Microstructures And Properties Of High-entropy Alloy/Ni-based Composites Prepared By Spark Plasma Sintering

Particle-reinforced metal matrix composites(PR-MMCs)effectively combine the properties of matrix metal with the excellent characteristics of adding particles.Compared with matrix alloys,the PR-MMCs have a higher specific strength,specific stiffness,thermal stability,heat/corrosion resistance,which are potential candidate materials for the aerospace industry and automotive industry.Because of the unique atomic structure,the high-entropy alloys(HEAs)exhibit unique properties different from traditional alloys.It shows an excellent performance in mechanical properties,magnetic properties,and corrosion resistance.Therefore,HEAs as reinforced particles of MMCs have attracted extensive attention,which have great application prospects.In the present study,a novel type of high-performance Ni-based composite with HEA additions was successfully prepared by mechanical alloying(MA)and spark plasma sintering(SPS).The microstructures of Ni-based composite were modulated by designing the composition and volume fraction of HEAs.The excellent mechanical properties and corrosion resistance in seawater solution have been acquired.The corresponding mechanisms of strengthening and corrosion were discussed.The results are shown below:1.The(CoCrFeNiMn)₉₀M₁₀(M=Al and Hf)HEA powders have been prepared by MA.The final milling products of(CoCrFeNiMn)₉₀Al₁₀ HEA powders have duplex face-centered-cubic(FCC)plus body-centered-cubic(BCC)solid solution structures.Differently,Hf addition induces the formation of amorphous phase and nanocrystalline,which are characterized as the HE-amorphous powders.Both as-milled powders display typical ferromagnetic behaviors,while(CoCrFeNiMn)₉₀Al₁₀ powders exhibit better saturation magnetization of 20.52 emu/g.Based on the summary and analysis of?,(?)H_mix,(?)S_mix,and VEC parameters,the formation rule of HEA powders has been preliminarily explored.Compared with the reported phase formation of as-cast HEAs,the solid solution phases in HEA powders are formed in a wide range of?-(?)H_mix.Moreover,the variation ranges of(?)H_mixand(?)S_mix of the amorphous phase in HEA powders are wider than those of HE bulk metallic glasses,and both extremes become lower.The VEC of as-milled HEA powder is consistent with that of as-cast HEAs.2.The(CoCrFeNiMn)₉₀Al₁₀/Ni-based composites with high relative density were acquired by SPS.The SPS-ed composites are composed of in-situ submicron Al-rich oxide particles,nano-precipitates and Ni matrix.The(CoCrFeNiMn)₉₀Al₁₀/Ni composites exhibit high hardness and excellent mechanical properties.The microhardness of 30 vol.%HEA/Ni is321 HV.The 20 vol.%HEA/Ni exhibits high yield strength of 231 MPa and ultimate tensile strength of 580 MPa,while maintaining fracture strain of 32.2%,realizing the combination of high strength and excellent plasticity.The notable improvement in high strength is mainly attributed to twins/stacking faults strengthening,precipitation strengthening,and grain-boundary strengthening.Combined with FCC characteristics of reinforcements,two factors,including the coherent interface between nano-precipitates and matrix and the nano and micron reinforcement coexisting as a hierarchical precipitate spatial distribution,promote excellent plasticity.With the significantly broadened passive region and low passive current density,the composites present good pitting resistance in seawater.3.The(CoCrFeNiMn)₉₀Hf₁₀/Ni-based composites with high relative density were prepared by SPS.The(CoCrFeNiMn)₉₀Hf₁₀ HEAs remain amorphous characterization during sintering.The amorphous particles with a size of less than 1 forms a large cluster region evenly distributed in the matrix.The microhardness of 20 vol.%HEA/Ni is the maximum(362 HV).The 10 vol.%HEA/Ni shows excellent mechanical properties with elongation of31.5%and fracture strength of 505 MPa.Through the analysis of fracture morphology,spherical-like amorphous characteristics particle reinforcements are embedded in the dimples of the Ni matrix,which can effectively limit the crack propagation.Compared with SPS-ed pure Ni,the corrosion rate of the composites in seawater solution is lower,which is beneficial to improving corrosion resistance.The broader passivation region and the stable passivation film reveal the excellent pitting resistance.4.The Fe₂₅Co₂₅Ni₂₅Al₁₀Ti₁₅/Ni-based composites with high relative density were acquired by MA and SPS.The composites comprising the FCC and BCC solid solutions,as well as Ni matrix,show enhanced hardness and strength.The microhardness of 20 vol.%HEA/Ni is as high as 353 HV.The fracture strength and elongation of 5 vol.%HEA/Ni are554 MPa and 28.9%,respectively.Compared with pure SPS-ed pure Ni,the composites possess better comprehensive corrosion resistance in seawater,which is mainly manifested by the lowest corrosion rate and the broader passivation region.The Co,Ni,and Ti elements in the reinforcements benefit oxidation,promoting the passivation film formation.