Primary microstructure characterization of Co-20Ni-9Al-7W-3Re-2Ti superalloy

The characterization of the primary microstructure of the new Co-based superalloy of Co-20Ni-9Al-7W-3Re-2Ti type was shown in this article. The investigated alloy was manufactured by induction melting process from pure feedstock materials. The fundamental technological problem related to Co-Al-W-X multicomponent alloys' casting process is a strong susceptibility to interdendritic segregation of alloying elements, especially tungsten and rhenium. The performed analysis revealed that the observed effect of alloying elements segregation is detectable and much stronger than for Co-9Al-9W and Co-20Ni-7Al-7W alloys, related to titanium, nickel and aluminium migration to inter-dendritic spaces. Consequently, the tungsten concentration gradient between dendritic and interdendritic zones is higher than for Co-9Al-9W and Co-20Ni-7Al-7W alloys. The same situation is in the case of rhenium and cobalt, but Co's concentration in the interdendritic zone is only slightly lower.


INTRODUCTION
Superalloys based on Ni or Co are high-performance materials usually used in high-temperature elements of land-based and aircraft turbines such as discs, blades, rotating shafts, nozzle guide vanes, and combustor liners [1]. Between them, the Co-based superalloys have applications as critical turbine engine parts, where hot corrosion, wear, and oxidation resistance is required [2][3][4]. Due to relatively low mechanical properties, especially at high temperature, the conventional Co-based superalloys are not dedicated to highperformance structural elements of turbines such as blades and disk. However, they can be used for staticloaded parts, e.g., vanes. The lower applications possibilities of carbides strengthened Co-based superalloys is related to temperature-depended limitation of strengthening effect by carbides precipitation in Co-based matrix [2]. The most popular and widely used alloys from this group are Haynes 188, Mar-M and stellite. Those alloys are strengthened by solid solution Coss and carbides of refractory elements. Still, its hightemperature properties, such as creep resistance, are lower than Ni-based superalloys due to lack of γ/γʹ structure [5][6][7]. Recent investigations showed that there is the possibility of beneficial γ/γʹ structure creation in Co-based superalloys. In the Co-Al system, the phase Co3Al (similar to Ni3Al phase in Ni-based systems) generally does not exist, and precipitation of an equilibrium B2-CoAl phase is more likely [8,9]. However, the Co3Al phase with L12 type of lattice was detected occasionally in some of the grains of Co-Al ferromagnetic shape memory alloys [10]. Simultaneously in the Co-X (X=W, Nb, Ta) systems, compounds Co3X with ordered L12 structure (γʹ) has been reported [11][12][13][14], but these phases are not stable at the higher temperature (> 600 ℃) [10,[15][16][17]. The Co3X L12 ordered phases are characterized by cuboidal shape, similar to Ni3(Al, Ti) phase in the Ni-based superalloys [18] and transform at the higher temperature to equilibrium disordered form with the same formula Co3X but with topological close-packed morphology D019 type of lattice) [19][20][21]. The metastability of Co3X phases with refractory elements can be weakened by forming triple phases such as Co3(Al, X). The presences of this type of compound were detected in the analysis of alloys form the Co-Al-W system. The additive and the appropriate proportion of Al and W in the structure, thereby stabilizing the L12 ordered structure of Co3(Al, W) phase up to 900 ℃ [15,16]. Mo's addition should get similar structural effects with additional decreasing of alloys density as an alloying element. However, the maximal level of Mo replacement is only up to 3 at %, because with the higher content of this alloying element, the formation of equilibrium Co3Mo phase with ordered D019 structure is expected. This type of precipitates is characterized by specific needle-like morphology favoring the cracking phenomena with a strongly brittle character [22]. Compared to Co3(Al, W), no data confirmed Co3(Al, Mo) phase with L12 type of structure. L12 ordering does not take place on ageing between 600 ℃ and 800 ℃ [16]. This transformation was detected in the cases of quaternary alloys of Co-Al-Mo-Nb/Ta type. It was revealed that Nb or Ta's small addition plays a critical role in stabilizing of γ-γʹ microstructure [18,22,23]. First principle calculations of Co3(Al, Mo, Nb) phase with the L12 structure revealed that this phase is mechanically stable and possesses intrinsic ductility. It is found additionally that the shear and Young's moduli of Co3(Al, Mo, Nb) are smaller than those of Co3(Al, W) [24]. Basing on those considerations, the new group of materials based on Co-Al-X systems (where X are refractory elements such as Wtungsten-containing alloys, and Mo, Nb or Tatungsten-free alloys) were developed [15,16,18,22,23]. The main strengthening element in those alloys is the ordered L12 phase with the overall formula Co3(Al, X). This new class of γʹ precipitates strengthened Co-based materials revealed higher solidus and liquidus temperatures and less segregation during solidification than traditional Ni-based superalloys [25]. The γʹ precipitates and the low-energy γ/γʹ interface provide high-temperature strength and stability to these alloys similar to the Ni-based superalloys. Intensive investigations in alloying strategy (increasing γʹ solvus temperature and growing strength and yield stress) get beneficial effects. At this moment, the creep properties and flow stress of tertiary, quaternary and quinary new Co-based superalloys are comparable to Ni-based superalloys of the first and second generation at a temperature approaching 900 ℃ [15,25]. But there are still challenges that require further research: decreasing the density of alloys and improving their creep and oxidation resistance [26,27]. In the case of increasing of γʹ solvus temperature and increasing strength and yield stress, only data for Co-Al-W are present in the literature. It can be assumed that to increase the γ′ solvus temperature and improve the microstructural stability, alloying elements, such as Ti, V, Ta, Nb, Mo, and Ni, should be added. Such elements increase the γ′ solvus temperature in the following order: Ta←Nb←Ti←V←Mo←Ni [28,29]. The latest conclusion presented in [29] revealed that Nb greatly destabilized the γ′ phase and is not suggested for alloy design in the Co-9Al-9W system. The morphologies and volume fractions of phases observed in these alloys' microstructures are highly sensitive to alloying with elements like Ni, Ti, Mo, B, Cr, and Ta [28,[30][31][32][33]. Co-based alloys' creep resistance is increased mainly with the addition of Cr, Mo, Ti, and Ta [34][35][36], and oxidation resistance can be improved primarily by expansion of Cr, Si and Ta [37][38][39]. The reduction of alloys density partially replaces W by Mo, Nb and Ta or developing tungsten free alloys. The aim of the research is to characterize the primary microstructure of a new cobalt-based alloy with the addition of Re and Ti in as cast condition. The role of rhenium is to increase the melting point of the alloy, while titanium should increase the temperature of the L12 solvus. It was assumed that the dominant part of rhenium will be located in the cobalt solid solution, while titanium will form the L12 phase.

MATERIALS AND METHODS
The nominal composition of the new cobalt-based superalloy Co-20Ni-9Al-7W-3Re-2Ti used in this investigation was shown in Table 1. The induction vacuum process was used for alloy preparation. The process was made in a VSG 02 Balzers type furnace, and the alloy was manufactured in Al2O3 crucible. The manually compacted molding sand Konmix MAPI was used to set the alumina crucible in the furnace coil. Argon of ALPHAGAZTM 1Ar (99,999% Ar) type was used to protection of liquid metal. Before the process starts, an operating chamber was washed by argon blowing (3 times). After this, the working pressure was decreased to a value of 10^-3Tr (~0,13 Pa). The furnace chamber pressure was increased to 600 Tr (8×10^4 Pa) by Ar filling in the next step. Technically pure metals were utilized as a stock material: electrolytic Co and Ni (both min. 99,98%), Al (purity 99,98%) and W, Re, Ti. Alloying Co, Ni, and Al elements were added to the crucible in the first step before the melting process. The other alloying elements -W, Re and Ti-was added to Co-Ni-Al's liquid solution after its high-temperature homogenization (ca. 1600 ℃). The final liquid alloy was thermally treated in a temperature range of 1650÷1750°C by 10 minutes. The final alloy was cast to the rods form under a protective argon atmosphere into cold graphite molds (Fig. 1). The actual composition of final alloy was shown at Tab. 2.  To ensure excitation of spectral lines of all analyzed elements during EDS analysis, the primary electron beam's variable energy was used. Phase identification is the EBSD method was based on comparing the experimental Kikuchi pattern with the theoretical pattern. The "macro" phase constituent analysis of the investigated alloy was developed by X-ray diffraction method using X`Pert 3 diffractometer. The light microscopic analysis of microstructure was made on Nikon Eclipse MA200 microscopy. The final sample was cut from the central part of the rod, then a metallographic sample was prepared, and the plates were ground, polished and etched. The electrolytic etching in a solution containing 25 ml H2O, 50 ml HCl, 15 g FeCl3 and 3 g CuCl2 × NH4Cl × 2H2O was used to microstructure disclosure.

RESULTS AND DISCUSSION
The phase composition analysis of Co-20Ni-9Al-7W-3Re-2Ti alloy in the as-cast state (obtained by XRD measurement) showed diffraction peaks corresponding only to Co solid solution lattice -ICDD pattern no 15-0806 (Fig. 2). No peaks ordered to expected compounds of Co-W, Co-Ti or other types were identified.
2 theta [deg.]  The primary microstructure (Fig. 3), visible in the longitudinal cross-section, consists of a very thin, peripheral chill zone (not shown in figure 3), and a wide columnar grain zone stretching to the core of the primary cast rod. The direction of columnar crystals growth is following the direction of heat dissipation from solidifying ingot. The rode's centre is occupied by a thick zone of refined, equiaxed grains, crystallized ahead of the columnar front.
(111) (200) (220) (311) Fig.3. LM picture of the primary microstructure of Co-20Ni-9Al-7W-3Re-2Ti alloy. The primarily revealed microstructure is typical for fast and directionally solidifying processes with heat dissipation effect, usually observed for casting into cold graphite molds. Detailed morphology of primary microstructure is visible in Fig. 4 and 5. Special attention was taken to characterize the inter-dendritic area, as the zones important from the point of view of chemical composition` homogenization during heat treatment processes.   More precisely, the phase constituent of precipitates in micro-areas was described by the EBSD method. Results of those investigations were shown in Fig 8. These investigations confirmed that the interdendritic zones are rich in carbides precipitates of (W, Ti) C2 type and intermetallic compounds such as TiAl, TiAl2 and TiAl3. The Ti-Al compounds have generally beneficial effect on high temperature properties of materials [40]. It should be assumed that interdendritic zones are characterized by a strongly lower concentration of tungsten and rhenium than the dendritic zone (assumption for solid solution). The same situation was detected in the case of cobalt, but the segregation effect was slightly lower. The contrary situation was observed for titanium distribution, where its concentration was ca. 3 times higher in the interdendritic zone (in at. %). In the case of Al, it is only ca. 0,5-time higher concentration. Ni concentration was only slightly higher. The effect of W, Al and Co segregation was practically not observed for as-cast Co-9Al-9W alloy [41]. A similar tendency to segregation of W, Al and Ni was observed in the case of Co-20Ni-7Al-7W alloy, but the scale of this effect was much lower [42]. Fig.8. Phase's composition in micro-areas of Co-20Ni-9Al-7W-3Re-2Ti alloy -EBSD point analysis.

CONCLUSIONS
The presented analysis revealed that multi-component alloy of Co-20Ni-9Al-7W-3Re-2Ti type obtained by vacuum induction casting process is characterized by the high level of homogeneity of chemical composition. The observed differences of alloying element concentration in dendrites core and interdendritic zone are detectable but generally negligible. The strongest tendency to interdendritic segregation was observed for titanium and much lower for nickel and aluminium. The main problem related to the segregation effect is the formation of W and Ti-rich carbides and intermetallic compounds from the Ti-Al system.