| High entropy alloy(HEA)is defined as a new type of metallic material with unique structure and excellent properties,and thus it has been widely concerned.In this study,HEAs containing refractory elements were prepared by arc melting method,which are CuYZr/Al,Ti,Hf,CuxYxZrTiHf(x = 0.6,1.0,in mole ratio)and AlxHfNbTiZr(x = 0,0.5,0.75,1.0,1.25 and 1.5,in mole ratio).The phase structures and morphologies of the tested samples were analysed,and their thermal parameters,mechanical properties and corrosion resistance in seawater were also investigated.The influence of composition on the structures and properties was systematically researched,and the main conclusions are as follows:1.Three CuYZr/Al,Ti,Hf HEAs mainly consist of hexagonal close-packed(HCP1)dendrites and interdendritic body-centered cubic(BCC)CuY-type compounds.In particular,Hf-rich HCP2 solid solution phase and a small amount of amorphous phase appear in CuYZrAlHf and CuYZrAlTi HEAs,respectively.Hf,Ti and Zr elements are concentrated in HCP1 and HCP2 solid solutions,indicating that HCP elements can promote the formation of HCP solid solution in HEAs.Compressive yield strength of CuYZrTiHf HEA is 1340 MPa,whose fracture mode is ductile behavior.CuYZrAlHf and CuYZrAlTi HEAs have the higher strength and hardness,but their fracture mode is a kind of brittle behavior.Hf-rich HCP2 phase and amorphous phase hinder dislocation motion,which play a significant strengthening effect,and further evidently reduce the plasticity of CuYZrAlHf and CuYZrAlTi HEAs.The electrochemical tests in seawater solution show that interdendritic CuY-type compound is seriously corroded,while HCP dendrites reveal much better corrosion resistance.In addition,CuYZrAlTi HEA possesses the better local pitting resistance.2.As-cast Cux YxZrTiHf HEAs consist of HCP solid solution phase and interdendritic BCC-CuY compound.By simultaneously changing Cu and Y contents,the volume fractions of the two phases are successfully adjusted.For Cu0.6Y0.6ZrTiHf HEA,the volume fraction of HCP phase is improved,while that of CuY phase is obviously decreased.No melting phenomenon occurs for these two HEAs within 1400 ?.Cu0.6Y0.6ZrTiHf HEA possesses the better plasity,but the hardness and compressive strength are slightly lower than that of CuYZrTiHf alloy.The volume fractions of CuY compound and HCP solid solution determine the mechanical properties of HEAs.Corrosion resistance tests show that Cu0.6Y0.6ZrTiHf HEA with secondary passivation reveals the better pitting resistance.Via slow pre-compression(10% of deformation amount),strength and hardness of CuxYxZrTiHf HEAs are slightly improved,while the plasticity of CuYZrTiHf HEA is significantly depressed.In addition,the seawater corrosion resistance of the CuxYxZrTiHf is distinctly improved by pre-compression.3.In the series of AlxHfNbTiZr(simply denoted as Alx)HEAs,Al0 and Al0.5 HEAs consist of single disordered BCC solid solution phase.In addition to the disordered one,ordered BCC solid solution is formed in Al0.75 ~ Al1.5 HEAs,and the lattice constants of these two phases are very close.The low mixing enthalpy value and valence electron concentration contributes to the formation of ordered solid solution.No melting phenomenon occurs for these HEAs within 1400 ?.Al addition induces significant change of the mechanical properties.With Al content increment,more brittle ordered BCC phase is formed.The strength and hardness are distinctly improved,but the plastic deformation capacity is decreased obviously.Strengthening mechanisms of AlxHfNbTiZr HEAs include solid solution strengthening,ordered solid solution strengthening and grain refinement strengthening.Al addition effectively enhances the specific strength by reducing the density and enhancing the strength.Via electrochemical tests,Al0 HEA shows good seawater corrosion resistance.After Al addition,the width of the passivation area decreases.Consequently,obvious pitting corrosion occurs and the seawater corrosion resistance declines. |
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