Computational Study On Melting,Freezing And Coalescence Of Titanium Nanoparticles

As a metal structural material,metal Ti has been widely used in aerospace,marine industry,semiconductor and biomaterials due to its excellent performance.At present,a large number of Ti products manufactured with additive manufacturing technology have been available.During the sintering process,Ti nanoparticles as the raw materials,its thermal stability and microstructure changes have an important impact on the performance of the products.Thus it is important to study the thermal stability of Ti nanoparticles.The molecular dynamics calculation method based on embedded atomic potential model is used to calculate the structural evolution of Ti nanoparticles with different sizes during melting and freezing;The simulated cells are placed at room temperature(300K)and gradually heated to 1600K.For the single Ti nanoparticles,the freezing process is also studied,that is,from 1600K indirect cooling to room temperature.In addition,the double Ti nanoparticles system constructed on the single Ti nanoparticles,the structural evolution of the double Ti nanoparticle system during the coalescence process is considered.Characterization analysis with the average energy of the atom,the pair distribution function and the atomic stack structure is implied.The structural change of Ti nanoparticles gradually spreads from surface atoms to internal atoms.The surface of the particles has a variety of defects such as atomic row misplacement,vacancy or interstitial atoms during this process.As the confusion increases,the nanoparticles transform from a hexagonal-close-packed stack to a body-centered-cubic stack,which eventually melts;During the cooling process,the system firstly transforms into a body-centered-cubic stack.As the temperature decreases,the confusion of the system decreases,and the body-centered-cubic stack transforms into a hexagonal-close-packed stack.It indicates that the hexagonal-close-packed stack of Ti nanoparticles is more stable at low temperatures,and the body-centered-cubic stack is more stable at high temperatures.The melting point of Ti nanoparticles increases as the particle size increases,but the hcp?bcc structure transition point,freezing point and the bcc?hcp structural transition point shows the law of oscillation.In the two particles' system,the two particles are merged at room temperature,and there is a connecting region appears.At the connection areas,regardless of the distance between the two Ti particles,there are two types of connection areas,such as a area resembling a diamond shape and a coherent interface.At low temperatures,the two particles remain in their respective crystal stack and crystal orientations away from the junction.As the temperature increases,the surface atoms will diffuse toward the connecting region.At a certain temperature,the double Ti nanoparticle system will change from a hexagonal-close-packed stack to a body-centered-cubic stack,the distance between the centroids of the two particles gradually decreases and eventually merges into one particle,which eventually melts.