Effects of Hot Isostatic Pressure on Microdefects and Stress Rupture Life of Second-Generation Nickel-Based Single Crystal Superalloy in As-Cast and As-Solid-Solution States
HE Siliang1, ZHAO Yunsong2, LU Fan1, ZHANG Jian2, LI Longfei1(), FENG Qiang1
1 Beijing Innovation Center for Materials Genome Engineering, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China 2 Science and Technology on Advanced High Temperature Structural Materials Laboratory, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China
Cite this article:
HE Siliang, ZHAO Yunsong, LU Fan, ZHANG Jian, LI Longfei, FENG Qiang. Effects of Hot Isostatic Pressure on Microdefects and Stress Rupture Life of Second-Generation Nickel-Based Single Crystal Superalloy in As-Cast and As-Solid-Solution States. Acta Metall Sin, 2020, 56(9): 1195-1205.
Due to the excellent high temperature comprehensive performance and cost effective, the second-generation nickel-based single crystal superalloy has been widely used in the high-pressure turbine blades of advanced aero-engines. Microdefects such as micropores and interdendritic eutectic are seriously harmful to the high temperature mechanical properties of nickel-based single crystal superalloys. Hot isostatic pressure (HIP) technology, which has been widely used in powder and casting superalloys, can effectively reduce the micropores, interdendritic eutectic and other structural defects formed in the turbine blades during manufacturing, and improve the service reliability of turbine blades. However, the effect of HIP process on the high temperature stress rupture life of nickel-based single crystal superalloys is still controversial, especially with regard to the initial microstructure state of the nickel-based single crystal superalloys, i.e. the as-cast microstructure state or the as-solid-solution state. In this work, a kind of second-generation nickel-based single crystal superalloy with as-cast state or as-solid-solution state was selected as the research object. Through two-stage heat/booster type heat treatment process, in combination with microdefects quantitative analysis, quantitative characterization of alloying element segregation and high temperature stress rupture tests at 980 ℃ and 250 MPa, the effects of HIP process on the microdefects and high temperature stress rupture life of the used superalloy with different initial microstructures were studied. The results indicated that the solid-solution treatment can significantly promote the diffusion of alloying elements, such as Re, W, Al, and Ta, reduce the area fraction of interdendritic eutectic, but significantly increase the average area fraction and size of micropores in the used alloy with as-cast state. While, HIP process can effectively reduce the average area fraction and size of microspores in the used alloy with as-cast state or as-solid-solution state, but cannot eliminate the interdendritic eutectic as remarkable as the solid-solution treatment. By HIP process of the used alloy with as-solid-solution state, the area fraction of micropores is reduced to 0.005%, the eutectic structure is basically eliminated, and the dendrite segregation of Re, W, Al, Ta and other elements is significantly alleviated, resulting in the higher stress ruputure life of the used alloy, about 40% over that of the used alloy with the standard heat treatment state. Performing HIP process on nickel-based single crystal superalloy alloy with as-solid-solution state is of benefit to the high temperature stress rupture life due to the reduction of microdefects and the homogenization of alloying elements, in comparison with performing HIP process directly on the alloy with as-cast sate.
Table 1 Heat treatment processes used in this work
Fig.2 Schematic of the heat treatment routes used in this work
Fig.3 Distributions (a1~d1) and morphological characteristics (a2~d2) of micropores in the used nickel-based SX superalloy before and after HIP treatment with the condition of 1300 ℃, 30 MPa, 2 h+1300 ℃, 100 MPa, 3 h, showing the effects of solid-solution treatment and HIP treatment on the micropores under different states
Fig.4 Average area fractions (a) and average diameters (b) of micropores in the used nickel-based SX superalloy before and after HIP treatment
Fig.5 The relative frequency distributions of micropore diameters in the used nickel-based SX superalloy before (a) and after (b) HIP treatment
Fig.6 The interdendritic eutectic microstructures in the used nickel-based SX superalloy before and after HIP treatment, showing that the eutectic microstructures were reduced obviously by solid-solution rather than by HIP treatments. Furthermore, the eutectic microstructure was completely eliminated in ASH specimen
Fig.7 Dendrite segregation coefficients (ki) of alloy elements in the used nickel-based SX superalloy before and after HIP treatment, showing that ki were reduced obviously by solid-solution and HIP treatments, especially for alloy elements Re, W, Al, Nb and Ta
Specimen
Re
W
Al
Nb
Mo
Cr
Co
Ta
Ni
AC
2.23
1.66
0.86
0.43
0.94
1.08
1.14
0.64
0.97
AS
1.66
1.28
0.94
0.97
0.98
1.02
1.03
0.86
0.96
ACH
1.75
1.41
0.94
0.85
1.00
1.05
1.05
0.81
0.97
ASH
1.37
1.17
0.97
0.94
0.91
0.99
1.00
0.95
0.99
Table 2 ki of alloy elements in the used nickel-based SX superalloy before and after HIP treatment
Fig.8 Typical microstructures of γ/γ' phases in the dendrite cores in the used nickel-based SX superalloy under different states
Specimen
Volume fraction of γ' / %
Size of γ' / nm
SHT
68.7±2.5
410±110
ACHS
66.1±2.3
401±91
ASHS
66.0±2.1
407±123
Table 3 Volume fractions and sizes of γ' phases in the dendritic cores of the used nickel-based SX superalloy under different states
Fig.9 The high temperature stress rupture lives of the used nickel-based SX superalloy under different states at 980 ℃ and 250 MPa, showing that the stress rupture lives could be obviously increased by HIP treatment
Fig.10 Schematic of the changes of interdendritic eutectic structures and micropores of the used nickel-based SX superalloy under different states during HIP treatment (P—pressure from HIP)
Fig.11 Microstructures around cracks in the interdendritic area from the longitudinal section of the fractured stress-rupture sample in the used nickel-based SX superalloy
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