Ion implantation of N-2(+) at 100 keV into metallic Al has been used to nucleate and grow nanocrystals of insulating AlN within the Al matrix. The AlN-nanocrystals are well aligned to the Al host lattice. With increasing N+ dose, the formation of a homogeneous layer of AlN is observed. The diffusion and growth process proceeds at temperatures below 70 degreesC (essentially at ambient temperature) and is enabled by a strain-enhanced diffusion of N into Al. The AlN-nanocrystals form rapidly within metallic Al, but not within the more stable Al2O3 phase.