Study On The Local Atomic Structure Of ZrCuAl Ternary Amorphous Alloys

Metallic glasses have been the research focus in the field of materials and physics in the world because of their high yield strength,high elastic strain limit,high corrosion resistance and other excellent properties.Howerer,because of the structural feature of the metallic glasses owning the short range order but lacking of the long range order and the limitations of the experimental techniques and simulation conditions,there are still many urgent problems to be revealed thoroughly,for example,the nature of glass transition(GT),the relationship between the structure and mechanical properties,the dependence of composition on the structure and properties of metallic glasses.The ZrCuAl ternary metallic glasses are chosen as the object to study the problems proposed above mainly using the Ab initio molecular dynamics(AIMD)and classic molecular dynamics(CMD)simulations in this paper.The important results are as following:(1)It was found that the addition of Al to the Zr-Cu binary alloy would lead to the formation of the more closely packed and stable clusters,promoting the GFA.The first peaks in the pair distribution functions(PDFs)of the supercooled Zr55Cu35Al10 liquid obtained from high-energy synchrotron X-ray experiments and AIMD simulations became higher and narrower as the temperature decreased,incidating increased short range order.By quantitatively analyzing the cluster,interconnecting zone(i-zone)and free volume atom pairs in the tight-bond cluster model,it was found that it was easier to form strong bonding between AI atom and other atoms comparing with Zr and Cu atoms.By calculating the DOSs and PDOSs related to the electronic structure of different clusters,it was found that the low energy state is mainly attributed to Al sp-electrons.(2)During quenching to the glass transition temperature(Tg)of the Zr55Cu35Al10 alloy,by quantitatively analyzing the cluster,i-zone and free volume atom pairs in the tight-bond cluster model,it was found that the fraction of the cluster and i-zone atom pairs increased while the fraction of the free volume atom pairs decreased continuously.The Zr-centered first shell clusters tend to transform to the coordination polyhedra with high coordination number(CN)owning low cluster energy and the Cu-and Al-centered clusters tend to become the coordination polyhedra with CN=12 with the lowest cluster energy in the short range order strcuture.The medium range order structure of the coordination polyhedra connection with the number of three shared atoms increased significantly.Furthermore,it was found that while the Zr55Cu35Al10 amorphous solid transformed to the supercooled liquid,the atom pair distance increased,cluster atom pairs becomed i-zone atom pairs and i-zone atom pairs becomed free volume atom pairs.As a result,the number of the free volume atom pairs increased and the number of the cluster atom pairs decreased obviously,leading to the increase of the free volume zones with high energy,and finally resulting in the dramatic variation of the strength and plasticity.(3)The cooling rate had a great influence on the amorphous alloy structure obtained by quenching the Zr55Cu35Al10 melt.The results from the CMD simulations presented that more Zr-centered high coordination polyhedra and Cu and Al-centered coordination polyhedra with CN=12 and coordination polyhedra connection with the number of three shared atoms were obtained at a lower cooling rate.(4)The composition had a great effect on the structure,energy and properties of the ZrxCu92-xAl8(x=55,60,65,and 70)ternary alloys.The analysis of the coordination polyhedra indicated that substituting Zr atoms for Cu atoms led to the reduction of high coordination polyhedra and precluded the formation of the icosahedral structure.The analysis of the cluster energy distribution showed that Zr-,Cu-,Al-centered clusters shifted towards higher energy,leading to the formation of the unstable clusters with high energy.As a result,the high Zr content Zr-Cu-Al metallic glasses were composed of more weakly bonded regions,exhibiting high plasticity and low GFA.The influence of the composition on the structure and GFA in the ZrxCu92-xAl8(x=20,30,40,50,60,70)termary alloys was investigated.The CMD simulation results showed that Zr30Cu60Al10 alloy has the highest Tg and GFA.According to the analysis of the short range order and medium range order structure,there were more Zr-centered high coordination polyhedra and Cu and Al-centered coordination polyhedral with CN=12 and coordination polyhedra connection with the number of three shared atoms in the Zr30Cu60Al10 amorphous alloy,promoting the GT.