The anomalous contraction on disordering Ni3Si

Zhou and Bakker (Acta Metall Mater 1994; 42:3009) reported the formation of progressively disordered L12 and also a distinct disordered fcc solid solution on ball-milling Ni3Si with the ordered L12 structure. The formation of disordered phases was accompanied by a decrease in lattice parameter, 0.53% for partially disordered L12 at a 1ro parameter of ∼0.56, and as much as 2.44% for the disordered fcc. These contractions were attributed to antisite defects. This behaviour is at variance with the generally observed small expansion on such disordering. Further, of the several L12 structures disordered by mechanical means, this is the only observation of the co-existence of a disordered L12 and a distinct disordered fcc phase. These results have been re-interpreted to show that the diffraction peak identified by them as characteristic of fcc in fact belongs to hexagonal Ni31Si12 phase, thus accounting for the major anomaly. The small decrease on partially disordering L12 is possibly due to an increase in the valence of Si.