增/降氧过程对高温合金粉末表面特性和合金性能的影响:粉末存储到脱气处理

Effects of Oxygen Increasing/Decreasing Processes on Surface Characteristics of Superalloy Powders and Properties of Their Bulk Alloy Counterparts:Powders Storage and Degassing

采用场发射扫描电镜(FESEM)、X射线光电子谱(XPS)、高角环形暗场扫描透射电镜(HAADF-STEM)以及程序升温脱附与质谱联用(TPD-MS)等表征手段研究了不同窄粒度范围镍基高温合金粉末的原始表面状态以及存储和脱气等增/降氧过程对合金组织和性能的影响。结果表明:不同粒度原始态粉末的表面组成均为Ni O/Ni(OH)_(2)、TiO_(2)、CoO和Cr_(2)O_(3),0~15μm粒径粉末(细粉)和150~180μm粒径粉末(粗粉)平均氧化膜厚度分别为3.32和10.90 nm。细粉和粗粉在空气环境中存储后氧含量逐渐增大,在3~10 d达到稳定值,分别约为250×10^(-6)和40×10^(-6)。存储后0~53μm粉末制备的块体合金氧含量升高,室温、650℃和750℃拉伸强度变化不大,但塑性下降,合金在650℃、890 MPa和750℃、530 MPa的持久性能均下降。0~15μm细粉加热过程中(室温~1000℃)会发生气体脱附,存在明显脱附峰的气体包括CO_(2)、H_(2)O和H_(2),粉末表面气体的脱附温度范围主要在100~600℃,脱附峰主要集中在300~600℃温度段,150~180μm粗粉加热过程脱附峰不明显。0~53μm的粉末采用300℃+600℃温度组合方式除气后制备的合金氧含量从初始状态的195×10^(-6)下降到113×10^(-6),合金力学性能更优,性能提升主要表现在合金的塑性升高。高温合金粉末增氧机制主要包括表面氧化和表面吸附,而降氧机制主要为粉末表面吸附的含氧气体的脱出。采用高温合金粉末升温脱附曲线中峰位所处温度精确定制除气保温温度组合,通过25℃+150℃+310℃+470℃低、中、高温组合多级除气处理使得空气中存储过的镍基高温合金粉末(0~53μm)氧含量可进一步降低到(87~96)×10^(-6)。

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),highangle 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),TiO_(2),CoO,and Cr_(2)O_(3)on their surfaces.The average thickness of the surface oxide layer for0-15μm fine and 150-180μm coarse powders is 3.32 and 10.90 nm,respectively.The oxygen content of the 0-15μm fine powders and 150-180μm 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(0-53μm)increased compared to that of the alloy from pre-storage powders,and the tensile strength at room temperature,650℃,and 750℃showed minor changes,but the ductility decreased and the stress rupture properties of the alloy at 650℃,890 MPa and 750℃,530 MPa decreased.During the heating process from room temperature(~25℃)to 1000℃,the gas desorption occurred on the 0-15μm fine powders,with desorption peaks of CO_(2),H_(2)O,and H_(2)observed.The gas desorption mainly occurred on the powders surface in the range of 100-600℃,and the desorption peaks are mainly located within 300-600℃.However,the desorption peaks were not obvious during the heating of the 150-180μm coarse powders.The oxygen content of the alloy consolidated from powders with particle size range of 0-53μm decreased from 195×10^(-6)in the initial state to 113×10^(-6)after the(300℃+600℃)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 25℃+150℃+310℃+470℃,the oxygen content of the powders(0-53μm)stored in ambient air was further reduced to within(87-96)×10^(-6).