R. Benedek

The effect of misfit on heterophase interface energies

Most previous atomistic simulations of heterophase interfaces have neglected misfit, the discrepancy between the interatomic length scales parallel to the interface of the two phases. The obstacles to quantitative calculations of interface energies in the presence of misfit are assessed. The most straightforward approach is to perform simulations for a supercell whose size is of the order of the cube of the smallest common periodic length scale (essentially the coincidence-site-lattice periodicity), which varies inversely with the misfit parameter. Such supercells typically contain at least thousands of atoms. First-principles simulations are highly accurate, but are feasible only for a few selected heterophase interfaces with large misfit. Classical interatomic potentials, on the other hand, are efficient numerically, but their accuracy has not been demonstrated in the context of heterophase interface calculations. An approximate formulation of the interface energy is presented here which can be evaluated numerically by first-principles calculations for supercells of only moderate size. This formulation explores the relationship between the interface energies for coherent and semi-coherent interfaces. A numerical application to an interface between tetragonal TiAl and perovskite Ti3AlC is presented.

Interface Structure and Energy Calculations for Carbide Precipitates in γ-TiAl

Ternary carbide precipitates improve the high-temperature creep strength of 2-phase TiAl alloys. The perovskite (P-type) Ti 3 AlC nucleates at relatively low temperatures (750 deg. C), whereas hexagonal (H-type) Ti 2 AlC precipitates occur at somewhat higher temperatures. Calculations are performed, based on first- principles-local-density-functional theory, of the interface structure and energy of these two carbides with a 7-TiAl matrix. Calculations are first done on coherent interfaces, and approximate corrections are then made for the effect of misfit. The perovskite is known to form needle-shaped precipitates oriented along the c-axis of the host. Our calculations yield a relatively low energy for the (100) perovskite-host interface, which is a favorable orientation owing to its low misfit, and because the terminating carbide layer for the coherent interface is pseudomorphic with the host. Predictions are given for the critical thickness for coherence and the critical nucleation size for a P-type precipitate. Calculations for interfaces of H-type platelets with the host show a much larger interface energy than that for the P-type precipitate.

Atomic Structure of a Polar Ceramic/Metal Interface: {222}MgO/Cu

{222}MgO/Cu is one of the most extensively characterized ceramic/metal interfaces, in view of the atom-probe field-ion-microscopy, Z-contrast scanning-transmission-electron-microscopy (STEM), and spatially-resolved electron-energy-loss-spectroscopy (EELS) measurements performed by the present authors, as well as the high-resolution electron microscopy (HREM) of this system by others. Atomistic simulations with local density functional theory (LDFT) and molecular dynamics (MD) have been performed to gain additional insight into the structure of this interface. This presentation describes an interface interatomic potential for {222}MgO/Cu derived from LDFT total energy calculations, and its application to structural properties, including the terminating species, the absence of dislocation standoff, and the symmetry of the interfacial dislocation network.