THE ROLE OF MELTING TEMPERATURE AND ELECTRON-PHONON COUPLING IN THE FORMATION OF CLUSTERED VACANCY DEFECTS FROM HEAVY-ION-GENERATED DISPLACEMENT CASCADES

The production of clustered vacancy defects (dislocation loops and sta cking fault tetrahedra) from heavy-ion-generated displacement cascades has been investigated by transmission electron microscopy in a series of Cu Ni and Ag-Pd alloys. The density of defects decreases as the so lute content of the alloy increases, but not in a simple manner. These results are interpreted in terms of changes in the lifetime of the th ermal spike or molten zone generated within the cascade. The factors w hich have been considered to affect the lifetime are the melting tempe rature, the degree of coupling between the electron and phonon systems , and the influence of solutes on the character of the molten zone. A comparison with stacking-fault energy data has also been made. It is d emonstrated that the results in the Ag-Pd correlate approximately with changes in the alloy melting temperature and stacking-fault energy, w hereas in Cu-Ni the results correlate with the change in the strength of electron-phonon coupling. Further tests of these models are made by examining previously published data from Cu alloys and Ni-Cr alloys. The Cu alloys are not affected by electron-phonon coupling and the cha nges observed appear to reflect the changes in melting temperature and solute effects. In the Ni-Cr system the density of states is large an d constant and the decrease in defect yield appears to reflect the cha nge in the strength of electron-phonon coupling as solute is added.