Introduction

Ni-based superalloys have come to dominate the high-temperature applications such as blades and disks in aircraft engines and industrial steam turbines [1]. Unfortunately, the possibility to develop temperature capability of these superalloys by increasing the fraction of γ′ phase, solvus temperature or by addition of refractory elements is now limited; hence, the alloy system is increasingly being viewed as mature. Moreover, due to limited corrosion properties, these alloys require usage of technologically challenging bond-coats in order to protect substrate material [2]. The recently discovered Co-based superalloys [3] have gained substantial interest as potential new generation of superalloys, alternative to commercial Ni-based superalloys in near future. Due to the γ–γ′ microstructure, these alloys resemble Ni-based superalloys; furthermore, owing to excellent properties at high temperature, the new group of superalloys may surpass the performance of analog Ni-based alloys, which are composed of 10-12 alloying elements and are capable of operating to 1150 °C [4, 5]. Due to inferior creep resistance compared to that of Ni-base alloys, the conventional Co-based superalloys are limited to static applications, such as vanes and combustors [6]. The remarkable creep resistance of alloys based on Ni–Al system is an effect of coherent cuboidal γ′ (L12) precipitates dispersed in a γ (fcc) matrix. The γ′-strengthened Co-based superalloys are potentially less prone to freckling than Ni-based superalloys [7] and possess higher melting temperatures [8, 9]. Nevertheless, they still exhibit poorer creep resistance due to a lower temperature of the γ′ phase dissolution. Further development of the new generation of superalloys requires substantial efforts dedicated to improvement in thermal stability, γ/γ′ lattice mismatch, volume fraction of the γ′, which contributes to the development of the mechanical properties at elevated temperatures. Many authors have focused on influence of alloying elements on stability and distribution of γ and γ′ phases in Co-based superalloys [10,11,12,13,14,15]. The Cr, Mn, Fe oraz Re dissolve in γ and decrease the solvus temperature. On the other hand, Ti, Ta, Nb, Mo and V are elements promoting γ′ phase.

Despite discussed high temperature properties, the oxidation resistance of these alloys remains a concern. Recent efforts of several authors have focused on the oxidation behavior of alloys based on the Co–Al–W system [16,17,18,19]. These alloys suffer from inferior oxidation resistance relative to that of commercial Ni-base superalloys. During the oxidation of Co–Al–W alloys, a brittle non-protective Co-based oxide is formed. This type of external layer is prone to spallation during high temperature exposure [8, 17]. The known way to improve oxidation resistance of ternary Co–Al–W alloys is addition of Cr and Si, although the γ′-phase stability steadily decreases with increasing content of these elements [11, 16, 20]. The deleterious effect of Cr and Si on the γ–γ′ microstructure may be neutralized by Ni addition, whose occurrence in the alloy improves the γ′ stability [21]. The newest data show detailed analysis of Co–Al–W alloys oxidation, including role of γ and γ′ phases in initial stage of oxidation [22]. Despite the oxidation properties, alloys based on Co–Al–W system have other limitations, resulting from W occurrence, i.e. high density and insufficient creep properties [23, 24], wherefore attempts of W-replacement in Co-based superalloys have been performed [25,26,27,28,29]. The new solutions are based on Mo and Nb addition to Co–Al system; these elements promote formation of γ′-Co3(Al, Mo) phase. Alloys based on Co–Al–Mo system are characterized by better creep resistance compared to that of Co–Al–W alloys; nonetheless, papers concerning the oxidation properties of the newly developed W-free superalloys are limited. The insufficient data concerning oxidation resistance of Co–Al–Mo–Nb alloys and information regarding these alloys at all were motivation of this paper. The aim of the study is introduction to oxidation resistance studies of new γ–γ′ Co-based alloys using thermogravimetry. This method was utilized in order to investigate the oxidation behavior of new superalloys under non-isothermal conditions. The another goal of study is to evaluate influence of Ni on the oxidation performance of Co–Al–W and Co–Al–Mo–Nb alloys.

Experimental

Materials

Four as-cast Co-based superalloys were chosen for the present work: Co–20Ni–7Al–7W, Co–10Al–5Mo–2Nb, Co–20Ni–10Al–5Mo–2Nb (at.%) alloys and reference Co–9Al–9W (at.%) alloy. The investigated Co-based superalloys were melted using induction vacuum furnace and casted into cold graphite molds under protective gas (Ar). The compositions of investigated superalloys were analyzed by EDS method. The chemical composition details of all four Co-based superalloys are shown in Table 1.

Table 1 Nominal and measured chemical composition of investigated alloys
Full size table

The ingot was subjected to turning, the specimens were cut from ingot base. Taking into account the microstructure, all investigated alloys were characterized by dendritic structure typical for metals solidifying under conditions of fast and directional heat dissipation. More detailed data concerning microstructure of base alloy is available [30]. The satisfactory chemical homogeneity was obtained for all ingots, however, in case of alloys containing Mo, tiny precipitates rich in Mo and Nb were observed. X-ray diffraction showed that these precipitates are Co3Mo and Co3Nb.

Methods

The thermogravimetric analysis of as-cast alloys under non-isothermal conditions was performed on NETZSCH STA 449 F3 Jupiter thermal analyzer from 40 to 1200 °C. The specimens used in thermogravimetric investigation were ground using SiC paper down to 1200 grid. The Oxidation of 5 × 5 × 10 mm samples contained in Al2O3 crucibles was carried out in air at heating rate 5 °C min−1. The specimens were cooled to room temperature and weighted in order to evaluate spallation of grown scale. Afterward, the surfaces of oxidized samples were investigated by SEM/EDS method. The analysis of grown scale was carried out Scanning Electron Microscope Hitachi S-4200N equipped with an energy dispersive spectrometer.

Results and discussion

Thermogravimetry

Figure 1 shows the TG and DTG curves related to the oxidation of as-cast Co–9Al–9W (at.%) alloy from 40 to 1200 °C. TG plot corresponding to this alloy was slightly growing till 930 °C and subsequently shot up. The gain of specimen mass at 930 °C was 0.17%, whereas the final mass change at 1200 °C reached almost 1%. Downward arrows indicate peaks present on the DTG curve. The first one, relatively indistinct peak occurred between 660 and 705 °C. The occurrence of the peak is connected with an acceleration of oxidation and subsequently slowing down, what could be attributed to a formation of new type of oxide on the surface. The literature data show that below 700 °C, the scale grown on pure Co is mainly composed of Co3O4 oxide; however, the authors reported the formation of CoO at the expense of Co3O4 from 700 to 900 °C [31, 32]. The Co3O4 oxide is not stable above 900 °C; therefore, at higher temperatures, transition of dual-phase scale composed of Co3O4 and CoO into single-phase CoO oxide film occurs. The investigated Co-based superalloys exhibit changes in the oxidation behavior at temperatures close to those of described in the literature, whereas the differences are result of different character of oxidation of alloys and pure metals.

Fig. 1
figure 1

TG–DTG profiles of oxidation of Co–9Al–9W (at.%) alloy

Full size image

The TG–DTG curves corresponding to Co–20Ni–7Al–7W (at.%) alloy examined in the temperature range 40–1200 °C are shown in Fig. 2. The mass change plot shows a slight gain of mass to 670 °C. In this case also occurred peaks, which have been indicated by downturn arrows. The peak between 670 and 690 °C detected in DTG curve implies the enhancement and subsequent slowing down of the oxidation rate. This phenomenon may be also attributed to selective CoO formation, discussed in the previous example. From 690 °C, the TG plot was increasing; nonetheless, the maximum of another peak of DTG curve was observed at approx. 857 °C. Afterward, the rate of oxidation slowed down, what may be observable as an visible threshold on the mass change profile and is implied by the DTG peak. In this case, it may be also explained by the formation of an another type of oxide, namely by transition of Co3O4 to CoO oxide, which is more stable over 900 °C [22,23,24,25,26,27,28,29,30,31]. The TG plot shot up after exceedance of 920 °C at which increase in mass reached 0.21%. At 1200 °C, the change of mass was 0.81%, what was slightly lower in comparison with that of basic Co–9Al–9W (at.%) alloy.

Fig. 2
figure 2

TG–DTG profiles of oxidation of Co–20Ni–7Al–7W (at.%) alloy

Full size image

Figure 3 shows the TG and DTG curves of Co–10Al–5Mo–2Nb (at.%). The first stage of oxidation occurred between 40 and 660 °C with 0.16% of mass gain that is attributed to initial scale growth. The DTG curve showed the peak corresponding to that of previous case, where possibility of Co3O4 oxide formation was discussed. The peak occurred in the temperature range 660–688 °C with the maximum at 670 °C. In the analogous way, the another peak has been indicated. The second one was present between 820 and 855 °C, which may be corresponding to Co3O4 into CoO transition, discussed in the previous examples. Above 910 °C at which mass gain was 0.44%, the rate of oxidation substantially improved resulting in the final mass change equal 2.25%.

Fig. 3
figure 3

TG–DTG profiles of oxidation of Co–10Al–5Mo–2Nb (at.%) alloy

Full size image

The TG and DTG curves (Fig. 4) attributed to the oxidation of as-cast Co–20Ni–10Al–5Mo–2Nb (at.%) alloy have comparable profile to that of Ni-free Co–10Al–5Mo–2Nb (at.%). The TG plot related to the investigated alloy was slightly growing, resulting in 0.07% of mass increase at 670 °C. The similar peaks those of previous examples were noticed. The DTG curve implied acceleration of the oxidation rate above 670 °C. In this case also occurred, the threshold on the mass changes profile at 830 °C. The mass was growing up to 910 °C, at which the mass change was approx. 0.27%; afterward, the TG plot shot up due to considerable acceleration of oxidation. At 1200 °C, the gain of mass reached 1.5%, considerably lower compared to that of Ni-free Co–20Ni–10Al–5Mo–2Nb (at.%) alloy.

Fig. 4
figure 4

TG–DTG profiles of oxidation of Co–20Ni–10Al–5Mo–2Nb (at.%) alloy

Full size image

The temperature range 600–950 °C is of interest for potential applications of Co-based superalloys. Illustration of mass changes corresponding to all investigated alloys is shown in Table 2. The highest scale growth as a function of temperature was observed for Co–10Al–5Mo–2Nb (at.%) alloy. From 600 to 900 °C, the changes of mass between Co–9Al–9W, Co–20Ni–7Al–7W and Co–20Ni–10Al–5Mo–2Nb (at.%) alloys were comparable; however, W-free alloys was characterized by higher gain of mass, contributed to oxidation. In case of oxidation of alloys, oxygen also reacts with alloying elements resulting in formation of oxides rich in Al, Nb, Mo, W or complex oxides [31].

Table 2 Results of chemical composition analysis in areas and points marked in Figs. 4 and 5
Full size table

Figure 5 shows losses of samples mass, attributed to the spallation of the scale from cooled specimens. The W-free alloys had considerably poorer resistance in this regard. In case of Co–9Al–9W (at.%), roughly 3% of the scale spalled as a result of cooling from 1200 °C to room temperature. Reduction of the scale mass was almost three times lower in case of Co–Al–W alloy with Ni. Approx. 5.8% of oxidized layer peeled off from Co–10Al–5Mo–2Nb (at.%) alloy surface due to cooling. In case of W-free alloys, the Ni content also caused better adhesion of scale under cooling conditions, resulting in 4.5% of the scale mass drop.

Fig. 5
figure 5

Losses of mass related to spallation of oxides under cooling

Full size image

Surface condition

The topography of the scale grown on different alloys after thermogravimetric investigations are shown in Fig. 6, whereas the chemical composition of these areas is shown in Table 3. The scale grown on Co–9Al–9W (at.%) alloy (Fig. 6a) is highly porous and chemically varied. The light contrast confirms the occurrence of elements with various atomic number, namely Co, Al, W and O, what is confirmed by the chemical composition analysis (Table 3). The occurrence of Al, W and O implies the formation of mixed oxides within the scale, which was discussed by consideration of thermogravimetric plots. The scale observed on Co–9Al–9W (at.%) does not include the top external layer, which spalled due to the cooling to room temperature. The another investigated alloy (Fig. 6b) was covered by a compact, smooth oxide film as a result of oxidation under non-isothermal conditions. In case of Co–20Ni–7Al–7W (at.%) alloy, the spallation occurred in a small extent; therefore, the outer layer has maintained. The external oxide coating was characterized by grain morphology and consisted of Co and O; thus, probably was CoO oxide, which is thermodynamically more stable above 900 °C than Co3O4. Chemical homogeneity of external layer does not exclude the occurrence of mixed oxides underneath. Furthermore, Ni oxidized to NiO tends to formation of CoO–NiO solid solutions at high temperatures, what may influence on protective properties of the scale [31, 33]. Moreover, another possible phenomenon is Al-oxidation to Al2O3 and subsequent reaction with CoO resulting in formation of CoAl2O4 spinel [34]. The scale of another investigated alloy (Fig. 6c) suffered from spallation the most. The SEM image shows an uncovered area of scale grown on Co–10Al–5Mo–2Nb (at.%) alloy. The chemical composition analysis (Table 2) shows the occurrence of Co, Al, Mo and O within the investigated area, hence the scale is composed of the different types of oxides. Furthermore, the formation of cobalt aluminates and molybdates may occur as well [34, 35]. The scale grown on Co–20Ni–10Al–5Mo–2Nb (at.%) alloy morphologically differs from that of previous example (Fig. 6d). In this case, also the image has been taken from the uncovered area of scale, whereas despite Co, Al, Mo and O, the occurrence of Nb and Ni was detected via EDS chemical composition analysis. The occurrence of many elements in the scale confirms the complex structure, which may be composed of oxides rich in Co, Al, Mo, Nb; moreover, it may contain the different types of complex oxides. In this case, also, formation of NiO and subsequent solution in CoO is possible.

Fig. 6
figure 6

SEM images showing the oxidized surface morphologies of alloys. a Co–9Al–9W; b Co–20Ni–7Al–7W; c Co–10Al–5Mo–2Nb; d Co–20Ni–10Al–5Mo–2Nb

Full size image
Table 3 EDS chemical composition analysis at the surface areas of Fig. 6
Full size table

Conclusions

The oxidation behavior of Co–9Al–9W, Co–20Ni–7Al–7W, Co–10Al–5Mo–2Nb and Co–20Ni–10Al–5Mo–2Nb (at.%) alloys under non-isothermal conditions was determined. The Co–20Ni–7Al–7W (at.%) was characterized by higher gain of mass as a function of temperature in comparison with that of Co–9Al–9W (at.%) in the temperature range 600–950 °C. However, adding of 20 at.% of Ni to Co–10Al–5Mo–2Nb (at.%) slowed down oxidation rate over whole temperature range. Furthermore, Ni influenced on adhesion of the oxide scale in terms of cooling to room temperature. The spallation of oxides occurred in less extent for alloys with 20 at.% of Ni. In most of cases, the DTG curve showed two characteristic peaks at about, respectively, 650 and 850 °C. The collation of obtained information with the literature data implies, that peaks are corresponding to formation of two-phase oxide layer composed of Co3O4 and CoO (700 °C) and transformation of Co3O4 to CoO (900 °C). The literature data concerning oxidation of pure Co was the base for consideration of oxides formation with increasing temperature, implied by thermogravimetry; however, it is worth noticing that oxidation behavior of discussed alloys may differ from pure Co due to occurrence of alloying elements. Generally, all investigated alloys suffered from accelerated oxidation over 900 °C. The substitution of W by Mo and Nb caused the cobalt-based superalloys to be more prone to the oxidation and spallation of oxide layer under cooling to room temperature. Further investigations should contain thermogravimetric analysis of Co-based superalloys under isothermal condition in the temperature range 700–900 °C.