Effects of stacking fault energy on the creep behaviors of Ni-base superalloy
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.
References (43)
- et al.
Materials selection in design of structures and engines of supersonic aircrafts: a review
Mater Des
(2013) - et al.
Superlattice intrinsic stacking faults in γ′ precipitates
Scripta Metal
(1985) - et al.
Precipitation hardening of superalloys by ordered γ′ particles
Prog Mater Sci
(1985) - et al.
Investigation of creep deformation mechanisms at intermediate temperatures in René 88DT
Acta Mater
(2005) - et al.
Heat treatment of UDIMET 720Li: the effect of microstructure on properties
Mater Sci Eng, A
(1999) - et al.
Creep behavior of a single crystal nickel-based superalloy containing 4.2% Re
Mater Des
(2012) - et al.
Influence of grain size and heat treatment on the microstructure and mechanical properties of the Nickel-Base superalloy U720 Li
Mater Sci Eng, A
(1997) - et al.
Creep mechanisms of U720Li disc superalloy at intermediate temperature
Mater Sci Eng, A
(2011) - et al.
A thermodynamic model for the stacking-fault energy
Acta Mater
(1998) The influence of stacking fault energy on creep
Acta Metall
(1982)