Microstructure And Mechanical Properties Of Different Particles Reinforced Ni-rich High Entropy Alloy Prepared By SLM

As a new multi-principal component alloy,the high-entropy alloy(HEAs)have many excellent properties such as high strength,high hardness,good ductility,and thermal stability due to their unique four core effects,and have become a research hotspot in the field of metallic materials.However,the studies on HEAs are still in the primary research stage,and the traditional arc melting and casting technology is still the main preparation method.However,the samples formed by traditional methods process are simple in shape and limited in size,and composition segregation easily appears,which limits their industrial application.Therefore,forming technology becomes very important to explore high-performance advanced HEAs.Selective laser melting(SLM),as a new additive manufacturing technology,has the advantages of discrete-accumulation rapid forming and can prepare complex metal components,which has become one of the most promising forming methods.Previous studies have shown that the HEAs prepared by SLM have good plasticity,but their strength and hardness are usually low.Introducing the concept of composite materials into HEAs brings hope to promote the formation of high-performance HEA composites.In this paper,Al0.5CoCr0.8FeNi2.5V0.2 HEA is taken as the research object,and the microstructures and mechanical properties are regulated by adding Fe-based arrorphous and Al5Si0.3C0.2B particles,respectively,and the effects of heat treatments on the microstructures and mechanical properties of the particle-reinforced HEA are further studied.The corresponding results are summarized as follows:Firstly,Fe-based amorphous alloy particles with different mass fractions(5 wt.%,10 wt.%,15 wt.%,20 wt.%;named 5MG,10MG,15MG,20MG,respectively)were prepared by the SLM technology to improve their mechanical properties.After adding Fe-based amorphous alloy particles,the grain structure changed from columnar crystals grown epitaxially to equiaxed crystals,mainly due to the different melting points of amorphous particles and matrix,which led to partial Fe-based amorphous deposition at the bottom of the molten pool and then hindered the epitaxial growth of grains.This transformation was obviously affected by the content of the amorphous phase,and the equiaxed crystals were completely transformed in the 20MG samples.The structure of the 10MG sample is still composed of coarse columnar crystals and subcellular crystals,maintaining a single FCC solid solution phase,and other precipitated phases caused by the aggregation of some amorphous particles exist in some grain boundaries.During deformation,all kinds of precipitates hinder dislocation movement,thus improving the strength of the materials.However,due to the incoherence of precipitates between the amorphous particles and matrix,internal stress is easily concentrated and then cracks tend to spread along subcellular boundaries,leading to the decrease of tensile ductility.The main strengthening mechanisms of the 10MG sample are solution strengthening,grain boundary strengthening,dislocation strengthening,and precipitation hardening.By calculating the contribution of each mechanism to the yield strength,the theoretical yield strength is～1012 MPa,which is close to the actual measured value of～941 MPa.Secondly,the influence of the heat treatments on the microstructures and mechanical properties of Fe-based amorphous particle-reinforced Ni-rich HEAs was investigated in order to improve their ductility.When annealed at a temperature below 1 173 K,the sample has no phase transformation and still maintains a single FCC solid solution structure.When the temperature is above 1173K,the B2 phase is produced in the FCC matrix.Fe-rich elements in the 20MG sample will diffuse to both sides of the molten pool after annealing.At this time,the blocking effect on the columnar growth of grains disappears,and the grains grow to both sides of the molten pool and change from equiaxed crystals to columnar crystals,and the ductility is greatly improved.In addition,when the particle-reinforced sample is annealed at 1073 K,nano M23C6 carbide will be precipitated at the grain boundaries.During deformation,nano M23C6 carbide plays a role as a hard second phase in hindering dislocation movement,achieving a good balance between strength and ductility.Finally,the effects of Al5Si0.3C0.2B particles with different mass fractions on the microstructure and mechanical properties of Ni-rich HEAs were studied.The results show that the addition of Al5Si0.3C0.2B particles can make the Ni-rich HEA changes from the single FCC phase structure to FCC+BCC dual-phase structure,and the volume fraction of the BCC phase can be changed by adjusting the particle amount.When the mass fraction of Al5Si0.3C0.2B reaches 10 wt.%,the transformation from the FCC to BCC solid solution is completed.It was found that the 10Al-HEA samples prepared by SLM have obvious cracks and Al-rich phases on the surface of the printed samples due to the high Al content.However,the forming quality of the 5 Al-HEA sample is good,and only common porosity defects are observed on the surface,which indicates that the SLM forming quality can be improved by reducing the Al/Ni molar ratio.At the same time,a double heterostructure structure can be found in the 5Al-HEA,which consists of coarse-grained FCC and ultra-fine-grained BCC structures.During deformation,the interaction between coarse-grained and fine-grained results in back stress strengthening.Then the HEA composites exhibit a tensile strength of～1377 MPa and a total elongation of～6.5%.