Evolution of Microstructures and Mechanical Properties of K439B Superalloy During Long-Term Aging at 800oC

doi:10.11900/0412.1961.2023.00141

Acta Metall Sin

2023, Vol. 59

Issue (9): 1253-1264 DOI: 10.11900/0412.1961.2023.00141

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Evolution of Microstructures and Mechanical Properties of K439B Superalloy During Long-Term Aging at 800^oC

ZHANG Leilei¹^,², CHEN Jingyang²(

), TANG Xin², XIAO Chengbo², ZHANG Mingjun², YANG Qing¹(

)

¹School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, China
²Science and Technology on Advanced High Temperature Structural Materials Laboratory, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China

Cite this article:

ZHANG Leilei, CHEN Jingyang, TANG Xin, XIAO Chengbo, ZHANG Mingjun, YANG Qing. Evolution of Microstructures and Mechanical Properties of K439B Superalloy During Long-Term Aging at 800^oC. Acta Metall Sin, 2023, 59(9): 1253-1264.

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Abstract

The K439B alloy is a novel equiaxed superalloy and is used for producing hot section components that need to resist high temperatures in aero engines and gas turbines as its temperature capacity exceeds 800^oC. In this study, the evolution of the microstructure and mechanical properties of K439B equiaxed superalloy after being subjected to long-term aging at 800^oC for 3000 h was examined. The predominant deformation mechanisms affecting room-temperature tensile and stress rupture properties at 815^oC and under 379 MPa stress following different aging durations for the K439B alloy were investigated.Results indicate that for heat-treated alloy, the morphology of the γ' phase is spherical, MC carbide is generated in the interdendritic region and grain boundaries, while M₂₃C₆ carbide is in the grain boundaries. During long-term aging at 800^oC, γ′ precipitates conform to the Ostwald ripening mechanism for growth and tend to take a cubic form; the coarsening rate of the γ′ phase is calculated to be 71.7 nm³/h; Additionally, the MC carbide deteriorates while the content of M₂₃C₆ carbide gradually increases. After long-term aging for 3000 h, the precipitated grain boundary phase comprises MC carbide, γ′ phase, and M₂₃C₆ carbide; the orientation relationship between γ′ phase and M₂₃C₆ carbide can be described as [111] _γ' //[111] _M $23$ _C $6$ and ( $2 2 ¯ 0$ ) _γ′ //( $2 2 ¯ 0$ ) _M $23$ _C $6$ . The heat-treated alloy demonstrates room-temperature tensile and yield str-engths of 1159.0 MPa and 911.5 MPa, respectively. Meanwhile, the stress rupture life at 815^oC and under 379 MPa stress is 150.4 h. As the size of γ′ precipitates increases, the dominant deformation mechanism shifts from dislocation slipping in the matrix to dislocation cutting through the γ′ phase after long-term aging, resulting in superior stacking faults appeared in the γ′ phase. Consequently, the room-temperature tensile strength and stress rupture life show reduction at 815^oC and under 379 MPa stress.

Key words: K439B long-term aging microstructure mechanical property deformation mechanism

Received: 03 April 2023

ZTFLH:

TG132.3

Fund: National Science and Technology Major Project of China(J2019-VI-0004-0117);National Key Research and Development Program of China(2022YFB3706804);Science and Technology on Advanced High Temperature Structural Materials Laboratory Fund(6142903210104);Science and Technology on Advanced High Temperature Structural Materials Laboratory Fund(6142903220101);AECC Science and Technology Innovation Platform Project(CXPT-2018-006)

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	ZHANG Leilei
	CHEN Jingyang
	TANG Xin
	XIAO Chengbo
	ZHANG Mingjun
	YANG Qing

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00141 OR https://www.ams.org.cn/EN/Y2023/V59/I9/1253

Fig.1 Low (a, c, e) and high (b, d, f) magnified OM images of microstructures of K439B alloy subjected to long-term aging at 800^oC for 0 h (a, b), 1000 h (c, d), 3000 h (e, f) (GB—grain boundary)

Fig.2 Typical microstructures of γ′ precipitates in the dendrite core of K439B alloy subjected to long-term aging at 800^oC for 0 h (a), 100 h (b), 500 h (c), 1000 h (d), 2000 h (e), and 3000 h (f)

Fig.3 Size distributions of γ′ precipitates in the dendrite core of K439B alloy subjected to long-term aging at 800^oC for 0 h (a), 100 h (b), 500 h (c), 1000 h (d), 2000 h (e), and 3000 h (f)

Table 1 Size, volume fraction, and shape factor of γ′ precipitates in the dendrite core of K439B alloy subjected to long-term aging at 800^oC

Fig.4 Relationship between size of γ′ precipitates in the dendrite core and aging time of K439B alloy at 800^oC ( $r ¯ 0$ —average radius of γ′ after aging for 0 h; $r ¯$ —average radius of γ′ after aging for time t; k—LSW theory coarsening rate constant; R²—coefficient of determination)

Fig.5 Evolution of carbide distributed in the interdendritic region (a, c, e) and at the grain boundary (b, d, f) of K439B alloy subjected to long-term aging at 800^oC for 0 h (a, b), 1000 h (c, d), and 3000 h (e, f)

Fig.6 Carbide in the heat-treated K439B alloy
(a) MC carbide distributed in the interdendritic region
(b) HRTEM image from the edge of the MC carbide (marked with an arrow in Fig.6a) and the atomic-scale HRTEM images from the regions 1# and 2# with corresponding fast Fourier transform (FFT) patterns (insets)
(c, d) MC carbide (c) and M₂₃C₆ carbide (d) at the grain boundary with corresponding SAED patterns (insets)

Fig.7 Grain boundary precipitated phase, SAED patterns of regions 1#-3#, and EDS compositional mapping of K439B alloy subjected to long-term aging at 800^oC for 3000 h

Table 2 Contents of M₂₃C₆ carbide in the interdendritic region and at grain boundary of K439B alloy subjected to long-term aging at 800^oC

Fig.8 Room temperature tensile properties (a) and 815^oC, 379 MPa stress rupture properties (b) of K439B alloy subjected to long-term aging at 800^oC (σ_b—tensile strength, σ_p0.2—yield strength, δ—elongation, τ^{—stress rupture life )}

Fig.9 Dislocation structures near room temperature tensile fracture surface of K439B alloy subjected to long-term aging at 800^oC for 0 h (a, b), 1000 h (c, d), and 3000 h (e, f)

Fig.10 Dislocation structures near 815^oC, 379 MPa stress rupture fracture surface of K439B alloy subjected to long-term aging at 800^oC for 0 h (a-c) and 3000 h (d-f)

Fig.11 Schematics of microstructural evolution of K439B alloy during long-term aging at 800^oC for 0 h (a), 1000 h (b), and 3000 h (c) (ID—interdendritic region)

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