Thermal-spike treatment of ion-induced grain growth: Theory and experimental comparison

Grain growth commonly observed during heavy-ion irradiation of initially fine-grained (≤100 Å diameter) thin films is modeled as a thermal-spike phenomenon in which temperature spikes caused by ions and recoils induce atomic jumps across grain boundaries, promoting boundary migration. In elemental and homogeneous alloy systems, in which grain growth is driven solely by the reduction of boundary surface area, the model predicts that the ion-induced grain-boundary mobility is linearly proportional to the quantity, ${\mathit{F}}_{\mathit{D}}^2$/Δ${\mathit{H}}_{\mathrm{coh}}^3$, in which ${\mathit{F}}_{\mathit{D}}$ is the ion and recoil energy deposited in elastic collisions and Δ${\mathit{H}}_{\mathrm{coh}}$ is the cohesive energy of the target. The model was evaluated with respect to data from two previously published ion-induced grain-growth experiments on elemental and coevaporated alloy films. The results were consistent with the thermal-spike model. Combining analytical results of the model with the experimental data it was possible to determine the value of the proportionality constant β relating the cohesive energy to the activation energy Q for grain growth (Q=-βΔ${\mathit{H}}_{\mathrm{coh}}$). The value of β for the coevaporated and elemental films, respectively, was 0.07 and 0.15, which is less than or about equal to the value previously determined for the thermal-spike treatment of ion beam mixing (${\mathrm{\beta }}_{\mathrm{IM}}$=0.14). The smaller value of β determined for the coevaporated films is consistent with the idea that atom migration across grain boundaries is easier than migration within the lattice. The thermal-spike treatment was also applied to ion-induced grain growth in multilayer films. The presence of concentration gradients in these systems adds another driving force affecting grain growth. In addition, the influence of the heat of mixing (Δ${\mathit{H}}_{\mathrm{mix}}$) on atomic mobility and boundary migration was incorporated in the model via a Darken effect.