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

The role of defects in the amorphization of ordered intermetallic comp ounds by high energy ball milling was addressed. The structural evolut ion with milling time was studied experimentally in the intermetallic compounds Ni3Si and CoZr. The results of these studies were compared w ith previous work on Ni3Al and Nb3Sn. In these four compounds, the sig nificant contributions to stored energy come from two major sources: ( 1) anti-site disorder DELTAG(disorder) and (2) grain boundary energy o f 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) greate r-than-or-equal-to DELTAG(a-c) where DELTAG(a-c) is the difference in free energy between the ordered crystalline and amorphous phases. Whil e the accuracy of the estimated DELTAG values is quite variable, the v alues 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 boun dary) greater-than-or-equal-to DELTAG(a-c). For CoZr which was complet ely amorphized, both DELTAG(disorder) > DELTAG(a-c) and DELTAG(grain b oundary) > DELTAG(a-c). Since no disordering of CoZr was observed expe rimentally, it is concluded that the nanoscale grain boundaries drive the amorphization reaction in ordered CoZr.