Journal of Materials Science & Technology, 2016, 32(10): 1003-1007
doi: 10.1016/j.jmst.2016.08.018
High Temperature Stress Rupture Anisotropy of a Ni-Based Single Crystal Superalloy
Guanglei Wang1,2, Jinlai Liu1, Jide Liu1, Tao Jin1,*,, Xiaofeng Sun1, Xudong Sun2, Zhuangqi Hu1,2
1 Superalloy Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 Key Laboratory of Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China
* Corresponding author. Fax: +86 24 23971758. E-mail address: tjin@imr.ac.cn (T. Jin).
Abstract:

High temperature stress rupture anisotropies of a second generation Ni-base single crystal (SC) superalloy specimens with [001], [011] and [111] orientations under 900 °C/445 MPa and 1100 °C/100 MPa have been investigated in the present study, with attentions to the evolution of γ/γ′ microstructure observed by scanning electron microscopy and the dislocation configuration characterized by transmission electron microscopy in each oriented specimen. At 1100 °C/100 MPa as well as 900 °C/445 MPa, the single crystal superalloy exhibits obvious stress rupture anisotropic behavior. The [001] oriented specimen has the longest rupture lifetime at 900 °C/445 MPa, and the [111] oriented sample shows the best rupture strength at 1100 °C/100 MPa. While the [011] oriented specimen presents the worst rupture lifetime at each testing condition, its stress rupture property at 1100 °C/100 MPa is clearly improved, compared with 900 °C/445 MPa. The evident stress rupture anisotropy at 900 °C/445 MPa is mainly attributed to the distinctive movement way of dislocations in each oriented sample. Whereas, at 1100 °C/100 MPa, together with the individual dislocation configuration, the evolution of γ/γ′ microstructure in each orientation also plays a key role in the apparent stress rupture anisotropy.
Key words: Ni-based single crystal superalloy; Stress rupture property; Anisotropy; Microstructure characterization; Deformation mechanism;
1. Introduction

Nickel-based single crystal (SC) superalloys have been one of key materials for advanced aeronautic and industrial gas turbine blades and vanes because of their excellent mechanical properties, creep as well as fatigue properties in particular, at elevated temperatures under the corrosive and oxidized condition. These outstanding mechanical properties of SC superalloys are mainly inherited from the particular γ/γ′ microstructure where ordered strengthening γ′ particles with cubic shape normally are coherently embedded in the disordered γ matrix. In other words, the nano-scale γ matrix channel, coherent γ/γ′ interface and strengthening γ′ precipitate can all effectively hinder movements of dislocations, enhancing these superb properties of SC superalloys[1], [2], [3] and [4].

Considering the fact that blades and vanes with complicated shapes are subjected to a multi-axial stress state during service, the creep or stress rupture anisotropy of SC superalloys has been receiving more and more attention. There have been extensive studies on this topic at immediate temperature regime (760 °C-850 °C), and the consensus is that the creep or stress rupture anisotropy at this condition is obvious due to the distinguishing deformation mechanism dominant in each orientation. While at high temperature regime, some apparent contradictory results have been reported, especially those of the [001] and [111] oriented specimens. Sass et al.[5] and [6] found that the CMSX-4 alloy exhibits a weaken creep anisotropy at 980 °C/350 MPa, yet the [111] oriented specimen displays a poor creep strength. Similarly, the creep anisotropy of SRR99 alloy at 1040 °C/165 MPa becomes unobvious, but the [111] oriented specimen shows a good creep elongation[7]. However, Liu et al.[8] investigated that the rupture lifetime of [111] oriented sample of an SC superalloy (without Re) is two times as much as that of the [001] oriented ones at 1010 °C/280 MPa. Yu et al.[9] reported that the [111] oriented specimen of the SC superalloy (without Re) has a far shorter creep lifetime than the [001] oriented one at 1040 °C/160 MPa because of the difference in the morphology of γ′-rafts and the movement of dislocations. For the LEK 94 alloy[10], [11] and [12], there is a higher increase in creep rate with strain in the [110] tensile creep test compared with the [001] tensile creep test at 1020 °C/160 MPa, and the phenomenon mainly arises from the large number of cutting γ′-rafts events and the high mobility of superdislocation segments in γ′-rafts in the [110] oriented specimen. Overall, the high temperature creep or stress rupture anisotropy of SC superalloys may be largely affected by the testing condition, and it also indicates that, for a specific SC superalloy, its creep or stress rupture anisotropy is needed to be further studied in detail.

Therefore, in this work, the influence of orientation (normally [001], [011] and [111]) was investigated on the stress rupture property of a second generation SC superalloy tested at 900 °C/445 MPa and 1100 °C/100 MPa, and attentions were paid to the evolution of γ/γ′ microstructure as well as the movement of dislocations after rupture, by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively, which are responsible for the stress rupture property of each orientated specimen under given testing conditions.

2. Experimental

An SC superalloy with composition of Ni-7Cr-8Co-2Mo-5W-7Ta-6Al-3Re-0.05C-0.004B (in wt%) was investigated in this study. SC rods with [001] orientation were produced with selecting crystal technique, while SC rods with [011] and [111] orientations were grown on pre-fabricated seeds. By electron backscattered diffraction (EBSD) technique, the cast bars within 5° deviation from normal orientation were selected and subjected to homogenization (1300 °C/2 h + 1310 °C/2 h, air cooling (AC)) and two-step annealing (1130 °C/4 h, AC + 900 °C/16 h, AC). This heat treatment produced cubic γ′ precipitates with the average edge of 0.45 µm and volume fraction of 69%, while the width of γ channels was only about 60 nm.

The cylindrical testing samples with a gauge length of 25 mm and a diameter of 5 mm were deformed to fracture in air at 900 °C/445 MPa and 1100 °C/100 MPa using constant-loading creep testing machines with clamping device, allowing rotation and ensuring uniaxial loading. In detail, the ratio of applied loading stress to engineering yield stress of specimens at 900 °C and 1100 °C is about 0.75 and 0.28, respectively, regardless of [001], [011] and [111] orientation. In addition, another [111] oriented specimen was interrupted after 2-h deformation at 900 °C/445 MPa and then cooled to room temperature.

Metallographic specimens along specific longitudinal sections of ruptured samples were first mechanically ground and polished, and then chemically etched by the etchant (4 g Cu2SO4 + 20 mL HCl + 20 mL H2O). The feature of γ/γ′ microstructure was observed by field emission scanning electron microscopy (FE-SEM, Inspect F50) operating at 30 kV.

Thin discs for TEM observation were cut along given sections, with at least 10 mm apart from the fracture surface. These foils were first mechanically ground to about 50 µm and then electrochemically thinned in the solution of 10 mL perchloric acid and 90 mL ethanol at -15 °C. The dislocation configuration in each oriented specimen was observed by TEM (JEOL 2100) operating at 200 kV.

3. Results
3.1. Stress rupture property

Stress rupture properties of the [001], [011] and [111] oriented specimens under two testing conditions are all listed in Table 1. The stress rupture property of each orientation is chiefly influenced by the testing condition. For example, the rupture lifetime and elongation of the [111] oriented sample at 900 °C/445 MPa are 127.1 h and 39.1% respectively, while at 1100 °C/100 MPa they are 522.6 h and 3.7% correspondingly. Similarly, compared with 900 °C/445 MPa, the rupture lifetime and elongation of the [011] oriented specimen are largely improved at 1100 °C/100 MPa, from 34.8 h and 5.8% to 116.3 h and 17.3% individually. Importantly, the SC superalloy exhibits obvious rupture anisotropies at these two testing conditions. In particular, at 900 °C/445 MPa, the [001] oriented specimen displays the longest rupture lifetime of 234.5 h; the [111] oriented sample has a less rupture time of 127.1 h; and the [011] oriented specimen presents the shortest lifetime of 34.8 h. In contrast, under 1100 °C/100 MPa, the rupture lifetimes of the [111], [001] and [011] oriented specimens are 522.6, 395.7 and 116.3 h, respectively. Also, the [111] oriented sample shows a rupture elongation maximum of 39.1% at 900 °C/445 MPa but a minimum of 3.7% at 1100 °C/100 MPa.


Table 1 Stress rupture properties of the SC superalloy specimens with three orientations: t characterizes the rupture lifetime and δδ marks the rupture elongation
SC superalloy [001] [011] [111]
t (h) δ (%) t (h) δ (%) t (h) δ (%)
900 °C/445 MPa 234.5 24.8 34.8 5.8 127.1 39.1
1100 °C/100 MPa 395.7 10.2 116.3 17.3 522.6 3.7

Table 1 Stress rupture properties of the SC superalloy specimens with three orientations: t characterizes the rupture lifetime and δδ marks the rupture elongation

3.2. 900 °C/445 MPa

The morphology of γ/γ′ microstructure for each oriented specimen ruptured at 900 °C /445 MPa has a discrete character, as seen in Fig. 1. The N type γ′-rafts[13] are formed in the [001] oriented sample, γ′ precipitates on the (100) section in the [011] oriented specimen retain cuboidal, but γ′ particles in the [111] oriented specimen directionally coarsen to some extent.


Fig. 1. SEM micrographs on (a) (100) plane of the [001] oriented specimen, (b) (100) plane of the [011] oriented specimen and (c) () plane of the [111] oriented specimen (c) after rupture at 900 °C/445 MPa. Stress parallel to vertical direction.

Dislocation configurations in these three oriented specimens after rupture at 900 °C/445 MPa are shown in Fig. 2. Irregular interfacial dislocation networks as well as combined dislocation pairs with anti-phase boundary (APB) in γ′-rafts, denoted by arrows in Fig. 2(a), are observed in the [001] oriented specimen, and in fact the directions of projections of these superdislocation segments are various, which indicates that several different slip systems are activated and participate in deformation. There are lots of stacking faults (SFs) found in γ′ precipitates in the [011] oriented specimen, as marked by arrows in Fig. 2(b). On the contrary, two sets of parallel dislocation segments are respectively observed in two of three matrix channels in the [111] oriented specimen after 2 h of deformation, as shown in Fig. 2(c), while after rupture, the two wider γ matrix channels are filled with dislocations, and many superdislocation segments with the parallel projection are found in γ′ precipitates, as seen in Fig. 2(d), indicating that one activated slip system dominantly take participate in deformation.


Fig. 2. Two-beam TEM bright field images on cross section of the (a) [001] oriented specimen, (b) [011] oriented specimen and [111] oriented specimen (c) after 2 h deformation and (d) after rupture at 900 °C/445 MPa.

3.3. 1100 °C/100 MPa

The γ/γ′ microstructures in three oriented samples ruptured at 1100 °C/100 MPa also have distinctive characters, as illustrated in Fig. 3, and they are generally different from those at 900 °C/445 MPa. The topological inversion of γ/γ′ microstructure that the initially isolated γ′ precipitates become the matrix finally[14] and [15] is observed in the [001] oriented specimen, as seen in Fig. 3(a). The γ′ rafts on the ( ) section in the [011] oriented specimen are observed perpendicular to the applied stress, as depicted in Fig. 3(b). Whereas, in the [111] orientation, initially isolated γ′ precipitates have transformed into the isolated γ′-rafts still embedded in the γ matrix, as illustrated by white arrow in Fig. 3(c).


Fig. 3. SEM micrographs on (a) (100) plane of the [001] oriented specimen, (b) () plane of the [011] oriented specimen and (c) () plane of the [111] oriented specimen after rupture at 1100 °C/100 MPa. Stress parallel to the vertical direction.

Fig. 4 exhibits morphologies of γ/γ′ interface dislocation networks and superdislocation segments in γ′-rafts in these three oriented samples after rupture at 1100 °C/100 MPa. These features are also different from those at 900 °C/445 MPa, but interfacial dislocation networks with individual feature are all observed in each specimen. In the [001] orientation, regular dislocation networks (Fig. 4(a)) are formed at the γ/γ′ interface, yet numerous superdislocation segments are found in the topological γ′ matrix (Fig. 4(b)). In the [011] oriented specimen, irregular dislocation networks are formed at the γ/γ′ interface, with a number of superdislocation segments in γ′-rafts, as signified in Fig. 4(c). However, together with interfacial dislocation networks, several superdislocation segments in γ′-rafts are investigated in the [111] oriented specimen.


Fig. 4. Two-beam TEM bright field images on (a) (001) plane and (b) (100) plane of the [001] oriented specimen, (c) (011) plane of the [011] oriented specimen, and (d) (111) plane of the [111] oriented specimen after rupture at 1100 °C/100 MPa.

4. Discussion

In the high temperature regime, where it highlights the effect of thermal activation on the stability of γ/γ′ microstructure in SC superalloys, the isolated γ′ precipitates inevitably become unstable and have a trend to rafting. The driving force of rafting is considered to originate from the difference between chemical potentials in matrix channels as a result of the interface misfit relaxation anisotropy, which attributes to the superposition of coherency stress and applied stress[16] and [17]. It is found that the rafting of γ′ phase in SC superalloys is also dependent on orientation, as the orientation plays a significant role in the chemical potential in each matrix channel[9], [18] and [19].

4.1. Stress rupture anisotropy at 900 °C/445 MPa

In the [001] oriented specimen, because the SC superalloy investigated here has a negative misfit in γ/γ′ microstructure[20], dislocation loops from different <110 > {111} slip systems activated in this oriented sample prefer to multiply in horizontal matrix channels as a result of coherency and external stresses[16] and [17], leaving dislocation segments with 60° character at the γ/γ′ interface. It is noted that the 60° dislocation segments cannot cross-slip. These dislocation segments can move slightly along the interfaces and interact with fitting partners under the action of thermal activation, gradually building up low-energy interfacial dislocation networks to minimize the misfit stress and dislocation line energy[3] and [16]. This process is sluggish under current condition due to the insufficient thermal activation, so the interfacial dislocation networks finally present irregularity, as shown in Fig. 4(a). At the same time, due to the quiet difference in chemical potentials between horizontal and vertical matrix channels, the rafting of γ′ precipitates takes place and lastly N type γ′-rafts are produced by stress-assisted cross-diffusion of γ- and γ′-forming solutes[3] and [16].

In the [001] oriented specimen, the plastic deformation is predominantly concentrated in the horizontal matrix channels since interfacial dislocation networks and continuous lamellar structure of γ′-rafts both effectively prevent dislocations from bypassing. It is reasonably theorized from the fact that certain different slip systems are activated in this oriented sample that dislocations, including superdislocation segments (dislocation pairs with APB) shearing into γ′-rafts, are more likely to interact with each other and block further movement, which results in dislocation hardening. So the [001] oriented specimen has a good deformation strength.

In the [011] oriented specimen, there are two types of matrix channels called roof channels (inclined to stress axis with 45° angle) and gable channel (parallel to stress axis). Roof channels experience a higher resolved shear stress as a result of superposition of external and coherency stresses[21], and thus the plastic deformation mainly concentrates in the roof channels. In fact, the leading screw dislocation segments in one roof channel can easily cross-slip toward the immobile 60° interfacial dislocations in the other roof channel and some of these coplanar matrix dislocations with different Burgers vectors can further react at the γ/γ′ interface once the local stress concentration is adequate, producing two partial dislocations[12], [22] and [23]. A typical reaction might then be given by[24]:

In general, a/3〈112〉a/3〈112〉 partial dislocation is able to enter the γ′ precipitates, leaving an SF behind it, and a/6〈112〉a/6〈112〉 partial dislocation remains at the γ/γ′ interface. Due to the high sensitivity to misorientation from the ideal [011] orientation, it is possible for the [011] oriented specimen that the conjugate slip systems, where one of these two slip systems is generally dominant, are activated and take parts in deformation. As a result, lots of coplanar matrix dislocations are easily formed, with screw or 60° character, and many of them with different Burger vectors can react as mentioned above and further shear into the γ′ precipitates, causing γ′ precipitates populated with SFs. Since significant amount of local strain is accumulated as SFs are produced in γ′ precipitates, this deform mechanism largely deteriorates the stress rupture property of [011] oriented sample. In addition, because the chemical potentials in two roof channels are nearly equivalent, γ′ precipitates in the [011] orientation are found to keep cubic on the (100) plane in Fig. 1(b).

Initially, in the [111] oriented sample, screw dislocations in matrix channels can easily cross-slip on {111} slip planes mainly due to the nearly equivalent Schmid factor for primary glide and cross-slip planes, which produces dislocation segments at the interface are parallel, at least their projections, as seen as in Fig. 2(c). In fact, two conjugate slip systems are more likely to be activated also due to misorientation, but one of them is in general dominant in deformation. This corresponds the finding that there are more dislocations in the matrix channel that is filled with dislocations than the others. Because the resolved shear stress on octahedral slip system is considered to be below the threshold stress required to operate the {111} deformation, γ′ precipitates are mainly sheared into by dislocation pairs with APB. Therefore, the poor rupture lifetime and good elongation of the [111] oriented specimen attribute to the one dominant slip system activated and the main propagation of dislocations through the connected γ-matrix. It is noted that the difference in chemical potentials in matrix channels as a result of the asymmetric plastic deformation is so slow that the slightly directional coarsening of γ′ precipitates occurs, as shown in Fig. 1(c) and Fig. 2(d).

4.2. Stress rupture anisotropy at 1100 °C/100 MPa

In the [001] oriented specimen, with the help of sufficient thermal activation, 60° interfacial dislocation segments can easily interact with each other to build up periodic low-energy networks[3] and [16]. The regular interfacial dislocation networks play an important role in the superior creep strength of SC superalloy, because they can effectively prevent matrix dislocations from approaching or shearing into γ′-rafts[25]. However, as the deformation continues, the γ′-edges gradually dissolve and γ′-forming solutes diffuse toward local dislocation concentrations in the γ-matrix, and thus neighboring γ′-rafts are connected by formation of γ′-junctions. This process is named as topological inversion[14] and [15]. The topological inversion of γ/γ′ microstructure seriously impairs deformation capability, because it largely accelerates the slip-climb motion of interfacial dislocation segments[26]. So the stress rupture property of the [001] oriented specimen is largely impaired by the topological inversion of γ/γ′ microstructure.

In the [011] oriented specimen, because of the quiet difference in chemical potentials between roof channels and gable channels, neighboring γ′ precipitates interlink with each other mainly in the [100] direction[19] and [21], and finally the rod γ′-rafts become perpendicular to the direction of external stress, as seen in Fig. 2(b). In fact, although two primary coplanar slip systems are likely to be activated in the [011] oriented specimen, one of these two slip systems tends to be prominent during deformation due to misorientation, and thus a small amount of new dislocation reaction products with a/2 < 110 > type are produced, leaving many long and straight dislocation segments remaining at the γ/γ′ interface. These long dislocation segments exhibit appropriate partners for the newly arriving dislocation segments, so the fitting dislocation pairs are easy to be formed, and further cutting into the rod-like γ′-rafts [12]. Therefore the [011] oriented specimens still has a relatively poor rupture lifetime, compared with the [001] and [111] oriented specimens.

For the [111] oriented specimen, γ′ precipitates keep isotropic growth mode when applied stresses are symmetrically distributed in three matrix channels. However, there inevitably is a misorientation from the accurate [111] orientation in the specimen. In fact, the plastic deformation is asymmetrically contributed in matrix channels. The difference in chemical potentials in matrix channels as a result of the asymmetric plastic deformation in the [111] orientation leads to the rafting of γ′ precipitates with the help of thermal activation. It is noted that, compared with the [001] and [011] oriented specimens, the driving force in the [111] oriented specimen is less significant, and this finally leads to the formation of irregular γ′-rafts but is still embedded in the γ matrix. Meanwhile, trailing segments as a result of movement of matrix dislocation loops are also dominant in the formation of interfacial dislocation networks, and thus dislocation networks are gradually formed at the γ/γ′ interface as several slip systems are activated with the action of sufficient thermal activation. The low resolved shear stress, irregular γ′-rafts embedded in γ matrix and interfacial dislocation networks can all provide resistances to the movement of dislocations, and therefore, the [111] oriented specimen exhibits a good strength and low plasticity.

5. Conclusions

(1) The SC superalloy specimens with [001], [011] and [111] orientations exhibit obvious stress rupture anisotropy at 1100 °C/100 MPa as well as at 900 °C/445 MPa.

(2) At 900 °C/445 MPa, in the [001] oriented sample, certain different slip systems are activated and the movement of dislocations mainly concentrates in the horizontal channels, producing a good rupture strength. Cutting γ′ precipitates by {111} deformation largely deteriorates the stress rupture property of the [011] oriented specimen. The poor hardening behavior and good elongation of the [111] oriented sample result from the one dominant slip system activated and the cross-slip movement of dislocations in matrix channels.

(3) At 1100 °C/100 MPa, although regular interfacial dislocation networks impede the movement of dislocations to some extent initially, the topological inversion of γ/γ′ microstructure, causing the accelerating movement of dislocation segments, impairs the rupture strength of the [001] oriented specimen. The relatively short rupture lifetime of the [011] oriented specimen is mainly due to high γ′-rafts cutting events as a result of limited slip systems activated. Whereas, the inclined γ′-rafts embedded in γ matrix, interfacial dislocation networks and the low resolved shear stress all contribute to the long rupture lifetime and poor elongation of the [111] oriented sample.

Acknowledgments

This work was financially supported by the National High Technology Research and Development Program of China (“863 Program”, No. 20102014AA041701) and the National Natural Science Foundation of China (No. 51331005) and (No. 51401210). The authors are also grateful to D.Q. Qi, Dr. X.G. Wang and Pro. J.J. Yu for the salutary discussion.

The authors have declared that no competing interests exist.
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The creep behavior of the Ni-base superalloy LEK 94 in the 1000 °C range has been investigated. Miniature tensile specimens were employed, which can be taken out of a thin walled blade or used when only limited amounts of material are available. We show that this miniature test technique yields reasonable creep data, including stress exponents and apparent activation energies . The values are close to 5 and the values are close to 500 kJ/mole, independent of the crystallographic direction. Creep rates are fastest for samples oriented in the [110] direction, followed by [001] samples. The lowest creep rates are observed for the [111] direction. This difference is related to a difference in resolved shear stresses and in the number of activated crystallographic slip systems. As a striking new result, it was observed that the onset of tertiary creep occurs earlier and rupture strains are significantly smaller in the case of [110] specimens as compared to [100] specimens, while the [111] specimens show an intermediate behavior. These effects are related to the alignment of cast porosity along the axis of primary dendrites. When cast micropores align perpendicular to the axis of the applied tensile stress, the onset of fast tertiary creep occurs early and small rupture strains are observed.
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[11] L.A. Jacome, P. Nortershauser, J.K. Heyer, A. Lahni, J. Frenzel, A. Dlouhy, C. Somsen, G. Eggeler. Acta Mater, 61 (2013), pp. 2926-2943
ABSTRACT The high-temperature and low-stress creep (1293 K, 160 MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [0 0 1] and [1 1 0] directions. In the early stages of creep, the [0 0 1]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [1 1 0] tensile loading. This may be related to the nature of rafting during [1 1 0] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [1 0 0] tensile creep.
DOI:10.1016/j.actamat.2013.01.052      URL     [Cited within:1]
[12] L.A. Jacome, P. Nortershauser, C. Somsen, A. Dlouhy, G. Eggeler. Acta Mater, 69 (2014), pp. 246-264
The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [001] and [110] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [110] as compared to [001] tensile loading were identified. [110] tensile creep is associated with a higher number of gamma' phase cutting events, where two dislocations with equal Burgers vectors of type (110) jointly shear the gamma' phase. The resulting (220)-type superdislocation can move by glide. In contrast, during [001] tensile loading, two dislocations with different (110)-type Burgers vectors must combine for gamma' phase cutting. The resulting (200)-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the gamma' phase cutting events. The higher [110] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in gamma') and a higher superdislocation mobility ((220) glide) in the gamma' phase. 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
DOI:10.1016/j.actamat.2014.01.021      Magsci     URL     [Cited within:3]
[13] A. Fredholm, J.L. Strudel. R.H. Bricknell, W.B. Kent, M. Gell, C.S. Kortovich, J.F. Radavich (Eds.), Superalloy 1984, TMS, Warrendale (1984), pp. 211-220
DOI:10.7449/1984/Superalloys_1984_185_197      URL     [Cited within:1]
[14] A. Fredholm, J.L. Strudel. J.B. Mariott, H. Herz, J. Nihoul, J. Ward (Eds.), High Temperature Alloys, their Exploitable Potential, Proceeding of the Petten, 1985, Elsevier Applied Science, London (1987), pp. 9-18
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[15] A. Epishin, T. Link, U. Bruchener, P.D. Portella. Acta Mater, 49 (2001), pp. 4017-4023
In undeformed superalloys, the disordered γ-solid solution of nickel is hardened by coherently embedded small cuboids of the ordered γ&prime;-phase (Ni<sub>3</sub>Al). During high-temperature creep the γ&prime;-phase coalesces, coarsens and finally surrounds the γ-phase, i.e., it becomes topologically the matrix. The kinetics of this so-called topological inversion during creep of the superalloy SRR99 at 980&deg;C and 200 MPa has been investigated quantitatively by analysis of scanning electron microscope images. The topological state of the γ/γ&prime;-microstructure was characterized by the parameter <em>R</em>: the ratio of area densities of transverse terminations of γ- and γ&prime;-lamellae. The topological inversion is explained by the formation of junctions connecting neighbouring γ&prime;-rafts and separating the γ-phase. One reason is the generation of dislocations in the γ-channels during primary creep. Another reason is the dissolution of γ&prime;-edges in the γ-phase, which is more diffusionally penetrative. The released γ&prime;-forming atoms move along the interface towards dislocation concentrations, resulting in the formation of junctions between the γ&prime;-rafts.
DOI:10.1016/S1359-6454(01)00290-7      Magsci     URL     [Cited within:2]
[16] T.M. Pollock, A.S. Argon. Acta Metall. Mater, 42 (1994), pp. 1859-1874
Directional coarsening has been investigated experimentally in two nickel-base single crystal alloys with high volume fractions of precipitates. The initial elastic stresses due to the positive or negative misfit of the coherent precipitates biased the orientation of the precipitates developed during coarsening. However, directional coarsening of the precipitates did not occur until some limited amount of creep deformation was initiated. With the application of an external stress, dislocations penetrated preferentially into the most highly stressed matrix channels and directional coarsening occurred by coalescence of the γ′ precipitates in the plane of the initially less highly stressed channels, where the misfit stresses were not altered by the presence of misfit relieving dislocations. The diffusional processes responsible for rafting apparently operate on the local scale of the precipitates, with preferential local dissolution of the γ′ and diffusional flow of alloying elements around the periphery of the precipitates.
DOI:10.1016/0956-7151(94)90011-6      URL     [Cited within:5]
[17] M. Fahrmann, E. Fahrmann, O. Paris, P. Fratzl, T.M. Pollock. R.D. Kissinger, D.J. Deye, D.L. Anton, A.D. Cetel, M.V. Nathal, T.M. Pollock, D.A. Woodford (Eds.), Superalloy 1996, TMS, Warrendale (1996), pp. 191-200
[Cited within:2]
[18] S. Tian, S. Zhang, C. Li, H. Yu, Y. Su, X. Yu, L. Yu. Metall. Mater. Trans. A, 43 (2012), pp. 3880-3889
The microstructure of a [001] orientated single crystal Ni-based superalloy was observed during tensile creep.By the stress-strain finite element method(FEM) for calculating the von Mises stress distribution in the coherent interface of the cubic γ/γ' phases,the influence of the applied stress on the regularity of γ' phase directional coarsening was investigated.Results show that the distribution of the von Mises stress in the cubic γ/γ' interfaces is changed by the applied tensile stress,which may bring the lattice contraction or expanding of the various crystal planes in the cubical γ' phase.Thereinto,the lattice contraction strain on(001) plane may repel the Al and Ti atoms with bigger radius,while the lattice expanding strain on(100) and(010) planes may trap the Al and Ti atoms to promote the directional growth of γ' phase into the mesh-like layer structure along the normal of the expanding lattice.This is thought to be the regularity of γ' phase directional coarsening during creep of the alloy.Furthermore,the driving force of the elements diffusing and γ' phase directional coarsening was proposed.
DOI:10.4028/www.scientific.net/MSF.689.154      URL     [Cited within:1]
[19] A. Gaubert, M. Jouiad, J. Cormier, Y.L. Bouar, J. Ghighi. Acta Mater, 84 (2015), pp. 237-255
Microstructure evolution during tensile creep of 〈 0 1 1 〉 mathContainer Loading Mathjax -oriented samples of first-generation Ni-based single-crystal superalloys was investigated both experimentally and numerically. Based on scanning electron microscopy and 3-D volume reconstruction using a dual-beam system, it was shown that the initially cuboidal microstructure first elongates along the cubic axis perpendicular to the tensile axis, and then coalesces along the two other cubic directions. We found that the normal to the platelets, initially close to a cubic axis, slightly rotates towards the tensile axis direction and the destabilization of the platelet microstructure appears to control the onset of the tertiary creep stage. In a second part, microstructure evolutions are analyzed using 3-D phase-field simulations. The inhomogeneous and anisotropic elastic and plastic driving forces are discussed, as well as the importance of a small misorientation of the tensile axis from the [011] direction. Surprisingly, contrary to the case of 〈 1 0 0 〉 mathContainer Loading Mathjax -oriented sample, complete rafting is not obtained in the simulations, suggesting that an additional mechanism at the level of individual dislocations could be missing. Finally, we have developed an energetic model for a perfectly rafted microstructure, which proves that plasticity in the γ phase is at the origin of the γ/γ ′ γ / γ ′ mathContainer Loading Mathjax interface rotation during creep.
DOI:10.1016/j.actamat.2014.10.034      URL     [Cited within:2]
[20] G.L. Wang. Stress rupture properties of a Ni-base single crystal superalloy, (Master thesis); Northeastern University (2013)
[Cited within:1]
[21] T. Kuttner, M. Feller-Kniepmeier. Mater. Sci. Eng. A Struct. Mater, 188 (1994), pp. 147-152
Tensile creep tests in the [011] direction have been performed on two specimens of SRR99 at 1173 K under a load of 300 MPa. The microstructure was analysed by transmission electron microscopy investigations after plastic strains of 0.1% and 0.29%. At both strains, the γ′ phase is not sheared by dislocations. Plastic deformation is concentrated in roof matrix channels lying at an angle of 45° to the load axis. Gable matrix channels that contain the load axis are almost free of dislocations. This experimental result was shown to be due to the superposition of coherency and external load stress tensors, leading to different levels of shear stress in the two types of matrix channel. The results obtained at 1173 K are compared with previous results at 1033 K.
DOI:10.1016/0921-5093(94)90366-2      URL     [Cited within:2]
[22] V. Sass, U. Glatzel, M. Feller-Kniepmeier. R.D. Kissinger, D.J. Deye, D.L. Anton, A.D. Cetel, M.V. Nathal, T.M. Pollock, D.A. Woodford (Eds.), Superalloy 1996, TMS, Warrendale (1996), pp. 283-290
[Cited within:1]
[23] M. Feller-Kniepmeier, T. Kuttner. Acta Metall. Mater, 42 (1994), pp. 3167-3174
An [011] orientierten Einkristallen der Legierung SRR99 wurden Kriechweesuche bei 1033 K unter einer Last von 680 MPa durchgeführt. Am. Minimum der Kriechrate werden vier oktaedrische Gleixtsyseme der matrix aktiviert. Oktaedrische Quergleitung der Gleiten werden nicht beobachtet. Als Floge der 05berlagerung von Koh01renzzpannung und 01βerer Last in engen Matrixkan01len, ist die Verfformung auf “Dach” Matrixkan01le konzertriert. W01hrend des sekund01ren kubischen γ/gg' Grenzfl01chen. γ' Phase und matrix k02nnen durch Super-Shocklly Partialversetzungen gemeinsam geschnitten werden. Die hohe Kriechgeswindigkeit [011] orientierter Proben wird auf die Spannungsonzentration in den Dachkan01len der matrix und auf geringe Zahl der activierten Gleitsysteme zurückgelfürt.
DOI:10.1016/0956-7151(94)90415-4      URL     [Cited within:1]
[24] G.L. Drew, R.C. Reed, K. Kakehi, C.M.F. Rae. K.A. Green, T.M. Pollock, H. Harada, T.E. Howson, R.C. Reed, J.J. Schirra, S. Walston (Eds.), Superalloys 2004, TMS, Warrendale (2004), pp. 127-136
[Cited within:1]
[25] J.X. Zhang, T. Murakumo, H. Harada, Y. Koizumi. Scr. Mater, 48 (2003), pp. 287-293
There is an important relation between the γ/γ ′ interfacial dislocations and the minimum creep rate. During creep (1100 °C, 137 MPa), the finer the interfacial dislocation networks are, the smaller the minimum creep rate is. This is attributed to the interaction between the γ/γ ′ interfacial dislocations and slip dislocations in the γ matrix.
DOI:10.1016/S1359-6462(02)00379-2      URL     [Cited within:1]
[26] A. Epishin, T. Link. Philos. Mag, 84(2004), pp. 1979-2000
[001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135 MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ-γ64 microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide-climb of (a/2) interfacial dislocations. A by-product of the interfacial glide-climb are vacancies which diffuse along the interfaces to growing pores or to a edge dislocations climbing in the γ64 phase. Climb of a dislocations in the γ64 phase is a recovery mechanism which reduces the constraining of the γ phase by the γ64 phase, thus enabling further glide of (a/2) dislocations in the matrix. Moreover the γ64 dislocations act as vacancy sinks facilitating interfacial glide-climb. The creep rate increases when the γ-γ64 microstructure becomes topologically inverted; connection of the γ64 rafts results in extensive transverse climb and an increase of the number of a dislocation segments in the γ64 phase.
DOI:10.1080/14786430410001663240      URL     [Cited within:1]
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Key words
Ni-based single crystal superalloy
Stress rupture property
Anisotropy
Microstructure characterization
Deformation mechanism
Authors
Guanglei Wang
Jinlai Liu
Jide Liu
Tao Jin
Xiaofeng Sun
Xudong Sun
Zhuangqi Hu