- Email: [email protected]
Influence of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations
Materials and Design 56 (2014) 816–821
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Influence of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations F.H. Latief ⇑, K. Kakehi Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan
a r t i c l e
i n f o
Article history: Received 7 June 2013 Accepted 28 November 2013 Available online 7 December 2013 Keywords: Ni-based superalloy Thermal exposure Aluminide coating Creep Microstructure Crystal orientation
a b s t r a c t The effect of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations has been studied in the present study. The specimens were coated by a pack aluminizing process conducted at 1000 °C for 5 h under an argon atmosphere. Long-term exposure was performed at 1100 °C for 500 h prior to the creep test. The creep properties were found to deteriorate after the long-term exposure as a result of the coarsening of c0 precipitates accompanied by the formation of a secondary reaction zone which is known to degrade the strength of aluminide coating on a Ni-based single crystal superalloy. The formation of a topologically close-packed phase is one of the degradation factors that are responsible for the reduction of the creep strength of the aluminide coating on a Ni-base single crystal superalloy both with and without long-term exposure. The anisotropic creep properties between the two side-surfaces are due to the different arrangements of f1 1 1gh1 0 1i slip systems. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Ni-based single crystal superalloys are widely used for the manufacturing of critical gas turbine components due to their high strength at elevated temperatures. The strength of Ni-based single crystal superalloys at elevated temperatures is influenced by two important factors: (1) solid solution strengthening by a high concentration of solute elements (Cr, Co, Mo, Re, and W), and (2) precipitation strengthening by dispersion of the c0 precipitates in the Ni-base superalloy matrix (c) [1]. Nevertheless, the high temperature creep behavior of Ni-based single crystal superalloys is strongly dependent upon other factors such as volume fraction, coarsening, and the average size and distribution of the c0 precipitates [2]. Critical gas turbine components are used in harsh environments can lower the strength of Ni-based single crystal superalloys. To prevent oxidation and corrosion during high temperature applications, a protective coating on the surface of gas turbine components made by single crystal superalloys is necessary [3]. Aluminide diffusion coatings have been used for gas turbine components as an oxidation resistant coating material that improves the lifetime of the component operating at high temperatures [3]. Several methods are used to produce aluminide coating, including one known as pack aluminizing process [4]. Microstructure instability during service at elevated temperatures, leads to the formation of topologically close-packed (TCP)
phases [5] and the secondary reaction zone (SRZ) [6]. In addition, the effect of crystallographic orientation is an interesting subject concerning in Ni-based single crystal superalloys. It was reported that the mechanical properties of Ni-based single crystal superalloys are affected by the crystallographic orientation of the specimen surface [7]. Acharya and Fuchs [8] studied the effect of long-term thermal exposure on the microstructure and creep properties of the Nibased single crystal superalloy CMSX-10, and they concluded that the creep strength of CMSX-10 during long-term aging was reduced. Xia et al. [9] also reported the influence of thermal exposure on the microstructure and stress rupture properties of the Ni-based alloy DZ951. However, we have found no reports that discuss the effects of thermal exposure and crystallographic orientation on the creep properties of coated Ni-based single crystal superalloys. The purpose of the present study was to investigate the influence of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations. We observed the microstructural changes before and after creep tests as a function of crystallographic orientation, and we briefly discuss the relationship between deformation mechanisms and creep properties.
2. Material and experimental procedures ⇑ Corresponding author. Tel./fax: +81 42 6772709. E-mail addresses: tmu.ac.jp (F.H. Latief).
[email protected],
hamdanlatief-fahamsyah@ed.
0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.11.075
A Ni-based single crystal superalloy with a chemical composition of 12Co, 3Cr, 2Mo, 6W, 6Al, 6Ta, 5Re, 0.1Hf and balance Ni (in mass%) was used as the substrate. A four-step solution heat
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
treatment was conducted, first at 1240 °C for 1 h, second at 1280 °C for 2 h, third at 1300 °C for 2 h, and fourth at 1320 °C for 8 h, followed by gas fan cooling. A two-step aging treatment was conducted, first at 1150 °C for 4 h and second at 870 °C for 20 h, followed by gas fan cooling. The crystal orientation of the Ni-based single crystal superalloy was determined by the X-ray Laue reflection method. The creep specimens were cut from cylindrical bars with a square cross-section area of 2.8 mm 2.8 mm and a gauge length of 19.6 mm as reported in our previous work [10]. The creep specimens were prepared by electric discharge machining (EDM) with {1 0 0} and {1 1 0} side-surfaces (Fig. 1). The specimens were mechanically polished down to 1200 mesh by silicon carbide (SiC) paper and ultrasonically cleaned in an acetone bath for 10 min prior to the aluminizing process. The polished specimens were embedded in an Al2O3 retort containing a mixture of 24.5 Al, 24.5 Fe, 49 Al2O3 and 2 NH4Cl (in mass%) and then heated at 1000 °C for 5 h in an argon environment during the aluminizing process. Some aluminized specimens were thermally exposed at 1100 °C for 500 h prior to the creep rupture test (designated EXAL) and some of them were not thermally exposed (designated AL). We also evaluated the bare specimens (designated as UNAL) in comparison with the aluminized specimens. The stress orientation of all of the specimens was within 4° of h0 0 1i. The creep rupture tests were performed at a temperature of 900 °C under a constant load of 392 MPa with a load direction parallel to the h0 0 1i direction. We examined the creep specimens with different surface orientations to determine the anisotropic creep properties. The heat-treated samples were polished for the examination of microstructures. A solution containing HCl and HNO3 at the ratio of 3:1 was used as the chemical etchant to selectively dissolve the c0 phase present in the microstructures, to facilitate the identification of the precipitates. The virgin material was observed by optical microscopy (OM) to determine the initial microstructure and the microstructures of the aluminized specimens after exposure, as well as before and after the creep rupture tests were observed by scanning electron microscopy (SEM). 3. Results The c/c0 two-phase structure was readily observed in the heattreated superalloy before the aluminizing process, as is the case with standard Ni-based single crystal superalloys (Fig. 2). The
817
Fig. 2. Optical and SEM micrographs of a Ni-base single crystal superalloy.
cuboidal c0 precipitates were regularly aligned along h0 0 1i during the aging treatment due to the elastic interaction between precipitates. The alloy used in this study contained about 70 vol.% cuboidal c0 precipitates that distributed uniformly in the matrix (c). From our observation, the average size of the cuboidal c0 precipitates was about 300 nm. Fig. 3 shows the cross-sectional microstructures of asaluminized specimens without (AL) and with thermal exposure (EXAL). Generally, three distinctive domains were observed in the as-aluminized specimens: (1) an outer protective layer composed of b-NiAl, and it has been confirmed in our previous paper [11], (2) an interdiffusion zone (IDZ), and (3) the substrate. The IDZ thickness of the AL specimens was similar for both surfaces at be about 5 lm (Fig. 3a and b). Moreover, the diffusion zone was extended after 500 h exposure at 1100 °C in the EXAL specimens for both side-surfaces (Fig. 3c and d). The IDZ of the EXAL specimens seemed to be widened and the secondary reaction zone (SRZ) was formed beneath the IDZ. The SRZ is defined as an intermediate layer formed by a cellular reaction, and a discontinuous precipitate reaction, which is similar to recrystallization [5]. SRZ formation may occur during the coating process or during exposure at elevated temperatures [12]. An SRZ is mostly formed under an IDZ as a result of the inward diffusion of Al from the coating layer to the superalloy substrate. The total diffusion zone thickness (i.e., the combination of IDZ + SRZ) of
Fig. 1. Arrangement of f1 1 1gh1 0 1i slip systems for (a) the {1 0 0} and (b) {1 1 0} side-surface orientations.
818
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
Fig. 3. Microstructures of as-aluminized specimens: (a and b) without thermal exposure and (c and d) with thermal exposure with the: (a and c) {1 0 0} side-surface and (b and d) {1 1 0} side-surface orientations.
24
UNAL Specimens
20
Creep strain (%)
{100}
16
{110}
12 8 4 0
0
100
200
300
400
500
Time (h)
600
700
800
Fig. 5. Creep curves of the bare specimens at 900 °C/392 MPa.
Fig. 4. Microstructures of c/c0 structure after long-term exposure for 500 h at 1100 °C: (a) {1 0 0} side-surface and (b) {1 1 0} side-surface.
the EXAL specimens was approximately 67 lm on the {1 0 0} sidesurface and 77 lm on the {1 1 0} side-surface (Fig. 3c and d). Voids appeared within the SRZ on the {1 1 0} side-surface (Fig. 3d), which may be attributed to the Kirkendall effect [13]. Small TCP phases with a needle-like shape [5] were found in both side-surfaces (Fig. 3c and d). The changes in the c0 precipitates after the 500 h exposure at 1100 °C are shown in Fig. 4. The size of the c0 precipitates increased due to thermal exposure, and the shape became irregular during thermal exposure for both surfaces. The creep rupture lives of the UNAL specimens were 732 h for the {1 0 0} side-surface and 697 h for the {1 1 0} side-surface (Fig. 5). With respect to the specimens with aluminide coating, the creep rupture lives of the specimens AL were 525 h for the {1 0 0} side-surface and 428 h for the {1 1 0} side-surface, whereas
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
24 20
Creep strain (%)
AL Specimens
(a)
{100} {110}
16 12 8 4 0
0
100
200
300
400
500
600
Time (h) 24
(b)
Creep strain (%)
20
EXAL Specimens
16
{110} {100}
12 8 4 0
0
10
20
30
40
50
60
70
80
90
100
Time (h) Fig. 6. Creep curves of the aluminized specimens at 900 °C/392 MPa: (a) without and (AL), (b) with pre-exposure (EXAL).
819
for the EXAL specimens were 58 h for the {1 0 0} side-surface and 49 h for the {1 1 0} side-surface (Fig. 6). In short, the thermal exposure performed at 1100 °C for 500 h prior to the creep rupture test resulted in an extreme reduction of creep rupture life. Nevertheless, the {1 0 0} specimens exhibited a longer creep rupture lives than the {1 1 0} specimens in all conditions. Fig. 7 shows the microstructures of ruptured AL and EXAL specimens. In the AL specimens, it was clear that the SDZ was formed on both surfaces (Fig. 7a and b). The SDZ is described as a layer that lies underneath the IDZ where the matrix has not transformed to bNiAl but it does have an increased level of aluminum. The SDZ can be distinguished from the IDZ as it remains particularly c0 and retains the orientation of the single crystal substrate [5]. The total diffusion zone (IDZ + SDZ) thickness was approximately 30 lm on the {1 0 0} side-surface and 37 lm on the {1 1 0} side-surface (Fig. 7a and b). TCP phases and voids occurred within the SDZ. The ruptured EXAL specimens were similar to their microstructures before the creep test, where the IDZ and SRZ were still observed on both surfaces (Fig. 7c and d). However, the total diffusion zone (IDZ + SRZ) after the creep test was more extensive compared to that observed before the creep test. The total diffusion zone (IDZ + SRZ) was about 82 lm on the {1 0 0} side-surface and 89 lm on the {1 1 0} side-surface (Fig. 7c and d). The numbers of TCP phases were somewhat increased after the creep test for both orientations. Voids were still observed on the {1 1 0} side-surface (Fig. 7d). Fracture surfaces of AL and EXAL specimens are shown in Figs. 8 and 9, respectively. Both figures show two modes of fracture, that is, the fracture along the single slip plane near the surface and the micro-facets fracture, which is associated with {1 1 1}-multiple-slip deformation inside of the specimen. In general, the fracture surface on the {1 0 0} side-surface revealed the {1 1 1} slip plane at the corner, whereas the fracture surface on the {1 1 0} side-surface exhibited the {1 1 1} slip plane at the edge, in agreement with the
Fig. 7. Cross-section images of ruptured aluminized specimens: (a and b) AL specimens and (c and d) EXAL specimens with the: (a and c) {1 0 0} side-surface and (b and d) {1 1 0} side-surface orientations.
820
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
(a)
(b)
(111)
(111)
(c) (111) (111) (111)
200 µm Fig. 8. Fracture surfaces of ruptured AL aluminized specimens without pre-exposure: (a) {1 0 0} side-surface, (b) {1 1 0} side-surface, and (c) magnified image of (a).
(a)
(b)
Fig. 9. Fracture surfaces of ruptured aluminized specimens with pre-exposure (EXAL): (a) {1 0 0} side-surface and (b) {1 1 0} side-surface.
arrangement shown in Fig. 1. The micro-facets fracture was clearly seen in the magnified view of the micro-facets fracture on the {1 0 0} side-surface (Fig. 8c). In the fracture surfaces of EXAL specimens (Fig. 9), the micro-facets fracture was not so as clear as in the AL specimens (Fig. 8). 4. Discussion Our comparison of the creep rupture lives of bare and aluminized specimens showed that the aluminizing process has a deleterious effect on the creep strength of the Ni-based single crystal superalloy used (Figs. 7 and 8). The reduction of creep lives in
the aluminized specimens may have been due to the inward diffusion of Al from the protective coating into the superalloy substrate. This diffusion might cause an increase in the Al concentration in the diffusion zone, resulting in the reduced creep strength and the premature failure of the sample [14]. In addition, the decrease in the creep lives of the aluminized specimens may be due to the weakening of atomic bonds, since aluminum is a low-melting metal its effect can be considered as similar to that of other low melting metals [15]. The decrease in the load-bearing section because of the formation of the coating layer could also be one of factors in the reduction of the creep strength [16]. The anisotropic creep properties
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
between the two side-surfaces were more tangible in the aluminized AL and EXAL specimens (Fig. 8). This creep anisotropy is primarily due to the different arrangements of the f1 1 1gh1 0 1i slip systems during creep deformation (Fig. 2), associated with the extension of the diffusion zone and the formation of TCP phases. We note that the mechanical properties of Ni-based single crystal superalloys are influenced by the morphology, size, volume fraction and distribution of c0 precipitates in the matrix. The coarsening of c0 precipitates occurs when aluminized specimens are subjected to high-temperature thermal exposure [17]. At high temperatures, deformation is controlled by the dislocation climb. When the c0 precipitates are coarsened during thermal exposure, the width of the c matrix channel is enlarged as well [18]. This will weaken the resistance to the dislocation movement, and consequently, the creep strength of aluminized EXAL specimens was reduced [19]. In the present study we found that the most noticeable effect of exposing the aluminized Ni-based single crystal superalloy to high temperature condition was the changes in microstructure. After 500 h at 1100 °C, the diffusion zone became larger and an SRZ was formed at this stage (Fig. 5). The formation of an SRZ is one of the serious problems in applications of coated Ni-based single crystal superalloy, as is TCP phase. An SRZ is usually found in NiAl coated specimens as a consequence of the inward diffusion of Al from the coating into the substrate. When Al diffuses inward toward the substrate and solutes in the c0 phase during long-term thermal exposure, the transformation from c to c0 will take place and it will break the c–c0 net-like structure in the substrate. We note that the solubility of the refractory elements such as Re, W and Mo is higher in the c/c0 structure than that in the coating layer, which causes the precipitation of the refractory elements from the c/c0 structure and leads to the formation of a TCP phase and SRZ [20]. The Al activity in the coating is prone to form an SRZ in a coated Ni-based single crystal superalloy [6]. Therefore, an SRZ is normally observed under the IDZ in a coated Ni-based single crystal superalloy specimen. The Ni-based single crystal superalloys containing high concentrations of refractory elements (such as Re) are potential candidates for the formation of an SRZ [21]. The formation of an SRZ will result in the lowering of the mechanical properties of the superalloy and coarsening of the c/ c0 structure [22]. However, an SRZ is not formed when the surface of the specimen is prepared by an electro-polishing technique prior to the aluminizing process [9]. Concisely, long-term thermal exposure has a deleterious effect on the creep rupture life of aluminide coating on a Ni-based single crystal superalloy. 5. Conclusions We investigated the influence of long-term thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy at different surface orientations, and we found that the thermal exposure of aluminized specimens at 1100 °C for 500 h prior to a creep test decreased the creep strength of the aluminized specimens. This is attributed primarily to the coarsening of c0 precipitates during the thermal exposure, accompanied with the formation of an SRZ. In addition, when aluminized specimens are exposed to high-temperature environments and stress, an interdiffusion of elements between the single crystal substrate and the coating layer occurs. However, the outward diffusion of
821
Ni from the substrate leads to the enrichment of refractory elements and promotes the formation of TCP phases which may also degrade the creep rupture lives of aluminized specimens both without and with thermal exposure. We also determined, the anisotropic creep properties induced primarily by different arrangements of the f1 1 1gh1 0 1i slip system. Acknowledgements We gratefully acknowledge Dr. H. Murakami of the National Institute for Materials Science, Tsukuba, for his support in the preparation of the aluminide coating, and Mr. M. Nakane for his contribution. References [1] Tian S, Su Y, Qian B, Yu X, Liang F, Li A. Creep behavior of a single crystal nickelbased superalloy containing 4.2% Re. Mater Des 2012;37:236–42. [2] Baldan A. Review progress in Ostwald ripening theories and their applications to the c0 -precipitates in nickel-base superalloys Part II Nickel-base superalloys. J Mater Sci 2002;37:2379–405. [3] Pomeroy MJ. Coatings for gas turbine materials and long term stability issues. Mater Des 2005;26:223–31. [4] Zhan Z, Liu Z, Liu J, Li L, Li Z, Liao P. Microstructure and high-temperature corrosion behaviors of aluminide coatings by low-temperature pack aluminizing process. Appl Surf Sci 2010;256:3874–9. [5] Rae CMF, Hook MS, Reed RC. The effect of TCP morphology on the development of aluminide coated superalloys. Mater Sci Eng A 2005;396:231–9. [6] Murakami H, Sakai T. Anisotropy of secondary reaction zone formation in aluminized Ni-based single-crystal superalloys. Scr Mater 2008;59:428–31. [7] Caron P, Khan T, Nakagawa YG. Effect of orientation on the intermediate temperature creep behavior of Ni-base single crystal superalloys. Scr Metall 1986;20:499–502. [8] Acharya MV, Fuchs GE. The effect of long-term thermal exposures on the microstructure and properties of CMSX-10 single crystal Ni-base superalloys. Mater Sci Eng A 2004;381:143–53. [9] Xia PC, Yu JJ, Sun XF, Guan HR, Hu ZQ. The influence of thermal exposure on the microstructure and stress rupture property of DZ951 nickel-base alloy. J Alloys Compd 2007;443:125–31. [10] Latief FH, Kakehi K. Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys. Mater Des 2013;49:485–92. [11] Latief FH, Kakehi K, Tashiro Y. Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air. J Ind Eng Chem 2013;19:1926–32. [12] Walston WS, Schaefter JC, Murphy WH. A new type of microstructure instability in superalloy-SRZ. In: Kissinger RD et al., editors. Superalloys. PA, USA: TMS, Warrendale; 1996. p. 9–18. [13] Smigelskas AD, Kirkendall EO. Zinc diffusion in alpha brass. Trans AIME 1947;171:130–42. [14] Latief FH, Kakehi K, Murakami H. Anisotropic creep properties of aluminized Ni-based single-crystal superalloy at intermediate and high temperatures. Scr Mater 2013;68:126–9. [15] Nikitin VI, Grigor’eva TN. Effect of aluminizing on the creep strength of a Nickel alloy in some environments. Mater Sci 1975;10:5–9. [16] Dryepondt S, Zhang Y, Pint BA. Creep and corrosion testing of aluminide coatings on ferritic–martensitic substrates. Surf Coat Technol 2006;201:3880–4. [17] Xia PC, Yu JJ, Sun XF, Guan HR, Hu ZQ. Influence of thermal exposure on gamma prime precipitation and tensile properties of DZ951 alloy. Mater Character 2007;58:645–51. [18] Aghaie-Simonetti M, Hajjavady M. The effect of thermal exposure on the properties of a Ni-base superalloy. Mater Sci Eng A 2008;487:388–93. [19] Liu JJ, Jin T, Yu JJ, Sun XF, Guan HR, Hu ZQ. Effect of thermal exposure on stress rupture properties of a Re bearing Ni base single crystal superalloy. Mater Sci Eng A 2010;527:890–7. [20] Wang Y, Guo HB, Peng H, Gong SK. Diffusion barrier behaviors of (Ru, Ni)Al/ NiAl coatings on Ni-based superalloy substrate. Intermetallics 2011;19:191–5. [21] Walston WS, Cetel A, MacKay R, O’Hara K, Duhl D, Dreshfield R. Joint development of a fourth generation single crystal superalloys. In: Green KA et al., editors. Superalloys. PA, USA: TMS, Warrendale; 2004. p. 15–25. [22] Suzuki AS, Kawagishi K, Yokokawa T, Harada H. Effect of Cr on microstructural evolution of aluminized fourth generation Ni-base single crystal superalloys. Surf Coat Technol 2012;206:2769–73.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Influence of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations F.H. Latief ⇑, K. Kakehi Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan
a r t i c l e
i n f o
Article history: Received 7 June 2013 Accepted 28 November 2013 Available online 7 December 2013 Keywords: Ni-based superalloy Thermal exposure Aluminide coating Creep Microstructure Crystal orientation
a b s t r a c t The effect of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations has been studied in the present study. The specimens were coated by a pack aluminizing process conducted at 1000 °C for 5 h under an argon atmosphere. Long-term exposure was performed at 1100 °C for 500 h prior to the creep test. The creep properties were found to deteriorate after the long-term exposure as a result of the coarsening of c0 precipitates accompanied by the formation of a secondary reaction zone which is known to degrade the strength of aluminide coating on a Ni-based single crystal superalloy. The formation of a topologically close-packed phase is one of the degradation factors that are responsible for the reduction of the creep strength of the aluminide coating on a Ni-base single crystal superalloy both with and without long-term exposure. The anisotropic creep properties between the two side-surfaces are due to the different arrangements of f1 1 1gh1 0 1i slip systems. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Ni-based single crystal superalloys are widely used for the manufacturing of critical gas turbine components due to their high strength at elevated temperatures. The strength of Ni-based single crystal superalloys at elevated temperatures is influenced by two important factors: (1) solid solution strengthening by a high concentration of solute elements (Cr, Co, Mo, Re, and W), and (2) precipitation strengthening by dispersion of the c0 precipitates in the Ni-base superalloy matrix (c) [1]. Nevertheless, the high temperature creep behavior of Ni-based single crystal superalloys is strongly dependent upon other factors such as volume fraction, coarsening, and the average size and distribution of the c0 precipitates [2]. Critical gas turbine components are used in harsh environments can lower the strength of Ni-based single crystal superalloys. To prevent oxidation and corrosion during high temperature applications, a protective coating on the surface of gas turbine components made by single crystal superalloys is necessary [3]. Aluminide diffusion coatings have been used for gas turbine components as an oxidation resistant coating material that improves the lifetime of the component operating at high temperatures [3]. Several methods are used to produce aluminide coating, including one known as pack aluminizing process [4]. Microstructure instability during service at elevated temperatures, leads to the formation of topologically close-packed (TCP)
phases [5] and the secondary reaction zone (SRZ) [6]. In addition, the effect of crystallographic orientation is an interesting subject concerning in Ni-based single crystal superalloys. It was reported that the mechanical properties of Ni-based single crystal superalloys are affected by the crystallographic orientation of the specimen surface [7]. Acharya and Fuchs [8] studied the effect of long-term thermal exposure on the microstructure and creep properties of the Nibased single crystal superalloy CMSX-10, and they concluded that the creep strength of CMSX-10 during long-term aging was reduced. Xia et al. [9] also reported the influence of thermal exposure on the microstructure and stress rupture properties of the Ni-based alloy DZ951. However, we have found no reports that discuss the effects of thermal exposure and crystallographic orientation on the creep properties of coated Ni-based single crystal superalloys. The purpose of the present study was to investigate the influence of thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy in different surface orientations. We observed the microstructural changes before and after creep tests as a function of crystallographic orientation, and we briefly discuss the relationship between deformation mechanisms and creep properties.
2. Material and experimental procedures ⇑ Corresponding author. Tel./fax: +81 42 6772709. E-mail addresses: tmu.ac.jp (F.H. Latief).
[email protected],
hamdanlatief-fahamsyah@ed.
0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.11.075
A Ni-based single crystal superalloy with a chemical composition of 12Co, 3Cr, 2Mo, 6W, 6Al, 6Ta, 5Re, 0.1Hf and balance Ni (in mass%) was used as the substrate. A four-step solution heat
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
treatment was conducted, first at 1240 °C for 1 h, second at 1280 °C for 2 h, third at 1300 °C for 2 h, and fourth at 1320 °C for 8 h, followed by gas fan cooling. A two-step aging treatment was conducted, first at 1150 °C for 4 h and second at 870 °C for 20 h, followed by gas fan cooling. The crystal orientation of the Ni-based single crystal superalloy was determined by the X-ray Laue reflection method. The creep specimens were cut from cylindrical bars with a square cross-section area of 2.8 mm 2.8 mm and a gauge length of 19.6 mm as reported in our previous work [10]. The creep specimens were prepared by electric discharge machining (EDM) with {1 0 0} and {1 1 0} side-surfaces (Fig. 1). The specimens were mechanically polished down to 1200 mesh by silicon carbide (SiC) paper and ultrasonically cleaned in an acetone bath for 10 min prior to the aluminizing process. The polished specimens were embedded in an Al2O3 retort containing a mixture of 24.5 Al, 24.5 Fe, 49 Al2O3 and 2 NH4Cl (in mass%) and then heated at 1000 °C for 5 h in an argon environment during the aluminizing process. Some aluminized specimens were thermally exposed at 1100 °C for 500 h prior to the creep rupture test (designated EXAL) and some of them were not thermally exposed (designated AL). We also evaluated the bare specimens (designated as UNAL) in comparison with the aluminized specimens. The stress orientation of all of the specimens was within 4° of h0 0 1i. The creep rupture tests were performed at a temperature of 900 °C under a constant load of 392 MPa with a load direction parallel to the h0 0 1i direction. We examined the creep specimens with different surface orientations to determine the anisotropic creep properties. The heat-treated samples were polished for the examination of microstructures. A solution containing HCl and HNO3 at the ratio of 3:1 was used as the chemical etchant to selectively dissolve the c0 phase present in the microstructures, to facilitate the identification of the precipitates. The virgin material was observed by optical microscopy (OM) to determine the initial microstructure and the microstructures of the aluminized specimens after exposure, as well as before and after the creep rupture tests were observed by scanning electron microscopy (SEM). 3. Results The c/c0 two-phase structure was readily observed in the heattreated superalloy before the aluminizing process, as is the case with standard Ni-based single crystal superalloys (Fig. 2). The
817
Fig. 2. Optical and SEM micrographs of a Ni-base single crystal superalloy.
cuboidal c0 precipitates were regularly aligned along h0 0 1i during the aging treatment due to the elastic interaction between precipitates. The alloy used in this study contained about 70 vol.% cuboidal c0 precipitates that distributed uniformly in the matrix (c). From our observation, the average size of the cuboidal c0 precipitates was about 300 nm. Fig. 3 shows the cross-sectional microstructures of asaluminized specimens without (AL) and with thermal exposure (EXAL). Generally, three distinctive domains were observed in the as-aluminized specimens: (1) an outer protective layer composed of b-NiAl, and it has been confirmed in our previous paper [11], (2) an interdiffusion zone (IDZ), and (3) the substrate. The IDZ thickness of the AL specimens was similar for both surfaces at be about 5 lm (Fig. 3a and b). Moreover, the diffusion zone was extended after 500 h exposure at 1100 °C in the EXAL specimens for both side-surfaces (Fig. 3c and d). The IDZ of the EXAL specimens seemed to be widened and the secondary reaction zone (SRZ) was formed beneath the IDZ. The SRZ is defined as an intermediate layer formed by a cellular reaction, and a discontinuous precipitate reaction, which is similar to recrystallization [5]. SRZ formation may occur during the coating process or during exposure at elevated temperatures [12]. An SRZ is mostly formed under an IDZ as a result of the inward diffusion of Al from the coating layer to the superalloy substrate. The total diffusion zone thickness (i.e., the combination of IDZ + SRZ) of
Fig. 1. Arrangement of f1 1 1gh1 0 1i slip systems for (a) the {1 0 0} and (b) {1 1 0} side-surface orientations.
818
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
Fig. 3. Microstructures of as-aluminized specimens: (a and b) without thermal exposure and (c and d) with thermal exposure with the: (a and c) {1 0 0} side-surface and (b and d) {1 1 0} side-surface orientations.
24
UNAL Specimens
20
Creep strain (%)
{100}
16
{110}
12 8 4 0
0
100
200
300
400
500
Time (h)
600
700
800
Fig. 5. Creep curves of the bare specimens at 900 °C/392 MPa.
Fig. 4. Microstructures of c/c0 structure after long-term exposure for 500 h at 1100 °C: (a) {1 0 0} side-surface and (b) {1 1 0} side-surface.
the EXAL specimens was approximately 67 lm on the {1 0 0} sidesurface and 77 lm on the {1 1 0} side-surface (Fig. 3c and d). Voids appeared within the SRZ on the {1 1 0} side-surface (Fig. 3d), which may be attributed to the Kirkendall effect [13]. Small TCP phases with a needle-like shape [5] were found in both side-surfaces (Fig. 3c and d). The changes in the c0 precipitates after the 500 h exposure at 1100 °C are shown in Fig. 4. The size of the c0 precipitates increased due to thermal exposure, and the shape became irregular during thermal exposure for both surfaces. The creep rupture lives of the UNAL specimens were 732 h for the {1 0 0} side-surface and 697 h for the {1 1 0} side-surface (Fig. 5). With respect to the specimens with aluminide coating, the creep rupture lives of the specimens AL were 525 h for the {1 0 0} side-surface and 428 h for the {1 1 0} side-surface, whereas
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
24 20
Creep strain (%)
AL Specimens
(a)
{100} {110}
16 12 8 4 0
0
100
200
300
400
500
600
Time (h) 24
(b)
Creep strain (%)
20
EXAL Specimens
16
{110} {100}
12 8 4 0
0
10
20
30
40
50
60
70
80
90
100
Time (h) Fig. 6. Creep curves of the aluminized specimens at 900 °C/392 MPa: (a) without and (AL), (b) with pre-exposure (EXAL).
819
for the EXAL specimens were 58 h for the {1 0 0} side-surface and 49 h for the {1 1 0} side-surface (Fig. 6). In short, the thermal exposure performed at 1100 °C for 500 h prior to the creep rupture test resulted in an extreme reduction of creep rupture life. Nevertheless, the {1 0 0} specimens exhibited a longer creep rupture lives than the {1 1 0} specimens in all conditions. Fig. 7 shows the microstructures of ruptured AL and EXAL specimens. In the AL specimens, it was clear that the SDZ was formed on both surfaces (Fig. 7a and b). The SDZ is described as a layer that lies underneath the IDZ where the matrix has not transformed to bNiAl but it does have an increased level of aluminum. The SDZ can be distinguished from the IDZ as it remains particularly c0 and retains the orientation of the single crystal substrate [5]. The total diffusion zone (IDZ + SDZ) thickness was approximately 30 lm on the {1 0 0} side-surface and 37 lm on the {1 1 0} side-surface (Fig. 7a and b). TCP phases and voids occurred within the SDZ. The ruptured EXAL specimens were similar to their microstructures before the creep test, where the IDZ and SRZ were still observed on both surfaces (Fig. 7c and d). However, the total diffusion zone (IDZ + SRZ) after the creep test was more extensive compared to that observed before the creep test. The total diffusion zone (IDZ + SRZ) was about 82 lm on the {1 0 0} side-surface and 89 lm on the {1 1 0} side-surface (Fig. 7c and d). The numbers of TCP phases were somewhat increased after the creep test for both orientations. Voids were still observed on the {1 1 0} side-surface (Fig. 7d). Fracture surfaces of AL and EXAL specimens are shown in Figs. 8 and 9, respectively. Both figures show two modes of fracture, that is, the fracture along the single slip plane near the surface and the micro-facets fracture, which is associated with {1 1 1}-multiple-slip deformation inside of the specimen. In general, the fracture surface on the {1 0 0} side-surface revealed the {1 1 1} slip plane at the corner, whereas the fracture surface on the {1 1 0} side-surface exhibited the {1 1 1} slip plane at the edge, in agreement with the
Fig. 7. Cross-section images of ruptured aluminized specimens: (a and b) AL specimens and (c and d) EXAL specimens with the: (a and c) {1 0 0} side-surface and (b and d) {1 1 0} side-surface orientations.
820
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
(a)
(b)
(111)
(111)
(c) (111) (111) (111)
200 µm Fig. 8. Fracture surfaces of ruptured AL aluminized specimens without pre-exposure: (a) {1 0 0} side-surface, (b) {1 1 0} side-surface, and (c) magnified image of (a).
(a)
(b)
Fig. 9. Fracture surfaces of ruptured aluminized specimens with pre-exposure (EXAL): (a) {1 0 0} side-surface and (b) {1 1 0} side-surface.
arrangement shown in Fig. 1. The micro-facets fracture was clearly seen in the magnified view of the micro-facets fracture on the {1 0 0} side-surface (Fig. 8c). In the fracture surfaces of EXAL specimens (Fig. 9), the micro-facets fracture was not so as clear as in the AL specimens (Fig. 8). 4. Discussion Our comparison of the creep rupture lives of bare and aluminized specimens showed that the aluminizing process has a deleterious effect on the creep strength of the Ni-based single crystal superalloy used (Figs. 7 and 8). The reduction of creep lives in
the aluminized specimens may have been due to the inward diffusion of Al from the protective coating into the superalloy substrate. This diffusion might cause an increase in the Al concentration in the diffusion zone, resulting in the reduced creep strength and the premature failure of the sample [14]. In addition, the decrease in the creep lives of the aluminized specimens may be due to the weakening of atomic bonds, since aluminum is a low-melting metal its effect can be considered as similar to that of other low melting metals [15]. The decrease in the load-bearing section because of the formation of the coating layer could also be one of factors in the reduction of the creep strength [16]. The anisotropic creep properties
F.H. Latief, K. Kakehi / Materials and Design 56 (2014) 816–821
between the two side-surfaces were more tangible in the aluminized AL and EXAL specimens (Fig. 8). This creep anisotropy is primarily due to the different arrangements of the f1 1 1gh1 0 1i slip systems during creep deformation (Fig. 2), associated with the extension of the diffusion zone and the formation of TCP phases. We note that the mechanical properties of Ni-based single crystal superalloys are influenced by the morphology, size, volume fraction and distribution of c0 precipitates in the matrix. The coarsening of c0 precipitates occurs when aluminized specimens are subjected to high-temperature thermal exposure [17]. At high temperatures, deformation is controlled by the dislocation climb. When the c0 precipitates are coarsened during thermal exposure, the width of the c matrix channel is enlarged as well [18]. This will weaken the resistance to the dislocation movement, and consequently, the creep strength of aluminized EXAL specimens was reduced [19]. In the present study we found that the most noticeable effect of exposing the aluminized Ni-based single crystal superalloy to high temperature condition was the changes in microstructure. After 500 h at 1100 °C, the diffusion zone became larger and an SRZ was formed at this stage (Fig. 5). The formation of an SRZ is one of the serious problems in applications of coated Ni-based single crystal superalloy, as is TCP phase. An SRZ is usually found in NiAl coated specimens as a consequence of the inward diffusion of Al from the coating into the substrate. When Al diffuses inward toward the substrate and solutes in the c0 phase during long-term thermal exposure, the transformation from c to c0 will take place and it will break the c–c0 net-like structure in the substrate. We note that the solubility of the refractory elements such as Re, W and Mo is higher in the c/c0 structure than that in the coating layer, which causes the precipitation of the refractory elements from the c/c0 structure and leads to the formation of a TCP phase and SRZ [20]. The Al activity in the coating is prone to form an SRZ in a coated Ni-based single crystal superalloy [6]. Therefore, an SRZ is normally observed under the IDZ in a coated Ni-based single crystal superalloy specimen. The Ni-based single crystal superalloys containing high concentrations of refractory elements (such as Re) are potential candidates for the formation of an SRZ [21]. The formation of an SRZ will result in the lowering of the mechanical properties of the superalloy and coarsening of the c/ c0 structure [22]. However, an SRZ is not formed when the surface of the specimen is prepared by an electro-polishing technique prior to the aluminizing process [9]. Concisely, long-term thermal exposure has a deleterious effect on the creep rupture life of aluminide coating on a Ni-based single crystal superalloy. 5. Conclusions We investigated the influence of long-term thermal exposure on the creep properties of an aluminized Ni-based single crystal superalloy at different surface orientations, and we found that the thermal exposure of aluminized specimens at 1100 °C for 500 h prior to a creep test decreased the creep strength of the aluminized specimens. This is attributed primarily to the coarsening of c0 precipitates during the thermal exposure, accompanied with the formation of an SRZ. In addition, when aluminized specimens are exposed to high-temperature environments and stress, an interdiffusion of elements between the single crystal substrate and the coating layer occurs. However, the outward diffusion of
821
Ni from the substrate leads to the enrichment of refractory elements and promotes the formation of TCP phases which may also degrade the creep rupture lives of aluminized specimens both without and with thermal exposure. We also determined, the anisotropic creep properties induced primarily by different arrangements of the f1 1 1gh1 0 1i slip system. Acknowledgements We gratefully acknowledge Dr. H. Murakami of the National Institute for Materials Science, Tsukuba, for his support in the preparation of the aluminide coating, and Mr. M. Nakane for his contribution. References [1] Tian S, Su Y, Qian B, Yu X, Liang F, Li A. Creep behavior of a single crystal nickelbased superalloy containing 4.2% Re. Mater Des 2012;37:236–42. [2] Baldan A. Review progress in Ostwald ripening theories and their applications to the c0 -precipitates in nickel-base superalloys Part II Nickel-base superalloys. J Mater Sci 2002;37:2379–405. [3] Pomeroy MJ. Coatings for gas turbine materials and long term stability issues. Mater Des 2005;26:223–31. [4] Zhan Z, Liu Z, Liu J, Li L, Li Z, Liao P. Microstructure and high-temperature corrosion behaviors of aluminide coatings by low-temperature pack aluminizing process. Appl Surf Sci 2010;256:3874–9. [5] Rae CMF, Hook MS, Reed RC. The effect of TCP morphology on the development of aluminide coated superalloys. Mater Sci Eng A 2005;396:231–9. [6] Murakami H, Sakai T. Anisotropy of secondary reaction zone formation in aluminized Ni-based single-crystal superalloys. Scr Mater 2008;59:428–31. [7] Caron P, Khan T, Nakagawa YG. Effect of orientation on the intermediate temperature creep behavior of Ni-base single crystal superalloys. Scr Metall 1986;20:499–502. [8] Acharya MV, Fuchs GE. The effect of long-term thermal exposures on the microstructure and properties of CMSX-10 single crystal Ni-base superalloys. Mater Sci Eng A 2004;381:143–53. [9] Xia PC, Yu JJ, Sun XF, Guan HR, Hu ZQ. The influence of thermal exposure on the microstructure and stress rupture property of DZ951 nickel-base alloy. J Alloys Compd 2007;443:125–31. [10] Latief FH, Kakehi K. Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys. Mater Des 2013;49:485–92. [11] Latief FH, Kakehi K, Tashiro Y. Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air. J Ind Eng Chem 2013;19:1926–32. [12] Walston WS, Schaefter JC, Murphy WH. A new type of microstructure instability in superalloy-SRZ. In: Kissinger RD et al., editors. Superalloys. PA, USA: TMS, Warrendale; 1996. p. 9–18. [13] Smigelskas AD, Kirkendall EO. Zinc diffusion in alpha brass. Trans AIME 1947;171:130–42. [14] Latief FH, Kakehi K, Murakami H. Anisotropic creep properties of aluminized Ni-based single-crystal superalloy at intermediate and high temperatures. Scr Mater 2013;68:126–9. [15] Nikitin VI, Grigor’eva TN. Effect of aluminizing on the creep strength of a Nickel alloy in some environments. Mater Sci 1975;10:5–9. [16] Dryepondt S, Zhang Y, Pint BA. Creep and corrosion testing of aluminide coatings on ferritic–martensitic substrates. Surf Coat Technol 2006;201:3880–4. [17] Xia PC, Yu JJ, Sun XF, Guan HR, Hu ZQ. Influence of thermal exposure on gamma prime precipitation and tensile properties of DZ951 alloy. Mater Character 2007;58:645–51. [18] Aghaie-Simonetti M, Hajjavady M. The effect of thermal exposure on the properties of a Ni-base superalloy. Mater Sci Eng A 2008;487:388–93. [19] Liu JJ, Jin T, Yu JJ, Sun XF, Guan HR, Hu ZQ. Effect of thermal exposure on stress rupture properties of a Re bearing Ni base single crystal superalloy. Mater Sci Eng A 2010;527:890–7. [20] Wang Y, Guo HB, Peng H, Gong SK. Diffusion barrier behaviors of (Ru, Ni)Al/ NiAl coatings on Ni-based superalloy substrate. Intermetallics 2011;19:191–5. [21] Walston WS, Cetel A, MacKay R, O’Hara K, Duhl D, Dreshfield R. Joint development of a fourth generation single crystal superalloys. In: Green KA et al., editors. Superalloys. PA, USA: TMS, Warrendale; 2004. p. 15–25. [22] Suzuki AS, Kawagishi K, Yokokawa T, Harada H. Effect of Cr on microstructural evolution of aluminized fourth generation Ni-base single crystal superalloys. Surf Coat Technol 2012;206:2769–73.