FeCrAIY. There was a precipitate-free region beneath the oxide layer. In this layer there was not enough aluminum to form CoAl precipitates. The presence of nickel was observed in this region. Nickel has diffused all the way from the substrate to the surface of the coating during creep at 1000 °C. EDX analysis shows that the diffusion zone was rich in cobalt and chromium.
Fig. 10. Scanningelectron micrograph of a creep-ruptured specimen, showing an oxide scale on the surface of the coating. An arrow in the upper right-hand corner points the scale. 4. DISCUSSION During heat treatment of the coated specimen, cobalt from the coating diffuses into the superalloy substrate which contains v'-NiaA1 precipitates. Addition of cobalt makes NiaAl precipitates unstable 3°. Instead, 13-(Ni,Co)A1 precipitates are formed in the region where cobalt diffuses. Since the concentration of aluminum (Table I) in the coating is higher than that in the substrate, aluminum diffuses into the substrate and a 7'-rich region is formed in the substrate below the diffusion zone. The diffusion of cobalt in 7' ordered phase is difficult and the concentration of cobalt decreases rapidly in the substrate below the diffusion zone. Cobalt solid solution in the coating is f.c.c, in structure and 13-CoAl precipitates in the coating are disordered 31 b.c.c, above 800 °C. C o - N i solid solution in the diffusion zone is also f.c.c.32 in structure. The self-diffusion coefficients of cobalt and nickel are about the same. In fact, bulk self-diffusion data 33 for f.c.c, and h.c.p, metals at high temperature show that they have similar 34 diffusion coefficients with D O~ 5 × 10 5 m 2 s- 1 and
2]8
T. K. CHAKI, A. K. S1NGH, K. SADANANDA
Q / R Tm ~ 18, where D Ois the pre-exponential factor and Q is the activation energy, Tm is the melting point in kelvins and R is the universal gas constant. The concentration of cobalt in the coating and that of nickel in the substrate are about the same (Table I). The diffusion of cobalt and nickel balances 3° each other and no Kirkendall pores have been observed at the interface between the diffusion zone and the substrate. However, the pores formed during processing at the interface between the coating and the diffusion zone might have a deleterious effect on long-term creep properties. As a matter of fact, more creep cavities were observed in the diffusion zone than in the interior of the gauge section. Kang et al. 17 have attributed the reduction in creep rupture life of an aluminidecoated Ren6 80 superalloy to cracking in the coating. CoCrA1Y used in the present study has a ductile-brittle transition temperature 13 at 650"C. The coating is very ductile at the test temperature of 1000 "C and no cracks were seen to initiate from the coating. Since the coating was deposited at a substrate temperature of 982 "C and the test temperature was 1000 '~C there is hardly any stress at the interface, during the test, due to mismatch in the thermal expansion coefficients of the coating and the substrate. The o- phase needles formed at the interface between the diffusion zone and the substrate can have a deleterious effect 35 on the mechanical properties of the superalloy. Castillo and Willett ~6 have observed fracture path associated with needle-like precipitates below the diffusion zone in chrome aluminide-coated Udimet 520, creep tested at 8 0 2 C and 414 MPa. In the present study, the precipitates do not have any effect on the creep life of Ren6 80 superalloy at 1000 C . No cracks or cavities were seen near a precipitates. The role of yttrium-rich precipitates at the interface between the coating and the diffusion zone is not understood at present. Yttrium is added in the coating to improve 36 its oxidation resistance and adhesive properties. No cavities have been found near yttrium-rich precipitates. 5. CON('LUSIONS
(1) The results of creep tests at 1000 C on cast Ren6 80 coated with EBPVD CoCrAIY alloy and subsequently heat treated show that the rupture life is close to the extreme case in which the coating does not bear any load. (2) A diffusion zone consisting of I3-(Ni,Co)A1 precipitates in ~-Co-Ni solid solution is formed at the interface between the coating and the substrate. 100 h of annealing at 1000 C in air produced a diffusion zone of about 16 I,tm thickness. (3) Yttrium-rich precipitates, about 0.1 gm in size, are formed at the interface between the coating and diffusion zone. (4) A protective oxide layer is formed on the surface of the coating in specimens annealed or creep tested in air at 1000 C . ACKNOWLEDGMENT
The present research is supported by the Office of Naval Research.
EFFECTS OF C o C r A 1 Y ON Ni-BASE SUPERALLOY
219
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T. K. CHAKI, A. K. SINGH, K. SADANANDA
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