退火温度对激光增材制造CoCrFeMnNi高熵合金耐点蚀性能的影响

Effect of Annealing Temperature on Pitting Resistance of CoCrFeMnNi High-entropy Alloy Fabricated by Laser Additive Manufacturing

目的研究激光增材制造CoCrFeMnNi高熵合金经不同温度退火后,微观组织演变对其在NaCl溶液中的耐点蚀性能的影响规律。方法采用激光选区熔化(SLM)技术制备CoCrFeMnNi高熵合金,通过X射线衍射(XRD)、光学显微镜(OM)和扫描电子显微镜(SEM)研究其退火后的微观结构。利用动电位极化和电化学阻抗谱(EIS)测试研究SLM成形高熵合金的耐点蚀性能,并通过X射线光电子能谱(XPS)分析钝化膜成分。结果经过不同温度退火后,高熵合金相组成并未改变,均为单一的面心立方结构固溶体。然而高熵合金的微观组织发生了明显转变,退火前微观组织由熔池、柱状晶及胞状亚晶所组成。随着退火温度的升高,熔池边界与亚晶结构逐渐消失,晶粒逐渐长大。SLM成形高熵合金在3.5%NaCl溶液中的腐蚀类型主要为点蚀。随着退火温度从700℃提高至1100℃,高熵合金的腐蚀电流密度先减小、后增加,700℃退火试样相较于打印态试样,腐蚀电流密度下降了97%。打印态和700℃退火试样的钝化膜中Co+Cr+Ni与Mn+Fe阳离子含量的比值分别为1.38和1.61,钝化膜中Cr本征氧化层厚度分别为5.43 nm和5.75 nm。结论高熵合金耐点蚀性能随退火温度的升高,先提升、后降低。胞状亚晶有利于阻碍点蚀坑的扩展,并促使形成稳定的钝化膜。高熵合金经700℃退火,在消除部分熔池边界的同时,保留了胞状亚晶,因此表现出最佳的耐点蚀性能。

Equiatomic CoCrFeMnNi high-entropy alloy(HEA)has good corrosion resistance and radiation resistance,and possesses high strength and excellent toughness at cryogenic temperature,which makes it a potential application material for advanced nuclear reactors.As one of the most widely applied additive manufacturing(AM)technologies,selective laser melting(SLM)provides a new idea for the green preparation of HEA,reducing the production cycle and the manufacturing cost.However,the corrosion resistance of the CoCrFeMnNi HEA prepared by laser additive manufacturing still lacks systematic study.The work aims to investigate the effect laws of microstructure evolution on the pitting resistance of the SLMed CoCrFeMnNi high-entropy alloy in NaCl solution after annealing at various temperature.Spherical CoCrFeMnNi HEA powder with an average diameter of 26.0μm fabricated by gas-atomized was used in this study.The high-density CoCrFeMnNi HEA with a relative density of 99.4%was fabricated by a SLM machine(Hanbang HBD-150D)under pure argon gas protection.The manufacturing process was set as:checkerboard scanning strategy,laser power of 180 W,scanning speed of 670 mm/s,hatching space of 70μm and layer thickness of 50μm.Then,the SLMed specimens were subject to annealing for 4 hours at 700℃,900℃and 1100℃respectively,followed by cooling in a furnace in an argon atmosphere.The phase of the HEA before and after annealing was analyzed by X-ray diffraction(XRD,Japanese Rigaku D/max2500pc).The microstructure was examined by optical microscopy(OM,Zeiss Axiovert 40MAT)and scanning electron microscopy(SEM,Tescan MIRA3 XMU).The pitting resistance of the SLMed HEA in 3.5 wt.%NaCl solution was analyzed by kinetic potential polarization and electrochemical impedance spectroscopy(EIS).The composition of the passivation film of the HEA formed in the NaCl solution was investigated by X-ray photoelectron spectroscopy(XPS,Thermo Scientific K-Alpha).The phase of the CoCrFeMnNi HEA before and after annealing was single-phase face-centered cubic,which indicated that the HEA had good phase stability at elevated temperature.However,the microstructure of the HEAs had undergone a significant transformation after annealing.The microstructure of the SLMed HEA consisted of melt pools,columnar crystals,and cellular subgrains due to the rapid heating and cooling in the manufacturing process.As the annealing temperature increased,the melt pool boundary and subgrain structure gradually disappeared and the grains gradually grew.Annealing treatment significantly affected the pitting resistance of the HEAs.With the increase of annealing temperature from 700℃to 1100℃,the corrosion current density of the HEAs decreased firstly and then increased.The corrosion current density of the sample annealed at 700℃decreased by 97%compared with that of the as-built sample.The HEA passive film mainly consisted of metal oxides and hydroxides.The cation ratios of the Co+Cr+Ni to Mn+Fe in the passivation film of the samples prepared by SLM and annealed at 700℃were 1.38 and 1.61,respectively,and the thickness of the Cr intrinsic oxide layer in the passivation film was 5.43 nm and 5.75 nm,respectively.The sample annealed at 700℃has the best pitting resistance.With the increase of annealing temperature,the pitting resistance of high-entropy alloy firstly increases and then decreases.Cellular subgrain is beneficial to hindering the expansion of pitting and promoting the formation of stable passivation film.The annealing at 700°℃provides the best pitting resistance by removing part of the melt pool boundary while maintaining the cellular subgrains,preventing further pitting pit expansion and improving the protection ability of passivation film.