Irradiation effects in Cu/W multilayers: Ion beam mixing and structural evolution

Irradiation effects in the immiscible Cu/W system are investigated for heli um 70 keV and krypton 340 keV irradiations at liquid-nitrogen temperature. Evolution of the [(3.1 nm Cu/4.2 nm W)x12] multilayer was characterized by x-ray diffraction measurements. Limits of solubility were observed in both matrixes for both irradiations. As expected, a strong thermal spike effect is evidenced in copper layers under krypton irradiation where almost no sol ubility is observed. For helium irradiations very similar limits of solubil ity are observed in both matrixes. These latter results can be analyzed by a model with two atomic fluxes: one due to ballistic displacement and the o ther due to chemically driven thermally activated transport. A structural s tudy shows a transition to an amorphous state for heavy ion irradiation in the large cascades above a critical solubility threshold. (C) 1999 American Institute of Physics. [S0021-8979(99)01821-6].