Some Thermodynamic Properties of NiAl Calculated by Molecular Dynamics Simulations
- PDF / 406,601 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 89 Downloads / 215 Views
SOME THERMODYNAMIC PROPERTIES OF NiAl CALCULATED BY MOLECULAR DYNAMICS SIMULATIONS P.C.
Clapp, M.J. Rubins, S. Charpenay, J.A. Rifkin, Z. Z. Yu Institute of Materials Science, Univ. Of Connecticut Storrs, CT 06268 and A.F. Voter, Theoretical Div., Los Alamos National Lab, Los Alamos, NM 87545
ABSTRACT Calculations of the surface free energy and anti-phase boundary energy as a function of low index orientations and temperature have been determined for equiatomic perfectly ordered bcc NiAl via molecular dynamics computer simulations. The simulations utilized an Embedded Atom Method calculation of the interatomic potentials and volume forces Values of about 0.95, 1.6, 1.9 in the Ni-As alloy system. and 2.0 J/m were found for surface energies of the 11001, APB respectively. and 11111 orientations:, 11121 11101, were determined for energies of about 0.24 and 0.38 J/m In addition, we 1110) and 11121 boundaries, respectively. have examined the phase stability and relative energies of the ordered bcc, fcc and bct phases at low temperature, and find a bct phase with c/a z 1.32 slightly lower in energy than the bcc, presaging the martensitic transformation that occurs at finite temperatures in more nickel rich alloys.
Introduction Thermodynamic properties such as surface energy or antiphase boundary energy are generally very difficult to determine experimentally with any degree of accuracy, and yet these properties play a fundamental role in understanding the strength and fracture resistance of materials. Although it is now beginning to be possible (with large amounts of supercomputer time) to calculate these quantities at 0 K for simple crystal structures using "first principles" variational methods, it will still be a substantial wait before computer speeds and budgets will permit such calculations "at temperature". In the interim, the best avenue for obtaining these essential thermodynamic parameters with reasonable accuracy appears to be using recent advances in determining atomic interactions semiempirically, combined with molecular dynamics(MD) computer simulations. Of the semi-empirical methods that include many-body volume forces, which would seem to be an essential feature in modelling metals, we have chosen to use the Embedded Atom Method (EAM) for several reasons. First, it seems to be well grounded in fundamental principles[l]. Second, it provides pair potentials and volume forces that are composition and crystal structure independent, which makes them very useful in computer simulations that encounter varying local structures and compositions. Third, the EAM potentials had already been calculated[2] for the alloy system Mat. Res. Soc.Symp. Proc.Vol. 133. c1989 Materials Research Society
30
of prime interest to us, although the empirical input was Al and almost entirely based on the fcc phases of Ni, 3-NiAl In fact, the only input from the ordered Ni 3 Al. ordered bcc phase was the values of the equilibrium lattice parameter and the cohesive energy. Given this last fact, it seemed important to test the accu
Data Loading...
