Unexpected field evaporation sequence in γ-TiAl: Interpreting field evaporation through dynamic bond-breaking processes

In atom probe tomography (APT), atoms from the surface of a needle shape specimen are evaporated under a high electric field and analyzed via time-of-flight mass spectrometry and position sensitive detection. 3D reconstruction of the atom positions follows a simple projection law, which can sometimes lead to artifacts due to deviation from an assumed ideal evaporation sequence. Here, we revisit the evaporation behavior of [001]-oriented γ -TiAl using a full-dynamics simulation approach empowered by molecular dynamics. Without any knowledge of charge states or assumptions about evaporation fields, we successfully reproduce the disrupted evaporation sequence observed in experiments, where Ti atoms are mostly reported to evaporate early. This effect has commonly been attributed to the lower evaporation field of Ti compared to Al. Instead, the seemingly counterintuitive “preferential” evaporation of the stronger-bonded Ti atoms can be explained by a two-step bond-breaking process where each bond-breaking step requires a smaller external force than that needed for Al for which evaporation happens by simultaneously breaking all bonds. On a more fundamental level, the determination of evaporation by the dynamic sequence of bond breaking – rather than by the sum of all bond energies, a measure that only applies when bonds break simultaneously – calls for a critical re-evaluation of the current models used to predict evaporation fields.