STRUCTURAL EVOLUTION IN NB3SN DURING MECHANICAL ATTRITION

Amorphization by mechanical attrition was observed for the Nb3Sn alloy composition on milling of Nb + Sn, Nb + NbSn2 or Nb3Sn crystalline powders. Mechanical alloying occurred first for Nb + Sn or Nb + NbSn2 powder mixtures such that the A15 structure Nb3Sn phase was synthesized. With continued milling, this phase transformed to the amorphous structure. Complete disordering of the ordered A15 Nb3Sn phase was found at milling times (about 1 h) well before amorphization was first observed (5-6 h). The estimated energy differences between the crystalline and amorphous Nb3Sn phases suggest that the driving force for the crystalline-to-amorphous transformation comes from a sum of the disordering energy and the grain boundary energy of the nanocrystalline grain structure that develops during milling. The hardness of hard brittle A15 Nb3Sn increases by about 20% before amorphization occurs. This hardness increase can be attributed to the refinement of the grain structure, and the data fit a Hall-Petch relationship. That plastic deformation can occur in nominally brittle Nb3Sn must be the result of a large hydrostatic component of the stress state in the powders during ball milling.