Multi-scale Mechanics And Corrosion Resistance Of A CoCrFeMnNi High-entropy Alloy Fabricated By Selective Laser Melting

Multi-component high-entropy alloys(HEAs)break the conventional alloy mold and have a good combination of strength and ductility,good thermal stability and strong corrosion resistance.Equiatomic Co Cr Fe Mn Ni HEA has become one of the most promising cryogenic materials due to an excellent combination of fracture toughness and strength at cryogenic temperatures.HEAs typically need good corrosion resistance in practical engineering applications.Traditional manufacturing processes have been extensively used to fabricate HEAs,such as as-casting,mechanical alloying and powder metallurgy.However,there are some problems in the HEAs fabricated by these manufacturing technologies,such as limited shape and size,coarse grain size and composition segregation,which limit the range of applicability for HEAs produced using these methods.As one of the most widely applied additive manufacturing(AM)technologies,selective laser melting(SLM)can manufacture high-precision and high-performance metal parts without the need for part-specific instruments.SLM provides a new idea for green preparation of high entropy alloy.It also reduces the design and production cycle and decreases the cost and time of manufacturing.Therefore,the research work of SLMed Co Cr Fe Mn Ni HEA was carried out in this paper.The process was optimizated to prepare high dense formed parts.The microstructure and formation mechanism of Co Cr Fe Mn Ni high entropy alloy formed by SLM were analyzed.Then the multi-scale mechanical behaviors of SLMed Co Cr Fe Mn Ni HEA were analyzed,including nano mechanics,micron mechanics and macro mechanics.The creep behavior at room temperature was studied by nano indentation method.The fracture toughness was investigated by micron scratch method.The strength and plasticity at room temperature and liquid nitrogen temperature was analyzed by macro-scale tensile test.The corrosion resistance of the SLMed Co Cr Fe Mn Ni HEA compared with the as-cast Co Cr Fe Mn Ni HEA in Na Cl solution and H₂SO₄ solution was investigated respectively.The influence of scanning strategy on temperature field and residual stress distribution in SLMed Co Cr Fe Mn Ni HEAwas simulated by finite element simulation.And the effects of laser power,scanning speed and laser energy density on the forming quality of Co Cr Fe Mn Ni high entropy alloy were investigated by processing experiment,to prevent cracking and reduce porosity.Compared with the S-shaped scanning strategy,the temperature field distribution of the forming parts using the checkerboard scanning strategy was more uniform,which was beneficial to reduce the stress concentration and prevent cracking.Laser energy density can not be used as the only indicator of SLM process optimization,and the coupling effect of various process parameters should be fully considered.The holes could be inhibited by optimized process,but it was difficult to eliminate them completely.The optimal SLM process for forming Co Cr Fe Mn Ni high entropy alloy is as follows:laser power of 150 W,scanning speed of 750 mm/s,layer thickness of 50?m,scanning spacing of 50?m.Under the condition,the relative density reaches the highest value(99.60%)with the laser energy density of 75.95 J/mm³.The microstructure and formation mechanism of Co Cr Fe Mn Ni high entropy alloy formed by SLM were analyzed.The traditional casting high entropy alloy was mainly composed of coarse dendritic grains with average grain size up to 90.3?m.However,the microstructure of as-build SLMed high entropy alloy was finer without composition segregation and the average grain size was7.83?m.The microstructure of the SLMed Co Cr Fe Mn Ni HEA contained hierarchical microstructure,including molten pool,columnar crystal,cellular subcrystal,nano-twin,dislocation and precipitated phase.Most columnar grains continuously grew epitaxially along the<101>and<001>directions across multiple molten pools.In addition,the columnar crystal contained a large number of cellular substructure,which were mainly composed of dislocation networks.Thermodynamically,the cellular subcrystals were in metastable state.Annealing resulted in the decrease of dislocation density.The creep behavior of SLMed Co Cr Fe Mn Ni high entropy alloy at room temperature was investigated by nanoindentation experiment.The creep displacement and the creep stress index(n)increased gradually with the peak load P_max.When P_max was constant,the creep displacement increased and n decreased with the increase of loading rate.Under the same conditions,the creep resistance of as-build sample was better than that of annealed sample.The room temperature creep of SLMed Co Cr Fe Mn Ni high entropy alloy is controlled by dislocation motion.The increase of loading rate significantly increased the nucleation rate of dislocation at loading segment,resulting in the increase of creep displacement.The fracture toughness of the annealed and as-build SLMed HEA was characterized by using the linear elastic fracture mechanics(LEFM)model via micro scratch method.The results showed that annealing was beneficial to improve the fracture toughness of SLMed HEA.Due to the shallow depth of scratches,it is necessary to consider the arc area at the top end of the scratche indenter to modify the LEFM model.Before and after the modification of the LEFM model,the plane stress-fracture toughness Kc of the HEA calculated was significantly different.The research of macro-scale tensile properties at room temperature(293 K)and ultra-low temperature(77 K)was carried out.It shows that the strength of SLMed Cr Co Fe Mn Ni HEA increased with the decrease of temperature,and good plasticity could be maintained.When the temperature of tensile test decreased from 293 K to 77 K,the ultimate tensile strength as-build Cr Co Fe Mn Ni HEA increased from 675 MPa to 1049 MPa,with an increase of up to 55%.Compared with the traditional cast Cr Co Fe Mn Ni high entropy alloy,SLMed high entropy alloy has smaller grains,higher dislocation density,and contains precipitates.Therefore,the tensile strength of SLMed high entropy alloy was significantly improved.The main deformation mechanism SLMed Cr Co Fe Mn Ni HEA was dislocation slip when stretched at room temperature.A large number of deformation twins occurred at ultra-low temperature tensile,which significantly improved the strength.As the dislocation density of SLMed high entropy alloy decreased after annealing at 600?,the annealed sample has lower tensile strength and higher elongation compared with the as-build sample.The corrosion behaviors of as-cast and the SLMed Co Cr Fe Mn Ni HEA were compared,by means of the polarization curve test and electrochemical impedance spectroscopy in 0.5 M H₂SO₄solution and 3.5 wt.%Na Cl solution respectively.The corrosion resistance of SLMed HEA was worse than that of as-cast HEA in 0.5 M H₂SO₄ solution,but the corrosion resistance of SLMed HEA could be improved by annealing.The annealed HEA had a 62%lower corrosion current density and higher corrosion resistance than the as-built HEA.This was because annealing released the residual stress and increased the percentage of low coincidence site lattice boundaries.The passive film of the annealed HEA had a higher cation ratio of Cr+Ni+Co/Fe+Mn(1.85)than the as-built HEA(1.40)and therefore higher corrosion resistance.Comparing with the as-cast HEA,more uniform composition and finer grains were observed in the as-build HEA.Therefore,the passive film formed on the as-build HEA has a more stable and stronger protective property in 3.5 wt.%Na Cl solution.The corrosion current density of the as-build Co Cr Fe Mn Ni HEA dropped by 58%compared to the as-cast HEA,indicating that the as-build HEA had better corrosion resistance than that of the as-cast one in 3.5 wt.%Na Cl solution.The corrosion mechanisms of the SLMed and as-cast HEAs were similar,and the corrosion process was controlled by charge transfer.The SLMed HEA sample has a higher Cr+Ni+Co/Fe+Mn atomic ratio than that of the as-cast HEA,confirms that the SLMed HEA has a stronger passive film.