Formation and evolution of porosity during high temperature creep of a nickel-based single crystal superalloy

A Re-containing single-crystal superalloy was used to research the high temperature low stress creep behavior. Transmission electron microscope, scanning electron microscope and some other research methods are employed. The results and analysis are summarized below: Two mechanisms for the steady creep are found in this experiment. The volume fraction of pores after creep test at 1100°C increased more than 2 times compared with that before test, but the increasing at 1000°C is relatively small, which reveals that temperature has an great influence on the formation of pore during creep; There are two types of pores associated with fracture during the creep process. One is the casting shrinkage located between the interdentritic, which is formed in the solidification of the alloy. Another type of pore is nucleated and growing during the creep deformation.

and guide blades of gas turbines. Under actual service conditions, blades are subjected to high temperature and high stress. Especially during engine operation, high temperature combined with centrifugal stress will lead to creep deformation of blades, which will seriously affect the service life of blades [1,2] . With the increase of turbine temperature in advanced aeroengines, high temperature creep fracture has been one of common failure modes of single-crystal(SC) superalloy blades. Creep behaviour of Ni-based SC superalloys under typical condition has been researched extensively. In order to design and predict the service life of components reasonably, the creep behavior of superalloy needs to be fully understood and mastered. In the past decades, the creep behavior of monocrystalline superalloys has received extensive attention. The results show that the creep behavior and fracture mechanism of SC superalloy is affected by experimental condition, including temperature, loading mode, stress and other factors. With the change of these factors, the creep behaviour and deformation mechanism show different patterns [3][4][5][6][7][8][9][10][11][12][13][14][15][16] .
In this paper, the high temperature creep of Ni-base SC superalloy under low applied stress at 1100℃ and 1000℃ was studied. The creep behaviour, fracture characteristics and creep deformation, especially the formation and growth of porosity in monocrystalline alloys were systematically researched and analysed. The results can provide experimental data for the application of SC alloys at high temperature and optimize the creep theory of Ni-base SC superalloys.

Experimental procedure
The material used in this article is a first-generation Ni-base SC superalloy containing the chemical components of Ni-Cr-W-Ta-Al-Co-Ti-Mo system. A series of constant load tensile creep experiment were carried out on a creep machine with 120~220 MPa stress.
The test temperatures were 1000℃ and 1100℃ respectively. The creep strain was measured by an extensometer attached to the shoulder of the creep sample, with a measurement accuracy of 5×10 -4~5 ×10 -5 . Most of the specimens were tensile fracture, and some of the specimens were subjected to the creep interruption experiment to observe the microstructure.  As can be seen from the figure, its high-temperature creep behavior mainly shows the following characteristics:

Results
(1) Under the test conditions of 1000℃ and 1100℃, the creep curve shows the creep characteristics of three stages: initial creep(first stage), steady creep(secondary stage) and accelerated creep(third stage).
(3) At 1000℃, the creep strain rate increased slowly after the alloy entered the third phase of creep. At 1100℃, the strain rate of creep increased quickly after the start of the accelerated stage, then, rupture occurred in a very short time.

Microstructural evolution
It's can be seen from    Table 1 shows that the volume fraction of pores after creep test at 1100°C increased more than 2 times compared with that before test, but the increasing at 1000°C is relatively small, which reveals that temperature has an great influence on the formation of pore during creep, and verified by the observation of the longitudinal microstructure morphology under other stress conditions at 1000°C and 1100°C.  In order to investigate the high temperature creep fracture mechanism of the alloy, it is necessary to further analyze the pores that cause the crack initiation. As mentioned above, there are two types of pores associated with fracture during the creep process. One is the casting shrinkage located between the interdentritic, which is formed in the solidification of the alloy. In this work, it is defined as I-type pore. Another type of pore is nucleated and growing during the creep deformation, which is defined as II-type pore.
the variation of the I-type pore during creep is presented in Fig. 6. It can be seen that the I-type pore grows with the increase of creep deformation and gradually evolves into a 6 E3S Web of Conferences 155, 01005 (2020) https://doi.org/10.1051/e3sconf/202015501005 HEET 2019 polyhedral shape, and its surface orientation conforms to a certain crystallographic law. At 1100°C, the cross-section of pore is mostly pentagonal orhexagonal, with plane paralleling to {100} plane or {110} plane, However, the cross-section of pore is mostly quadrilateral at 1000°C, with plane paralleling to {100} plane. Combined with the analysis of specimen under other conditions, it shows that the higher the experimental temperature, the higher the degree of polyhedralization of pore and the clearer the polyhedral contour. The reason for the above phenomenon is that the surface energy has obvious anisotropy at higher experimental temperatures, which is the characteristic of pores when grown up in the form of vacancy aggregation. From the micro-fracture mechanism, the structure at the apex angle of the polyhedral pore is equivalent to a lot of micro-notches, and the stress concentration effect around the notch makes it easy to crack during creep process, especially when the specimen is necked and the stress is increased. The initiation of microcracks at the corners of the pore,and its connection with surrounding cracks after expansioncan be seen in Fig. 7.
The degree of polyhedralization of pores is low at 1000°C, and the phenomenon of crack initiation at sharp corners is not obvious compared with the experiment at 1100°C.  8 shows the initiation and cracking of II-type pore during creep. II-type pores are more likely to occur at the γ/γ' phase interface of the dendritic region and gradually grow up during the creep process. At 1000°C, the number of II-type pores is small due to poor atom diffusion capacity, and no cracking is observed in the third stage of creep, so the impact on the creep process is minimal. In contrast, the growth trend of II-type pores at 1100°C is obvious. In the post stage of creep, the pores in the necking region can be extended along any path in the γ/γ′ structure under stress, and together with I-type pore to cause the rapid cracking, which is also directly causing the small proportion , faster creep rateand small total elongation in thethird stage of creep test at 1100°C.

Analysis and discussion
The creep behavior of SC superalloys usually shows distinct deformation and fracture mechanisms according to changes in stress and temperature. In summary, under the condition of low temperature and high stress, dislocations will be cut into the γ′ phase by indicating that the interface dislocations climbing mechanism is suppressed at 1000°C, and the main deformation mechanism after creep steady state is dislocations cutting γ′ phase. In contrast, there is only a small amount of superlattice dislocations formed at1100°C creep, indicating that the stress ranges of 120-150 MPa are not enough to press a large amount of dislocations into γ′ phase. On the other hand, pre-existing pores in the alloy undergo significant polyhedral growth, and a large number of new creep pores appear, which means that the diffusion capacity of elements is significantly improved after the temperature raising to 1100°C, and the interface dislocations climbing mechanism begins to become the main deformation mechanism in the middle and post stage of creep. Only few amounts of deformations occur during the whole creep steady state, which indicates that the contribution of the interface dislocations climbing mechanism to creep deformation is small, and its main influence is extruding vacancies and continuous accumulation at pores. When the cross-sectional area of pores reaches critical value, the alloy rapidly enters the third stage and fractures. The simultaneous fracture of both types of pores also causes the elongation of the alloy to be significantly reduced.

conclusions
In this article, a Ni-based SC superalloy was used to research the high temperature creep under low applied stress at 1100°C, 120-150 MPa, and 1000°C, 160-220 MPa. The main findings of the research are summarized as below: 1. At 1000°C, the accelerated stage of creep lasts longer and deformation rate changes slowly, the elongation is obviously larger than 1100°C. The deformation rate of the accelerated stage quickly increases and ruptures within a short time at 1100°C. By contrast, the total elongation decreases by around 50% compared with that of 1000°C;