A model was proposed, for the nucleation of collapsed vacancy clusters in irradiated metals, that was based upon the principle that a vacancy loop could be nucleated in a cascade which had melted and recrystallized. The thermal conduction equation was solved by using a discretization method, and the initial temperature and vacancy distributions were given by a computer simulation. The model simulated heat propagation, local melting, absorption and release of latent heat, and redistribution of the density within the melt. The concentration of vacancies in the depleted zone increased under the influence of a temperature gradient. The simulation of hundreds of cascades furnished the distribution of zones as a function of the vacancy concentration and the number of vacancies. It was assumed that critical values of these parameters had to be exceeded in order to produce a visible vacancy loop. However, if the concentration exceeded a certain value under sufficiently fast cooling (strong electron-phonon coupling), the melted zone could not crystallize completely and instead solidified into a semi-amorphous core. This prevented collapse into a vacancy loop. The model was used to calculate the yield and mean site of vacancy loops in ion-bombarded samples of the above materials. It was found that physically reasonable values of the various critical parameters gave a good agreement with experimentally determined values of yield and size. Moreover, the dependence upon alloy content could be explained and it was noted that electron-phonon coupling could have a marked effect upon loop yields.

V.G.Kapinos, D.J.Bacon: Physical Review B, 1995, 52[6], 4029-43