Session Q14 - Multiscale Modeling of Materials II.
FOCUS session, Wednesday morning, March 14
Room 210, Washington State Convention Center, Seattle, WA

11:00
Q14.001 First -Principles Theory of 10**5 atom Alloy Precipitates*
Alex Zunger (NREL , Golden, Colorado 80401 ,USA)

When cooled below the miscibility temperature,alloys decompose into precipitates rich in one of the two components.These precipitates are observed in TEM to have remarkable shapes and sizes ,and are considered important for material strength ( in metal alloys) and electron transport (in semiconductor alloys). Since a precipitate system contains N= 10**3 to 10**6 atoms,in the past,simple energy models( e.g,elastic springs plus an isotropic chemical energy) have been used to describe their evolution.. We are interested in applying LDA-quality energy functionals to this problem. .Since the system of precipitate +medium is structurally coherent,all that is needed is the knowledge of the energy of N atoms (A and B) as a function of their lattice configuration ,so that the energy of any shape and size of the precipitate can be calculated.However,one must know the energy corresponding to the (coherently) relaxed system.We show that this can be conveniently done via the "mixed Basis Cluster Expansion" containing pair as well as multi-body and strain terms deduced from LDA calculations of simple prototype A-B systems.This functional is then subjected to a Monte Carlo simulation of typically 10**5 atoms. I will show how this reproduces remarkably the shape VS size VS temperature behavior noted experimentally for Al-Zn alloys ,and how the physical accuracy of the underlying LDA hamiltonian permits a transparent analysis of the factors that control precipitate shapes. Thus,this method allows one to extend the atomistic LDA size scale to mesoscopic dimensions without compromizing the quality of the energy expression.

This work was done in collabiration with Stefan Muller, Li.W.Wang and C.Wolverton and was supported by DOE-OS-DMS.