Influence of Solution Temperature on Compositions Segregation andMSF.747-748.690

Influence of Solution Temperature on Compositions Segregation and

Creep Behavior of a Single Crystal Nickel-based Superalloy

S.G. Tian a , Y .C. Xue b

and Z. Zeng

School of Materials Science and Engineering, Shenyang University of Technology,

Shenyang, China

tiansugui2003@https://www.360docs.net/doc/78459908.html,, b xueyongchao2007@https://www.360docs.net/doc/78459908.html,

Keywords: Single crystal nickel-based superalloy, Creep, Deformation mechanism, Microstructure evolution, Dislocation.

Abstract. By means of solution treatment at various temperatures, creep properties measurement and microstructure observation, the effects of heat treatment on composition segregation and creep properties were investigated. Results show that the various segregation extents of the elements are displayed in the alloys solution treated at different temperatures, and the segregation extent of the elements is improved with the solution temperature elevated, which may obvious improve the creep resistance of the alloy. And no rafted structure of the γ′ phase is detected in the alloy during creep at medium temperature. The deformation features of the alloy during creep at medium temperature are that the slipping of dislocations is activated in the γ matrix channels, and dislocations shearing into the γ′ phase may be decomposed to form the configuration of partials + stacking faults, which may hinder the cross-slipping of the dislocations to improve the creep resistance of the alloy. Introduction

Microstructure of single crystal nickel-based superalloy consists of the cubical γ′ phase embedded coherently in the γ matrix, and creep resistance of the superalloy increases with the concentration of the refractory elements (W+Ta+Mo) [1,2], but the segregation of the elements in the inter-dendrite /dendrite regions have an important effects on the creep properties of alloy. During creep at elevated temperature, the cubical γ′ phase in alloy is transformed into the rafted structure along the direction vertical to stress axis, and the dislocation networks are formed in the γ/γ′ interface, in the further, the creep dislocations in the γ matrix may climb over the rafted γ' phase by means of the dislocation networks [3]. Investigations on the creep behavior of SRR99 alloy at 980℃ indicated [1] that the slipping of the screw dislocation may be activated in the γ matrix during primary creep, no dislocation shearing into γ′ phase is detected in alloy during steady state creep, but some dislocations may shear into the γ′ phase in the latter stage of creep [4].

It must be considered that the work of the aircraft engine in service, from starting to stable course process, undergoes the various stages from medium temperature / higher stress to high temperature / lower stress, and the superalloys with different compositions display the various creep properties in the ranges of different temperatures [5]. Although the creep behavior of single crystal superalloys at high temperature / lower stress had been reported [6-8], a few literatures reported the creep behavior of the single crystal nickel base superalloy at medium temperature, and deformation mechanisms of the alloy during creep at medium temperatures are still unclear.

Hereby, in this paper, by means of measuring creep properties and microstructure observation, the creep behavior of single crystal nickel base superalloy at medium temperature is investigated, and deformation mechanisms of the superalloy during creep at medium temperature are briefly discussed by means of the dislocation analysis.

Experimental procedure

A [001] orientation single crystal nickel-base superalloy had been prepared by means of selecting crystal method in a vacuum directional solidification furnace under a high temperature gradient, the nominal chemical composition of the single crystal nickel-based superalloy is given as Ni-Cr-Co-W-Mo-Al-Ta (Wt. %). The growth direction of the single crystal bars was determined to be within 7° deviating from [001] orientation. In this paper, two temperatures of the solution treatment are selected to investigate the effects of solution temperatures on composition segregation and creep properties, the used heat treatment regime of the single crystal bars are given as follows: 1280℃×2h, A.C + 1315℃×4h, A.C / 1325℃×4h, A.C + 1080℃×4h, A.C + 870℃×24h, A.C. After being fully heat treated, the bars of the alloy were cut into the plate-like creep specimen with cross-section of 4.5 mm×2.5 mm and the gauge length of 20.0 mm. A uni-axial constant load tensile testing was performed in a creep testing machine (GWT504 model), under different conditions, to plot the creep curves. In the further, the apparent creep activation energies and apparent stress exponents of the SC superalloy in the ranges of the applied stresses and temperatures are calculated according to the creep curves. And the microstructures of the alloys at different states are observed under SEM and TEM, for investigating the creep behavior and deformation mechanism of the single crystal nickel-base superalloy. Results and discussion

Effects of heat treatment on composition segregation. The dendrite morphology of as-cast single crystal superalloy in the cross-section is shown in Fig. 1, this indicates that the secondary dendrite is grown along directions parallel to [100] and [010] orientations. The spacing of the primary dendrite is measured to be about 270~300μm, and the spacing of the secondary dendrite is measured to be 80~100μm. And the segregation coefficients of the alloy in different regions can be calculated according to the formula given as follows:

%1001

2×?=

C C C K Here, K is segregation coefficient, C 1 is content of elements in the dendrite arm region, and C 2 is content in the inter-dendrite regions.

The elements distribution in the inter-dendrite and dendrite arm regions of the alloy is measured by means of SEM/EDS analysis, and given in Table 1. It can be understood from Table 1 that the elements Al, Cr and Co in as-cast alloy are richer in the interdendrite regions, and element Cr has a bigger segregation coefficient which is measured to be 25.00%. The elements Ta, W and Mo are richer in dendrite arm regions, the refractory elements Mo has the bigger negative segregation coefficient which is measured to be -33.22%. After solution treated at 1315℃ and 1325℃, respectively, the segregation extent of the elements are obviously improved, especially refractory elements Mo, W and Ta. The segregation coefficient of Cr decrease to 8.06% and 1.94%, and the segregation coefficient of Mo decrease to -13.15% and -4.44%, respectively.

Fig. 1 Dendrite morphology of alloy

on (001) plane

[100]

[010]

Table 1 Distribution and segregation coefficient of elements in inter-dendrite/dendrite regions

Areas Al Ta W Cr Mo Co Ni As-cast

Inter-dendrite 4.08 8.13 8.64 6.75 2.15 11.51 Bal. Dendrite 4.96 10.67 11.06 5.40 3.22 10.57 Bal. K (%) 17.74 -23.81 -21.80 25.00 -33.22 8.93 Bal. 1315℃

Inter-dendrite 4.67 11.29 9.15 5.36 2.18 9.95 Bal. Dendrite 4.23 12.58 10.25 4.96 2.51 9.70 Bal. K (%) -9.42 -10.25 -10.70 8.06 -13.15 2.58 Bal. 1325℃

Inter-dendrite 4.40 10.83 8.90 5.25 3.01 10.32 Bal. Dendrite 4.30 11.24 9.50 5.15 3.15 10.11 Bal. K (%)

-2.27

-3.65

-4.20

1.94

-4.44

2.08

Bal.

Creep behaviors of the alloy. Creep curves after the

alloy solution treated at 1315℃ and 1325℃, respectively, are shown in Fig. 2, indicating that the

alloys solution treated at different temperatures display the various creep features and lives at 760℃/750MPa.

The creep curve of alloy treated at 1315℃ displays a higher strain rate during steady state creep and shorter creep lifetime about 125 h, as marked by number 1.

The creep curve of the alloy solution treated at 1325℃ is marked by number 2, which displays a lower strain rate during steady state creep, the lifetime increases to

244 h. This indicates that the creep resistance of alloy

may be improved by solution treated at higher temperature due to decreaseing segregation extent of

the elements.

The creep curves of the alloy, after being solution treated at 1325℃, under different conditions are measured as shown in Fig. 3. The creep curves under the applied stress of 750 MPa at different temperatures are shown in Fig. 3(a), thereinto, the creep curve of the alloy at 740℃ displays a lower strain rate during steady state creep and longer life. The strain rate of the alloy during steady state creep is measured to be about 0.007 %/h, the lasting time during steady state creep is about 400 h. As the creep temperature increases to 760℃, the strain rate and lasting time of the alloy during steady state creep, and the creep life are measured to be 0.024 %/h, 155 h and 244 h, respectively. With the creep temperature enhancing to 780℃, the strain rate of the alloy during steady state creep is measured to be about 0.058 %/h, the creep life decrease to 133 h.

The creep curves of the alloy under the applied different stresses at 760℃ are shown in Fig. 3(b), under the applied stress of 780 MPa, the strain rate of the alloy during steady state creep is measured to be 0.033 %/h, and the creep lifetime of the alloy is 198 h. With the applied stress increasing to 800 MPa, the strain rate of the alloy during steady state creep is measured to be 0.044 %/h, the creep life is measured to be 145 h. This indicates that the creep life of the alloy decreases with the increase of the applied temperatures and stresses, and the creep life of the alloy displays an obvious sensitivity on temperature when creep temperature is more that 760℃. But no obvious sensitivity is detected in the alloy in the ranges of the applied stresses at 760℃.

Fig. 2 Effects of solution temperature

on creep properties of the alloy 0501001502002500

121620

121 - Solution at 1315o

C 2 - Solution at 1325o

C T - 760 o C

σ - 750 MPa

Time, h

S t r a i n , ε (%)

The transient strain of single crystal superalloy occurs at the moment of the applying load, and the density of dislocations increases as the creep goes on, which results in the strain strengthening of alloy to decrease the strain rate. Once the creep enters the steady state stage, the strain rate of the alloy keeps constant, and the strain rate can be described by Dorn law [8]. According to the data in the creep curves in Fig. 3, the dependences of the strain rates of the alloy during steady state creep on the applied temperatures and stresses are shown in Fig. 4(a) and (b). In the further, in the ranges of the applied temperatures and stresses, the apparent creep activation energy and apparent stress exponent of the alloy during steady state creep are calculated to be Q = 483.6 kJ/mol and n = 10, respectively. And it may be deducted according to the stress exponent that the dislocations shearing into the γ′ phase is thought to be the main deformation mechanisms of the alloy during steady state creep at medium temperature.

Fig. 3 Creep curves of alloy at different conditions.

(a) Applied different temperatures at 750 MPa, (b) applied different stresses at 760℃

Fig. 4 Dependence of the strain rates during steady state creep on applied temperatures and stresses.

(a) Strain rates & temperature, (b) strain rates & applied stress

Microstructure evolution during creep. After being fully heat treated, microstructure of the single crystal nickel-based superalloy consists of the cubical γ′ phase embedded coherently in the γ matrix, and arranged regularly along <100> direction, the size of the cubical γ′ phase is 0.4 ~ 0.5μm (the picture omitted). The morphologies in the different regions of the specimen after crept for 244 h up to fracture at 760℃/750MPa are shown in Fig. 5, the normal direction of the observed sample is [100] orientation, the dark areas in the picture corresponds to the γ′ phase, and the white areas corresponds to the γ matrix phase, which indicates that the various morphologies are displayed in the

S t r a i n ε, %

1/T,10-4K

-1

I n (εs s )

In(σ

120180240300360

05101520

σ - 750MPa

1 - 780℃

2 - 760℃

3 - 740℃

Time, h

S t r a i n , ε (%)

(a)

100150200250

101520Time, h

S t r a i n , ε (%)

T - 760℃

1 - 750MPa

2 - 780MPa

3 - 800MPa

(b)

different regions of sample due to the difference of the deformation extent. Therefore, the deformed feature of the alloy in the different regions may be evaluated according to the configuration of γ′ phase.

The various microstructure is displayed in different regions of the alloy after crept 244 h up to fracture, the schematic diagram of observing regions on sample is shown in Fig. 5(a), the γ′ phase in the region A keeps still the cubical morphology due to no strain and the lower diffusion rate of the elements during creep, as shown in Fig. 5(b). The morphology of the γ′ phase in the regions B is shown in Fig. 5(c), although the γ′ phase keeps still the cubical configuration, the size of the one increases slightly, and displaying the wave-like twisted feature due to plastic deformation. The much more plastic deformation occurs in the region C near the fracture, which displays the obvious twisted configuration of the cubical γ′ phase so that the orientation of the cubical γ′ phase is contorted to the inclined direction relative to the applied stress axis as shown in Fig. 5(d), and the thickness size of the cubical γ′ phase in the region increases to 0.8 μm. This indicates that the size and twisted extent of the cubical γ′ phase increases with the plastic deformation, but no rafted structure of the γ′ phase is detected in the alloy during creep at 760℃.

Fig. 5 Morphologies in the different regions of alloy crept up to fracture under the applied stress of 750MPa at 760℃. (a) Schematic diagram of marking observed locations in specimen, (b), (c) and

(d) being SEM morphologies corresponding to A, B and C regions, respectively

Deformation feature during creep. The deformation feature of the alloy after crept for 244 h up to fracture at 760℃/750 MPa is shown in Fig. 6, this indicates that the original cubical γ′ phase in the alloy has been transformed into the sphere-like configuration in the corner regions, and linked together to form the bunch-like structure as shown in the region D of Fig. 6(a).

Fig. 6 Microstructure of the alloy crept for 244 h up to fracture under the applied stress of 750 MPa at 760℃. (a) γ′ phase transformed into the bunch-like structure, (b) slipping direction of dislocations

marked by arrow

(a)

(b)

The super-dislocations which shear into the γ′ phase is marked by black arrow, and the stacking fault formed in the alloy is marked by letter E. Dislocations with the cross-slipping feature are located in the γ matrix between the cubical γ′ phases, as shown in the region F of Fig. 6(a), thereinto, the slipping directions of dislocations with double-oriented feature are marked by arrows. And the denser dislocations with single-oriented slipping feature are activated in the matrix as shown in Fig. 6(b), thereinto, the slipping direction of dislocations is marked by arrow.

Therefore, it may be concluded that the deformation feature of alloy during creep is significant amount of dislocations slipping in the matrix channels and some dislocations shearing into the γ′ phase, and the dislocations shearing into the γ′ phase may be decomposed to form the configuration of the partials plus stacking faults.

In general, it is considered that the creep resistance of alloy is related to the solubility of the elements in γ′ and γ phases, and the relationship of the solubility and creep resistance of alloy may be expressed as 2/1AC ss =τ, the creep resistance of alloy increases with the atomic concentration. In specially, the γ′ phase with Ll 2 structure has an important effect on hindering dislocations movement, therefore, the alloy displays a better creep resistance due to the higher volume fraction of γ′ phase and higher solubility of the refractory elements in γ′/ γ phases. This analysis is well agreement with the experimental results. Conclusions

1. For as-cast supperalloy, composition segregation of elements occurs in the different regions of the alloy, and the segregation extent of the elements may be decreased, with the solution temperature elevated, to improve the creep resistance of alloy.

2. In the ranges of the applied temperature and stress, the alloy which is solution treated at 1325℃ possesses a better creep resistance, and displays an obvious sensitivity on the applied temperature. The apparent creep activation energy of the alloy during steady state creep is calculated to be Q = 48

3.6kJ/mol.

3. Deformation features of the alloy during creep are the dislocations slipping in the γ matrix channels and shearing into the γ′ phase. References

[1] L. Müller, U. Glatzel, M.F. Kniepmeier, Modelling thermal misfit stress in nickel-base superalloy containing high volume fraction of γ′ phase, Acta Metall. Mater. 40 (1992) 1321-1327. [2] P. Caron, P.J. Henderson, T. Khan, et al., Effects of heat treatment on the creep behaviour of a single crystal superalloy, Scripta Metall. 20 (1986) 875-880.

[3] J.X. Zhang, J.C. Wang, H. Harada, et al., The effect of lattice misfit on the dislocation motion in superalloys during high-temperature low-stress creep, Acta Mater. 53 (2005) 4623-4633.

[4] S.G. Tian, H.H. Zhou, J.H. Zhang, et al., Formation and role of dislocation networks during high temperature creep of a single crystal nickel-base superalloy, Mater. Sci. Eng. A 279 (2000) 160-165.

[5] J. Cormier, X. Milhet, J. Mendez, Non-isothermal creep at very high temperature of the nickel-based single crystal superalloy MC2, Acta Mater. 55 (2007) 6250-6259.

[6] A. Sengupta, S.K. Putatunda, L. Bartosiewicz, et al., Tensile behavior of a new single crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures, J. Mater. Eng. Perform. 3 (1994) 664-672.

[7] X. Zhang, T. Jin, N.R. Zhao, et al., Temperature dependence of deformation mechanism in single crystal Ni-base superalloy, Trans. Nonferrous Met. Soc. China 15 (2005) 759-763.

[8] S.G. Tian, H.H. Zhou, J.H. Zhang, et al., Dislocation configuration in single crystal nickel-base alloy during primary creep, Acta Metal. Sinica 34 (1998) 123-128.

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Influence of Solution Temperature on Compositions Segregation and Creep Behavior of a Single Crystal Nickel-Based Superalloy

10.4028/https://www.360docs.net/doc/78459908.html,/MSF.747-748.690