Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650 °C to 800 °C

doi:10.1016/j.jmst.2019.03.002

Abstract

Abstract:

The stress rupture properties and deformation mechanisms of K4750 alloy at 650 °C, 700 °C, 750 °C and 800 °C were investigated. As the decrease of temperature and stress, the stress rupture life gradually increased. A Larson-Miller Parameter (LMP) method was used for analyzing the stress rupture life under different conditions. The linear fitting formula between stress (σ) and LMP was derived as σ = 3166.455 - 119.969 × LMP and the fitting coefficient was 0.98. After testing, the dislocation configurations of all stress rupture samples were investigated by transmission electron microscopy (TEM). The temperature and stress had a significant impact on the deformation mechanism, thereby affected the stress rupture life of K4750 alloy. As the increasing stress at a given temperature, the deformation mechanism gradually transformed from Orowan looping to stacking fault shearing. Based on experimental results, the threshold stress at 650 °C, 700 °C, 750 °C and 800 °C for the transition of deformation mechanism was estimated to be about 650 MPa, 530 MPa, 430 MPa and 350 MPa, respectively. Below the threshold stress, γ' phase effectively hindered dislocation motion by Orowan looping mechanism, K4750 alloy had a long stress rupture life. Slightly above the threshold stress, Orowan looping combining stacking fault shearing was the dominant mechanism, the stress rupture life decreased. As the further increase of stress, stacking fault shearing acted as the dominant deformation mechanism, the resistance to dislocation motion decreased rapidly, so the stress rupture life reduced significantly.

Key words: Nickel based superalloy, Stress rupture properties, Dislocation, Deformation mechanisms, Transmission electron microscopy

Meiqiong Ou, Yingche Ma, Weiwei Xing, Xianchao Hao, Bo Chen, Leilei Ding, Kui Liu. Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650 °C to 800 °C[J]. J. Mater. Sci. Technol., 2019, 35(7): 1270-1277.

Figures/Tables 10

Fig. 1. (a) Relationship between stress-rupture life and stress and (b) relationship between stress-rupture life and elongation at 650 °C, 700 °C, 750 °C and 800 °C with various stress levels.

Table 1 Parameters for linear equation of stress rupture data shown in Fig. 1(a).

Temperature (°C)	A	B	SE	R²
650	2.97488	-0.05402	0.01038	0.933195
700	3.00201	-0.11378	0.00738	0.991817
750	2.90082	-0.12672	0.00878	0.989806
800	2.87024	-0.18052	0.01502	0.991996

Fig. 2. Applied stress as a function of Larson-Miller Parameter (LMP) for K4750 alloy.

Fig. 3. Dislocation configurations of K4750 alloy after stress rupture tests at (a, b) 650 °C/650 MPa and (c, d) 650 °C/680 MPa, showing Orowan looping and stacking faults (SF) shearing γ′ phase.

Fig. 4. TEM images of stress rupture specimens after testing: (a, b) 650 °C/705 MPa; (c, d) 650 °C/740 MPa, showing γ′ phases were sheared by dislocations with leaving SF inside these γ′ phases, and slip bands were generated in (c).

Fig. 5. Dislocation configurations of K4750 alloy after stress rupture tests: (a, b) 700 °C/400 MPa, double Orowan loops around single γ′ phase and the large Orowan loops around two or several γ′ phases; (c) 700 °C/530 MPa, SF started to be generated in some γ′ phases; (d, e) 700 °C/590 MPa, many stacking faults were formed in γ′ phases; (f) 700 °C/620 MPa, almost all of γ′ phases were sheared by dislocations with leaving SF inside them.

Fig. 6. Dislocation configurations of K4750 alloy after stress rupture tests: (a, b) 750 °C/320 MPa, some large Orowan loops surrounded several γ′ phases; (c) 750 °C/360 MPa and (d) 750 °C/400 MPa, Orowan looping process; (e, f) 750 °C/430 MPa, a few γ′ phases started to be cut by dislocations with leaving SF inside them.

Fig. 7. TEM images of stress rupture specimen after testing at 750 °C/505 MPa, showing (a) slip bands, (b) extended SF passing both γ matrix and γ′ phases, (c) dislocation pairs, (d) Orowan loops surrounding a few γ′ phases.

Fig. 8. TEM images of stress rupture specimens after testing: (a, b) 800 °C/150 MPa, Orowan looping process and dislocation climbing up γ′ phases; (c) 800 °C/250 MPa and (d) 800 °C/300 MPa, Orowan looping process; (e, f) 800 °C/350 MPa, large Orowan loops were formed and a few γ′ phases were sheared with leaving SF inside them.

Fig. 9. Schematic illustration of relationship among dominant deformation mechanism, temperature and stress.

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