Phase formation in Zr-Fe multilayers : effect of irradiation

Abstract

We have conducted a detailed in situ study of phase formation in Zr–Fe metallic multilayers using irradiation and thermal annealing. Metallic multilayers with near equiatomic and Fe-rich overall compositions and with repetition thicknesses ranging from 7.4 to 33 nm were either irradiated with 300 keV Kr ions at various temperatures (from 17 to 623 K) or thermally annealed at 773 K while being observed in situ. The kinetics of multilayer reaction were monitored by following the diffraction patterns. For near equiatomic samples, irradiation causes complete amorphization. The dose to amorphization increases in proportion to the square of the wavelength, indicating a process controlled by atomic transport. Amorphization was also achieved by 900 keV electron irradiation at 25 K showing that displacement cascades are not required. The critical dose to amorphization was independent of temperature below room temperature and decreased above room temperature. The activation energy for this second process is 0.17 eV. For the temperature range studied, diffraction from Zr disappears first, indicating that amorphization takes place in the Zr layer by atomic transport of Fe from the Fe layers. These results are consistent with a combination of simple ballistic mixing at low temperature and either simple diffusion or radiation-enhanced diffusion at higher temperatures. Thermal annealing of the equiatomic samples at 773 K produced the same reaction products with slower kinetics. Ion irradiation of Fe-rich samples did not cause complete amorphization and intermetallic compounds Zr3Fe and ZrFe2 were observed in longer wavelength samples. Amorphization of Fe-rich samples was more sluggish, likely because there was competition with formation of other phases. The reaction kinetics were not proportional to square of wavelength for Fe-rich samples, indicating a process that depends on more than atomic transport. Thermal annealing at 773 K of a long wavelength, 57% Fe sample resulted in intermetallic compounds Zr3Fe and ZrFe2 which amorphized during subsequent irradiation. The ease of amorphization of equiatomic samples relative to Fe-rich samples can be explained by a narrower, single minimum free energy curve for the amorphous phase.