Single crystal (SX) superalloys have been widely used for their excellent high temperature capability. To further enhance the efficiency of advanced turbine engines, the level of refractory elements such as tantalum, tungsten, and rhenium in the advanced SX alloys has been gradually increased, while the grain boundary reinforcement elements such as carbon and boron have been removed from SX superalloys. However, with increasing addition of refractory elements, there is a greater tendency for the formation of grain defects (strays and freckles etc.)^{[1] and [2]}. Therefore, carbon has been reintroduced into SX superalloys^[3], and its beneficial effects, such as reducing both casting defects^{[4], [5] and [6]}, microporosities^[7] and recrystallization^[8] during solidification have been reported. When the carbon content was changed, the MC carbides exhibited various morphologies which had a vital effect on the creep and fatigue performance of the alloy ^{[9], [10], [11] and [12]}. Hence, it is important and necessary to figure out how the morphology of MC carbides evolves.

Usually, the morphology of MC carbides depends on solidification parameters and alloy compositions. It was reported that the morphology of MC carbides varied from faceted to script-like with decreasing G/R ratio ^{[13], [14], [15] and [16]}, where G is the thermal gradient at the solid/liquid interface and R is the solid growth rate. The carbon content also affected the carbide morphology, i.e., increasing the carbon content induced the transformation from blocky carbides to script-like carbides ^{[8], [17], [18] and [19]}. In addition, the morphology of MC carbides in the alloys containing higher amounts of hafnium was more prone to be blocky ^{[17] and [20]}. However, most of the characterizations of the carbides morphology were two-dimensional. Little attention has been paid to the three-dimensional feature of MC carbides. Moreover, the mechanism for the morphology evolution of MC carbides with increasing carbon content has not been analyzed. The purpose of the present paper is therefore to elucidate the transformation behavior of the MC carbides based on the scanning electron microscopy (SEM) observation of deep-etched samples, which gives three-dimensional information of the carbide morphology.

Two kinds of directionally solidified Ni-base superalloys, K417G and DD33 (SX) were used in the present experiment. Hereinafter these materials are denoted by K417G-DS and DD33, respectively. The actual chemical compositions are given in Table 1. K417G-DS was directionally solidified (DS) into 16 mm × 200 mm bars, which consist of two kinds of grain structure by high rate solidification (HRS) process. The macrostructure of K417G-DS bars is illustrated in Fig. 1. The structure at the bottom of bar is equiaxed grains, while the structure at the top of bar is columnar grains. The DD33 master alloys with three different amounts of carbon additions, made just before pouring, were DS into 16 mm × 200 mm SX bars by HRS with the same casting parameters. The actual carbon content of DD33 alloys in the experimental is given in Table 2.

Table 1

Table 1

Table 1 Actual chemical compositions of alloys in the experimental C Cr Co Mo W Re Al Ti Ta V Fe Hf K417G-DS 0.12 9 10 - - - 5.5 4.5 - 0.8 0.4 - DD33 0.01 2.5 9 2.5 6 4 6 0.5 8 - - 0.15

Table 1 Actual chemical compositions of alloys in the experimental

Fig. 1

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	Fig. 1 Macrostructure of K417G-DS bar.

Table 2

Table 2

Table 2 Actual carbon content of DD33 alloys in the experimental DD33-1 DD33-2 DD33-3 DD33-4 40 × 10-6 140 × 10-6 220 × 10-6 320 × 10-6

Table 2 Actual carbon content of DD33 alloys in the experimental

To investigate carbide morphologies in K417G-DS alloy, the samples were sectioned perpendicularly to the DS direction and taken from equiaxed grain zone and columnar grain zone, respectively. The samples for comparison of the carbide morphologies in DD33 alloys with different carbon content were sectioned perpendicularly to the DS direction and taken from the same location on each SX bar to get similar solidification conditions. To investigate the three-dimensional features of the MC carbides, the samples were deeply etched for 1 min with the etchant (30% hydrogen peroxide, 70% hydrochloric acid). SEM was employed to observe the morphology of the MC carbides. The compositions of MC carbides with different morphologies were measured by energy dispersive X-ray spectroscopy (EDX).

Fig. 2(a)-(d) are the optical images showing the carbides morphology in the equiaxed grain zone and columnar grain zone in the K417G-DS alloy. The volume fraction of MC carbides increases from equiaxed grain zone to columnar grain zone ( Fig. 2(a) and (b)). The morphology of carbides is blocky in the equiaxed grain zone ( Fig. 2(c)), while the morphology of carbides transforms into script-like ( Fig. 2(d)). The cooling rate increases gradually during the transformation from the equiaxed grain solidification to columnar grain solidification. Moreover, the morphology of MC carbides varied from faceted to script-like with decreasing G/R ratio. These results agree well with previous studies ^{[13], [14], [15] and [16]}.

Fig. 2

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	Fig. 2 SEM micrographs showing the morphologies of MC carbides in the K417G-DS alloy: (a) and (c) equiaxed grain zone, (b) and (d) columnar grain zone.

The compositions of carbides with different morphologies in the K417G-DS alloy were measured by using EDX and are compared in Fig. 3. It is clear that the blocky and dendritic carbides are rich in Ti and Mo, and contain small amount of V, Cr and Ni. Less amount of Ti was detected in the dendritic carbides than in blocky carbides.

Fig. 3

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	Fig. 3 Comparison of the compositions of MC carbides with different morphologies in the K417G-DS alloy.

Fig. 4(a)-(d) are the SEM images showing the evolution of carbides morphology in the DD33 alloys with the change of carbon content. The volume fraction of MC carbides increases gradually with increasing carbon addition, and when the carbon content is 40 × 10^-6, the carbides are blocky, as seen in Fig. 4(a). When increasing carbon content from 40 × 10^-6 to 140 × 10^-6 then 220 × 10^-6, the morphology of the carbides gradually changes from blocky into rod-like ( Fig. 4(b) and (c)). Eventually, the carbides grow into script-like at the carbon content of 320 × 10^-6 carbon content, as shown in Fig. 4(d). These results agree well with previous studies ^{[8] and [17]}.

Fig. 4.

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	Fig. 4. SEM micrographs showing the morphologies of MC carbides in the DD33 alloys with different carbon contents: (a) 40 × 10-6, (b) 140 × 10-6, (c) 220 × 10-6, (d) 320 × 10-6

Further SEM observations on deep-etched samples show salient features of carbides morphology. The results are shown in Fig. 5. One can see that the blocky carbides shown in Fig. 4(a) are actually octahedral in a three-dimensional perspective. The growth steps were observed from the exposed surfaces, and are presented in Fig. 5(a). In the DD33-2, DD33-3 and DD33-4 alloys, where rod-like or script-like carbides were observed, the MC carbides actually took a dendritic structure. Increasing the carbon content from 140 × 10^-6, through 220 × 10^-6, to 320 × 10^-6 leads to the formation of well-developed dendrites of carbides ( Fig. 5(b)-(d)). Interestingly, the flake-like carbides formed at the tip of dendritic carbides were also observed in the alloy with carbon content of 320 × 10^-6 carbon, as seen in Fig. 5(e).

Fig. 5

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	Fig. 5 SEM images showing the three-dimensional morphologies of MC carbides in the DD33 alloys with the carbon contents: (a) 40 × 10-6, (b) 140 × 10-6, (c) 220 × 10-6, (d)-(e) 320 × 10-6

The compositions of carbides with different morphologies were measured using EDX and are compared in Fig. 6. It is clear that the blocky carbides are rich in Ta, Hf, and Ti, while the dendritic carbides contain Ta and Ti, and small amount of Mo, Co and Cr. No Hf and less Ti than that in blocky carbides were observed in the dendritic carbides.

Fig. 6

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	Fig. 6 Comparison of the compositions of MC carbides with different morphologies in the DD33 alloys.

During DS casting of Ni-base superalloys, solidification usually occurred in the order of γ dendrites, MC carbides and γ /γ ′ eutectics ^{[18] and [21]}. MC carbides were produced preferentially in the interdendritic regions where the liquid was enriched in Ta, Ti, Hf and C. Based on our present observations, the morphology evolution mechanism of MC carbides with increasing carbon content is proposed and illustrated in Fig. 7.

Fig. 7

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	Fig. 7 Schematic illustration of the morphology evolution of MC carbides with the carbon content variations

It is known that MC carbides take an fcc structure with the most stable plane of {111}. The octahedron consisting of {111} faces is therefore the equilibrium shape ^[15]. Carbides may nucleate at the inclusions or unmelting carbides clusters during melt superheating treatment ^[22]. Moreover, in the equiaxed grain zone of the K417G-DS alloy, the growth rate of the carbides is relatively low because of the lower cooling rate in equiaxed grain solidification. Correspondingly, with low carbon content, the supersaturation of carbon in the remaining liquid at the last stage solidification is lower, which also leads to the relatively lower growth rate of the carbides. Therefore, the octahedral MC carbides will precipitate as seen in Fig. 7(a) due to the near-equilibrium solidification. The misfit of carbides with matrix may have an influence on the morphology of carbides, and the larger misfit energy makes MC carbides containing Hf tend to be blocky ^[7]. That is the reason why more Hf and Ti were detected in these blocky MC carbides than in dendritic carbides.

With increasing carbon content or the cooling rate, the degree of the carbon supersaturation and the growth rate of the carbides become larger. Moreover, the carbides grow along the < 001> directions more quickly than other directions for the least atoms in the {001} faces of the fcc crystal. The primary dendrite arms of MC carbides along octahedral tips were observed ( Fig. 7(b)). In the DD33-4 alloy with carbon content of 320 × 10^-6, the increasing degree of carbon supersaturation promotes the formation of dendritic structure. The secondary and tertiary dendritic arms were observed in the Fig. 5(e). The growth steps were also observed at the tips of the carbides, and the development and connection of these secondary and tertiary dendrite arms form the flake-like MC carbides finally ( Fig. 7(c)).

(1)The morphology of MC carbides in the K417G-DS alloy varies from faceted to script-like with increasing cooling rate. With the carbon content increasing from 40 × 10^-6 to 320 × 10^-6 in single crystal nickel-based superalloy DD33, the morphology of carbides changes from blocky, rod-like into script-like.

(2)Scanning electron microscopy observation of deep-etched samples indicates that these carbides actually evolve from octahedral to dendritic and then into well-developed dendrites accordingly in a three-dimensional manner. The formation of flake-like carbides is due to the development and connection of these secondary and tertiary dendrite arms.

(3)The development of dendritic carbides is closely related to the growth characteristics of fcc crystals and the carbide growth rate. Higher carbon content leads to a greater supersaturation in the later stage of solidification, which induces the growth of well-developed secondary and tertiary dendrite arms.