Structure and Energetics of Vacancies, Antisites and Divacancy Complexes in the Ni-Al System
- PDF / 329,613 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 88 Downloads / 144 Views
Structure and Energetics of Vacancies, Antisites and Divacancy Complexes in the Ni-Al system Zhao Yang Xie and Diana Farkas Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 ABSTRACT We studied the energetics and structure of simple point defects and defect clusters in the Ni-Al system. Atomistic computer simulation was used for this purpose with embedded atom interatomic potentials. In both, NisAl and NiAl the energetics are such that excess Ni is accommodated by anitisite defects more easily than excess Al. Structural vacancies are therefore more likely with stoichiometry deviations in the Al-rich side than in the Ni-rich side. The interaction of vacancies of different types to form divacancies was studied. Several configurations were found with a very similar energy to that of the two monovacancies far apart. Some configurations show attraction and others show repulsion. The vacancy interaction distance is limited to a few interatomic distances. The relaxation around these defects was studied in detail and the results can be interpreted in terms of the larger size of the Al atom.
Introduction There is great interest in understanding the effects of compositional stoichiometry deviations and increasing disorder on the behavior of intermetallic compounds in the Ni-Al system. These effects are very important in mechanical behavior and alloy response to ternary additions. For example, it has been shown that B additions can only ductilize Ni-rich Ni 3AI [1]. There is also great interest in understanding the changes in mechanical behavior of B2 NiAI with deviations from stoichiometry [2]. Point defects and point defect interaction , and in particular structural vacancies are very important in this respect. The purpose of the present work is to obtain energies and relaxation behavior corresponding to various point defects that may play a role in accommodating stoichiometry deviations and disorder. Computer simulation results are presented for vacancies and antisite defects in NisA1 and NiAI. The interaction of vacancies of different types to form divacancies was also studied. The interatomic potential used in the present calculation was derived by Voter and Chen [3] within the spirit of the embedded atom model [4], and has proved successful in a variety of applications. They reproduce the experimental lattice constant, cohesive energy, and bulk modulus, as well as the elastic constants, the vacancy formation energy, diatomic bond length and bond energy, stacking-fault and antiphase boundary energy in Ni 3A1. The potentials also give reasonable values for the cohesive energy, lattice parameter and elastic constants of NiA1. The computer simulation program for the calculation is based on the DEVIL code developed by Norgett, Perrin, and Savino [5]. The method does not include the entropy contributions to free energy and therefore gives vacancy structures at zero temperature.
Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
306
Data Loading...
