Review
TCP phases growth and crack initiation and propagation in nickel-based single crystal superalloys containing Re

https://doi.org/10.1016/j.jallcom.2018.02.133Get rights and content

Abstract

Based on crystal slip theory, the rafting of γ′ precipitate phases, the atomic diffusion of Re, the growth mechanism of topologically close-packed (TCP) phases and the microcrack initiation and propagation near TCP phases during creep deformation in a single crystal superalloy were studied at 1100 °C and 140 MPa. The results show that the γ′ rafting process is accompanied by moving dislocations. The diffusion along with dislocations is the main reason for the enrichment of Re in the TCP phases. In the state of uniaxial tensile stress along the [001] direction, the octahedral and dodecahedral slip systems may open, and the hexahedral slip system may close. TCP phases grow along the slip systems (111)[-1-12], (-1-11)[112], (-11-1)[1-1-2] and (1-1-1)[-11-2]; the Schmidt factors of which are larger than the others. Meanwhile, the four slip systems have the maximum resolved shear stress. Microcracks usually generate near TCP phases and propagate along the TCP phase growth direction and the slip system families. The angle is approximately 70.5° between the TCP phase growth direction and the slip plane. In other words, cracks generate and propagate along the slip systems (-1-11)[112] and (111)[-1-12]. The microcrack propagation direction corresponds to the theoretical calculation result and the zigzag propagation of a macrocrack in the tenon of an aero-engine single crystal turbine blade.

Introduction

Nickel-based single crystal superalloys have been widely used in the hot part of aero engines because of their excellent mechanical properties [[1], [2], [3], [4]] that are related to the two-phase microstructure, with the long-range ordered L12 γ′ phase that appears as cubes coherently embedded in a face-centered cubic solid solution γ matrix [5,6]. An important feature of modern nickel-based single crystal superalloys is the widespread addition of refractory elements that can significantly improve the creep properties at high temperatures. In the development of single crystal superalloys, the refractory element Re has the greatest effect on improving the mechanical properties of single crystal superalloys, which is the so-called “rhenium-effect” [7]. Compositions of 3% Re and 6% Re are nearly the main features and differences of the second and third generation single crystal superalloys. The strengthening mechanism of Re is due to the short-range order Re clusters in the γ matrix [8,9]. The strengthening effect of Re clusters, which can hinder the movement of dislocations, reduce the diffusion coefficient, inhibit the rafting process of γ′ phases and increase the lattice misfit between γ/γ′ phases, is more excellent than isolated solute atoms. However, A. Mottura and Ernst Fleischmann have an opposite conclusion, that Re clusters do not exist in matrix alloys, and therefore, Re clusters are not the reason for the Re effect in Ni-based superalloys [10,11]. At present, the segregation configuration of Re is problematic, the study of Re on migration is conducive to understanding the Re effect.

During the high-temperature creep process, the cuboidal γ′ phases may combine and form a rafted structure. One of the main reasons for rafting is the diffusion of elements [12]. The addition of Re, which is one of the slowest diffusing elements, can reduce the diffusion coefficient. Meanwhile, Re can stabilize or increase the segregation abilities of other alloying elements in the γ/γ′ phases [13]. In this manner, Re-enriched needle-shaped TCP phases [14,15] are precipitated during high-temperature long-term aging or endurance tests. The formation of TCP phases, essentially caused by element diffusion [16], has a great relationship with temperature, time and stress [[17], [18], [19]]. From the mechanics point of view, TCP phases belong to brittle inclusions, which can significantly reduce the endurance life of nickel-based single crystals. At high temperatures, TCP phases largely deplete refractory elements in the vicinity [15], block the rafting structure of the γ′ phase and greatly reduce the retarding effect of the γ′ phase on dislocation movement [20]. At high temperatures, stress concentration and then microcracks usually generate near TCP phases [21]. Most studies to date have concentrated on the generation location and propagation process of the microcracks, and the main conclusions are summarized as follows [2,[22], [23], [24], [25], [26]]: (1) Microcracks usually generate at the interface between the TCP phase and the γ matrix or the region of stress concentration interaction; (2) Cracks propagate along the γ channels or the γ/γ′ interface after rafting; (3) Cracks first propagate on the (001) and then along the (111) planes during a 760 °C/800 MPa creep process; (4) Cracks mainly propagate along the TCP/γ′ interface; and (5) Microcracks are often formed at the intersection of two sets of slip lines. However, few researchers have done a systematic study on the formation mechanism of TCP phases and the initiation and propagation of cracks near TCP phases, combining theory, experiment and engineering.

In this paper, standard samples were used to research the rafting of γ′ phases, the diffusion of Re elements, the formation mechanism of TCP phases and the initiation and propagation of cracks near the TCP phases of nickel-based superalloys, combining theory, experiment and engineering.

Section snippets

Experimental procedures

The material considered here is the nickel-based single crystal superalloy, classified as a second generation nickel base superalloy. The chemical composition is listed in Table 1. All samples were prepared from [001] oriented (less than ±7°) bars cast in a directional solidification vacuum furnace under a high thermal gradient. The heat treatment regime of the alloys was given as follows: 1290 °C, 1 h + 1300 °C, 2 h + 1315 °C, 4 h, air cooling (A.C.) + 1120 °C, 4 h, A.C. + 870 °C, 32 h, A.C.

Creep morphology evolution

The creep curve consists of three stages: initial creep, steady-state creep and tertiary creep at 1100 °C and 140 MPa, as shown in Fig. 2. The morphologies of the original specimen, initial creep, steady-state creep and tertiary creep were observed at a magnification of 10000, as shown in Fig. 3. Dislocations in the γ channel and at the γ/γ′ interface are shown in Fig. 4.

After starting the experiments, the instantaneous strain (elastic deformation) comes into being, and then, the dislocations

The morphology and growth mechanism of the TCP phase

At 1100 °C and 140 MPa, the TCP phase is essentially a needle-shaped topologically close-packed phase that is rich in refractory elements, such as Re, and readily precipitates at high temperatures and low stress, as shown in Fig. 5(a)-(d). The EDS spot is given in Fig. 5(a)-(d), and the average content of the main elements in percentage for five measurements is given as follows: 29.93 wt% W (0.17 error%), 25.26 wt% Ni (0.34 error%), 18.18 wt% Re (0.19 error%), 9.23 wt% Co (0.26 error%),

Elastic-plastic deformation

The crystal elastic behavior originates from the tendency to resist separation, compression and shear movement between internal lattice atoms. The interaction between atoms is given byf(r)=du(r)drwhere u(r) is the interatomic potential energy, and r is the distance between atoms. The crystal elastic potential energy increases under the applied load. The distance between atoms changes and atoms deviate from the equilibrium position slightly. Interatomic equilibrium is disturbed. The internal

Conclusions

Based on crystal slip theory, the rafting of the γ′ precipitate phase, the diffusion of Re, the growth mechanism of the TCP phases and the microcrack initiation and propagation near the TCP phases are studied. The conclusions are summarized as follows:

  • (1)

    The creep curve consists of three stages: initial creep, steady-state creep and tertiary creep at 1100 °C and 140 MPa. In the initial creep stage, dislocations come out in the γ matrix, and the γ′ phase is transformed into the rafted structure. In

Acknowledgment

The research was supported by the National Natural Science Foundation of China (NO. 51375388) and the Natural Science Basic Research Plan in Shaanxi Province of China (NO. 2016JM1020).

References (40)

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