Atomical simulations of structural changes of a melted TiAl alloy particle on TiAl (001) substrate

Abstract

Atomic packing structures of a melted TiAl alloy nanoparticle on TiAl(001) substrate at different temperatures are investigated by molecular dynamic simulation within the framework of embedded atom method. In order to obtain a melted TiAl alloy nanoparticle, a larger TiAl alloy bulk in nano-size is initially constructed, subsequently it is heated up to 1500 K and finally melted. A smaller sphere is extracted from the center of the melted bulk to serve as the melted nanoparticle. Periodic boundary conditions are employed in the x and y directions when constructing the sheet-like TiAl alloy substrate. In this simulation, the melted nanoparticle at 1500 K is laid on a TiAl(001) substrate, separately, at 1100, 1000, 900, …, 200 and 100 K as integral systems, and then they experience rapid solidification process. With the analysis of atomic arrangements of the nanoparticle and substrate surface layer by layer, it is found that temperature greatly affects the atomic packing structure of the nanoparticle. When the temperature of the substrate is 1100 K, most atoms in the nanoparticle disorderly pack, indicating that the nanoparticle is still melted at this temperature. At 1000 K, nearly all the atoms in the nanoparticle occupy TiAl lattice points, indicating that the nanoparticle is already solidified at this temperature. With the substrate temperature decreasing, most atoms in the nanoparticle are still of orderly pack. Meanwhile, a pyramid-like inner region, which takes TiAl(001) crystallographic plane as undersurface and TiAl [101], [101], [011], and [01 1] crystallographic axis as edges, abruptly emerges in the nanoparticle. Different atomic packing structures are observed inside and outside this region. Atomic layers composed of atoms inside this region are parallel to the (001) crystallographic plane of TiAl alloy substrate while atomic layers composed of atoms outside this region arranges along other different directions, which therefore leads to four interfaces separating the inner region from other parts of the nanoparticle. At low temperatures, this inner region still exists but its volume decreases with temperature decreasing. Besides, more and more atoms in the upper part of the nanoparticle gradually pack disorderly, which makes it more difficult to distinguish the inner region. In addition, the melted nanoparticle has very limited influences on the central and bottom parts of the substrate. However, thermal motion of atoms of substrate surface which touches the nanoparticle is intensified, thus leading to more obvious lattice distortion.