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Acta Metall Sin  2023, Vol. 59 Issue (9): 1265-1278    DOI: 10.11900/0412.1961.2023.00197
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Effects of Oxygen Increasing/Decreasing Processes on Surface Characteristics of Superalloy Powders and Properties of Their Bulk Alloy Counterparts: Powders Storage and Degassing
ZHENG Liang1(), ZHANG Qiang1,2, LI Zhou1, ZHANG Guoqing1()
1Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China
2Beijing Advanced Innovation Center for Materials Genome Engineering, University of Science and Technology Beijing, Beijing 100083, China
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ZHENG Liang, ZHANG Qiang, LI Zhou, ZHANG Guoqing. Effects of Oxygen Increasing/Decreasing Processes on Surface Characteristics of Superalloy Powders and Properties of Their Bulk Alloy Counterparts: Powders Storage and Degassing. Acta Metall Sin, 2023, 59(9): 1265-1278.

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Oxygen content of Ni-based superalloy powders is higher than those of their bulk alloy counterparts due to the larger specific surface area of the former, which is detrimental to the performance of powder metallurgy (PM) and additive manufacturing (AM) superalloys. Therefore, at present, research in this field is primarily focused on understanding the mechanism of oxygen content increase of the powders and approaches of oxygen decrease. Storage and degassing treatment are typical processes of increasing and decreasing of oxygen content in superalloy powders, respectively. Studying the effects of these processes is of great significance for guiding the optimization of powder treatment processes and further improving alloy properties. The original surface state of powders with different narrow particle size ranges, as well as the effects of oxygen increasing/decreasing processes, i.e. storage and degassing, on the microstructure and mechanical properties of alloys were investigated using field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and temperature programmed desorption with mass spectrometry (TPD-MS). The results indicate that the surface composition of the original powders with different particle sizes has no significant difference, all samples exhibit NiO/Ni(OH)2, TiO2, CoO, and Cr2O3 on their surfaces. The average thickness of the surface oxide layer for 0-15 μm fine and 150-180 μm coarse powders is 3.32 and 10.90 nm, respectively. The oxygen content of the 0-15 μm fine powders and 150-180 μm coarse powders gradually increases in ambient air environment and stabilize at about 250 × 10-6 and 40 × 10-6, respectively, within 3-10 d. The oxygen content of the bulk alloy consolidated from the post-storage powders (0-53 μm) increased compared to that of the alloy from pre-storage powders, and the tensile strength at room temperature, 650oC, and 750oC showed minor changes, but the ductility decreased and the stress rupture properties of the alloy at 650oC, 890 MPa and 750oC, 530 MPa decreased. During the heating process from room temperature (~25oC) to 1000oC, the gas desorption occurred on the 0-15 μm fine powders, with desorption peaks of CO2, H2O, and H2 observed. The gas desorption mainly occurred on the powders surface in the range of 100-600oC, and the desorption peaks are mainly located within 300-600oC. However, the desorption peaks were not obvious during the heating of the 150-180 μm coarse powders. The oxygen content of the alloy consolidated from powders with particle size range of 0-53 μm decreased from 195 × 10-6 in the initial state to 113 × 10-6 after the (300oC + 600oC) combined degassing process. Alloys prepared from powders that underwent combined degassing exhibited higher mechanical properties, with the performance improvement mainly reflected in the ductility index of the alloy. The oxygen increase mechanism of superalloy powders mainly includes surface oxidation and surface adsorption, while the oxygen decreases mainly due to the desorption of oxygen-bearing gases on the powder surface. The temperatures of the peak position in the desorption curves of superalloy powders were selected to accurately customize the holding temperature of the degassing process. As a result, through multi-stage degassing treatment at 25oC + 150oC + 310oC + 470oC, the oxygen content of the powders (0-53 μm) stored in ambient air was further reduced to within (87-96) × 10-6.

Key words:  Ni-based superalloy powder      oxygen content      powder surface characteristic      existing form of oxygen      particle size range      degassing process      powder storage      mechanical property     
Received:  04 May 2023     
ZTFLH:  TG132.32  
Fund: National Natural Science Foundation of China(52071310);National Natural Science Foundation of China(52127802);National Science and Technology Major Project(Y2019-VII-0011-0151);Key Laboratory Fund(6142903200303);Key Laboratory Fund(6142903220302)

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Fig.1  Surface morphologies of FGH96 superalloy powders with different narrow particle size ranges
(a) 0-15 μm (b) 75-100 μm (c) 150-180 μm
Fig.2  XPS survey spectra of FGH96 superalloy powders with different narrow particle size ranges
Fig.3  Detailed XPS high-resolution spectra of Ni2p (a), Ti2p (b), Co2p (c), Cr2p (d), C1s (e), and O1s (f) on the surface of FGH96 superalloy powders with different narrow particle size ranges
Fig.4  XPS etch depth analyses of different elemental concentrations for FGH96 superalloy powders with different narrow particle size ranges
(a) 0-15 μm (b) 150-180 μm
Fig.5  Relative intensity (I / Iω ) of Ni-metal (Nimet) and oxygen (O) with different ion etching depths by XPS, indicating surface NiO/Ni(OH)2 layer thickness of FGH96 superalloy powders with narrow particle size ranges of 0-15 μm (a) and 150-180 μm (b)(I—intensity of Nimet or O at different etch depth, Iω —intensity of Nimet or O at etch depth of 50 nm)
Fig.6  Cross-sectional images of high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) (left) and energy dispersive spectrum (EDS) elemental mapping (right) on surface oxide layer of FGH96 superalloy powders with different particle size ranges
(a) 0-15 μm (b) 150-180 μm
Fig.7  Curves of oxygen content of FGH96 superalloy powders with different narrow particle size ranges stored in ambient air (20oC, 40%-50%R.H.) for different time
Fig.8  SEM images (a, d), EBSD images (b, e), and average grain sizes (c, f) of hot isostatic pressed (HIPed) bulk alloys consolidated from original (HIP-1, with oxygen content of about 120 × 10-6) (a-c) and stored (in the atmospheric environment for 90 d, HIP-2, with oxygen content of about 200 × 10-6) (d-f) FGH96 superalloy powders with particle size range of 0-53 μm (PPB—prior particle boundary)
Fig.9  Tensile stress-strain curves (a) and mechanical properties of HIP-1 and HIP-2 bulk alloys at ambient temperature (25oC) (b), 650oC (c), and 750oC (d) (YS—yield strength, UTS—ultimate tensile strength, El—elongation, A—reduction of area)
Fig.10  Low (left) and high (right) magnified SEM images of tensile fracture surfaces of HIP-1 (a, c, e) and HIP-2 (b, d, f) bulk alloys at ambient temperature (25oC) (a, b), 650oC (c, d), and 750oC (e, f)
Stress rupture conditionAlloyRupture life / hEl / %A / %
650oC, 890 MPaHIP-129.1 ± 1.227.7 ± 1.125.2 ± 0.9
HIP-222.0 ± 0.522.5 ± 1.921.6 ± 1.2
750oC, 530 MPaHIP-132.4 ± 0.814.1 ± 0.316.3 ± 0.4
HIP-228.1 ± 0.610.1 ± 1.710.9 ± 1.3
Table 1  Stress rupture properties of HIP-1 and HIP-2 bulk alloys at 650oC, 890 MPa and 750oC, 530 MPa
Fig.11  Temperature programmed desorption with mass spectrometry (TPD-MS) degassing curves of FGH96 superalloy powders with different narrow particle size ranges at heating rate of 20oC/min
SampleDegassing parameterOxygen content / 10-6
25oC degassing25oC, 2 h195
300oC degassing25oC, 2 h + 300oC, 5 h140
600oC degassing25oC, 2 h + 600oC, 5 h124
300oC + 600oC degassing25oC, 2 h + 300oC, 5 h + 600oC, 5 h113
Table 2  Degassing parameter setup of FGH96 superalloy powders (0-53 μm original powders stored in ambient for 15 d, degassing vacuum condition ≤ 10-3 Pa) and the oxygen contents of relative alloys
Fig.12  Microstructures of HIPed alloys consolidated from FGH96 superalloy powders with different degassing processes, indicating the severity of PPBs defects
(a) 25oC degassing (b) 300oC degassing (c) 600oC degassing (d) 300oC + 600oC degassing
Fig.13  Tensile properties of FGH96 alloy at ambient temperature (25oC) (a), 650oC (b), and 750oC (c) HIP consolidated from powders with different degassing processes
Fig.14  Tensile fracture surfaces of HIPed FGH96 alloy consolidated from air stored powders after 25oC (a, b, e, f, i, j) and 300oC + 600oC (c, d, g, h, k, l) degassing
(a-d) 25oC tensile (e-h) 650oC tensile (i-l) 750oC tensile
Fig.15  Optimization and precise customization of multi-stage degassing process parameters for Ni-based superalloy powders and oxygen content of the manufactured alloys
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