Search for nucleation of phase embryos in binary alloys by impurity atoms

A search was made to detect the possibility that individual impurity atoms in two-phase, binary alloys nucleate embryonic crystals. The alloy system s tudied was Ni-Al and the impurities were In-111 probe atoms. Local surround ings of the probes characteristic of the crystal phases were detected throu gh quadrupole interactions with Cd daughter nuclei using the method of pert urbed angular correlation of gamma rays. Analysis of site fractions of the probes led to two alternative interpretations that could not be distinguish ed on the basis of the present measurements: 1.) In-111 probes in the two-p hase domain between Ni2Al3 and NiAl segregate to Ni2Al3 with a segregation energy of about 0.10 eV; or 2.) In-111 probes nucleate embryonic crystals o f Ni2Al3, with the driving force for nucleation being an attraction between In atoms and Ni vacancies that is known to be strong in NiAl.