Threshold displacement and interstitial-atom formation energies in Ni 3 Al
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R. S. Averback Department of Materials Science and Engineering, University of Illinois at Urbana, Illinois 61801
Urbana-Champaign,
(Received 7 December 1989; accepted 20 March 1990)
Threshold displacement energies for atomic displacements along (110), (100), and (111) directions, and formation enthalpies of several symmetric interstitial atom configurations were calculated for Ni3Al by computer simulation using "embedded atom method" potentials. The Ni-Ni (100) dumbbell in the plane containing only Ni atoms has the lowest interstitial-atom enthalpy although the enthalpies of other configurations are similar. Interstitial configurations involving Al atoms all have much higher enthalpies. The anisotropy of the threshold energies in Ni3Al is similar to pure metals and no significant difference in threshold energy was observed for (110) replacement chains in rows containing all Ni atoms or alternating Ni-Al atoms. Various metastable interstitial atom configurations were observed, including crowd-ions. In addition, the spontaneous recombination volume for some configurations can be much smaller than in pure metals. The consequences of these results for radiation induced segregation and amorphization are discussed.
I. INTRODUCTION
Computer simulation has been found to be a powerful method for investigating the mechanisms of defect production1'2 and for calculating point defect properties.3 Although these studies have been performed mostly on pure metals, advances in computing capabilities and the development of realistic, yet tractable, interatomic potentials now make it possible to study a number of alloy systems as well.4 We have begun such studies and present here calculations of the threshold energy for Frenkel pair production and the formation enthalpy of interstitial-type defects in the ordered alloy, Ni3Al. Ni3Al is of interest for several reasons. First, Ni3Al provides an interesting ordered structure (Ll2) for threshold energy investigations since along some close packed directions (110) the atoms are all Ni whereas along others, they are alternating Ni and Al. The effect of the mass mismatch for the propagation of RCSs is not known although it was considered briefly by Vineyard and co-workers years ago.5 Replacement sequences along other close packed directions, (100), contain pure Al and pure Ni chains. The point defect properties in Ni3Al are also of interest since the high ordering energy may influence point defect configurations, the spontaneous recombination volume, and point defect diffusion mechanisms. Finally, reliable a)
Fellow of the Consejo Nacional de Investigaciones Cientificas y Tecnicas Argentina. J. Mater. Res., Vol. 5, No. 7 Jul 1990
http://journals.cambridge.org
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"embedded atom method" (EAM) potentials are available for the Ni-Al system.6 Vacancy properties in Ni3Al were studied previously,6 so we restrict our investigation here to the interstitial-atom properties. One of the motivations for undertaking this investigation was the finding that radiation-induced s
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