Microstructure Evolution And Mechanical Properties Of CoCrFeNi-based High Entropy Alloy Prepared By Directional Solidification

High entropy alloys(HEAs)consist of more than four kinds of elements and the content of those elements are equal mole or near equal mole.Multi principal elements usually resulted in the alloy system with high entroy and lower Gibbs free energy,therefore,the solidified phases of HEAs are usually FCC solid solution and BCC solid solution but not intermetallics.The novel solid solution structure leads to HEAs with excellent mechanical properties such as high strength and ductility,good corrosion resistance and wear resistance.Therefore,HEAs were regardeds as a newly potential alloys.HEAs with multi-principal elements resulted in the phase formation,microstructure evolution,strengthening mechanism are significantly different from trdaitional alloys.However,the study about this region is lack and needs further systematic discussed.The mechanical properties of most alloys are mainly depednded on the component C0,solidification velocity V,temperature gradient G.However,the important parameters during solidification such as solidification velocity V and temperature gradient G can not be controlled easily in traditioal casting method.Directional solidification is an effective method to control the solidification velocity,temperature gradient and the morphology of S-L interface during solidification.Furthermore,it can eliminate(or restrict)the horizontal grain boundary of the alloys,which is considered as an important method to improve mechanical properties.Therefore,directional solidification is a promising method to study the microstructure evolution and mechanical properties of HEAs.In this paper,directional solidification process was applied to reveal the relationship between phase formation and microstructure evolution as well as mechanical properties of HEAs.The CoCrFeNi based HEAs were directionally solidified with different solidification velocities and different elements addition.The phase formation,microstructure evolution and mechanical properties were investigated deeply.The CoCrFeNi HEAs can be well directionally solidified by directional solidification process.The microstructure of the directionally solidified CoCrFeNi HEAs exhibits transition from cellular,to cellular-dendritic,and then to dendritic growth with increasing of the growth rate from 5μm/s to 50μm/s.The approximate criterion growth rate for cellular-dendrite transition is calculated to be 26.1μm/s.For directionally solidified CoCrFeNi HEAs,when the growth rate increases from 5μm/s to 50μm/s,the yield strength improves from 233 MPa to 383 MPa,and the ultimate tensile strength improves from 431 MPa to 596 MPa.Meanwhile,the elongation of the samples prepared at all growth rates exceeds 34.3%.The fracture morphology shows that with increasing of growth rate,the fracture mechanism changes from the mixed mode of ductile and brittle to ductile mode.This result is mainly attributed to the increase of transverse grain boundaries.With the addition of equal mole percent of element Mn,the morphologies of solid-liquid interface of CoCrFeNi Mn HEAs at different solidification velocities exhibit dendritic,this result indicated that the addition of element Mn significantly facilitated the transition of cellular to dendritic.The solidified phases of Co Cr Fe Mn Ni HEA are Fe Ni3-enriched FCC1 solid solution and Mn Ni-enriched FCC2 solid solution.When the solidification velocity increased from 5μm/s to 100μm/s,the preferred orientation of Fe Ni3 transformed from(001)to(101),and the preferred orientation of Mn Ni transformed from(010)to(110).The fracture morphologies indicated that the fracture mechanism is ductile fracture.The addition of equal mole percent of element Cu facilitated the cellular to dendritic tranistion and resulted in serious Cu segragation of CoCrFeNi Cu HEA.The solidified phases of CoCrFeNi Cu are CoCrFeNi enriched FCC1 solid solution phase and Cu-Ni enriched FCC2 solid solution phase.The fracture mechanism is ductility fracture and brittle fracture,the ductility fracture usually occurs in the Cu-Ni enriched region and the brittle fracture usually occurs in CoCrFeNi enriched region.With the addition of 8% mole percent of element Al,the solidified phases of(CoCrFeNi)92Al8 HEAs are not single FCC solid solution phase but Al-Ni-enriched BCC solid solution phase surrounded by FCC solid solution phase.The phase formation mechanism of(CoCrFeNi)92Al8 HEAs is significantly depended on the solidification route.The lower velocity 5 μm/s leads to Al-Ni-enriched BCC solid solution phase was serious segregated near the grain boundaries and solidified as the secondly phase.The strengthening mechanisms are grain boundary strengthening and precipitation strengthening.The microstructure of directionally solidified CoCrFeNi Tix HEA is dendric,the solidified sample exists numerous casting defects.The directionally solidified phase is not solid solution but intermetallic compounds.The Co3 Ti and Ni3 Ti intermetallics were enriched in the inter-dendritic region,and resulted in a serious Ti-segregation.Near equilibrium directional solidification demonstrated that Al CoCrFeNi2.1 is not a fully lamellar eutectic HEA.The solidification route of Al CoCrFeNi2.1 could be described as L→L+FCC→L+FCC+BCC→FCC+BCC.When the solidification velocity is low,the microstructure consists of primary Cr Fe Co enriched FCC phase and eutectic Cr Fe Co/Al Ni lamellar phase.When the solidification velocity is 100 μm/s,the microstructure is nearly full eutectic Cr Fe Co/Al Ni lamellar phase.The phase diagram of Al CoCrFeNi2.1 is simplified as Al Ni-Co Cr Fe binary phase diagram to explain the novel solidification behavior.When the solidification velocity increases from 5 μm/s to 100 μm/s,the fracture strength increases from 696 MPa to 1012 MPa.Meanwhile,the elongation decreases from 33.3% to 14.5%.