Study on Creep Anisotropy at 760°C of a Single Crystal Superalloy

In this paper, the creep properties of a second generation single crystal superalloy DD412 with [001], [011] and [111] orientation at 760°C were studied. The experimental results show that the creep properties of DD412 alloy with different orientations have significant anisotropy under the stress of 580MPa-620MPa at 760°C. The [001] and [111] oriented alloys have remarkably better creep performance than [011] oriented alloy. And the creep performance of [111] oriented alloy is superior to that of [001] oriented alloy under the condition of 760°C/620MPa, while the tendency is inverse under the condition of 760°C/580MPa. The dislocation configurations of the alloy with different orientations during creep process are significantly different, resulting in creep anisotropy. For [001] and [111] oriented alloy, there are large number of dislocations in the γ channel that intersect and form local high-density dislocation aggregation, which has a strong blocking effect on the movement of dislocations. For [011] oriented alloy, the <110>{111} dislocations intersect each other to form <112>{111} partial dislocations and stacking faults, cutting into the γ′ phase, which lead to rapid growth of creep deformation and the poor creep performance of [011] oriented alloy.


Introduction
With the development of aeroengines ， the working temperature of the turbine blade continuously growing, and the requirements for the comprehensive properties of the superalloys are also continuously ameliorated [1].Ni-base single crystal alloy completely eliminates the grain boundary, and has high strength at high temperature, excellent thermal resistance, good creep fatigue resistance, excellent corrosion resistance, good service reliability and structural stability.As it can meet the service requirements under harsh conditions for the turbine blades, Ni-base single crystal salloy has become the main material for gas turbine blades and manufacturing advanced aeroengine [2][3].Since the 1980s, the first generation of single crystal alloy such as CMSX-2, SRR99, PWA1480 and René N4 have appeared, in the wake of the addition of Re and Ru elements, as well as the theoretical level of alloy design and the progress of casting technology, the sixth-generation Ni-base single crystal alloy were developed [4][5][6][7][8][9], which has higher temperature resistance than the previous generation single crystal alloy [10].And the second-generation Ni-base single crystal alloys such as PWA1484, CMSX-4, René N5 have been used as aeroengine turbine blades and vanes most widely.Compared with polycrystalline materials, a feature of single crystal alloys is their anisotropy.It is different for the mechanical properties along different crystal orientations [11].As the inherent mechanical properties anisotropy of single crystal superalloys is very important for the service properties and reliability of turbine blades, it has been highly concerned by materials experts and turbine blade designers.The turbine blades will be subjected to long-term effects of high-temperature environment and centrifugal tensile stress, and creep phenomenon will inevitably occur during service process.Therefore, high creep resistance is a fundamental property that turbine blade materials must possess [12].For single crystal superalloys, significant creep occurs above 0.56 Tm (approximately 650℃) [13].According to the distribution of temperature and stress in turbine blades under real service conditions, the creep that occurs in nickel based single crystal superalloys can be roughly divided into two categories: medium temperature high stress creep and high temperature low stress creep [14].Extensive research has been conducted on the creep anisotropy of single crystal superalloys under the condition of medium temperature and high stress, such as Mar-M247, CMSX-4, DD499, and DD6  [18].Therefore, the anisotropy of creep properties is different among different single crystal superalloys, and there is no such orientation presenting always the best performance.DD412 alloy is a second-generation Ni-base single crystal superalloy with excellent comprehensive properties, containing 3wt% Re, and has been used as turbine blades of aeroengines.However, there is still a lack of research on the anisotropy of the creep properties of DD412 alloy.Meanwhile, for the turbine blade rabbet, 760℃ is one of the typical working ambient temperatures.Therefore, it is very necessary to investigate the creep anisotropy at 760℃ of DD412 alloy to reveal the creep mechanism of the alloys with different crystal orientations, and therefore lay a solid foundation for engineering application of the DD412 alloy.

Experimental
The nominal chemical composition of DD412 alloy used in this experiment is (mass fraction/%): Ni-5Cr-10Co-5.6Al-16.5(Mo+W+Ta)-3Re-0.1Hf.The double vacuum induction melting method were used to prepare the single crystal bars of DD412 alloy.Firstly, the master alloy ingot was prepared by VIDP-400 vacuum induction melting furnace, and then was directionally solidified to prepare Φ 15mm×200mm single crystal bars with [001], [011] and [111] orientation by seed crystal method respectively.The X-ray backscatter Laue method was used to measure the primary crystal orientation of single crystal bars.Then, the bars with a deviation between the nominal crystal orientation and the axial direction less than 6° were selected for the creep test.The heat treatment for the selected single crystal bars with three different orientations were the same, 1320℃/4h/AC+1120℃/4h/AC+ 870℃/24h/AC.The single crystal bars as heat-treated were processed into creep samples according to the diagram shown in Fig. 1, and the axial direction of creep samples are parallel to the crystal orientations of bars.

Figure 1. Schematic diagram of creep sample processing (unit is mm)
The DD412 samples with three different orientations were subjected to uniaxial constant load tensile creep tests in the GWT504 high temperature creep testing machine, and the specific test conditions are shown in Table 1.After creep tests, metallographic specimens were taken along the working section of the creep specimen in the directions perpendicular (cross-section) and parallel (longitudinal) to the stress, and the microstructure was observed using Zeiss Merlin Compact field emission scanning electron microscopy.In addition, transverse transmission electron microscopy samples were taken in the directions perpendicular to the stress for dislocation configuration observation (sampling positions are shown in Figures 2 and 3).The transmission samples were prepared using an electrolytic double spray thinning instrument, and the double spray solution was 10% perchloric acid+90% alcohol.The double injection process uses liquid nitrogen cooling, with a temperature of about -30℃ and a voltage of about 25V.Afterwards, FEI Talos F200X field emission transmission electron microscopy was used to observe the dislocation configuration under different conditions, and the relationship between creep mechanism and orientation was analysed.It can be seen that the microstructures of DD412 alloy as heat-treated consist of cubic γ' phase embedded coherently in the γ matrix phase, and the alloy with different orientations has obviously various microstructure characteristics on the section perpendicular to the crystal growth direction.The size of γ' phase in DD412 alloy as heat-treated is about 0.4m.After creep at 760℃/620MPa for several hundred hours, the γ' phases in DD412 alloy with different orientations have no obvious growth and rafting, due to that the creep temperature is low and elements diffusion is slow relatively.And there is no new phase precipitated in DD412 alloy after creep.However, it can be seen that the γ' phase deforms and slip bands appear in [011] oriented alloy, resulting from significant deformation during creep.The microstructures of DD412 alloy after creep at 760℃/580MPa are similar to that at 760℃/620MPa, which are not shown in this paper.

Dislocation configuration of DD412 alloy with different crystal orientations after 760℃ creep test
In order to deeply analyse the microscopic mechanism of creep anisotropy of DD412 alloy, the dislocation configurations of samples with different orientations after creep were observed, as shown in Figure 7.For [001] oriented alloy, there are a large number of dislocations in the γ channel that intersect each other and form dislocation tangle, which has a strong blocking effect on the movement of dislocations.The Schmidt factor of the <110>{111} slip system for [001] oriented alloy is 0.41, and there are eight sets of <110>{111} slip systems that can be activated simultaneously.In fact, the number of activated slip systems is always less than eight, resulting from that the sample inevitably deviates from the accurate [001] orientation.

Figure 2 .Figure 3 .
Figure 2. Schematic diagram of sampling position for ruptured specimen

Figures 5 and 6
Figures5 and 6show the microstructures of DD412 alloy with [001], [011], and [111] orientations asheat-treated and after creep at 760℃/620MPa, respectively.It can be seen that the microstructures of DD412 alloy as heat-treated consist of cubic γ' phase embedded coherently in the γ matrix phase, and the alloy with different orientations has obviously various microstructure characteristics on the section perpendicular to the crystal growth direction.The size of γ' phase in DD412 alloy as heat-treated is about 0.4m.After creep at 760℃/620MPa for several hundred hours, the γ' phases in DD412 alloy with different orientations have no obvious growth and rafting, due to that the creep temperature is low and elements diffusion is slow relatively.And there is no new phase precipitated in DD412 alloy after creep.However, it can be seen that the γ' phase deforms and slip bands appear in [011] oriented alloy, resulting from significant deformation during creep.The microstructures of DD412 alloy after creep at 760℃/580MPa are similar to that at 760℃/620MPa, which are not shown in this paper.
for [111]  oriented alloy is 0.27, much smaller than that for [001] and [011] oriented alloys, and results in lower resolved shear stress in slip system.Therefore, the dislocations in [111] oriented alloy move slowly and propagate gently, resulting in the high resistance to creep deformation.
[15~18].Mackay et al. found that the creep anisotropy of Mar-M247 alloy under 774℃/724MPa conditions was very good for the alloy with [001] and [111] orientation, but very poor for the alloy with [011] orientation [15].Sass et al. found that the CMSX-4 alloy exhibited significant anisotropy under 850℃/500MPa, and the creep strength decreased in order of [001], [011] and [111] orientation [16].Han et al. found that the creep anisotropy of DD499 alloy under 760℃/790MPa conditions was very good for the alloy with [001] orientation, but poor for the alloy with [011] and [111] orientation [17].Wang et al. found that the creep properties of DD6 were obviously anisotropic at 760℃, creep duration decreased in the order [111], [001] and [011] orientation under the same test conditions

Table 1 .
Creep test condition for DD412 alloy with different orientations at 760℃

Table 2 .
Under the condition of 760℃/620MPa, the durations of DD412 alloy with orientations [001] and[111]are 285.3h and 454.0h when the plastic strains reach about 1% respectively, while the creep life of [011] alloy is only 121.7h with 8.271% plastic strain.The creep deformation of [011] oriented alloy grows rapidly, and has almost no steady-state creep stage, while that of [001] and[111]oriented alloy Creep data for DD412 alloy with three different orientations at 760℃/620MPa [18]]alloy is smallest, while that of [011] oriented alloy is largest.It can be seen that the creep performance of [001] and [111] oriented alloys is remarkably better than that of [011] oriented alloy under the condition of 760℃/620MPa, and [111] oriented alloy has best creep performance among the three alloys, which is same as the case of DD6 alloy[18].

Table 3
Creep data for DD412 alloy with three different orientations at 760℃/580MPa [15]dislocations in[001]oriented alloy crept at 760℃/580MPa are much more than that in[001]oriented alloy crept at 760℃/620MPa, which means more slip systems are activated due to smaller deviation to [001] orientation.The formation of the interaction of multiple<110>{111} slip systems can result in obvious work hardening, and the alloy enters the steady-state creep stage, which is the reason for the excellent creep performance of the [001] alloy.For [011] oriented alloy after creep failure, as shown in Figure7(b) and 7(e), there are a lots of parallel dislocation bands in one direction with high density of stacking faults appeared in γ´ phase, which have the weak hindrance to dislocation movement.Compared to [001] oriented alloy, the Schmidt factor of the <110>{111} slip system for the [011] alloy is also 0.41, but only four sets of <110>{111} slip systems that can be activated simultaneously.The <110>{111} dislocations intersect each other to form <112>{111} partial dislocations and stacking faults, cutting into the γ´ phase, which lead to rapid growth of creep deformation.On the other hand, lattice rotation happens in [011] oriented alloy during creep, which further decrease the creep properties of [011] oriented alloy[15].Therefore, the creep performance of [011] oriented alloy is poor.
As shown in Figure7(c) and 7(f), for [111] oriented alloy, there is a certain amount of dislocation entanglement as well as some stacking faults in the γ channel, which means some <110>{111} dislocations have decomposed to <112>{111} partial dislocations.Moreover, there is almost no dislocation entering into γ´ phase, and no stacking fault appears in γ´ phase.The Schmidt factor of the