Many metallic alloys contain nanoparticles created by solidstate precipitation and, to date, these nanoparticles show simple chemical structures (either homogenous or core-shell). Creating more-complex nanoparticles in metallic alloys (e.g., particles with a core surrounded by multiple concentric shells) would be interesting for two reasons. First, this would represent a proof of concept of a generic, solid-state approach based on a simple heat treatment, applicable to a large variety of metallic and nonmetallic systems with particular characteristics (i.e., a matrix with solubility for solute elements varying with temperature whose diffusion kinetics are mismatched). [1,2] Second, in the field of structural metallic alloys, such complex, tailored nanoparticles can better fulfill the often contradictory requirements for particle stiffness, lattice-parameter mismatch, and shearability, which control the elastic interactions of nanoparticles with dislocations (and hence the alloy's strength) and their coarsening resistance via intrinsic diffusivities and interfacial free energies (and hence the alloy's aging resistance). [3][4][5][6] While core/multishell nanoparticles have been synthesized in liquid suspensions, [7][8][9][10][11] they have never been created within a solid matrix to the best of our knowledge.