Elsevier

Materials & Design

Volume 64, December 2014, Pages 316-323
Materials & Design

Effects of stacking fault energy on the creep behaviors of Ni-base superalloy

https://doi.org/10.1016/j.matdes.2014.08.007Get rights and content

Highlights

  • The decrease of SFE could promote the dislocation dissociation.

  • The creep mechanisms were significantly affected by the SFE of the alloys.

  • The creep properties of the alloys improved with the decrease of SFE by facilitating the microtwinning process.

Abstract

Cobalt in a 23 wt.% Co containing Ni-base superalloys was systematically substituted by Ni in order to study the effects of stacking fault energy (SFE) on the creep mechanisms. The deformation microstructures of the alloys during different creep stages at 725 °C and 630 MPa were investigated by transmission electron microscopy (TEM). The results showed that the creep life increased as the SFE decreased corresponding to the increase of Co content in the alloys. At primary creep stage, the dislocation was difficult to dissociate independent of SFE. In contrast, at secondary and tertiary creep stages the dislocations dissociated at γ/γ interface and the partial dislocation started to shear γ precipitates, leaving isolated faults (IFs) in high SFE alloy, while the dislocations dissociated in the matrix and the partials swept out the matrix and γ precipitates creating extended stacking faults (ESFs) or deformation microtwins which were involved in diffusion-mediated reordering in low SFE alloy. It is suggested that the deformation microtwinning process should be favorable with the decrease of SFE, which could enhance the creep resistance and improve the creep properties of the alloys.

Introduction

Ni-base superalloys are an important class of high temperature structural materials, which have been widely used as turbine blades and discs in aircraft engines and industrial gas engines owing to their excellent high temperature mechanical properties, such as good tensile and creep properties [1], [2]. In order to increase the performance of the engines, the engineers explore more challenging materials by improving their creep resistance at elevated temperature. Therefore, the high temperature creep deformation mode is a major concern for the researchers to design new alloys. As we know, Ni-base superalloys mainly consist of γ matrix and γ (Ni3Al) precipitates which are coherent, ordered L12 crystal structure embedded in the disordered solid-solution face centered cubic (fcc) γ matrix. In Ni-base superalloys the γ′ precipitates contribute to their unique high temperature creep properties through acting as effective barriers to the moving dislocations. Dislocation bypassing γ′ precipitates via Orowan loop, cooperative climbing, and dislocation shearing γ′ precipitates [3], [4] are three main processes during creep deformation depending on microstructure, temperature and loading stress. When dislocations shear γ′ precipitates during plastic creep deformation, there will produce different high energy configurations such as anti-phase boundary (APB) and complex stacking fault (CSF) which are energetically unstable. These high energy configurations would convert into superlattice stacking fault (SSF) (including superlattice extrinsic stacking fault (SESF) and superlattice intrinsic stacking fault (SISF)) which possess lower energy in γ′ precipitates during creep deformation [5]. Deformation microtwin running across the whole grain could also be created after the shearing of γ′ precipitates during creep deformation [6], [7], [8].

The previous studies [6], [7], [8], [9], [10], [11] indicated that Ni-base superalloys retained high creep resistance depending on several microstructural factors, including the volume fraction, particle size, distribution of γ′ precipitates, and the chemical composition of γ′ precipitates and the matrix. In polycrystalline Ni-base superalloys, the grain sizes also played a vital role in affecting their high temperature creep properties [12], [13]. Furthermore, SFE affected by Co content also had crucial effect on the creep deformation of Ni-base superalloys [14], [15]. However, due to the complicated chemical compositions of Ni-base superalloys, the creep deformation mechanisms might be influenced by other compositions and γ′ content in the studies [14], [15]. Thus, it is necessary to investigate the influence on the creep mechanisms by SFE relating to Co content when other compositions and γ′ content are relatively stable in Ni-base superalloys.

In our previous study [16], the SFE of Ni-base superalloys with 5 wt.%, 15 wt.% and 23 wt.% Co content were measured to be 40.1, 33.3 and 24.9 mJ/m2, which indicated that the SFE decreased with the increase of Co content in the alloys under certain conditions. The result was in agreement with the previous studies [17], [18], [19], [20]. Then in the present paper the three alloys respectively marked as Alloy1, Alloy2 and Alloy3 were prepared in order to study SFE on the creep mechanisms at 725 °C/630 MPa.

Section snippets

Experimental details

The nominal compositions of three Ni-base superalloys for creep tests are listed in Table 1. For all tested three alloys, 20-kg ingots were cast using vacuum induction melting (VIM). These ingots were then hot extruded into 35 mm bars at about 1160 °C. The extruded samples were heat-treated at 1100 °C/4 h (air cooling) followed by aging at 650 °C/24 h (air cooling) and 760 °C/16 h (air cooling). Constant load tensile creep experiments were performed at 725 °C/630 MPa. The samples before creep tests for

Initial microstructures before creep tests

Fig. 1 shows the heat-treated microstructures of the alloys with various Co contents. It can be seen that all the alloys had a similar grain size of about 35 μm, as shown in Fig. 1(a)–(c). The high magnification microstructures of the three alloys all revealed a sparse distribution of primary γ′ precipitate (⩾300 nm), secondary γ′ precipitate (about 100 nm) and tertiary γ′ precipitate (typically less than 30 nm) which were shown in Fig. 1(d)–(f). The γ′ volume fractions of the three alloys

Effects of SFE on the dislocation dissociation

From other researchers’ works [24], [25], it can be deduced that SFE has great effect on the process of the dissociation of the perfect a/2 〈1 1 0〉 matrix dislocation. Burton [24] assumed that the dislocation network model could be applied to predict the creep rates of single phase metals and alloys, the SFE of which was high. However, the creep rates of the materials with lower SFE fell below the predicted values. It was deduced that the dislocation dissociation process other than the

Conclusions

The effects of SFE on the deformation mechanisms by substituting Co by Ni in three Ni-base superalloys during creep tests at 725 °C/630 MPa were investigated. The following conclusions could be obtained:

  • (1)

    The decrease of SFE as well as the increase of Co content in the alloys had little effect on the alloy’s microstructures and γ′ contents, however it can promote the dislocation dissociation and the stacking fault formation in the matrix.

  • (2)

    At primary creep stage, the Orowan looping combining a/2 〈1 1 

Acknowledgements

This work was partly supported by “Hundred of Talents Projects”, the National Basic Research Program (973 Program) of China under grant No. 2010CB631206 and the National Natural Science Foundation of China (NSFC) under Grant Nos. 51171179, 51128101, 51271174 and 11332010.

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