Study Of Structures And Electronic Properties For Ti-Al Binary Clusters By First-principles Calculations

Titanium,aluminum and their alloy materials,which have excellent mechanical and chemistry properties,have a large range of applications in industries.However,due to its low ductility at room temperature,the application of titanium-aluminum materials in the aerospace and automotive fields is severely limited.In order to improve the low ductility of titanium-aluminum alloy at room temperature and improve their mechanical properties at high temperatures,it is necessary to conduct further research on nano-scale titanium-aluminum materials.In order to understand the structures and evolve mechanism of TiAl alloy from single-atom to large-size bulk phase,the geometrical,energetic and electronic properties of pure titanium clusters,pure aluminum clusters and titanium-aluminum clusters were studied by plane wave pseudopotential method within the density functional theory frame.The following studies are performed:(1)The initial structures of linear Tin(n=2-8),non-linear Tin(n=2-8)and Alm(m=2-8)clusters are constructed.In order to obtain the ground state structure of these cluster,these initial structures are optimized,and the total energy is calculated.We also analyze the binding energy and second-order difference of the ground state structures.It is found that for linear Tin(n=2-8)clusters,when the contained number is odd,a trimer is formed at the end of the structures with dimers.With the increase of titanium atom number n,the binding energy of the cluster increases,indicating that the cluster is more stable.For non-linear Tin(n=2-8)clusters,the larger titanium atom number n is,the steadier the clusters is.Starting with the Ti4 cluster,geometry structure changes from two-dimension to three-dimension.The Ti2?Ti5 or Ti7 cluster is more stable than other clusters.For Alm(m=2-8)clusters,the larger the aluminum atoms number m is,the stabler the clusters.Starting with Al5 cluster,geometry structure changes from two-dimension to three-dimension.The Al3 or Al7 cluster is more stable compared to the other clusters.(2)The lowest energy structures,stability,and electronic properties of TinAlm(n+m=2-8)are obtained.We find that one cluster containing more than four atoms,has a three-dimensional configuration,except for the Al4 cluster.With the increase of the atom number,the geometry structures of the clusters change from triangular pyramid to quadrangular pyramid,and then to pentagonal pyramid.Titanium atoms are closer to the center of TinAlm clusters compared to aluminum atoms.The number of Ti atoms and Al atoms in the Ti-Al clusters determines its structure and properties.When the number of the Ti atoms is larger than of Al,the properties of the clusters are more similar to that of pure titanium clusters.The increase of the number of the Ti atoms can improve the cluster's stability.For the Ti3iAli,TiiAl3i and TiiAli clusters,the clusters become more stable as i increases.Ti4Al4 and Ti2Al2 are more stable than the other clusters.All of the TinAlm(n+m=2-8,m? 0)clusters are more likely to dissociate one aluminum atom,in other words,TinAlm-1 type dissociation occurs.For pure titanium clusters,the Ti4,Ti5 and Ti7 cluster prefer to have Tin-2 type dissociation,while the remaining titanium clusters prefer to Tin-1 type dissociation.The charges in TinAlm clusters tend to transfer from titanium atom to aluminum atom,and transfer from the interior part of the cluster to the outer part.