Crystallization of Ni60Ta40 and Ni60Nb40 metallic glasses at temperatures well below their respective calorimetric glass transition temperatures has been studied by differential scanning calorimetry, nanodiffraction, and electron microscopies. Nanometer-scaled metastable FCC phases were found to grow out from the glass structures. The Z-contrast images reveal that the nanocrystals are enriched in Ta or Nb in comparison with the surrounded amorphous structures. The experimental evidence is suggestive of the decoupled cooperative diffusion of Ni atoms in these fragile glasses.