The Design And Performance Optimization Of Refractory High-Entropy Alloys

High-entropy alloys(HEAs)generally include at least four principal elements.The basic features of HEAs are the high mixing entropy,the random solid solution matrix,and good comprehensive performance,such as high hardness,high fracture toughness,excellent wear and corrosion resistance and so on.HEAs exhibit excellent thermodynamic stability and mechanical properties especially at high temperatures,which hold great potential in high temperature applications.In present work,a series of NbTaV-(Ti,Mo,W)refractory HEAs were designed by combing Calculation of Phase Diagrams(CALPHAD)modeling,first-principles densuty functional theroy(DFT)calculations,and empirical parameters.The ingots were synthesized by vacuum arc melting,then their microstructure,mechanical properties,and strengthening mechanisms were investigated.The following conclusions are reached:(1)The as-cast NbTaV-(Ti,Mo,W)refractory HEAs have a single body-centered cubic solid solution phase,respectively.At room temperature,the HEAs show high strength and good ductility in which MoNbTaTiV has a yield strength of 1.4 GPa and a fracture strength 2.45 GPa with a fracture plastic strain of 30%.At 1273 K,the hardness of NbTaTiVW is 315 HV,which is almost 2.5 times that of two Ni-based superalloys,In 718 and In 718 H.For the NbTaV-(Ti,Mo,W)HEAs,the ductility increased with the addition of Ti while the strength increased by adding the Mo or W.(2)Compared with the empirical parameters(Miedema model),CALPHAD calculations not only can simulate the equilibrium and non-equilibrium phase diagrams,but also can show the thermodynamic properties of both BCC and liquid phases.In addition,the observed elemental segregation in each alloy qualitatively agrees with CALPHAD prediction.(3)The crystal lattice parameters of NbTaV-(Ti,Mo,W)HEAs obey the rule of mixtures(ROM).The calculated atomic volume and elastic properties(e.g.,bulk and shear moduli)for the alloys obey the ROM within the computation uncertainties.(4)For the most single-phase refractory HEAs with BCC structure,the hardness values are approximately three times the yield strength.Solid solution strengthening(SSS)is the most important contribution to the high strength and hardness.Subsequently,a semi-empirical models was built to predict the yield strengh and hardness for single-phase HEAs.