On the structure and chemistry of Ni₃Al on an atomic scale via atom-probe field-ion microscopy

The atom-probe field-ion microscope (APFIM) is employed to study the structure and chemistry of boron-doped Ni₃Al on an atomic scale. In this study annealed melt-extracted wire specimens were analyzed using time-of-flight mass spectroscopy along the 〈100〉 direction exposing the 100 fundamental and superlattice planes. Not only is the depth resolution equal to the interplanar spacing of 0.18 nm, but the transitions between these planes are unambiguously identified by characteristic changes in the field-evaporation rate. The identification of the plane transitions allows, for the first time, to precisely count the number of detected atoms per plane in this material. The extent of the interruption associated with the transition from a pure nickel plane to a mixed nickel-aluminum plane is not significantly different from the reverse transition. From the small number of Al atoms encountered in the supposed pure Ni planes and the symmetry of the cubic system, it is inferred that variations in the measured composition of the mixed planes are not a result of actual composition fluctuations in this alloy, as has been previously argued.