Molecular Dynamics Simulation Of Solid - State Transformation And Melting Properties Of Monocrystalline Zirconium

Phase transiton is the transformation between possible phases of a certain material under a specified temperature and pressure or other physical conditions. It is essentially featured by changes of the structure of materials at the micro scale that lead to notable changes of their properties at the macrosopic scale. In nuclear industry, with superior properties including the low neutron absorption cross section, the high melting point as well as the high hardness, the zirconium (Zr) is widely used, which is stable with the close-packed-hexagonal structure (theα phase) at the ambient temperature and transforms to the body-centered-cubic structure (theβphase) at a high temperaute before melting. This solid transformation of the β-to-α phase and the melting of the Zr are two representative types of temperature-induced phase transiton, of which the behavior were explored with the molecular dynamics (MD) method, a powerful tool that bridges the micro structure of a material with its macrosopic properties.Among all the possible phases of a material, the most stable phase at a given temperature and pressure is the one with the lowest free energy, the decisive factor of the phase stability that can be used to determine the transition points and to depicit the phase diagram. However, as a function of the entire phase space, the free energy can not be obtained directly by MD simulations, which has given rise to the development of several indirect approaches, among which the thermodynamic integration method has the most popularity. It converts the calcu-lation of the free energy of the target system to the integration of a ensemble average, which can be calculated easily with MD simulations, of the coupled systems on a reversible thermo-dynamic path that links the target system to a reference system, of which the free energy can be obtained analytically. Howerer, different coupled systems on the thermodynamic path have different potential functions established by the combination of the potential functions of the target system and the reference system with corresponding couple parameters. These coupled systems with variable potential functions can not be tackled with the common MD codes. In this dissertation, this problem has been solved and the thermodynamic integration method has been implemented on the basis of the LAMMPS codes.With this thermodynamic integration method, the free energies of thea phase and the(3phase as functions of the temperatures were calculated respectively, which indicated the tran-siton temperature as1310K, in considerable agreement with the experimental result of1136K. By tracing the trajectories of atoms in the crystal, it was revealed that the β-to-α phase transiton proceeded as follows:the {110} planes of the β structure became the close-packed planes of the a structure after a shuffle of every second planes combined with a contraction in the [001] direction and an extension in the [0-11] and the [011] directions, following the identical mechanism proposed by Burgers.Melting of crystals is a first order phase transiton accompanied by the the sudden change of volume. By monitoring the variation of average atom volume at various temperatures in the heating and cooling process, the equilibrium melting point of Zr at0pressure was calculated as2010K, in good accordance to the experimental result of2128K. With a series of analogous simulations at different pressures, it was suggested that the melting point of Zr first increases slightly and then decreases slowly as the pressure rises, which is different from many metals such as the copper and the aluminum with monotonic increasing melting points under rising pressures but similar to its peer titanium from the same group, implying the unique property of metals in this transitional group. By deleting specific proportions of atoms from the perfect crystals, the effect of the initial concentration of porosity on the melting point of the Zr was studied, showing that the melting point first decreases markedly but subsequently fluctuates around an equilibrium temperature as experimental results expect.