Co-Re-based Alloys - Wiley Online Library

COMMUNICATIONS

DOI: 10.1002/adem.200700132

Co-Re-based Alloys: A New Class of High Temperature Materials?** By Joachim Rösler, Debashis Mukherji and Torsten Baranski Ni-based superalloys are the dominant material class in the hot section of gas turbines. This is so, because they uniquely combine three key properties at a favourable performance to cost ratio: toughness, strength at elevated temperature and oxidation resistance. However, there is also an important limitation. Given gas temperatures in excess of 1500 °C and peak material temperatures of up to 1100 °C, the latter are already very close to the melting point of superalloys at around 1300 °C. It is very likely that material development will continue to push the maximum allowable service temperature of superalloys upwards. However, there is a limit and given the long term nature of material development, it is essential to search now for solutions beyond the temperature capability of Ni-based superalloys. For this reason, substantial international research is underway, focussing on ceramics,[1,2] intermetallic materials such as NiAl,[3,4] refractory metals such as Nb-based and Mo-based alloys,[5–8] as well as noble metals mimicking the c/c′-microstructure of superalloys.[9–12] The difficulty faced in all these new developments is that these materials have at least one pronounced weakness regarding the above mentioned key properties, and the challenge is to find acceptable solutions. In fact, there is substantial progress. For example, much improved oxidation resistance has been reported for certain refractory alloys and significant patent activity is noticeable in this field.[5–8] Nevertheless, none of the evaluated material systems has come to the point were an acceptable property mix has been reached, allowing for demanding applications in the hot section of gas turbines. For this reason, it seems sensible to further search for alternative solutions, which have not been explored so far. One, namely the development of Co-Re-based high temperature alloys, is addressed here and the article is structured as follows. Firstly, the rational for focussing on Co-Re-based alloys is given. Four alloys are then evaluated to gain first in-

– [*] Prof. Dr. Joachim Rösler, Dr. Debashis Mukherji, Dipl.-Ing. Torsten Baranski Institute für Werkstoffe, Technical University Braunschweig, Langer Kamp 8, D-38106 Braunschweig, Germany E-mail: [email protected]; [email protected] [**] We would like to thank the Deutsche Forschungsgemeinschaft for financial support of this research in the frame of the DFG research group “Beyond Ni-base Superalloys”.

876

sight in the microstructure evolution of this material class. Finally, these results are summarized and directions for further alloy development are discussed. The Co-Re System Co-based alloys have their place in turbines since many decades. They are used nowadays primarily because of the ease of manufacturing (air casting instead of vacuum casting) and repair. They exhibit also excellent thermal shock resistance, making Co-based alloys a good choice for vane applications. However, they lack the strength of c′-hardened Nibased superalloys and are therefore not suitable for highly stressed components. This is probably the reason why the development of Co-based alloys has essentially ceased some decades ago, whereas superalloy development is still ongoing. One major event in the long history of Ni-based superalloys was the introduction of rhenium as an additional alloying element, starting in the early 1980’s.[13,14] While the so-called second generation single crystal alloys contained about 1 % rhenium (all values are in atomic percent unless otherwise stated), 2 % is typical for third generation alloys. However, these latter developments were also facing a severe problem, as brittle phases started to occur due to the presence of rhenium.[15] For this reason, a further increase of the Re content in Ni-based superalloys by a significant amount is unlikely, even though it would pay off despite the relatively high price of rhenium, provided a corresponding property gain can be achieved. Comparing the Ni-Re and Co-Re phase diagrams, it is noticed that rhenium increases the melting temperature in both systems alike. However, the important difference is that its solubility is limited in nickel, as mentioned above, while complete miscibility occurs in the Co-Re system.[16] This opens a unique possibility, as it allows to steadily elevate the melting range, changing the character of the material from that of a contemporary Co-based alloy to that of a high melting point material. In other words, there is a good chance to tune the properties as required and find a suitable balance between the need for toughness and ductility on the one hand and strength at temperatures beyond the capability of Ni-based superalloys on the other hand. Of course, material cost, density and oxidation resistance, which are adversely affected by the addition of rhenium, have to be considered at the same time. Nevertheless, it seems worthwhile to explore the potential of Co-Re-based alloys as a new high temperature material class for service temperatures beyond 1200 °C. Before starting such a development effort, it is sensible to inspect the “classical” Co-based gas turbine materials first.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ADVANCED ENGINEERING MATERIALS 2007, 9, No. 10

Rösler et al:/Co-Re-based Alloys: A New Class of High Temperature Materials

– Co-17Re-22Cr-2.6C (alloy CoRe1) – Co-17Re-23Cr-1.2Ta-3.0C (alloy CoRe2) – Co-17Re-22Cr-1.4Si-2.6C (alloy CoRe3)

to 3 mm thick plates. As a sufficiently high melting range is a key requirement, the alloys CoRe1 and CoRe4 were tested by differential thermal analysis to a temperature of 1450 °C, which is the maximum furnace temperature of the used NETZSCH DSC 404C analysis system. Onset of melting was not observed up to that temperature, confirming that the solidus temperature is significantly increased compared to conventional Ni- and Co-based alloys due to the addition of rhenium. The microstructures of the alloys were studied in the ascast and solution heat treated condition, using the parameters given in Table 1. Figure 1 shows the as-cast microstructure of alloy CoRe1. A dendritic microstructure is noticed. The micro-hardness (520 HV 0.003) of the dendrites exceeds that of the interdendritic regions (370 HV 0.003) significantly (see insets in Fig. 1). This effect can be associated to the alloying element rhenium, which segregates to the dendritic core, leading to solid solution strengthening there. Occasionally, light-grey areas are observed in the centres of the dendrites (see arrows in Fig. 1). They exhibit the highest hardness and are particularly Re-rich. It is not entirely clear at present whether the contrast is merely due to segregation or to the formation of a Re-rich phase. Solution heat treatment according to Table 1 leads to substantial homogenisation and the light-grey areas disappear entirely (Fig. 2). However, the microstructure appears now “freckled” at low magnification and precipitates are observed at grain boundaries (Fig. 2(a,b)). The latter are Table 1: Heat treatment parameters

Alloys

Solution heat treatment

CoRe1, CoRe4

1350°C/5h + 1400°C/5h + 1450°C/5h, Argon quenched

CoRe2, CoRe3

1350°C/7.5h + 1400°C/7.5h, Argon quenched

– Co-31Re-23Cr (alloy CoRe4) Rhenium contents of 17 % and 31 % were selected as they span the expected compositional range of this element. Carbon has been added to alloy CoRe1 in order to study the Cr23C6 formation and compare the precipitation reaction with that of conventional Co-alloys. As service temperatures beyond 1200°C are anticipated, carbide stability at elevated temperatures is an issue. For this reason, alloy CoRe2 was designed to study the role of tantalum as strong MC-carbide forming element. As mentioned above, chromium alone will not suffice to ensure adequate oxidation resistance. Consequently, it is the objective of alloy CoRe3 to gain first insight as to how silicon influences the microstructure evolution of this material class. Finally, the extent to which Re can be added is explored with alloy CoRe4. Experimental Results The four alloys CoRe1-CoRe4, weighing about 400 gr each, were melted in a vacuum arc furnace and subsequently cast


Fig. 1. Alloy CoRe1 in the as-cast condition (scanning electron microscopy image). Microhardness values (HV 0.003) of the dendritic and interdendritic regions are given. Areas, displaying a light-grey contrast, are marked by arrows and their hardness is shown as well.


http://www.aem-journal.com

877

COMMUNICATIONS

They typically contain 25 % chromium for corrosion and oxidation protection. Furthermore, they rely on solid solution hardening by addition of about 2.5 % tungsten and precipitation strengthening via carbide formation. About 1.5 to 3 % carbon (corresponding to 0.3–0.6 weight percent) is added for the latter purpose, leading predominantly to the formation of chromium-rich M23C6 type carbides. In some alloys, more stable MC-carbide formers such as tantalum are also incorporated for additional strength.[17,18] About 10 % nickel is usually present in order to stabilize the fcc a-phase of cobalt. In Co-Re-based alloys, Re takes the role as solid solution strengthening element and it is worth noting that rhenium is well known as potent solid solution hardener of refractory alloys.[19,20] It is special amongst the refractory metals as it possesses highest strength in unalloyed form and is the only element with substantial ductility at ambient temperature, stemming from its hcp crystal structure. For example, a tensile strength of 330 MPa at 1400 °C is reported in[21] for pure rhenium. Consequently, tungsten is not essential as alloying addition in this case. On the other hand, it appears to be sensible to add chromium in comparable quantities as in contemporary Co-based alloys. Even though chromium alone will not suffice for oxidation protection at very high temperatures, it is an important element as it effectively protects against oxidation and environmental embrittlement at intermediate temperatures. In presence of aluminium, it can also assist the formation of a protective Al2O3-scale, thus reducing the required aluminium content.[22] Similarly, the precipitation hardening concept by carbide formation can be adopted for Co-Re-based alloys. Putting these considerations together, the following nominal alloy compositions have been selected to start the development effort:

COMMUNICATIONS

Rösler et al:/Co-Re-based Alloys: A New Class of High Temperature Materials carbides, which have probably formed by a eutectic reaction upon solidification and remained undissolved during the solutioning heat treatment. The “freckles” in the grain interior are associated with carbides as well, having a blocky morphology (Fig. 2(c)). Surprising on first sight is the fact that the micro-hardness of the matrix increases due to the heat treatment at 1450 °C from about 370–650 HV 0.003 to a value of about 800 HV 0.003. The reason is apparent at higher magnification: Fine lamellar carbides have formed in the grain interior in addition to the blocky carbides. The fine lamellar carbides can be best seen in the etched condition (Fig. 2(d)). Their diffuse contrast in the polished condition leads to the above mentioned “freckled” appearance of the microstructure at low magnifications. A similar microstructure is unknown for conventional Co-alloys, and it is at present unclear under which conditions these precipitates form. However, it is reasonable to assume that they are responsible for the high hardness value. Remarkable is the fineness of the lamellar structure with distances between the platelets well below 1 lm, and it may be speculated that rhenium plays a role in this respect in slowing down diffusion processes. Addition of tantalum in alloy CoRe2 causes certain changes in the microstructure. Even though the microstructure is still dendritic after solidification (Fig. 3(a)), the appear-

(a)

(b)

(c)

(a)

(d)

(b)

Fig. 2.(a-d) Alloy CoRe1 after solution heat treatment in polished (a-c) and etched condition (d). A low magnification overview is given in (a). Carbides at grain boundaries are visible in (a, b). Blocky and lamellar carbides in the grain interior are shown in (c) and (d), respectively. Microhardness (HV 0.003) values are indicated in (c). Aqua Regia was used as etchant.

878


Fig. 3.(a,b) Alloy CoRe2 in the as-cast condition. Fig. b shows the marked area in fig. a at higher magnification. Microhardness (HV 0.003) values of dendritic, interdendritic and rhenium rich (light-grey) areas are given in (b) (scanning electron microscopy images).



Rösler et al:/Co-Re-based Alloys: A New Class of High Temperature Materials In contrast to the minor influence of silicon on the microstructure evolution, increasing the rhenium content to 31 % in alloy CoRe4 has a profound effect. In the as-cast condition, a dendritic microstructure with pronounced Re-segregation is observed as before (Fig. 7(a)). Once again, distinction between segregation and formation of a second phase is difficult and identification of the crystal structure by X-ray or TEM diffraction is required to clarify this point. However, it is apparent that the area fraction of the Re-rich domains has

Fig. 5. A bright filed transmission electron microscopy image of alloy CoRe2 in the solution heat treated condition. Extremely fine carbide particles in-between the lamellar carbides are noticed.

(a)

(b)

Fig. 4. Alloy CoRe2 in the solution heat treated condition. Ta-rich MC-carbides are visible in the grain interior and at grain boundaries. Furthermore, a Re-rich phase is noticed (scanning electron microscopy image).


Fig. 6.(a,b) Alloy CoRe3 in the solution heat treated and etched condition. (a) Fine lamellar carbide precipitates. (b) Carbides and Re-rich phase (light-grey) at the grain boundaries (scanning electron microscopy images).



879

COMMUNICATIONS

ance of the light-grey Re-rich areas is now more prominent. Inspection of Figure 3(b) suggests, that the light-grey contrast is due to pronounced Re-coring (area marked “1” in Fig. 3(b)) and to the formation of a second Re-rich phase (area marked “2” in Fig. 3(b)). As before, the micro-hardness of these areas is very high (see Fig. 3(b)). Additionally, tantalum rich MCcarbides with Chinese script morphology are noticed in the interdendritic regions, which is also typical for contemporary Co-alloys containing tantalum. Solutioning reduces the amount of the interdendritic MC-carbides. However, they do not disappear entirely. Homogenization also takes place and the contrast associated with Re-coring disappears, whereas a Re-rich phase is now undoubtedly present (Fig. 4). It is located alike in dendritic and interdendritic regions. In addition, round TaC particles appear, which are difficult to distinguish from the Re-rich particles in Fig. 4. A particularly remarkable feature of alloy CoRe2 is the fact, that carbides are formed on three different length scales. As shown above, relatively coarse blocky and script like carbides are present with typical dimensions of 1-10 lm. In addition, lamellar carbides with interlamellar spacing of some hundred nanometres are formed as shown in Figure 2(d) for alloy CoRe1. Inspecting the matrix between these carbide platelets in the transmission electron microscope shows that even finer carbides are present with characteristic dimensions of some ten nanometres (Fig. 5). It has to be emphasized that investigation of these alloys has just started, and as a consequence, very little is known about the stability of the carbides and the conditions under which they form. Nevertheless, it appears that there is significant strengthening potential due to these precipitation reactions at different length scales. The addition of silicon to alloy CoRe3 has little influence on the microstructure and it resembles that of alloy CoRe1 in the solution heat treated condition. Very fine lamellar carbides are again visible in the etched condition (Fig. 6(a)) along with blocky carbides and grain boundary carbides as seen in alloy CoRe1 (Fig. 2). However, one difference is the occasional presence of a Re-rich phase at the grain boundary regions, often occurring along with carbides (Fig. 6(b)).

COMMUNICATIONS

Rösler et al:/Co-Re-based Alloys: A New Class of High Temperature Materials significantly increased and their micro-hardness is extremely high (1395 HV 0.005). It is to be noted that no carbon was added to this alloy and, consequently, carbides do not appear. Solutioning leads unambiguously to a two-phase microstructure, consisting of a brighter Re-rich phase and a darker Co-rich phase (Fig. 7(b)). The compositions of the two phases were measured by energy dispersive X-ray spectroscopy (EDX), which gives the composition (in at. %) of matrix as 54.4 % Co, 17.3 % Cr and 28.3 % Re and that of the Re-rich phase as 39.8 % Co, 24.6 % Cr and 35.6 % Re. The micro-hardness values of the two phases are shown in Figure 7. Formation of a second phase is not expected from the binary Co-Re phase diagram. It may be due to the presence of chromium[23] and diffraction experiments, as mentioned above, are needed for further clarification. Summary and Future Perspectives Even though investigation of Co-Re based alloys as a material class for high temperature applications is at a very early stage of development, the obtained results are encouraging. They show that substantial elevation of the melting temperature can be achieved by addition of rhenium. Furthermore, the microstructural investigations point to interesting options for alloy strengthening. Firstly, substantial solid solution

(a)

(b)

Fig. 7.(a,b) Microstructure of alloy CoRe4 in the as-cast (a, polished) and solution heat treated condition (b, etched). A two-phase microstructure is noted after solutioning (scanning electron microscopy images).

880


hardening is noticed due to the presence of rhenium. Secondly, carbides are available at different length scales for precipitation strengthening. Thirdly, a composite structure, consisting of harder (Re-rich) and softer (Co-rich) phases is observed, which may strengthen by load transfer. However, there are also a number of open issues at this early phase of alloy development. One concern is the stability of the carbides and their interaction with dislocations during creep deformation. Very likely, thermo-dynamically more stable carbides than Cr23C6 are needed to ensure acceptable properties during long term operation. As shown here, addition of tantalum to from MC-type carbides is a possibility and their stability during elevated temperature exposure has to be carefully examined in the future. Other elements such as titanium, zirconium or hafnium, having an even higher enthalpy of carbide formation than tantalum, may be also explored, especially since the melting temperature of the Co-Re matrix is higher than that of contemporary Co-based alloys, thus allowing for higher solutioning temperatures. Another open issue pertains to the oxidation resistance, which is adversely affected by addition of refractory metals such as rhenium. As mentioned above, chromium is helpful to improve the oxidation resistance in a number of ways. However, it is not sufficient as Cr2O3-scales are not stable enough beyond 1000 °C. Options to improve their stability exist, for example by addition of manganese.[24] Yet, it is likely that Al2O3- or SiO2-scales are needed in view of the intended application temperatures. In fact, aluminium is found already today in certain cobalt alloys for this reason.[25] Given the limited solubility of aluminium in cobalt and rhenium and the need for a sufficiently high aluminium concentration to ensure formation of a dense Al2O3-scale, one has then to tolerate the formation of relatively brittle aluminide phases within the matrix. Alternatively, silicon can be used as alloying element and the example of alloy CoRe3 shows that small amounts of Si can be dissolved without significant influence on the microstructure. However, it is apparent that larger quantities are required to ensure formation of a protective SiO2-scale and Re-rich silicides are likely to form under these circumstances. This is analogous to Mo-based refractory alloys for high temperature applications containing typically 2–15 % Si,[7] where molybdenum silicides form within the molybdenum rich matrix. In this context, it is noteworthy that further addition of boron significantly improves the oxidation resistance of these alloys,[6] presumably because of the formation of a relatively low viscosity glassy borosilicate phase on the specimen surface. Whether an analogous approach can be used in case of the Co-Re system remains to be seen. In summary, we note that there are many open issues to be tackled and, thus, the question posed in the title cannot be conclusively answered at present. Nevertheless, Co-Re is a promising alloy system, as it fundamentally allows for the design of ductile high melting point materials and as the presented microstructural investigations show a wealth of op-



Rösler et al:/Co-Re-based Alloys: A New Class of High Temperature Materials

Received: May 29, 2007 Final version: July 16, 2007

– [1] F. W. Zok, C. G. Levi, Adv. Eng. Mater. 2001, 3, 15. [2] B. Kanka, H. Schneider, J. Eur. Ceram. Soc. 2000, 20, 619. [3] M. Rosefort, C. Dahmen, A. Bührig-Polaczek, W. Hu, H. Chen, Y. Zhong, G. Gottstein, D. Hajas, J. M. Schneider, Adv. Eng. Mater. 2006, 8, 730. [4] R. Darolia, W. S. Walston, M. V. Nathal, in Superalloys 1996 , TMS, 1996, 561. [5] B. P. Bewlay, M. R. Jackson, J.-C. Zhao, P. R. Subramanian, M. G. Mendiratta, J. J. Lewandowski, MRS Bull. 2003, 28, 646. [6] D. M. Dimiduk, J. H. Perepezko, MRS Bull. 2003, 28, 639. [7] D. M. Berczik, in United States Patent 1997, 5,595, 616. [8] P. Jéhanno, M. Heilmaier, H. Kestler, Intermetall. 2004, 12, 1005. [9] Y. Yamabe, Y. Koizumi, H. Murakami, Y. Ro, T. Aruko, H. Harada, Scr. Mater. 1996, 35, 211. [10] P. J. Hill, G. B. Fairbank, L. A. Cornish, JOM 2001, 53, 19. [11] R. Süss, D. Freund, R. Völkl, B. Fischer, P. J. Hill, P. Ellis, I. M. Wolff, Mater. Sci. Eng. 2002, A338, 133.

[12] S. Vorberg, M. Wenderoth, B. Fischer, U. Glatzel, R. Völkl, JOM 2004, 56, 40. [13] F. A. Schweizer, D. N. Duhl, in United States Patent 1980, 4,222, 794. [14] M. F. Henry, United States Patent 1983, 4,388, 124. [15] W. S. Walston, J. C. Schaeffer, W. H. Murphy, in Superalloys 1996, TMS, 1996, 9. [16] T. B. Massalski, in Binary Alloy Phase Diagrams 1986, ASM. [17] J. S. Haydon, A. M. Beltran, J. H. Wood, in United States Patent 1990, 4,938, 805. [18] Y. Fukui, T. Kashimura, in United States Patent 1984, 4,437, 913. [19] ASM Specialty Handbook “Heat Resistant Mater.”, ASM International, 1986, 361. [20] G. Leichtfried, J. H. Schneilbel, M. Heilmaier, Metall. Mater. Trans. 2006, 37A, 2955. [21] J. B. Lambert, J. J. Rausch, in ASM Handbook, 2, ASM International, 1990, 557. [22] C. S. Giggins, F. S. Petit, Solid State Sci. 1971, 118, 1782. [23] E. M. Sokolovskaya, M. L. Tuganbaev, G. I. Stepanova, E. F. Kazakova, I. G. Sokolova, J. Less-Common Met. 1986, 124, L5. [24] W. J. Quadakkers, V. Shemet, L. Singheiser, in Deutsches Patent- und Markenamt 2001, DE 100 25 108 A1. [25] S. T. Magyar, E. C. Hirakis, M. L. Gell, E. J. Felten, in United States Patent 1978, 4,078, 922.

______________________




881

COMMUNICATIONS

tions for alloy strengthening. Therefore, further exploration of this alloy system along the lines outlined above is a worth while endeavour.