Study On Effect Of Lattice Distortion On Crystal Structures And Elastic Properties Of TiZr-based Multiple Principal Element Alloys From Density Functional Theory

With the development of society,science and technology,traditional alloy is more and more difficult to meet the needs of human beings.The multiple principal element alloy(MPEA),as a novel design concept,has attracted extensive attention in the field of materials science and engineering.To date,most MPEAs are face-centered cubic and body-centered cubic structures,while hexagonal-close packed(HCP)structures are pretty rare.Due to its low symmetry,HCP MPEAs are considered to have unique mechanical,physical,and chemical properties,so are becoming a research hot spot to exploit new MPEAs.Severe lattice distortion is one of the four core effects of MPEAs and has a profound impact on the excellent properties,which has been the most concerned problem in the MPEAs field.Due to the complexity of multi-component and disordered solid solution structures,traditional experimental methods are facing with great challenges in satudy of lattice distortion.Ab initio studies can easily and effectively predict the structures and properties of MPEAs.Therefore,the effects of lattice distortion on crystal structures and elastic properties of TiZr-based MPEAs are studied systematacially through the density functional theory in conjunction with special quasi-random structure.The main contents in this study are as follows:Based on the density functional theory and special quasi-random structure,the energetics of TiZr M(M=Nb,Hf)and TiZrHf M(M=Nb,Sc)MPEAs with different structures are studied systematically,aiming to reveal the impacting factors and underlying mechanisms of structural selection.Although the formation enthalpy(?H_form)can predict the crystal structures of MPEAs effectively,the internal factors that affect the size of?H_form are not clear yet.Based on mixing characteristics of the multi-component solid solution for MPEAs,?H_form can be further decomposed into two parts,including the structure transformation energy(?E_trans)of elements and mixing energy(?E_mix).The influence of?E_trans and?E_mix on?H_form is studied in particular,and the key role of them in the structural selection of MPEAs is pointed out.Obviously,?E_trans is an effective part of?H_form and plays an important role in the structural selection.The importance of?E_mix is very prominent in quaternary TiZrHf Nb MPEA,and its influence on the structural selection is further discussed.The smaller?E_transand?E_mix,the lower?H_form and more stable structures of MPEAs.Furthermore,the influence of lattice distortion energy(?E_dis)on the structural stability of MPEAs is investigated.The results show that?E_dis is closely related to the formation of the final structure for MPEAs.The lower?E_dis may be related to the lower?H_form,thus promots to the structure formation and stability of MPEAs.According to the standard deviation of bond length distribution,the degree of local lattice distortion(LLD)is quantified efficiently.The?E_dis is associated with LLD.The smaller the degree of LLD in the MPEAs,the lower the?E_dis.This study provides a quantitative perspective for the structural selection of solid solution,which is significant for the structure optimization and performance design of MPEAs.Employing density functional theory calculation based on special quasi-random structure,this work studies the influences of LLD on elastic properties for both novel hexagonal TiZrHf(Sc)MPEAs.Compared to the pristine structures,the lattice stability of distorted random structures is improved evidently.Moreover,LLD obviously lessens the shear elastic properties,which may be ubiquitous phenomenon irrespective of lattice types of MPEAs.These results also uncover the apparent improvement in malleable behavior and shear anisotropy for HCP MPEAs.The degree of LLD is further studied via the standard deviation of bond length distributions as an effective indicator of LLD.More significant distortion uncovered in TiZrHf alloy corresponds consistently to the stronger effects of LLD in TiZrHf alloy.Bader's charge and charge density distribution also demonstrate the underlying impact on LLD for both HCP MPEAs,so electronic nature is a significant factor for LLD.The present study provides guideline for designing mechanical properties of TiZrHf–based HCP MPEAs engineering materials.