Analysis of the evaporation and re-condensation processes induced by pulsed beam treatments

A physical model is proposed to describe the evaporation phenomena taking place at the surface of pulsed energetic beam treated samples. Samples of a NiTi alloy and a TiAl based alloy were subjected to high current pulsed electron beam (HCPEB) treatment for an increasing number of irradiation (5, 10 and 20 pulses). The results of their surface chemistry analysis have been confronted to the model. For the sample treated with 5 pulses, the experimental composition profiles revealed a selective evaporation of Ni over Ti in the NiTi alloy and Al over Ti, Cr, Nb in the TiAl based alloy, which is nicely reproduced by the simulated composition profiles. As the number of pulses increases, the results show that a part of the vapour re-condensed onto the surface. This leads to an inversion of the composition profile at the top surface of the samples, which cannot be predicted by the theoretical model. This cyclic process of evaporation and recondensation leads to the cyclic variation of compositions in the surface layer.