Nucleation of embryos of a second phase by individual impurity atoms

Perturbed angular correlation of gamma rays (PAC) was used to monitor the atomic environments of indium atoms in Ni–Al alloys at temperatures and compositions in the two-phase field between Ni2Al3 and NiAl. In earlier PAC measurements at room temperature, the fraction of signal that is characteristic of the Ni2Al3 phase greatly exceeded the fraction predicted by the lever-rule, and it was suggested that indium could nucleate small embryos of the Ni2Al3 phase in the NiAl single-phase field. Measurements at elevated temperatures are now reported. PAC signals from the Ni2Al3 phase are observed to extend into the NiAl single-phase field from 0.5 to 1.0 at.% beyond published phase boundaries that were established by experiments on samples prepared similarly to those in this study. Thus, the new observations support the hypothesis that individual solute atoms can nucleate small crystals of a second phase within a single-phase field. In addition, phase boundaries were calculated from the PAC data, assuming that the lever rule was obeyed, and compared with boundaries from the literature.