MECHANICAL MILLING OF ORDERED INTERMETALLIC COMPOUNDS - THE ROLE OF DEFECTS IN AMORPHIZATION

The role of defects in the amorphization of ordered intermetallic compounds by high energy ball milling was addressed. The structural evolution with milling time was studied experimentally in the intermetallic compounds Ni3Si and CoZr. The results of these studies were compared with previous work on Ni3Al and Nb3Sn. In these four compounds, the significant contributions to stored energy come from two major sources: (1) anti-site disorder DELTAG(disorder) and (2) grain boundary energy of nanoscale grains DELTAG(grain boundary). The occurrence, or not, of amorphization by ball milling was consistent with estimated values of free energy such that DELTAG(disorder) + DELTAG(grain boundary) greater-than-or-equal-to DELTAG(a-c) where DELTAG(a-c) is the difference in free energy between the ordered crystalline and amorphous phases. While the accuracy of the estimated DELTAG values is quite variable, the values are consistent with the results. For Ni3Si no amorphization was observed and DELTAG(disorder) + DELTAG(grain boundary) < DELTAG(a-c). In the cases of Ni3Al which exhibited partial amorphization, and Nb3Sn which was completely amorphized, DELTAG(disorder) + DELTAG(grain boundary) greater-than-or-equal-to DELTAG(a-c). For CoZr which was completely amorphized, both DELTAG(disorder) > DELTAG(a-c) and DELTAG(grain boundary) > DELTAG(a-c). Since no disordering of CoZr was observed experimentally, it is concluded that the nanoscale grain boundaries drive the amorphization reaction in ordered CoZr.