AP-FIM STUDY ON THE MICROSTRUCTURES OF TiAl INTERMETALLIC COMPOUNDS FXBRICATED BY INGOT METALLURGY AND CENTRIFUGAL SPRAY DEPOSITION

The microstructures of TiAl intermetallic compounds prepared by ingot metallurgy and centrifugal spray deposition (CSD) were compared by field ion microscopy and atom probe (AP-FIM). The samples used have nearly stoichiometric composition of TiAl but there is Mn addition for ingot metallurgy (IM) samples and Mn and Nb additions for CSD samples. The field ion image of TiAl showed two regions of ordered f.c.t crystal structure (γ-TiAl) and disordered α-Ti3Al, and the distribution of Ti and Al atoms in TiAl alloy showed a lamellar structure which was confirmed by AP profiles. AP profiles showed that the major constituent of IM TiAl alloy is γ-TiAl with some Ti-rich regions. The compositions of these Ti-rich regions are nearly Ti3Al and Ti2Al from the calculation of AP data. The sample of CSD has a similar microstructure as compared with IM sample. The effects of alloying elements (Mn and Nb) on the ordering extent ,their distribution, and their sites occupied in the lattice were explored for both TiAl alloys. But there are no obvious Ti3Al or Ti2Al phases as shown in IM sample. In addition, the distributions of Mn and Nb concentrations in the CSD sample have a characteristic of wave form.