| High-entropy alloys(HEA),as the novel type of multi-principal alloys materials,have a single solid solution phase structure and excellent corrosion resistance,mechanical resistance as well as high temperature oxidation resistance by virtue of unique composition design.These advantages make HEA have widely application prospects in the fileds of structural and functional materials.At present,the preparation of high-entropy alloys uses these traditional process methods such as casting,forging,rolling,which is a serious challenge for the fabrication of some industrial components with complex shapes.However,additive manufacturing(AM)technology(also known as 3D printing)has the advantages of rapid formation of sophisticated structural parts and AM technology can effectively address the problem mentioned above.Selective Laser Melting(SLM)is currently one of the most advanced metal additive manufacturing(MAM)technologies.In the formation process,it does not require any fixtures,nor is it limited by the geometry complexity of parts,so it has been broad applied.However,the study on the preparation of HEAs by SLM remains preliminary step,and there are still many difficulties in how to select the SLM process parameters as well as how to regulate the microstructure and properties.Therefore,this paper chose the gas atomized CoCrFeNiMn high-entropy alloy powder as the research object,and used SLM method to additively manufacture the bulk CoCrFeNiMn high-entropy alloy.In combination with microstructure characterization analysis,hardness test,tensile performance test and in-situ electrochemical hydrogen charging slow strain rate tensile methods,the influences of formation process of SLM and post treatment(including annealing and hot isostatic pressing)on microstructure,mechanical properties,hydrogen embrittlement resistance of bulk CoCrFeNiMn HEA specimen were studied.The corresponding deformation and fracture mechanism were discussed,and the main results were as follows:(1)Volume energy density(VED)has a key influence on the microstructure and mechanical performance of the bulk CoCrFeNiMn HEA manufactured by SLM process.The best SLM forming process parameters:laser power is 175 W,laser scanning speed is 500mm/s,layer thickness is 0.05 mm,scanning distance is 0.07 mm,layer rotation is 67°,and the corresponding VED is calculated to be 100 J/mm~3.The relative density is 98.68%,and the actual density is 7.9493 g/cm~3.The phase structure is a simple single face-centered cubic(FCC)solid solution,(111)is the highest diffraction peak.The microscopic morphology of different surfaces is different.The elements within each area are uniformly distributed.The overall level of hardness is above 200 HV.Fracture mechanism is a ductile-brittle mixed fracture mode and the fracrue surface is covered by dimples,cleavage planes,pores and cracks.(2)The density of the SLM printed CoCrFeNiMn HEA after annealing treatment is98.46%and the actual density is 7.9312 g/cm~3,which is slightly lower than that of the original SLM printed HEA.After hot isostatic pressing(HIP)treatment,the density is significantly increased(99.01%)and the actual density is 7.9760 g/cm~3 as well as internal micropores,cracks and other metallurgical defects are basically eliminated.The phase structure is still a single face-centered cubic(FCC)solid solution,but there are second phase in both annealed and HIPed HEA specimens.After post treatment,the grain orientation of annealed and HIPed samples in{001}and{101}increases while{111}orientation decreases.Post treatments have a positive effect on the ductility enhancement of the SLM printed CoCrFeNiMn HEA.The elongation of the 3D built HEAs after annealing and HIP is increased by 55.2%and 78.6%respectively.The SLM printed CoCrFeNiMn high-entropy alloy exhibits a strong temperature dependence.At the liquid nitrogen temperature(77 K),the anti-tensile performance is significantly improved,and the main strengthening mechanism is the synergistic effect of dislocation movement and nano-twin deformation.The fracture mechanism at ambient and cryogenic temperatures is ductile-brittle mixed mode,with ductile dominant and a small amount of brittle.(3)The presence of hydrogen atoms will adversely affect the strength and ductility of the SLM printed CoCrFeNiMn high-entropy alloy.However,post treatment including annealing and HIP can reduce the hydrogen embrittlement sensitivity of the SLM printed parts as well as improve the resistance to hydrogen embrittlement.The fracture surface observation in the hydrogen charging solution shows that the fracture mechanism is a ductile-brittle combination mode.The hydrogen embrittlement cracking mechanism is mainly the synergistic effect of adsorption-induced dislocation emission(AIDE)and hydrogen-enhanced local plastic(HELP). |
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