Molecular Dynamics Study On Liquid-Liquid Phase Transition And Solidification Of Confined Ti And Its Alloys

Due to the size effect and interface effect,the structure and thermodynamic properties of the melt in the confined space often have new characteristics different from those of the free melt,which will have a genetic effect on the phase transition behavior and the structural properties of the low-dimensional confined materials obtained by solidification,thus making the thermal,optical and electrical properties of the low-dimensional materials change obviously,making them show great application prospects in the fields of electronic devices,energy storage,composite materials and so on.It is of great theoretical significance to systematically study the structure and phase transition behavior of melt in confined space and establish a theory describing the structure and phase transition characteristics of low-dimensional melt for developing advanced nano-material preparation technology and improving the processing technology of traditional materials.In this paper,the liquid-liquid phase transition and solidification behavior of Ti and its alloys in confined space are studied by molecular dynamics simulation,and the induction of liquid-liquid phase transition of melt Ti by confined walls is found,revealing the close relationship between liquid-liquid phase transition and layering transition.At the same time,the layer-by-layer solidification behavior of Ti-Al alloy in confined space is explored,and the important role of confined walls in the crystallization of Ti-Al alloy is analyzed.The main contents of this paper are as follows:(1)The liquid-liquid phase transition of confined melt Ti was studied.The abnormal change of curve slope of volume and potential energy under different slit sizes confirms the existence of liquid-liquid phase transition of melt Ti,which is characterized by the discontinuous distribution of density,which is,layering.The system will form different layers of structure under different slit sizes,and the system density will fluctuate with the change of slit sizes.Slit sizes,temperature and pressure are all factors that affect layering,and the temperature has little effect on layering.Slit sizes and pressure play an important role in the liquid-liquid phase transition of melt Ti,which will affect the location and quantity of melt layers.In order to better understand the layering mechanism,the phase diagram of the layering transition is drawn.In the temperature-slit size phase diagram,the layering transition is controlled by slit sizes because the temperature has little influence on the layering transition.In the pressure-slit size phase diagram,the layering transition of the low-pressure zone is controlled by slit sizes,and the layering transition of the high-pressure zone is controlled by pressure.The abnormal change of coordination number and diffusion coefficient further verifies the existence of liquid-liquid phase transition.(2)The effects of cooling rates and confined space on the solidification behavior of Ti-Al alloy were studied.The results show that when the cooling rates are 0.005-0.02 K/ps,the melt Ti-Al can form a crystal structure during solidification,and when the cooling rates are 0.0110 K/ps,an amorphous structure can be formed.When the system is placed in a confined space,with the decrease of the slit size,the system will solidify layer by layer from the position near the confined walls.When the influence range of the confined space can radiate the whole system,the system will be layered in the Z direction,while on the XY plane,the atoms of the system will be arranged regularly to form a crystal structure.The interaction between the confined walls and Ti-Al atoms and the geometric constraint effect of the confined walls lead to the crystallization of Ti-Al alloy.By analyzing the angular distribution function during cooling,it is found that the crystal structure is composed of fcc and hcp.The similarity of the angular distribution function of the Al atom and Ti atom proves the similarity of their local structures,indicating that the system is not prone to lattice distortion,which also reasonably explains the crystallization phenomenon of the system.With the progress of solidification,the coordination number of the system increases first,then decreases,and then tends to be stable,which is related to the crystallization of the system.In this paper,the structural evolution law and phase transition behavior of metal melt in confined space are systematically expounded,which provides the necessary theoretical basis and guidance for the preparation and application of low-dimensional confined materials in the future.