This study highlights the capability of high-energy synchrotron X-rays to investigate polymorphous phase transformation upon nanostructuring and diffusion bonding of Al and Mg through high-pressure torsion. The measurements providing diffraction peak profiles at a series of local positions over the sample volume allow mapping of gradual yet significant structural changes in the Al-Mg nanocrystalline alloy. The diffraction peaks involve apparent grain refinement broadening and compositional broadening with increasing shear strain. The results reveal strain-dependent transformations between f.c.c and h.c.p. phases with a sharp dissolution at strains of ���2500 and subsequent homogenization towards the formation of an Al supersaturated solid solution with the Mg concentration of ���14���15 at.%.