Microstructure And Mechanical Properties Of Additive Manufacturing Fe_23.3Co_25.1Cr_18.8Ni_22.6Ta_8.5Al_1.7 Eutectic High Entropy Alloy

Eutectic high entropy alloys（EHEAs）have high strength,good ductility and desirable castability.EHEAs containing refractory elements exhibit excellent phase stability and mechanical properties under high temperature conditions,and have strong application potential in the aerospace field.EHEAs prepared by casting show coarse grains,and the shape of the material is limited by the process,which restricts the development of EHEAs properties.Additive manufacturing（AM）can fabricate parts with complex shapes and shorten the production cycle.AM process can achieve a high cooling rate,and obtain materials with uniform composition,fine structure and excellent mechanical properties.AM has been widely used in traditional metals such as titanium alloys and superalloys,but it is still in its infancy in EHEAs.In this thesis,Fe_23.3Co_25.1Cr_18.8Ni_22.6Ta_8.5Al_1.7 eutectic high entropy alloy has been fabricated by selective laser melting（SLM）and selective electron beam melting（SEBM）.The effects of AM processes on the microstructure and defects were investigated.The growth mechanism of equiaxed and columnar grains during AM was discussed.The effect of hot isostatic pressing（HIP）on the solidification structure and mechanical properties was explored.The main results are as follows:（1）The microstructure of SLMed EHEA is composed of FCC phase dendrites and interdendritic network Laves phase.The competitive growth of dendrite and eutectic under high cooling rate of SLM process is the main reason for the formation of the solidified microstructure.By calculating the effect of undercooling on the growth rate of dendrite and eutectic,the growth rate of eutectic is faster when?<129 K,and the solidification structure is dominated by eutectic growth.When?>129 K,the solidification structure of eutectic high entropy alloy is mainly dendritic growth.（2）Fe_23.3Co_25.1Cr_18.8Ni_22.6Ta_8.5Al_1.7 EHEA has poor formability under SLM process,and cracks are easily generated during SLM.Such cracks are cold cracks,caused by the accumulation of high thermal stress in the SLM.Preheating the substrate can inhibit the crack propagation,but cannot completely eliminate macroscopic cracks.（3）The high preheating temperature in the SEBM process reduces thermal stress,inhibits the formation of cold cracks in EHEA.The process window for fabricating EHEAs by SEBM is:power P>300 W,linear energy density≈（0.15～0.28）J/mm.The microstructure of SEBM EHEA is composed of lamellar eutectic and FCC phase dendrite,and the eutectic and dendrite alternate in the build direction.Increasing the electron beam power and increasing the scanning speed can refine the lamellar space of the eutectic,and increasing the electron beam power can suppress the equiaxed grain growth.（4）The SEBMed EHEA has the ultimate tensile strength of 1260 MPa and the elongation of 1.1%.The tensile strength of the material at 800℃is595 MPa,and the elongation is 23.8%.The high temperature fracture mechanism is mainly due to the crack aggregation formed by the fracture of the micropores at the eutectic/dendritic interface and the Laves phase fracture in the eutectic region.（5）The density of HIPed EHEA increased to more than 99.7%.The tensile elongation at room temperature of the material is increased to 2.4%,and the strength is comparable to that of SEBM alloy.The tensile strength of the material is 589 MPa at 800°C,and the tensile elongation is 28.1%.The improvement of the mechanical properties of the material after HIP is due to the elimination of defects in the material and the formation of fine equiaxed Laves phases.