Electron-irradiation induced creep in amorphous alloys

Electron-irradiation induced creep rates in amorphous alloys, a-${\mathrm{SiO}}_{2}, {\mathrm{Fe}}_{79}{\mathrm{B}}_{16}{\mathrm{Si}}_{5}, {\mathrm{Cu}}_{60}{\mathrm{Ta}}_{40}$, and ${\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$, were measured at room temperature using a miniaturized beam-bending apparatus within a transmission electron microscope operated at 200 keV. The creep rates of these amorphous samples increased nearly linearly with both e-beam current density and applied stress, while a reference crystalline (c-)${\mathrm{SiO}}_{2}$ sample failed to creep under the same conditions. The irradiation-induced creep compliance of a-${\mathrm{SiO}}_{2}$ was $\ensuremath{\sim}15$ times larger than that of ${\mathrm{Fe}}_{79}{\mathrm{B}}_{16}{\mathrm{Si}}_{5}$ and over 1,000 times larger than that of the two Cu alloys. Molecular dynamics computer simulations were employed to simulate electron irradiation induced creep using interatomic potentials representing amorphous ${\mathrm{Cu}}_{75}{\mathrm{Zr}}_{25}, {\mathrm{Ni}}_{85}{\mathrm{P}}_{15}$, and ${\mathrm{SiO}}_{2}$ as model systems. The irradiation induced creep compliances calculated for ${\mathrm{Cu}}_{75}{\mathrm{Zr}}_{25}$ during 200 keV electron irradiation provided good quantitative agreement with the two Cu-based alloys, but that for a-${\mathrm{SiO}}_{2}$ was $\ensuremath{\sim}180$ times too small. These results indicate that unlike neutron or ion-beam induced creep in a-${\mathrm{SiO}}_{2}$, creep under electron irradiation is dominated by the effects of ionization, owing largely to the far higher ratio of electronic stopping to nuclear stopping for electrons than for ions.