MICROCHEMICAL AND MICROSTRUCTURAL INVESTIGATIONS ON 2-PHASE INTERMETALLIC TIAL/TI3AL ALLOYS BY FIELD-ION MICROSCOPY AND ATOM PROBE (APFIM)

The microstructure and the chemical compositions of the coexisting TiAl and Al3Ti phases in as-cast Ti-48at%Al and Ti-46at%Al-1at%Cr-0.2at%Si alloys are investigated by field ion microscopy and supported atom probe microanalysis (APFIM). The alloys exhibit a fine-scaled lamellar microstructure. gamma-TiAl possesses a high degree of ordering, whereas alpha(2)-Ti3Al is off-stoichiometric. The alpha(2)/gamma interfaces and gamma/gamma(twin) twin relations are studied in detail. The true twin boundaries gamma/gamma(twin)T in gamma-TiAl lamellae are mono-phased, whereas the pseudo-twin boundaries consist of a layered gamma/alpha(2)/gamma(twin)P structure. Antiphase boundaries (APBs) are present in conventionally cast alloys containing chromium. Additions of Cr decrease the APB energy of TiAl and raise the Al concentration in the Ti3Al phase.