The structural evolution of a mixture of elemental Ni and Ru powders during high energy ball milling has been studied as a function of milling time using X-ray diffraction and transmission electron microscopy. On the Ni-rich side of the phase diagram (Ni70Ru30) we observe, during the milling, the diffusion of Ru in the Ni and an expansion of the Ni fcc lattice. After about 20 hours of milling, all the Ru has diffused into the Ni and we are left with an oversaturated Ni(Ru) fcc structure with a lattice parameter about 3.6% larger than the one of pure Ni. The Ni does not seem to diffuse into Ru during the process. On the Ru-rich side (Ni43Ru57) we observe initially the same diffusion process taking place but more rapidly. After about 8 hours of milling, the incorporation of Ru in the Ni has driven the fcc Ni lattice to its limit of stability (about 3.9% expansion) and the fcc structure transforms, upon further milling, into a new hcp structure. In the later stage of the process the remaining Ru diffuses into this new phase.