Phase formation in Zr-Fe multilayers: Effect of irradiation

We have conducted a detailed in situ study of phase formation in Zr-Fe meta llic multilayers using irradiation and thermal annealing. Metallic multilay ers with near equiatomic and Fe-rich overall compositions and with repetiti on thicknesses ranging from 7.4 to 33 nm were either irradiated with 300 ke V 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 s amples, irradiation causes complete amorphization. The dose to amorphizatio n increases in proportion to the square of the wavelength, indicating a pro cess controlled by atomic transport. Amorphization was also achieved by 900 keV electron irradiation at 25 K showing that displacement cascades are no t required. The critical dose to amorphization was independent of temperatu re below room temperature and decreased above room temperature. The activat ion energy for this second process is 0.17 eV. For the temperature range st udied, diffraction from Zr disappears first, indicating that amorphization takes place in the Zr layer by atomic transport of Fe from the Fe layers. T hese results are consistent with a combination of simple ballistic mixing a t low temperature and either simple diffusion or radiation-enhanced diffusi on at higher temperatures. Thermal annealing of the equiatomic samples at 7 73 K produced the same reaction products with slower kinetics. Ion irradiat ion of Fe-rich samples did not cause complete amorphization and intermetall ic compounds Zr3Fe and ZrFe2 were observed in longer wavelength samples. Am orphization 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 proc ess that depends on more than atomic transport. Thermal annealing at 773 K of a long wavelength, 57% Fe sample resulted in intermetallic compounds Zr3 Fe and ZrFe2 which amorphized during subsequent irradiation. The ease of am orphization of equiatomic samples relative to Fe-rich samples can be explai ned by a narrower, single minimum free energy curve for the amorphous phase . (C) 1999 American Institute of Physics. [S0021-8979(99)01610-2].