Metallic crystalline/amorphous core-shell nanoparticles consisting of a crystalline Pd core (c-Pd) surrounded by an amorphous Fe25Sc75 shell (a-FeSc) are prepared by inert-gas condensation. A phase transformation of the c-Pd by a solid-state diffusion process resulting in an amorphous core (a-PdSc) surrounded by an amorphous FeSc shell is observed if the core-shell structure is irradiated at ambient temperature with 300 keV electrons. The amorphization process seems to involve the diffusion of irradiation-induced defects and is presumably driven by the large negative heat of mixing of Pd and Sc, as well as by the excess enthalpy of the interfaces between the c-Pd regions and the surrounding a-FeSc. The structural transformation reported here opens a new way to producing metallic amorphous core-shell nanoparticles of different chemical compositions and probably novel properties.