Atomistic simulation of the generation of vacancies in rapid crystallization of metals

Abstract The generation of vacancies at a crystal-liquid interface propagating under conditions of undercooling below the equilibrium melting temperature is investigated in molecular dynamics simulations performed under well-controlled temperature and pressure conditions for two representative metals, bcc Cr and fcc Ni. The results of the simulations reveal that, for both metals, vacancy concentrations produced in the course of the steady-state propagation of crystallization fronts can exceed the equilibrium values at the corresponding temperatures by orders of magnitude. Different trends in the temperature dependences of vacancy concentration observed for the two metals, namely, the continuous increase with increasing undercooling for Ni and a nonmonotonous temperature dependence in Cr, are related to the qualitatively different temperature dependences of the crystallization front velocity predicted for the two metals. The general character of the computational predictions is confirmed in simulations performed for Ni with four different interatomic potentials and for both metals with (001), (011), and (111) interface orientations. A detailed analysis of atomic rearrangements at the crystallization front suggests that the level of vacancy supersaturation is largely defined by the ability of atoms to migrate within the interfacial region and to fill the numerous vacant sites produced through the simultaneous construction of several atomic crystal planes within the interfacial region. While the majority of the transient vacancies generated during the construction of the crystal planes are annihilated by the atomic flux coming from the liquid side of the interface, a small fraction of the vacancies are trapped behind the crystallization front. Under conditions of strong undercooling, the fast movement of the interface and decreased vacancy mobility prevent the equilibration of vacancy concentration in the newly built crystalline region, thus creating a strong vacancy supersaturation. Analysis of the temperature dependence of the atomic rearrangements occurring at the crystallization front suggests incomplete relaxation of the disordered phase at and in front of the crystal-liquid interface rapidly advancing under conditions of strong undercooling.