In the past few decades, nickel-based single crystal superalloys (SXs) have been widely used in hot section components (mainly gas turbine blades) because of their excellent performance against high temperature creep. The SXs consist a two-phase microstructure including a regular embedded phase of cuboidal L12 ’ precipitates and an face-centered cubic (FCC) matrix, leading to complicated deformation mechanisms. Together with the anisotropy of SXs, and inherently complex nature of creep and dislocation motion in the structures, this makes it the subject of numerous scientific investigations to estimate and model the mechanical properties of single crystal superalloys.
Various models were proposed to explain the creep mechanism of single crystals and to predict the creep rupture lives. In earlier works, the elastic-plastic constitutive relationships developed since the 1960s are strain rate independent [1
]. Later, a viscoplastic approach was proposed to gain unique solutions [5
]. Based on such plastic constitutive frame and the damage mechanics, many other models with different damage parameters were proposed [8
]. Furthermore, mechanism based models [13
], microstructure sensitive models [16
] and multi-scale models [19
] have also been built in recent decades. All these models can either elucidate certain physical mechanism to a degree or predict the creep rupture lives of SXs with certain degree of accuracy.
However, there is enough experimental evidence showing that far more factors can contribute to the creep behaviors and thus the creep rupture life of SXs in practical applications. Tian [22
] studied the creep behavior of SXs with different content of element Re by microstructure observation and creep test at 1060–1100
C and 120–150 MPa, and found that alloys with 4.2 wt. % Re displays a lower strain rate during steady state creep and longer creep rupture life than alloys with 2 wt. % Re. Sass [23
] conducted creep test of a second generation nickel-based single crystal named CMSX-4 at 1123 K/500 MPa, 1123 K/650 MPa and 1253 K/350 MPa, respectively, and suggested that the creep resistance declines in sequence of ,  and  at 1123 K. However, the anisotropy between  and  is strongly reduced, although the  orientation remains weak at 1253 K. The decline of stacking fault energy can ameliorate the creep properties by facilitating the microtwinning process [24
] and the large lattice misfit leads to denser
interfacial dislocation network, which contributes to small minimum creep rate in the secondary creep stage [25
]. Furthermore, macroscopic creep deformation and microscopic mechanisms are affected by relatively small changes in creep test conditions in SXs (CMSX-4) [26
]. For example, the microstructure is stable below approximately 900–950
C (depending on the specific alloy), while above these temperatures, a rafted structure of
platelets formed, which can either boost or deteriorate creep performance relying on the details of test conditions [28
]. Therefore, creep behaviors and creep rupture properties are correlated with chemical composition, crystallographic orientation and microstructure of SXs as well as certain creep test conditions.
The creep of SXs as one case of the most classic problem of mechanics, except for the material and the creep test condition, geometry of the investigated structure is also a decisive factor to be concerned; however, there is very limited work focusing on this issue. The creeping behavior in the thin wall SXs structures was examined [32
], and the creep response presented to be larger than in test specimens typically used to characterize the material. For the issue of stress multiaxiality caused by the notches and film-holes of specimens, a number of research works were conducted. Ref. [33
] showed that the notched specimens exhibit a longer creep rupture life than the smooth specimens under the same minimum-section stress, which was the so-called notch strengthening effect. Ref. [34
] studied the evolution of plasticity for SXs cooled blade using specimens with a single hole, and the results showed that the stress fields form four banded stripes around the hole, on which the initiation and propagation of the cracks are experimentally proved to be dependent. The finite element analysis (FEA) simulation was carried out by Yu [35
] to investigate the creep damage evolution in a 14 film-hole specimen of SXs and the experiment revealed that the creep damage is localized in the film-holes region, where the fracture will occur easily. Ref. [36
] found that specimens of SXs with different slant-angles of film-holes can exhibit different damage distributions as well as different crack propagation directions. All these indicated that local geometrical features of specimens may play a remarkable role in creep rupture properties of SXs components.
To date, most studies have focused on understanding the physical phenomena regarding the film cooling process and finding the most effective film-hole configurations with a minimal amount of coolant because film cooling techniques were widely used as an active cooling method for higher thermal efficiency in practical applications. However, little research work was done to investigate the effect of film-hole configurations on creep behavior of SXs.
The objective of this paper is to investigate the effects of multi-row film-hole configuration of a second generation nickel-based single crystal superalloy DD6, both in experimental and numerical approaches. The plate specimens with 0–4 rows of small holes of 0.3 mm diameter were used to model the air-cooled turbine blades. In addition, creep test at 980 C/300 MPa were carried out in crystallographic orientation  to examine the influence of these geometric features on stress distributions and the stress rupture behavior of DD6 specimens. The fracture positions and morphology of specimens in the experiment procedure were analyzed and compared with the FEA simulation results. Both global stress distribution of specimens and local stress state features around film-holes characterized by multiaxiality factor and maximal shear stress were presented. A discussion of the geometric configuration parameters of film-holes and their effects on the stress distribution and rupture lives were given.