Phase decomposition induced by ultra-precision raster milling (UPRM) and the surface properties of a furnace-cooled Zn–Al-based alloy were studied using a range of methods, including backscattered electron microscopy, X-ray diffraction and nanoindentation. It was found that UPRM resulted in plastic deformation and phase decomposition on the surface layer of the alloy at a thickness of about 250 nm. The effects of the feeding rate of the UPRM on phase transformation and surface properties are discussed.