Transformations in primary crystallites in (Fe,Ni)-based metallic glasses

Fully amorphous melt-spun alloys of compositions in the series Fe-Ni-B and Fe-Ni-Zr-B are annealed to achieve dispersions of nanocrystals in a residual amorphous matrix. The conditions for forming bcc (alpha) or ccp (gamma) phases are investigated. Nanoindentation reveals that the partial devitrification may or may not suppress evident shear-banding, depending on crystallite size. The martensitic transformation from gamma to alpha phase does not occur in isolated nanocrystals. When the alloys are fully crystallized, however, the transformation does occur spontaneously on cooling, but at onset temperatures greatly reduced by the refinement of the grain size. The implications of the results for optimization of the mechanical properties of metallic glasses are considered.