Microstructure And Properties Of CoCrMoNbTi And AlCoCuFeNi High Entropy Alloys By Additive Manufacturing Technology

High-entropy alloys,as a burgeoning alloy system,have attracted considerable research attention because of their unique advantages in terms of hardness,strength,phase stability,corrosion resistance,etc.The research on high-entropy alloys is currently in its nascent stage.For component design using refractory elements,the high-entropy alloy system reveals excellent high-temperature performance,indicating its potential application in high-temperature structural material.On the other hand,high-entropy alloys reported previously were fabricated by the arc melting technique.The limitations of size and geometry complexity,together with the composition segregation,restrict the industrial application of high-entropy alloys.Additive manufacturing of parts in a layer-by-layer fashion provides a new method for complex geometry fabrication.Rapid solidification during the additive manufacturing process can help avoid composition segregation,thus achieving the preferred properties.The scope of this research lies in alloy component design and material formation.In this study,a novel CoCrMoNbTi high-entropy alloy system was developed,and the effects of alloy components and formation method on the phase composition,microstructure,and mechanical properties were investigated.Additionally,pre-alloyed AlCoCuFeNi powders were used to fabricate the parts via additive manufacturing technologies(selective laser melting and selective electron beam melting).The fabricating mechanism and microstructure evolution of the high-entropy alloys with complex shape,high relative density,and high performance were studied in detail.The relationship between the forming process and its microstructure and mechanical properties was reported for high-entropy alloys.First,CoCrMoNbTi refractory high-entropy alloy system was developed based on the principles of high-entropy alloy design.The phase composition,microstructure,structural stability,mechanical properties,and high-temperature performance behavior were studied in detail.The influence of Ti element addition on the microstructure and mechanical properties was also investigated.This study showed that CoCrMoNbTi0.4 represents a single body-centered cubic(BCC)solid-solution phase with a hardness of 829 HV0.5.The compression strength and fracture strain were found to be 1578 MPa and 3.3%respectively.The addition of Ti element led to precipitation of Laves phase with(Cr,Co)2(Nb,Ti)structure and enhanced the hardness and compression strength;however,plasticity was degraded upon Ti element addition.After high-temperature heat-treatment,the hardness of this alloy increased to 959 HV0.5 to reveal its high structural stability and high-temperature softening resistance.The as-cast alloy possessed compression strength of 680 MPa and 17%fracture strain under high-temperature deformation at 1200 ?.For the heat-treated samples,the compression strength and fracture strain at 1200 ? reached 634 MPa and 50%,respectively.Based on the study of the Co-Cr-Mo-Nb-Ti alloy,the forming process,microstructural evolution,and mechanical properties of CoCrMoNbTix refractory high-entropy alloy coating,prepared by laser melting deposition(LMD),were further studied.The LMDed CoCrMoNbTi0.4 alloy coating prepared with optimized alloy composition and process parameters showed good outer surface quality and good metallurgical bonding.The coating was composed of a single BCC solid solution phase.The surface morphology of the coating was isoaxonal,composed of fishbone-like eutectic tissue.Moreover,the cross section of the coating from the bottom to the top was planar,cellular dendrite and equiaxed crystal,respectively.As the laser beam moved,the dendrites in the molten pool grew along the direction of heat loss.Under the combined action of solid solution strengthening from the BCC phase and fine grain strengthening from the LMD rapid solidification,the microhardness of the coating was improved(above 890 HV0.5),that was better than that of 45#steel plate.To explore the industrial application potential of high-entropy alloys further,the mechanism of AlCoCuFeNi high-entropy alloy fabrication by selective additive manufacturing was studied systematically.Pre-alloyed AlCoCuFeNi powder was used as raw material,and the influence of process parameter on microstructure and mechanical properties was investigated in detail.The experimental result showed that the AlCoCuFeNi samples achieve densities as high as 99.0%via selective laser melting after proper process parameter optimization and melt pool simulation.The microhardness and compression strength were 541 HV0.2 and 1576 MPa,respectively.However,the samples tended to be fragile with low plasticity.In terms of microstructure,the fabricated parts were homogeneous with a single BCC(B2)solid-solution phase.The very fine grains changed from equiaxial at the contour to columnar in the center,and from random orientation turns to epitaxial grain growth.After heat treatment,Cu-rich face-centered cubic(FCC)participation was generated from the BCC substrate.This phenomenon dramatically increased the ductility of the AlCoCuFeNi high-entropy alloy.Particularly,the fabricated part after heat treatment at 1000 ? showed compression strength of 1600 MPa with yielding strength of 871 MPa and fracture strain of 16.6%.Based on the above-mentioned results of SLMed AlCoCuFeNi high-entropy alloy,selective electron beam melting(SEBM)technology was used to achieve rapid forming and heat treatment effects simultaneously.SEBMed AlCoCuFeNi HEA with ultrafine grain BCC(B2)+ FCC dual-phase was obtained.The orientation relation between FCC participation and BCC(B2)matrix phase yields:[1 11]FCC//[001]BCC,(111)FCC//(001)BCC.The preheating effect and interlaminar melting of SEBM improved the quality of the fabricated part,resulting in density greater than 99.8%.The microstructure of the cross-section had a mixed grain structure with orientated column grains and interlayer equiaxed grains,which showed excellent comprehensive mechanical properties.The compressive yield strength and fracture strength of the SEBM fabricated parts were 1248 MPa and 2579 MPa,respectively.The fracture strain was 25.8%.Through the above research,the microstructure,phase composition,and the mechanical properties of high-entropy alloys,controlled by the forming process parameters,were studied in detail.The research provided an important theoretical basis and scientific guidance for the control of microstructural and mechanical properties of high-entropy alloys fabricated by additive manufacturing technologies.