Disordering in the Ni-Al system under low dose ion-irradiation: a computer simulation study
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ABSTRACT Starting from the radiation damage cascades, as obtained using the binary collision approximation, we derive the spatial distribution of the energy deposited into the lattice by the primary knock-on atom (PKA). We follow the time evolution of the cascade core in two ordered intermetallics Ni 3AI and NiAl, using the molecular dynamics (MD) method with the embedded potential and with the liquid droplet model (LDM) in which a simplified version of the heat equation is solved. Moreover, the MD was used to determine the evolution of the local structure and to identify the disordered zones. The LDM allows us, after a calibration with MD results, to estimate the dependence of the molten volume on the PKA energy up to 1 MeV. The results show good agreement with other published simulations and the available transmission electron microscopy (TEM) observation experiments. INTRODUCTION The order -disorder transition under irradiation has been extensively studied for many years. It is well accepted that under irradiation, two contradictory processes compete for establishing a steady state configuration: i) chemical disordering imposed by replacement sequences and cascade mixing, ii) ordering promoted by thermally activated point defect jumps. In order to determine qualitatively the contribution of these two fundamental
characteristics of cascades in ordered alloys, several radiation experiments have been performed at different temperature regimes, in different ordered intermetallics using mainly the TEM as observation technique [1-4]. The experimental studies have showed that the radius of the observed disordered zones is proportional to the third root of the PKA energy at low irradiation doses and low temperatures. Computer simulations, especially those using the molecular dynamics (MD) method, have been successful in giving a clear description of the earlier stages of the cascade evolution and their possible link to phenomena like order-disorder or crystalline-amorphous transformations. Recently, Diaz de la Rubia et al. [5], have concluded from MD simulation of 5 keV cascades in Ni 3AI and Cu 3Au that the bulk of the disordered zone corresponds to the extend of the molten zone. In the same manner Gao and Bacon [6] have shown from cascade simulation with PKA energies up to 5 keV in Ni 3A1 that the regions containing 90% of the final antisite defects have approximately the same size as the melted regions at the beginning of the heat spike. They suggested that the disordered zone in a cascade may be taken at first approximation to be equal to the size of the maximum volume of the melt. Zhu and Averback [7], demonstrated in NiAl that the core of the cascade has high chemical disorder and does not relax back to the original structure after melting. The main goal of this work is to determine the effect of the deposited energy on the cascade size and shape after the cooling down of the thermal spike according to two models of 371 Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
computer simulations: M
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