Molecular dynamics study of displacement cascades in Ni₃Al II. Kinetics, disordering and atomic mixing

Abstract

The 76 displacement cascades in Ni₃Al at 100 K simulated in part I have been analysed to investigate the influence of the energy E _p of the primary knock-on atom on the kinetics of defect production, the degree of disorder and the atomic mixing of the different atomic species. The Ni atoms are more readily displaced during the ballistic phase, but they also return to lattice sites at a faster rate than do the Al atoms. The characteristic time of this recombination increases with increasing E _p, an effect related to the development of a highly defective ‘molten’ zone, which has a radius given by R _melt ≈ 3a_o E _p ^1/3. The final concentration of antisite defects in this region is 2–5% for most cascades, consistent with an integrated long-range order parameter of 0.85–0.9 over the same region, and the value of this parameter in the cascde centre falls to 0.5–0.7 for all E _p. The chemical short-range order is almost entirely lost at the peak in the number of ballistic displacements, particularly at high cascade energies, and in the final state it declines with increasing E _p. A high proportion of the atomic mixing occurs in the ballistic phase and is larger for the Ni atoms, implying that the phenomenon is not purely a liquid-like process. This conclusion is at variance with the 5 keV data of Diaz de la Rubia et al. obtained in 1992 and of Diaz de la Rubia, Caro and Spaczer obtained in 1993. The cascade thermal diffusivity given by a continuum treatment of Abromeit and Wollenberger in 1992 has been evaluated and is found to be approximately independent of E _p. The mechanisms of radiation-induced disordering are assessed, and the importance of ‘wrong’ recombination in the highly defective region produced in the thermal spike is emphasized. Replacement collision sequences play only a small part in disordering.