Creep deformation behaviour of Rhenium free Ni-based single crystal superalloys LSC-15

In this paper, creep deformation behavior of Ni-based single crystal superalloys LSC-15 were studied. LSC-15 does not include Rhenium and has been developed by IHI Corporation Japan. Creep tests were performed at 1000 and 1050 ◦C under several stress levels. The creep deformation behaviour was different between test temperatures at 1000 ◦C and 1050 ◦C. Moreover, the relationship between the minimum creep rate and stress was different at the various temperatures. The stress exponent values at 1000 ◦C and 1050 ◦C, were n = 6 and 12 respectively. This difference was due to differences in the formation of dislocation network. At 1000 ◦C, when the minimum creep rate, the dislocation network formed completely independent of stress level. On the other hand, at 1050 ◦C, the dislocation network had not developed fully at the minimum creep rate and the formation of dislocation network depended on the stress level. Therefore, stress dependency at 1050 ◦C is higher than that at 1000 ◦C.


Introduction
Gas turbine components require thermal fatigue and creep resistance at high temperature.In general, more refractory elements such as rhenium improve the creep resistance of the materials for advanced turbine blade [1][2][3][4].It has also been reported that the γ /γ rafting behaviour is effective for creep resistance in case of the single crystal (SX) superalloys [5,6].However, the price of rhenium, which is primarily consumed for the production of nickel base superalloys, has been substantially increasing for several years.To overcome these situations, several researchers have attempted to develop SX superalloys with the lower rhenium content which has the same level of creep resistance as the second generation superalloys with rhenium [7,8].In Japan, IHI Corporation developed the Re-free SX superalloy.The high γ /γ lattice misfit of this alloy improves the high temperature creep strength even though the alloy doesnot include rhenium.Therefore, it is supposed that this alloy has different creep deformation behaviour at high temperature from the first and second generation SX superalloys.In the present study, we investigated the creep deformation behaviour of the Refree SX superalloy at high temperature.

Experimental procedure
The alloy LSC-15(Ni-6.0Co-7.0Cr-1.5Mo-10.0W-6.0Al-5.5Ta-0.1Hfwt.%) was investigated in this paper.The alloy does not include Rhenium and has been developed by IHI Corporation Japan.The single-crystal superalloy in the fully heat-treated condition was provided by IHI a Corresponding author: nobuyasu tsuno@ihi.co.jpTransmission Electron Microscope (TEM).Thin plates were cut from the crept specimens parallel to longitudinal (110) planes to observe the microstructure evolution by SEM.These plates were mechanically polished and then etched in a γ dissolving agent prior to SEM observations.The thin foils for TEM analysis were obtained by cutting discs from the gage length perpendicular to the tensile axis.These were electro-polished using a twin jet with a solution of 10% perchloric acid in ethanol at 30 V and 0 • C.

Results and discussion
The microstructure of the alloy after aging heat treatment is shown in Fig. 1.The microstructure of alloy has homogeneous distribution of cuboidal primary γ .When creep properties at 1050 • C were compared between the second generation superalloy and Re-free SX superalloy, rupture times are at the same level.On the other hands, when they are compared at 1000 • C, the creep strength of the Re-free SX superalloy was lower than that of the second generation SX superalloys.Figure 4 shows   5 shows that the microstructures of the interrupted crept specimens by SEM.All specimens had γ /γ rafted structures.The γ and γ size were summarised in Table 1.These sizes were also the same among the specimens.The γ /γ interface microstructures of the interrupted crept specimens are shown in Fig. 6.All crept 1µm

Figure 4 .Figure 5 .Figure 6 .
Figure 4.The stress dependence of the minimum creep rate of LSC-15 at two temperatures.

Figure 7 .
Figure 7.The relationship of γ /γ interfacial dislocation spacing and minimum creep rate.