The microstructural changes and phase decomposition at the surface of an ultra-precision machined Zn-Al based alloy were studied using optical microscopy, back-scattered electron microscopy, electron back-scattered diffraction and X-ray diffraction techniques. It was found that with increasing DoC the phase decomposition was accelerated and further increasing DoC might cause a microstructural recovery on the surface of the UPM specimen. UPM induced phase decomposition was discussed in relation to the structural evolution of the chips. (c) 2007 Elsevier B.V All rights reserved.