The site preference of Cu in Ni3Al with a composition of Ni75Al23Cu2 has been determined by atom-probe field ion microscopy (APFIM). The sample was analyzed in the three major low index planes with atomic layer resolution. From the data chain obtained in the  direction the site occupancy of Cu atoms was determined by analyzing whether they were evaporated from the NiAl mixed planes or the Ni pure planes. It has been determined from an extensive number of quantitative data that the fraction of the Cu atoms in the Ni sites of the Ni75Al23Cu2 intermetallic compound is 94-10+6%. The present finding substantiates the recently proposed mechanism which explain the ductilization of Ni3Al by adding a few atomic percent of transition elements, such as Pd and Cu.
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