High-throughput Design And Calculation Of Mg-containing Lightweight High-entropy Alloys

A high-entropy alloy generally consists of five or more alloying elements,each of which has a molar percentage between 5%and 35%,forming a single-phase solid solution alloy with a simple crystal structure.Since its inception in 2004,high-entropy alloys have received much attention due to their attractive properties.Subsequently,through experimental research,more high-entropy alloys with different compositions were found,such as CoCrFeNiMn alloy with ultra-high strength and hardness and W_21.1 Nb_20.6Mo_21.7Ta_15.6V₂₁ alloy with excellent high temperature resistance.At present,the developed high-entropy alloys are still limited to high-entropy alloys composed of transition elements,especially Cr,Fe,Ni,Mn,Co,etc.,and the specific gravity tends to be large.On the other hand,the formation mechanism of high-entropy alloys still not clear.The reduction of the density of high-entropy alloys and the development of so-called lightweight high-entropy alloys by alloying and alloy design is an important research direction in the field.Lightweight high-entropy alloys are designed to achieve density reduction of alloys while producing low-density alloying elements（light elements）such as Mg,Al,Li,Si,etc.,while producing high-entropy alloys having the same or even higher performance.In general,there are usually two ways to reduce the specific density of the alloy:（1）adding more than three light elements at the same time;（2）adding two to three light elements and increasing their mole percentage in the alloy.At present,it has been shown that the Al_1.5CrFeMnTi high-entropy alloy formed by replacing Al and Ti elements in CoCrFeNiMn alloy has a density of 5.31g/cm³.In addition,the Al₂₀Li₂₀Mg₁₀Sc₂₀Ti₃₀ high-entropy alloy prepared by ball milling has a density of only 2.67 g/cm³ due to the use of light elements as components,and the hardness of this lightweight high-entropy alloy is as high as 5.9 GPa.It can be seen that it is very meaningful to study the formation mechanism of high-entropy alloys and design lightweight high-entropy alloys.However,due to the high number of high-entropy alloy components,the traditional experimental trial and error method does not have the ability to investigate tens of thousands of alloy components.Therefore,the computational material science method based on supercomputer has become a high-through put design method to study high-entropy alloys.The Coherence Potential Approximation,Multi-cluster Expansion and Special Quasi-random Structure theory are the three most important calculation methods and theories for studying high-entropy alloys.The Special Quasirandom Structures theory uses small volume primitive cells to approximate the disordered atomic distribution in high-entropy alloys by simulating correlation functions,and often combined with the first principles calculations to study the properties of high-entropy alloys.In this paper,we utilised the Special Quasi-random Structure theory,established the optimal quasi-random structure of Mg containing five-element lightweight high-entropy alloy Mg_0.2（BCDE）_0.8 by adjusting the reasonable calculation parameters by experiments.On this basis,combined with the first principle method（VASP）,the enthalpy of formation of the alloy was calculated,the thermodynamic stability of the alloy was studied,the most stable alloy composition was obtained,and the influence of crystal structure and component elements on the stability of the alloy was analyzed.The main findings are:（1）For the Mg_0.2（BCDE）_0.8 series five-element high-entropy alloy,setting the appropriate calculation range and correlation function cut-off value is the key to find the optimal SQS model.After analysis and testing,when the calculation considers only the pair correlation function of the second neighboring atoms located in the second and third shells,and the degree of mismatch to the correlation function is less than 0.03,the optimal SQS model of the alloy can be obtained.（2）The formation enthalpy of MgAlAgX（X=Nd,Gd,Er）Y（Y=Ca,Sn,Zn,Si,Pb）five-element atomic ratio high-entropy alloy was calculated.After comparison,it was found that the FCC structure of MgAlAgGdCa alloy has the best thermodynamic stability.（3）In MgAlAgX（X=Nd,Gd,Er）Y（Y=Ca,Sn,Zn,Si,Pb）,the FCC structure has better thermodynamics stability than the BCC structure under the two crystal structures of the same component alloy.（4）In MgAlAgX（X=Nd,Gd,Er）Y（Y=Ca,Sn,Zn,Si,Pb）,the alloy containing Nd has poor thermodynamic stability,while the alloy containing Er element has better thermodynamic stability.The Ca element contributes to the stability of the alloy,and the non-metallic element Si reduces the thermodynamic stability of the alloy.