γ/γ′ interface strengthening of Re in single crystal superalloys

LUO Yu-shi(骆宇时), LI Jia-rong(李嘉荣), LIU Shi-zhong(刘世忠), HAN Mei(韩 梅), CAO Chun-xiao(曹春晓)

National Key Laboratory of Advanced High Temperature Structural Materials,

Beijing Institute of Aeronautical Materials, Beijing 100095, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Based on analysis of three alloys with various Re contents, the effects of Re on the interfacial structure characteristic and interfacial dislocations of γ/γ′ in single crystal superalloys were investigated. A high resolution transition electron microscopy (HRTEM) technique was used to detect the structure characteristic of γ/γ′ interface. The interfacial dislocations of γ/γ′ were analyzed with TEM. The results show that as Re content increases, the interfaces of γ/γ′ become orderly, the atomic arrangements at interfaces are more uniform, the number of mismatch dislocations increases and transition areas between γ and γ′ phases become narrow. That Re changes the interfacial structure promotes the formation of dense, regular and homogeneous interfacial dislocation networks in short time during creep at 1 100 ℃ and 140 MPa, which results in strengthened γ/γ′ interface at elevated temperature.

Key words: single crystal superalloy; γ/γ?; interface strengthening; dislocation; Re

1 Introduction

The strength of single crystal superalloys at elevated temperature has been found to be notably improved by the addition of Re. In order to understand the hardening mechanisms of Re, Some microstructure studies have been carried out. It was found that Re generally replaces other refractory elements such as Mo or W in the alloy, it partitions mainly into the matrix γ phase[1,2] and is known to retard the γ′ coarsening[3]. Re has also been reported to form clusters within the γ matrix, and these clusters are thought to be a more potent source of strengthening than other elements in solid solution[4]. Moreover, Re as well as the other refractory metals can alter γ/γ′ lattice misfit[5]. The general characteristics of these studies are as follows: investigating on Re distribution and focusing on solution strengthening for the γ phase. For a long time the strengthening effects of Re on γ/γ′ interface has been neglected.

As the γ/γ′ interface is the channel of elements diffusion, the structure of interface influences element diffusion rate and dislocation motion during creep. An important step towards a better understanding of the influence of Re on the mechanical properties is the detailed investigation of its effects on the γ/γ′ interface of the single crystal suprealloys. Unfortunately, few studies about it have been conducted.

The purpose of this study is to identify the effects of Re contents on the γ/γ′ interface structure and to study the interface strengthening effect of Re in single crystal superalloys.

2 Experimental

Specimens of single crystal superalloy were produced in a vacuum induction furnace with a selector technique for [001] orientation. The chemical compositions of the experimental superalloys are listed in Table 1. The crystallizing orientation was determined by Laue-back reflection X-ray technique. All specimens used to creep tests were within 10? of the [001] orientation. Subsequently, the heat treatment was carried out. A high resolution transition electron microscopy (HRTEM) technique was used to detect structure characteristic of γ/γ′ interface for heat treatment specimens. Creep tests were performed at 1 100 ℃ and 140 MPa in air. After crept for 12 h, these specimens were cut to thin slices perpendicular to the [001] direction. The thin foils were prepared by electropolishing. A JEM-2000 FX TEM was used for the observation of interfacial dislocations configuration.

Table 1  Chemical compositions of experimental alloys(mass fraction, %)

3 Results and analysis

Fig.1 shows the images of atoms structure at interface after Fourier translation from HRTEM observation on three experimental single crystal superalloys after heat treatment. Coherent structures are clearly observed at interfaces in all three tested alloys. There are some mismatch dislocations at coherent interfaces. As shown in Fig.1, the atom arrangements at interfaces in alloys 2Re and 4Re are orderly and uniform but disorderly at the alloy 0Re interface. Phase transition areas of alloy 0Re are wider than those of alloys 2Re and 4Re. Moreover, more mismatch dislocations exist at interfaces in alloys 2Re and 4Re (shown in Figs.1(b) and (c)). The results shown in Fig.1 indicate that Re changes the atomic arrangement structure of γ/γ′ interface in single crystal superalloys.

Fig.2 shows the morphologies of the γ/γ′ interfacial dislocations after 12 h creep. Obviously, dislocations networks are formed at interfaces for all three alloys. As shown in Fig.2, the interfacial dislocation networks in alloy 0Re are irregular and thin, but in alloy 2Re they are regular and dense, furthermore, in alloy 4Re, the dislocations in the squared networks are denser.

In order to identify the effects of Re on γ/γ′ interfacial dislocation spacing and distribution, the interfacial dislocation spacing was measured. An approximately linear relationship exists between the interfacial dislocation spacing and Re content. As shown in Fig.3, the dislocation spacing decreases with the

Fig.1 Images of atoms structure at interface: (a) Alloy 0Re; (b) Alloy 2Re; (c) Alloy 4Re

Fig.2 Morphologies of γ/γ′ interfacial dislocation networks: (a) Alloy 0Re; (b) Alloy 2Re; (c) Alloy 4Re increase of the Re content.

Fig.3 Interfacial dislocation spacing as a function of Re content

An important characteristic of the γ/γ′ interfacial dislocation networks is the difference in dislocation spacing distributions (shown in Fig.3). The interfacial dislocation spacing in alloy 0Re is distributed in a wide range, whereas, the dislocation spacing in alloy 2Re as well as alloy 4Re has a narrow distribution range. Described results above confirm that with the increase of the Re content, the interfacial dislocations tend to become homogeneously arranged.

4 Discussion

The three alloys exhibit considerably different creep properties from creep tests. The alloy 4Re has the longest creep life. In contrast, the alloy 0Re demonstrates the shortest creep life, just 1/5 of that of alloy 4Re. Great difference of creep life can be interpreted from the different interfacial structure between two superalloys.

Intensive studies have shown that the addition of Re leads to the large γ/γ′ lattice misfit[5-7]. HRTEM observation also indicates that the number of mismatch dislocation increases with the increase of Re content. More mismatch dislocations at phase interface mean that stronger elastic interaction exists in atomic arrange at interface. The stronger elastic interaction between interface atoms results dense and regular dislocation networks formed rapidly during creep[6,7]. After interfacial dislocation formation, the interfacial dislocations tend to become homogeneously arranged due to the stronger elastic interaction between the dislocations closer to each other.

As shown in Fig.2, after 12 h creep, the interfacial dislocation networks in alloy 0Re are irregular and thin, just like the deformed meshes; but in alloys 2Re and 4Re, the dislocation networks are regular and dense, almost in the squared networks. In general, the dislocation networks in the squared form can prevent the glide dislocations more effectively than the irregular meshes, because the formation of new segments by dislocation reactions during creep will result in an increasing possibility of a glissile dislocation in γ channel to pass through the interfacial dislocation networks[8-12]. As a result, dislocation networks in Re-bearing alloys can prevent the glide dislocations from cutting the rafted γ/γ′ structure more effectively than that of alloy 0Re.

As shown in Fig.4, the dislocation spacing distributes in larger range and inhomogeneously in alloy 0Re. Because the inhomogeneously distributed interfacial dislocations may let the glissile dislocations to pass through the networks at those positions with considerable spacing[12], in this respect, the homogeneous distribut- ion of the interfacial dislocations in alloys 2Re and 4Re can effectively prevent the glide dislocations from cutting the γ/γ′ rafted structure.

Fig.4 Distribution of γ/γ? interfacial dislocation spacing: (a) Alloy 0Re; (b) Alloy 2Re; (c) Alloy 4Re

As the effect of interfacial dislocations which effectively prevent the glide dislocations from cutting the γ/γ′ rafted structure exhibits interface strengthening, it can be found from above analysis that the interface strengthening effect in alloy 4Re is the strongest but the weakest in alloy 0Re for the three experimental single crystal superalloys. So a conclusion can be drawn that Re strengthening the γ/γ′ interface is the important reason of Re improving the creep life.

5 Conclusions

1) The addition of Re leads to the phenomenon that the interfaces of γ/γ′ become orderly, the atomic arrangements at interfaces are more uniform, the number of mismatch dislocations increases and transition areas between γ and γ′ become narrow.

2) Re promotes the formation of dense, regular and homogeneous interfacial dislocation networks during creep at 1 100 ℃ and 140 MPa.

3) That Re strengthening the γ/γ′ interface is the important reason of Re improving the creep life.

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(Edited by YANG You-ping)

Corresponding author: LUO Yu-shi; Tel: +86-10-62496338; E-mail: luoyushi1978@sohu.com