First-Principle Molecular Dynamics Study On The Correlation Of Short-Range Structural Evolution Of ZrCo-based Metallic Glass

Until now,the formation and deformation of Zr-based and Co-based amorphous alloys have been a hot topic in the research of bulk amorphous alloys.Zr-based amorphous alloys usually exhibit good glass-forming ability,excellent mechanical properties,corrosion resistance and good biological activity,while the Co-based amorphous alloys display good wear resistance,high permeability,low coercivity,low loss and excellent temperature stability,etc.Therefore,ZrCo-based alloys possess the advantages of both alloy systems,which have drawn a lot of attentions.However,most studies on ZrCo-based glass-forming compositions mainly focuses on their glass-forming ability and mechanical properties,while their structure origin was less investigated.In this work,the structural evolutions of ZrCo and ZrCoAl melts and amorphous alloys were was studied by first-principle molecular dynamics experimental method in order to find out the relationship between structural characteristics and mechanical properties.Firstly,the microstructural evolutions of the binary ZrxCo100-x(x=10,30,50,70,90)melts and amorphous alloys were simulated in detail.It was found that at 2000 K,the first peak of g(r)curve gradually shifts to the right side when the ratio of Zr and Co gradually increases from 1:9 to 9:1,while exhibits a double peak characteristic(marked as P11 and P12).Therefore,the shift of the first peak of g(r)curve is not a simple position shift but is accompanied by the decrease of the intensity of the P11 peak and the increase of the intensity of the P12 peak.According to the total g(r)and gij(r)curves,the numbers of the Co-centered and Zr-centered clusters gradually decrease and increase with increasing the ratio of Zr and Co,respectively.When the temperature decreases to room temperature,ZrxCo10-x(x=10,30,50,70,90)melts transform into fully amorphous phase,leading to the formation a double peak characteristic of the second peak of total g(r)curve.Besides,the double peak characteristic of the first peak of total g(r)curve for the fully amorphous samples become more obvious,implying that some dominant liquid structural features of melts can be kept into the amorphous states.Furthermore,according to the coordination number parameters and chemical short-range orders(SROs)analysis,the double peak characteristic of the first peak of total g(r)curve is due to the competition between Co-centered and Zr-centered clusters.Based on the bond pair analysis,the five-fold symmetry can be reduced upon cooling,leading to the easy formation of amorphous phase.Secondly,since it is difficult to obtain the binary ZrCo amorphous alloy and the verification of the structural heterogeneity in the amorphous state should be conducted in the amorphous samples,a little Al was added into the ZrCo alloy in order to increase their glass-forming ability.Here,two types of Zr-Co-Al glass-forming systems,i.e.Zr65Co30Al5 and Zr10Co85Al5 alloys,were selected as model alloys.The results show that the clusters centered around the Zr and Co atoms may govern the chemical SROs in Zr65Co30Al5 alloy,resulting in the splitting of the first peak in the total g(r)curves.At the same time,coordination number analysis showed more obvious structural heterogeneity.Bond pair analysis and Voronoi polyhedron studies also confirm that different symmetries play an important role in structural inhomogeneity.The results show that Zr65Co30Al5 has higher glass-forming ability than Zr10Co85Al5.Zr65Co30Al5 was proved to have better bending ductility than Zr10Co85Al5 by(dynamic)mechanical testing,which should be attributed to obvious structural heterogeneityThirdly,in order to verify the effect of the structural heterogeneity during deformation,the microstructural evolution of the Zr56Co28Al16 bulk amorphous alloys were analyzed by first principles molecular dynamics combined with synchrotron radiation X-ray diffraction during compression.Synchrotron radiation experiments show that the structure of the alloy becomes much denser during compression.The simulation data are compared with the synchrotron radiation experimental results,which well corresponds with each other.The splitting of the first peak is caused by the interaction of Zr-Zr,Zr-Co and Zr-Al curves by analyzing the results of the pair distribution function.The results of chemical SROs show that the Co atom exists as the central atom,the Zr atom works as the coordination atom,and the A1 atom only coordinates with the Zr atom as the central atom.The results of bond pair analysis show that the final formation of the amorphous phase is dominated by quantic symmetry,and the fourth and sixth pairs are called the secondary locally symmetric short-range structures.Then Voronoi polyhedron and coordination number are used to further explain the different formation behaviors of clusters around different central atoms,which reflects the structural heterogeneity.As a result,the structural heterogeneity can easily induce the formation of multiple shear bands,leading to the good compressive ductility.