Research On Corrosion And Wear Behavior Of A CrMnFeCoNi High Entropy Alloy Prepated By Laser Melting Deposition

In this thesis,CrMnFeCoNi high entropy alloy（HEA）prepared by laser melting deposition technology was taken as the research object.According to the differences in the number of thermal cycles at different positions during the additive manufacturing process,the HEA samples were divided into three regions:Top,Middle and Bottom.The microstructure of the samples in different regions were carefully characterized.The corrosion behaviors in 3.5 wt.%NaCl and 0.5M H₂SO₄ solution,and the friction and wear behavior in air and the corrosion solution of CrMnFeCoNi HEA with different thermal cycles were studied.Finally,the tensile and frictional wear behaviors of the samples were also investigated in the extreme environment of liquid nitrogen.The corrosion resistance of CrMnFeCoNi HEA with different thermal cycles in 3.5wt.%NaCl and 0.5M H₂SO₄ solutions was examined by means of polarization curves,impedance spectra,scanning electron microscopy（SEM）and X-ray photoelectron spectroscopy（XPS）.The polarization curve results show that the corrosion resistance of each sample in the two corrosive solutions gradually decreases with increasing the number of thermal cycles.The polarization curve results show that more obvious passivation occurs in the 0.5M H₂SO₄solution.The sample（Top）has the smallest self-corrosion current density among the studied three samples.Impedance spectrum results show that the sample（Top）has the largest impedance value,capacitance arc diameter and charge transfer resistance in the two corrosion solutions among the studied three samples.SEM observations indicated that the pitting pits of the sample（Top）in 3.5 wt.%NaCl solution were less in number and smaller in size than the sampleMiddle and Bottom.However,all the three samples showed regular corrosion along the grain direction,and no obvious elemental selective corrosion occurred in 0.5M H₂SO₄ solution.The surface products of the sample after corrosion were analyzed by X-ray Phtelectrn Spectrscpy（XPS）.The ratio of（Cr+Ni+Co）/（Fe+Mn）atoms on the surface of the sample（Top）are relatively high in the 3.5 wt.%NaCl solution,and the Cr and bound water contents in the passivation film on the surface are relatively high in the 0.5M H₂SO₄ solution,demonstrating its better corrosion resistance.The hardness of the CrMnFeCoNi HEA gradually increases with increasing the number of thermal cycles,and the sample（Bottom）has the highest hardness value among the three samples.The wear resistance was examined based on the friction coefficient,mass loss,surface morphology and surface roughness of each sample in air,3.5 wt.%NaCl and 0.5M H₂SO₄ solutions.Compared with the abrasive test results in the air,the friction coefficient and surface roughness of each sample are reduced,and the mass loss is increased in corrosion solution.As the number of thermal cycles increases,the friction coefficient,mass loss,surface roughness of the sample decrease,suggesting the best wear resistance of the sample（Bottom）.The tensile test results of the additively manufactured CrMnFeCoNi HEA at room temperature and liquid nitrogen showed that the strengths of all the three samples increased whereas the elongation gradually decreased with increasing the number of thermal cycles.Under the cryogenic temperature environment,the dimple size of the fractured surface became large and shallow,the second phase particles were reduced,and the strength and elongation of each sample were increased.The wear test results under liquid nitrogen conditions showed the same trend with the room temperature wear test results.That means the sample（Bottom）possesses the smallest friction coefficient,mass loss and surface roughness,indicating its best wear resistance.However,due to the vaporization of liquid nitrogen,small abrasive particles gathered and grew up to participate in the friction,forming a large number of grooves in the surface,and resulting in an increase in enhanced quality loss and surface roughness of the sample compared with those at room temperature.