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Constitutional and Thermal Defects in Nickel Aluminides

Published online by Cambridge University Press:  10 February 2011

P. A. Korzhavyi ,
I. A. Abrikosov  and
B. Johansson
P. A. Korzhavyi
Affiliation:
Condensed Matter Theory Group, Physics Department, Uppsala University, S–75121 Uppsala, Sweden
I. A. Abrikosov
Affiliation:
Condensed Matter Theory Group, Physics Department, Uppsala University, S–75121 Uppsala, Sweden
B. Johansson
Affiliation:
Condensed Matter Theory Group, Physics Department, Uppsala University, S–75121 Uppsala, Sweden
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Abstract

The formation energies of intrinsic point defects and the interaction energies of possible defect pairs in NiA1 are calculated from first principles within an order-N, locally self-consistent Green's function method in conjunction with the multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground-state properties of the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms in Ni-rich and Ni vacancies in Al-rich NiAl) are shown to form ordered structures in the ground state, in which the defects of the same kind tend to avoid each other at the shortest separation distance on their sublattice. A mean-field theory is applied to calculate the equilibrium concentrations of thermal defects. The statistics of thermal defects is interpreted in terms of dominant composition-conserving complex defects which are shown to be triple defects in Ni-rich and nearly stoichiometric NiAl. In the Al-rich region a novel thermal excitation dominates where two constitutional Ni vacancies are replaced by one antisite Al atom. The number of vacancies, as well as the total number of point defects decrease with temperature in Al-rich NiAl. The boundary between the two regions is treated analytically. The vacancy concentration exhibits a minimum in its temperature dependence at the boundary. Similar analysis is applied to study constitutional and thermal defects in Ni3Al. The calculated effective vacancy formation energy in Ni3Al as a function of concentration is in excellent agreement with recent experimental data.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 552: Symposium KK – High-Temperature Ordered Intermetallic Alloys VIII , 1998 , KK5.35.1
Copyright
Copyright © Materials Research Society 1999

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References

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