【Citation Format】
HE Jiabao, WANG Liang, ZOU Mingke, et al. Research Progress on the Effect of Deviation [001] Orientation on Creep of Nickel-based Single Crystal Superalloys[J]. Special Casting & Nonferrous Alloys,2024,44(3):299-304.
Citation:HE J B，WANG L，ZOU M K，et al. Research progress in influence of deviation from ［001］ orientation angle on creep behavior of nickel-based single crystal superalloys［J］. Special Casting & Nonferrous Alloys，2024，44（3）：299-304.
Due to their excellent properties, nickel-based single-crystal superalloys have been widely used in important components such as aero engines, gas turbines, turbine blades and guide vanes. In the actual manufacturing of turbine blades and other parts, nickel-based single-crystal superalloys with a directional solidification direction of [001] orientation are mostly used. However, due to the complex structure of the blade and the harsh service environment, the blade will produce a wide temperature gradient, local multi-directional axial stress and thermal stress during the service process, resulting in damage, so the force direction of the blade often deviates to a certain extent. And in order to avoid resonance in the engine, consider using non-axial blades, that is, the axial deviation of the blades from the [001] orientation at a certain angle. Therefore, it is very important to study the mechanical properties of nickel-based single crystal superalloys that deviate from the [001] orientation at a certain angle.
Creep damage caused by centrifugal stress on the blade is one of the main failure mechanisms of single crystal turbine blades. The stress applied when creep occurs is generally less than the tensile yield strength, and the strain rate is very small, and the lower limit temperature of the superalloy is 0.56Tm in the range of 10-10~10-3 s-1. The γ′ phase in nickel-base superalloys is mainly the Ni3(Al,Ti) phase of L12 structure. During high-temperature creep, the γ′ phase will undergo obvious directional coarsening along a specific orientation, forming a raft-like structure, which is a phenomenon unique to nickel-based single crystal alloys. Therefore, nickel-based single-crystal superalloys have creep anisotropy, so the creep behavior of the blade is closely related to the axial deviation direction.
Meng Jie's research team from the Institute of Metal Research, Chinese Academy of Sciences published a paper entitled "Research Progress on the Creep of Nickel-based Single Crystal Superalloys by Deviating from [001] Orientation" in the journal Special Casting and Nonferrous Alloys, Vol. 44, No. 3, 2024. The research progress of creep behavior of nickel-based single crystal superalloys deviating from the [001] crystallographic orientation at different angles is introduced, and the relationship between creep behavior and the angle deviating from the [001] orientation is reviewed. In general, the orientation deviation angle has little effect on the creep behavior of nickel-based single-crystal superalloys under high temperature conditions, and the creep properties of nickel-based single-crystal superalloys with little deviation from the [001] orientation and close to the [011] orientation are better than those with little deviation from the [001] orientation and close to the [111] direction under medium temperature conditions.
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1 Effect of orientation deviation on creep deformation process
The creep deformation process of nickel-based single-crystal superalloys can generally be divided into three stages. The first stage is the primary creep phase, which is the stage in which the strain rate decreases over time, the second stage is the service life of the superalloy that undergoes a stable strain rate, and the third stage increases the strain rate sharply until fracture.
YU J ET AL. STUDIED THE EFFECT OF ORIENTATION DEVIATION ON THE CREEP PROPERTIES OF DD6 nickel-based single-crystal superalloys, and found that at 760 °C and 785 MPa, within 20° of deviation from the [001] direction, compared with the values of sample A1 (17° near the [001]-[011]-[011] boundary), samples D1 (11° near the [001]-[111] boundary) and G1 (17° near the [001]-[111] boundary) were compared with the values of sample A1 (17° near the [001]-[011]-[011] boundary). ) is very high, see Figure 1.
Fig.1 Creep curves of DD6 single crystal superalloy at 760 °C and 785 MPa
SASS V et al. found that the creep rate of the first stage was very sensitive to small deviations in [001]. MACKAY R A ET AL. BELIEVE THAT IN THE STANDARD STEREO PROJECTION TRIANGLE ORIENTED TO [001], [011], [111], WITHIN 25° OF THE [001] ORIENTATION, THE CRYSTALS ORIENTED CLOSE TO THE [001]-[011] BOUNDARY HAVE A LOWER STEADY-STATE CREEP RATE, WHILE THE CRYSTALS CLOSE TO THE [001]-[111] BOUNDARY HAVE A LARGER CREEP RATE.
HEEP L et al. studied the tensile creep behavior of nickel-based single-crystal superalloys of the CMSX-4 type under medium temperature and high stress, and found that when the reaction dislocation is driven by the Peach-Köhler force, a high creep rate is generated. Li Yifei used optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) to study the creep behavior of the third-generation DD33 nickel-based single-crystal superalloy under the conditions of high temperature and low stress and medium temperature and high stress. Single crystal specimens A, B, C with θ angles of about 12°, ρ angles of 4°, 20° and 38° were selected with single crystal specimens A, B, C and ρ angles of about 4°, θ angles of 17° and 18.8°, θ angles of about 12°, ρ angles of 4°, 17°, 42° and 38°, single crystal specimens M, N, O, P, and single crystal specimens L and Q with θ angles of about 14°, ρ angles of 4° and 38°, respectively. Fig. 2 and Table 1 are the creep strain rate-strain curves and creep results of specimens A~E deviating from the [001] orientation of 10°~20° at 1 100 °C × 150 MPa, respectively. It can be seen that the steady-state creep rate of the sample deviating from the [001] orientation of 10°~20° is basically the same as that of the [001] orientation sample, and there is no obvious difference in creep rate. Fig. 3 and Table 2 are the creep strain rate-strain curves and creep results of the sample deviating from the [001] orientation of 10°~15° (L~Q) at 850 °C × 650 MPa, respectively. It can be seen that the samples with similar θ angles and different ρ angles have completely different initial creep strain (ɛp), initial creep strain rate (p) and steady-state creep rate (s) under medium temperature and high stress, and the samples L and M near the [001]-[011] edge have the lowest ɛp, p and s.
Fig.2 Creep strain rate-strain curves of specimens A~E deviating from the [001] orientation of 10°~20° at 1 100 °C × 150 MPa
Fig.3 Creep strain rate-strain curves of samples L~Q deviating from the [001] orientation of 10°~15° at 850 °C × 650 MPa
RAE C M F ET AL. NOTED THAT AT LOWER TEMPERATURES (E.G., 750 °C or 850 °C), crystals within 20° of the [001] orientation are highly dependent on the slight deviation of the alloy from the [001]-[011] boundary. At higher temperatures (e.g. 950 °C), creep deformation has little to do with the angle of deviation. Under the condition of 750~850 °C, the creep in the initial stage is strongly dependent on the degree of deviation of the alloy from the [001]-[011] orientation within the range of 20°. Alloys oriented close to [001]-[011] have low initial creep strain due to the low difficulty in forming a[112] dislocation bands. The alloy with a close orientation to [001]-[111] exhibits a large initial creep strain due to the large number of cutting γ′ phases in the a[112] dislocation zone.
GUNTURI S SK ET AL. PERFORMED A DEAD-LOAD CREEP TEST ON THE NICKEL-BASED SINGLE-CRYSTAL SUPERALLOY CMSX-4 AT 750 °C AND FOUND THE OPPOSITE RESULT. It has a high creep rate in the direction away from the [001]-[111] boundary, and a relatively low initial creep rate in the direction close to it.
In summary, it can be concluded that the general law of creep deformation of nickel-based single crystal superalloy deviation [001] orientation: under high temperature (low stress) conditions, the orientation deviation angle does not exceed [001]20°~25°The creep behavior of nickel-based single crystal superalloys is small in the range, which is basically the same as that of the [001] oriented samples, and the creep degree and creep rate of nickel-based single crystal superalloys with orientation deviating from [001] and close to the direction of [011] are lower than those deviating from the [001] orientation and close to the direction of [111], and the creep resistance is better. Nickel-based single-crystal superalloys that deviate from the [001] orientation and are close to the [111] orientation may have a greater degree of creep due to the formation of more a[112] dislocation bands and the cutting of γ′ phases.
2 Effect of orientation deviation on creep life
LEVERANT G R ET AL. SUGGEST THAT CREEP LIFE IS SIGNIFICANTLY SHORTENED WITH INCREASING DEVIATION FROM [001] ORIENTATION. Yue et al. proposed a two-parameter creep damage model to simulate the lifetime of DD3 nickel-based single crystal alloy blades. The environmental parameters were set as follows: the temperature rose from 538 °C at the root of the blade to 964 °C at the tip of the leaf, the leaf withstood a centrifugal force of 45 000 r/min, and the lower end of the blade was fixed. Figure 4 shows the variation of leaf life with angular α (when α=0, the crystal orientation is strictly [001]). It is found that with the increase of axial declination, the dispersion of life becomes larger, and when the deviation angle reaches 15°, the difference in life is 6 times. There is a 50% change in lifetime when two uncontrolled crystal orientation changes.
Fig.4. Effect of uncontrolled orientation and declination α of blades on creep life
CHATTERJEE D ET AL. FOUND THAT UNDER HIGH TEMPERATURE AND LOW STRESS, THE CREEP STRAIN OF [001] SINGLE CRYSTAL WAS WITHIN 1% OF ALMOST 80% OF ITS CREEP LIFE IN THE INITIAL STAGE. When the temperature is constant, tr (life at break) decreases linearly with the logarithm of stress. In addition, the creep curves obtained at different stress values show that the creep strain is similar at a given t/tr, an observation that can be used to estimate a typical long creep lifetime.
Yu J et al. also studied the effect of orientation on the creep lifetime of DD6 nickel-based single crystal superalloys, and found that at 760 °C and 785 MPa, the [001] direction and the [001]-[011] boundary had a longer creep lifetime than the [001]-[111] boundary within 20° deviation from the [001] direction. SASS V et al. found that the longevity near the [001]-[011] boundary was better than that near the [001]-[boundary. Zhang Zehai et al. cut a sample that deviated from the [001] orientation at a certain angle on the [001] oriented Ni-Co-Cr-Mo-W-Al-Ti-Ta nickel-based single crystal superalloy slab, and the deviation degree was within 20°, and the test results are shown in Table 3. It can be seen that the specimens at all angles have a long creep life, except for A1 (deviation from [001]4.4°) and A8 (deviation from [001]19.0°), which have a short creep life. Under the medium temperature and high stress of 750 °C × 750 MPa, the specimens with different angles deviated from each other obtained a long creep life and low creep elongation. Fig. 5 shows the creep curves of alloys deviating from the [001] orientation at different angles under medium temperature and high stress conditions, which shows that the orientation deviation has little effect on the overall creep life after entering the steady-state stage.
Fig.5 Creep curves with different orientations at 750 °C × 750 MPa
Li Yifei found that ×the third-generation nickel-based single-crystal superalloy DD33 has a high creep life in the samples close to the [001]-[011] boundary, while the alloy far away from the [001]-[011] boundary has a short creep lifetime, and the creep life is less affected by the deviation angle of [001]-[011] under high temperature and low stress (1 100 °C×150 MPa). Li et al. studied the creep properties of a nickel-based single-crystal superalloy with a deviation of 15° from the [001] orientation at medium temperature (760 °C×793 MPa). The results show that the alloy with orientation close to the [001]-[101] boundary has the longest creep lifetime, while the alloy close to the [001]-[111] boundary has the shortest creep lifetime, as shown in Fig. 6. Although the angle of orientation deviation from [001] is about 15°, the deformation of the specimens with orientation close to the [001]-[101] boundary is mainly controlled by the {111} [110] slip system, while the deformation of the specimens with orientation close to the [001]-[111] boundary is mainly controlled by the {111} [112] slip system.
Fig.6 Strain-time curves of DD413 at 760°C×793 MPa (A is the sample with orientation close to the [001]-[101] boundary, B is the alloy sample with orientation inside the reverse pole diagram, and C is the sample with orientation close to the [001]-[111] boundary)
SASS V et al. studied the creep behavior of CMSX-4 nickel-based single-crystal superalloy deviating from [001] orientation by 1°, 8°, 10° and 14° at 800 °C, and found that the creep life of the crystal deviated from the [001] direction by 8° was significantly shortened, and when the deviation angle was 10°, the creep life of the alloy was significantly prolonged. Sun et al. used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the mesothermal creep properties and microscopic mechanisms of DD6 single crystal superalloys with different orientation deviations, as shown in Fig. 7. The results show that the secondary slip system is easy to start during the creep at medium temperature for the alloy with an orientation deviation of 6°, and the overdislocation and stacking layer fault of the γ′ phase cut into the near fracture area of the alloy have different orientations and a high creep life (about 243 h), while the alloy secondary slip system with a deviation of 7° is difficult to start, and the overdislocation and stacking layer fault have a single orientation, and only the main slip system is activated, which is γ/γ′ The stress concentration at the interface of the two phases cannot be effectively released, and the cracks accumulate in this area and connect with each other to form macroscopic cracks, resulting in a sharp shortening of the creep life of the alloy (about 40 h).
Fig.7. Creep curves of alloys with different orientations deviation at 760 °C × 810 MPa
ZHANG S et al. tested the creep fracture properties of a first-generation single crystal alloy at 975 °C × 255 MPa, and found that the specimens with a deflection direction away from [001] were roughly distributed along a line between the boundaries of [001]-[011] and [001]-[11] in the projection triangle. When the deflection angle exceeds 30°, the fracture life is rapidly reduced, and lattice rotation is observed during creep. The rotation process can be divided into two steps, first rotating towards the [001]-[11] boundary, and then rotating along the boundary towards [001] or [11]. When the directional deviation from [001] exceeds about 25°, the initial creep stage is dominated by single slip, and there is strong asymmetric deformation.
Experiments conducted by KEAR B H et al. on the first generation of MAR-M200 nickel-based single crystal superalloy show that the orientation has little effect on the creep lifetime at 982 °C. At 760 °C, the [001] orientation has the best creep life, while at 872 °C and 982 °C, the [111] orientation shows the best creep life.
In summary, crystals with orientation close to the [001]-[011] boundary have a higher creep lifetime at medium temperature and high stress, while crystals with orientation close to the [001]-[111] boundary have a lower creep lifetime, and the creep lifetime of the crystals is less anisotropic at high temperature and low stress. Further research is needed on the creep lifetime of crystals with little difference in orientation angle (only 1°~5° difference) or crystals that deviate from [001] specific angles.
3 Creep deformation mechanism
SASS V et al. studied the creep deformation behavior of nickel-based single-crystal superalloy CMSX-4 at 800 °C deviating from different orientations [001], and found that for single crystals, the easy sliding deformation will cause the sample axis to move in the sliding direction. In the range of 750~850 °C, the main deformation mechanism of the primary creep of nickel-based single-crystal superalloys is the synergistic shear of γ′ particles slipping through {111} [112]. The direction of rotation of the specimen axis of all test crystals is consistent with the typical direction of rotation of the {111} [112] slip-deformed single crystal. The secondary slip system is activated only after the double slip boundary [001]-[011] or [001]-pole is reached. Cross-slip results in deformation strengthening of the material and is thought to initiate a transition from primary to secondary creep. In the process of secondary creep, the main deformation mechanism changed from {111} [112] single slip to {111} [110] multiple slip. Therefore, the magnitude of the primary creep strain depends on the amount of lattice rotation required to achieve multi-slip.
CARON P ET AL. INVESTIGATED THE CREEP DEFORMATION BEHAVIOR OF CMSX-2 AT 760 °C AND 750 MPa. It was found that when the size of the γ′ was 0.45 μm, the matrix was rapidly formed {111} dense dislocation network due to the slippage of [110], resulting in the rapid formation of a dense dislocation network at the γ/γ′ interface. A dense dislocation network can inhibit dislocation shearing into γ′ phases. When the size of the γ′ phase decreases to 0.23 μm, the creep mechanism is mainly [112] dislocation shear γ/γ′ interface and 1/3 [112] dislocation, in which the superlattice partial dislocation is separated by the inner and outer dislocations and cut into the γ′ phase.
Some researchers believe that the {111}-slip system and stratification fault formation of [112] are important deformation mechanisms for creep at medium temperature and high stress, especially in the initial stage of creep. Li Yifei studied the dislocation configuration of samples L (θ=13.9°, ρ=4°), N (θ=11.7°, ρ=17°) and P (θ=12.1°, ρ=38°) under the conditions of creep rupture at medium temperature and high stress of 850 °C × 650 MPa, see Fig. 8. It can be seen that under medium temperature and high stress, the specimens close to the [001]-[011] boundary start two sets of [112]{111}slip systems, but the density of the lamination is low, which can produce a certain work hardening effect and have good performance, while the specimens far away from the [001]-[011] boundary only start one group of [112]{111}slip systems, and the density of the lamination fault is very high, which will produce a high initial creep strain and increase the creep rate, which is not conducive to the performance. As a result, crystals closer to the [001]-[011] boundary exhibit better creep strength than crystals closer to the [001]-[11] boundary.
Fig.8 Dislocation configuration after creep rupture at 850 °C×650 MPa
Lin Dongliang et al. found that at medium temperature, the volume fraction and size of fine γ′ increased with the increase of solution temperature. When the solution temperature increases, the creep rate of the second stage decreases, the service life is extended, and the creep resistance of the alloy is greatly improved. Therefore, it is believed that the medium-temperature creep properties of the alloy depend on the volume fraction, size and spacing of the fine γ′. However, some researchers believe that one of the characteristics of high-temperature creep is the migration of γ′, which may reduce the anisotropy of creep and improve the creep resistance of the material, which is different from the creep situation where the γ′ phase size affects creep in the case of medium temperature. At high temperatures, the γ′ phase rapidly rafts, the Orowen stress decreases, and the width of the matrix channel increases, dislocation movement becomes easier, and the anisotropy of creep is reduced.
In summary, the creep behavior of Ni-based single-crystal superalloys with a small deviation from [001] orientation is affected by temperature, stress and deformation mechanism. Under medium temperature conditions, the change of orientation deviation has a significant effect on the creep behavior of single crystal superalloys, but has little effect on high temperatures.
4 Conclusion and outlook
The long-lasting creep properties of nickel-based single-crystal superalloys have obvious anisotropy, among which [001] oriented single-crystal superalloys are widely used due to their good comprehensive properties, but deviation from [001] direction will also have a certain impact on the creep properties of the alloys.
On the whole, under the conditions of medium temperature and high stress, the alloys near the [001]-[011] direction generally have the law of low creep rate and high creep life, and the deviation is more obvious when the degree of deviation is small, while the overall creep life of the alloy near the [001]-[111] direction is short, and the creep deformation in this direction should be avoided as much as possible in practical application, but the orientation deviation has little effect on the creep deformation behavior under high temperature conditions, while the creep behavior and temperature of nickel-based single crystal superalloys deviating from the [001] direction, stress, and deformation mechanisms.
For different nickel-based single-crystal superalloys, further research is still needed on how to determine the boundaries of orientation deviation on creep temperature and stress, how to determine the influence of specific orientation deviation on creep properties under near-service conditions, and how to establish a model of the impact of orientation deviation on creep life and explore the creep mechanism.