Creep deformation of DD15 single crystal superalloy at 980°C/250MPa

The creep deformation behavior of DD15 alloy was studied under the condition of 980°C/250MPa. The microstructure characteristic and dislocation feature were analyzed after the different creep strain. The creep strain was 0.2%, 0.5%, 1%, 2% and rupture, respectively. The result shows that the creep properties at 980°C/250MPa of DD15 alloy was very good. The first stage creep life was relatively short. When the strain of the alloy was 0.2%, the creep deformation process had already been into the second stage. The second stage creep life was relatively long, which is the majority of the whole life. When the alloy strain was 2%, the creep was in the tertiary stage in which the creep rate increased significantly. The γ′ precipitates were rafted for all the specimens with different strain, and the dimension of raft row in the stress direction slightly grew up as the strain rised. The TCP phase did not precipitated for all samples with different strain. The above results indicate that DD15 alloy has excellent microstructure stability. The dislocation network formed on the two-phase interfaces, and a few dislocations cut into γ′ precipitates in creep specimen deformed with 0.2% strain. With increase of creep strain the dislocation network of sample turned denser.


Introduction
Ni-based single crystal alloy is commonly applied to prepare aeroengine turbine blades because it has excellent comprehensive high temperature properties [1][2] .Although it has a very good overall performance, the creep deformation damage is one of the main type of failure model during the blade operation [3] .The creep deformation characteristic of Ni-based single crystal alloy is connected with the operating temperature and bearing stress [4] .At low temperature and large stress, the dislocation cutting mode works, while the dislocation by-passing mechanism are operative under elevated temperature and minor stress [5] .A lot of research about creep behavior of the first, second and third generation single crystal superalloys has been studied in recent years [6][7][8][9][10][11] .However, there are relatively few studies on creep property and dislocation characteristic for the 4th generation single crystal Ni-based superalloy.DD15 alloy is a newly developed the 4th generation superalloy by BIAM [12][13] .It is significant to study creep deformation under high temperature to provide data support for the alloy application.

Experimental procedures
DD15 single crystal superalloy was produced using the vacuum melting furnace.The elemental components of single crystal sample are listed in the Table 1.The direction deviations of single crystal samples were measured by X-ray diffractometer, and the sample with orientation deviations less than 15 degree was chosen as subsequent tests.The specimens were treated with solid solution (1335℃/6hour, Air Cool) and a two-stage aging (1140℃/4hours, Air Cool + 870℃/20hour, Air Cool).The creep sample was prepared and tested at 980℃/250MPa.The size of creep specimen is illustrated in Figure 1.The microstructure evolution of five specimens was analyzed when the strain was 0.2%, 0.5%, 1%, 2% and rupture, respectively.The microstructure characteristic and dislocation feature were analyzed after the different creep strain.

Creep properties
The deformation curves of creep sample under 980℃/250MPa with different strain are illustrated in Figure 3.The creep property data is listed in Table 2.The results in Fig. 3 and Table 2 indicate that the creep properties at 980℃/250MPa of DD15 alloy is very good.The first stage creep life is relatively short.When the creep strain of the alloy was 0.2%, the creep deformation process was already into the second stage.The second stage creep life of this alloy is relatively high, which is the majority of the whole life [5] .When strain of the alloy was 2%, the creep was in the tertiary stage in which the creep rate increased significantly.

Microstructure characteristic
The cubic γ′ precipitate increasingly turned to a raft shape due to the directional diffusion of alloying elements.So the elements redistribution in γ matrix and γ′ precipitate took place [8] .Ta, Al, Hf spread toward the perpendicular channel so that the γ′ precipitates grow up vertical to [001] orientation under influence of misfit and loading stress.Meanwhile, Re, Mo, W, Co and Cr spread to the horizontal channels to make the γ channel width increase.A raft characteristic gradually took shape.The longer creep life is, the higher γ′ phase rafting degree is.The microstructure stability is a key technical index   3.4.Dislocation morphologies Figure 5 illustrates dislocation feature of creep specimens after various strain.It can be found that the dislocation network came into being on the two-phase interfaces.The rafted γ′ precipitates in specimen deformed 0.2% strain were sheared by some dislocations.The dislocation network in the sample became finer and denser with increasing strain.It is known that the dislocation by-passing mechanism is operative under elevated temperature and minor stress [10] .The deformation behavior was 1/2[110] dislocation movement within γ phase octahedral slip systems [11] .Taking shape of dislocation net can be attributed to two sets dislocation reaction with various Burgers vectors.The dense dislocation network would be able to block the subsequent dislocation to shear γ′ phase [14] .

Figure 1 .
Figure 1.Size of the creep specimens

Figure 2
Figure 2 Microstructure of specimen after heat treatment (a) OM; (b) SEM

Table 2 .
Creep life of the sample under 980℃/250MPa