Chemical Short-range Ordering And Its Strengthening Effects In TiZrHfNb High-entropy Alloy By First-principles Calculations

As a new class of metallic materials developed in recent years,high-entropy alloys(HEAs)possess unique characteristics of multi-principal component,high configuration entropy and sluggish diffusion.HEAs exhibit unusual physical,chemical and mechanical properties due to their high configuration entropy.Refractory high-entropy alloys(RHEAs)have shown outstanding softening resistance at elevated temperatures,as well as high irradiation resistance and corrosion resistance,which endow them promising prospect in applications under extreme service environments.Recent results reveal that local composition heterogeneity or chemical short-range ordering(CSRO)might be the intrinsic characteristic for the atomic structure of HEAs.It was found that CSROs play important roles in deformation and mechanical properties of HEAs.Nevertheless,due to the complex chemistry of HEAs,it is challenging to characterize CSROs by experimental methods.Alternatively,computational simulations provide an effective mean to study CSROs in these highly concentrated HEAs.More recently,it was reported that addition of interstitial atoms in the bodycentered-cubic(BCC)TiZrHfNb RHEA can regulate the CSROs and then simultaneously improve the strength and plasticity.It is known that the CSROs are the key for the enhanced mechanical properties;however,the formation and strengthening mechanism of the CSROs in the TiZrHfNb RHEA remain unclear.As such,this dissertation aims to address these issues via first-principles calculations.Formation of the CSROs and its influences on defect structures and mechanical properties were investigated systematically.Particularly,emphasis was placed on CSRO effects on formation of interstitial atoms(i.e.,B,C,N and O)and strengthening mechanisms of the CSROs.The main contents and conclusions of this dissertation are as follows:(1)CSROs and their strengthening effects in the TiZrHfNb RHEA were studied comprehensively.Influences of the CSROs on the structural stability,modulus,hardness and ideal strength of the TiZrHfNb RHEA were studied by the Warren-Cowley(WC)CSRO parameter and similar atomic environment method,as well as synchrotron small angle x-ray scattering experiments.The results show that the energy of the system decreases,while the elastic modulus,hardness and ideal strengthen increase,with the enhancement of(Ti,Zr)-CSROs.The atomic configuration of the(Ti,Zr)-CSROs with a WC parameter of-0.435 is most stable,and its energy is 5.9 meV/atom lower but modulus 11.7%higher than that of the special quasirandom structure(SQS),respectively.Electronic structure analyses reveal that the development of(Ti,Zr)-CSROs reduces d electrons at the Fermi level,which lowers the system energy and thus stabilizes the BCC HEA.More importantly,severe local lattice distortions and more electron transfer from highenergy to low-energy states under applied strains are found to be the intrinsic reasons for the CSRO strengthening in the TiZrHfNb HEA.(2)Effects of CSROs on formation of vacancy and stacking faults in the TiZrHfNb RHEA were studied.As compared with the SQS,existence of the CSROs reduces the vacancy formation enthalpy from 1.65 to 1.22 eV and the vacancy migration energy from 0.63 to 0.39 eV.Hence,the TiZrHfNb RHEA has a higher equilibrium vacancy concentration and jump frequency than pure Nb,which results in a higher theoretical diffusion coefficient.Nevertheless,presence of the CSROs causes a large fluctuation for the vacancy migration energy in the RHEA,which hinders the vacancy migration along a straight line.The vacancy tends to migrate along the direction with the lowest migration energy in the RHEA,which makes the path of vacancy migration rugged and increases the distance of vacancy migration.As a result,the diffusion of vacancy becomes difficult and its diffusion rate is reduced in the TiZrHfNb RHEA due to the developed of CSROs.In addition,the change of unstable stacking fault energy with the CSROs is very small,and its mean value(369.5±11.4 mJ/m2)is significantly lower than that of pure Nb(i.e.,665 mJ/m2),indicating that dislocations movement in the RHEA is prone to be in the form of plane slip,which provides theoretical bases for understanding the deformation mode observed in experiments.(3)Effects of CSROs on the formation of interstitial atoms and mechanical properties of the TiZrHfNb RHEA were explored.Formation enthalpy of interstitial B,C,N and O atoms at octahedral interstitial sites of the BCC-TiZrHfNb RHEA is-0.24,-1.40,-3.79 and-5.85 eV,respectively.The decrease of the formation enthalpy of interstitial atoms in the order of B,C,N and O well explains their significant difference of solid solubility in this RHEA.As the enhancement of the(Ti,Zr)-CSROs,the formation enthalpy of O decreases,indicating that O atoms tend to occupy the octahedral interstitial positions near the(Ti,Zr)-CSROs.The formation enthalpy of B,C and N is less affected by the CSROs.The solid solution of O atoms near the(Ti,Zr)-CSROs increases the Young’s modulus by 16%and reduces the Pugh parameter by 5%.The electronic structure analysis reveals that the preference of O to combine with the(Ti,Zr)-CSROs origins from the orbital hybrid between p electrons of O atoms and d electrons of the(Ti,Zr)atoms.As such,the bonding electrons between O and(Ti,Zr)atoms are in lower energy orbitals than those between O and(Hf,Nb)atoms,leading to the lower energy of the system.(4)Formation and strengthening mechanisms of(O,Ti,Zr)-ordered oxygen complexes in the TiZrHfNb RHEA were revealed.By systematically analyzing the formation enthalpy of interstitial O and N atoms in different distribution states in the TiZrHfNb RHEA,it is found that,when multiple O atoms located near(Ti,Zr)CSROs to form(O,Ti,Zr)-ordered oxygen complexes,O atoms have the lowest formation energy(-5.85 eV).Nevertheless,when multiple N atoms are randomly distributed in the system,N has the lowest formation energy(-4.04 eV).It is thus confirmed that the(O,Ti,Zr)-ordered oxygen complex is a stable structure in the TiZrHfNb RHEA.Also,the(O,Ti,Zr)-ordered oxygen complex can improve the modulus and Pugh parameter of the RHEA simultaneously,which originates from the inhomogeneous distribution of valence electrons induced by covalent bonds between ordered oxygen atoms.As elaborated above,CSRO effects in the BCC TiZrHfNb RHEA were carefully studied,and the formation and strengthening mechanisms of CSROs,particularly the formation of(O,Ti,Zr)-ordered oxygen complexes,were revealed.The findings provide not only in-depth understanding of CSRO effects in HEAs,but also a new avenue for regulating the mechanical properties of HEAs.